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Absolute

CHEMISTRY Vol. 1.2

NEET – UG & JEE (Main)

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PREFACE Target’s “Absolute Chemistry Vol - 1.2” is compiled according to the notified Std. XI syllabus for NEET-UG & JEE (Main). The content of this book is framed after reviewing various state syllabi as well as the ones prepared by CBSE, NCERT and COBSE. The sections of Theory, Quick Review, Formulae, MCQs and Topic Test form the backbone of every chapter and ensure adequate revision. These MCQs are framed considering the importance given to every topic as per the NEET-UG & JEE (Main) exam. They are a healthy mix of theoretical, numerical, multi-step reactions and graphical based questions. The level of difficulty of these questions is at par with that of various competitive examinations like AIIMS, CPMT, JEE, NEET-UG, TS-EAMCET (Med. and Engg.), BCECE, Assam CEE, AP EAMCET (Med. and Engg.) & the likes. Also to keep students updated, questions from most recent examinations such as NEET-UG MHT CET, KCET, WB JEE, JEE (Main), of years 2016 to 2018 are exclusively covered. In the development of each chapter, we have ensured the inclusion of important points that are enclosed within a box. These topics will enhance students’ knowledge and provide them an edge to score better in the highly competitive exams. We are confident that this book will cater to needs of students across a varied background and effectively assist them to achieve their goal. We welcome readers’ comments and suggestions which will enable us to refine and enrich this book further. Please write to us at: mail@targetpublications.org

All the best to all Aspirants! Yours faithfully, Authors Edition: Second

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No. Topic Name Page No.

8 Redox Reactions 1

9 Hydrogen 38

10 s-Block Elements (Alkali and Alkaline Earth Metals) 84

11 Some p-Block Elements 138

12 Organic Chemistry - Some Basic Principles and Techniques 205

13 Alkanes 311

14 Alkenes 354

15 Alkynes 398

16 Aromatic Hydrocarbons 429

17 Environmental Chemistry 475

'Chapters 1 to 7 are a part of Absolute Chemistry Vol - 1.1'

Index

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

Alkynes: i. Alkynes are aliphatic unsaturated hydrocarbons containing at least one carbon-carbon triple (C C)

bond in their structure. ii. They contain four hydrogen atoms less than the corresponding alkanes and two hydrogen atoms less than

the corresponding alkenes. iii. General formula of alkynes is CnH2n 2 (where, n is an integer). iv. The carbon-carbon triple bond is also called acetylene linkage and hence alkynes are commonly called as

acetylenes. Eg. HC CH (Acetylene or Ethyne) ; H3C C CH (Propyne).

Common system: This naming system is not much in use and does not have any specific relation with the compounds named.

Except for acetylene, the common names of the other alkynes are no longer used.

Formula (CnH2n – 2) Common name C2H2 (HC CH) Acetylene C3H4 (CH3C CH) Allylene C4H6 (CH3C CCH3) Crotonylene

Derived system: In this system, alkynes are named as derivatives of acetylene, the first member of the alkyne series. IUPAC system: Following rules are used to obtain the IUPAC names of the alkynes: i. Select the longest and continuous chain of carbon atoms containing the carbon-carbon triple bond. ii. Number the carbon atoms from that end which is nearest to the triple bond. iii. The name of the alkyne is derived by replacing suffix ‘ane’ from the name of the parent alkane by ‘yne’. iv. The locant indicating the position of triple bond is placed immediately before the suffix ‘yne’. v. The position of the branches (alkyl groups) is indicated by proper numbers. vi. Compounds with more than one triple bond are called diynes (two), triynes (three) and so forth. Common names and IUPAC names of some of the alkynes:

No. Alkyne Derived name IUPAC name i. HC CH Acetylene Ethyne ii. CH3 C CH Methyl acetylene Propyne iii. CH3 CH2 C CH Ethyl acetylene But-1-yne

15 Alkynes

15.0 Alkynes: Introduction 15.1 Nomenclature of alkynes ** 15.2 Structure of triple bond ** marked section is for NEET-UG

15.3 Isomerism in alkynes 15.4 Methods of preparation of alkynes 15.5 Physical properties of alkynes 15.6 Chemical properties of alkynes

Alkynes: Introduction15.0

Nomenclature of alkynes 15.1

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Chapter 15: Alkynes

CH3 CH C CH CH3

iv. CH3 C C CH3 Dimethyl acetylene But-2-yne v. CH3 CH2 C C CH3 Ethyl methyl acetylene Pent-2-yne

vi.

Isopropyl acetylene 3-Methylbut-1-yne

vii. n-Propyl acetylene Pent-1-yne

viii. n-Butyl acetylene Hex-1-yne

ix. Methyl n-propyl acetylene Hex-2-yne

x. CH3 – CH2 – C C – CH2 – CH3 Diethyl acetylene Hex-3-yne Note: If the compound contains double bond as well as triple bond then preference is given to the double bond. Eg. CH2 = CH C CH But-1-en-3-yne Structure of triple bond: i. Ethyne is the simplest molecule of alkyne series. ii. In ethyne, each carbon atom is sp hybridized forming two sp-hybrid orbitals. iii. These two sp-hybrid orbitals lie along a straight line and the bond angle between them is 180. iv. The two unhybridized 2p-orbitals are perpendicular to each other and also to the axis of the hybridized

orbitals. v. In the triple bond formation, one sp-hybridized orbital of one carbon atom overlaps axially (head on) with

the similar sp-hybrid orbital of the other carbon atom to form -bond. vi. Each of the two unhybridized orbitals of one carbon overlaps sidewise with the similar orbital of the other

carbon atom to form two weak -bonds. vii. The remaining sp-hybrid orbitals of each carbon atom overlaps with 1s-orbital of hydrogen to form

CH bond. The structure of the triple bond is illustrated by the orbital structure of ethyne as shown in the figure.

viii. Thus, carbon-carbon triple bond is made up of one -bond and two -bonds. If one -electron cloud is

visualized as lying above and below the internuclear axis representing -bond, the other -electron cloud lies in front and behind the line. The overall orbital picture of the molecule is, that of a cylindrical electron cloud symmetrically distributed around the internuclear axis.

CH3 2 2CH C CH

CH3 2 3CH C CH

CH3 2 2CH C C – CH3

Structure of triple bond15.2

Unhybridizedorbitals

C

2p 2p

-bond

-bond

-bond -bond sp

1s

C

2p2p -bond

H sp H sp

1s

sp

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

ix. The electron diffraction studies have shown that, C C bond length is 120 pm and CH bond length is 106 pm, the HC C bond angle is 180 and the strength of the C C bond is 823 kJ mol1.

x. Electronic structure or Lewis electron dot formula of acetylene is given as follows: Where, ‘’ represents electron of carbon, ‘’ represents electron of hydrogen Note: i. The C C bond is stronger than C = C and C C bond since the electron cloud between two carbon atoms

is cylindrically symmetrical about the internuclear axis. ii. The strength of the triple bond in alkyne (823 kJ mol–1) is more than that of the double bond (681 kJ mol–1)

in alkene and C – C single bond (348 kJ mol–1) in alkanes. Isomerism: Alkynes exhibit the following structural isomerism: i. Chain isomerism: It is due to the difference in the type of carbon skeleton. These isomers differ in the

chain of carbon atoms. Eg. The molecule having molecular formula C5H8 shows the following chain isomers: ii. Position isomerism: It is due to the difference in the position of triple bond in the carbon chain. Alkynes

having four or more than four carbon atoms show position isomerism. Eg. a. b. iii. Functional isomerism: It is exhibited by alkynes and dienes, which have the same molecular formula.

Alkynes contain one triple bond while dienes contain two double bonds. Eg. a. b.

Pent-1-yne

CH3 CH2 CH2 C CH 5 4 2 1 3

3-Methylbut-1-yne

4 3 2 1 CH3 CH C CH

CH3

H C C H 120 pm 106 pm

180

Isomerism in alkynes15.3

HC C H

But-2-yne

4 3 2 1 CH3 C C CH3 CH3 CH2 C CH

But-1-yne

4 3 2 1

CH3 CH2 CH2 – C CH Pent-1-yne

5 4 3 2 1 CH3 CH2 C C – CH3

Pent-2-yne

5 4 3 2 1

CH3 CH2 C CH But-1-yne

H2C = CH CH = CH2

Buta-1,3-diene

CH3 C CH Propyne

H2C = C = CH2

Propa-1,2-diene(Allene)

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iv. Ring chain isomerism: It is exhibited by alkynes and cycloalkenes, which have the same molecular formula.

Eg. a. b. Note: Alkynes do not exhibit geometrical isomerism. Action of water on calcium carbide: Reactants : Water and calcium carbide Products : Acetylene (alkyne) and calcium hydroxide Reaction :

Note: i. Calcium carbide required for this purpose is obtained by heating calcium oxide (from limestone) and

coke in an electric furnace at 2275 K. CaCO3 Heat CaO + CO2 Calcium Calcium carbonate oxide CaO + 3C 2275 K CaC2 + CO Calcium carbide ii. Impurities such as phosphine (PH3) and hydrogen sulphide (H2S) are removed by passing acetylene

through acidified copper sulphate solution. 2PH3 + 3CuSO4 Cu3P2 + 3H2SO4 Cupric phosphide (Black ppt) H2S + CuSO4 CuS + H2SO4 (Black ppt) iii. Similar to ethyne, propyne can be prepared by the hydrolysis of magnesium carbide. By dehydrohalogenation of geminal dihalides or vicinal dihalides: Reactant : Ethylene dibromide/Ethylidene dibromide (vicinal dihalide/geminal dihalide) Strong base : i. Alcoholic caustic potash ii. Sodamide in liquid ammonia Products : Acetylene (alkyne), potassium bromide and water and sodium bromide and ammonia

Methods of preparation of alkynes 15.4

HO H + C C + H OH H C C H + Ca(OH)2

Calcium carbide

Ca AcetyleneWater Water Calcium hydroxide

CH3 C CH Propyne

Cyclopropene

CH3 CH2 – C CH But–1-yne

Cyclobutene

Mg2C3 + 4H2O CH3 – C CH + 2Mg(OH)2 Magnesium

carbide Propyne

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

Reaction: i. From vicinal dihalide:

ii. From geminal dihalide: Note: i. When ethylene or ethylidene dibromide is boiled with alcoholic potash, acetylene is formed. But yield

is very less, because the intermediate compound vinyl bromide is less reactive. Hence, bases stronger than NaOH/KOH, such as KNH2/NaNH2 are used in the second step.

ii. Alternatively, acetylene can be prepared from ethylene or ethylidene dibromide on heating with sodamide. It is known as double dehydrohalogenation.

By the action of zinc on tetrahalogen derivatives of alkanes (dehalogenation): Reactants : Ethylene tetrabromide and zinc Solvent : Alcohol Products : Acetylene (alkyne), zinc bromide Reaction :

Br CH2 CH2 Br + 2NaNH2 H C C H + 2NaBr + 2NH3

Ethylene dibromide Acetylene

H C C H + 2NaBr + 2NH3CH3 CH Br + 2NaNH2

Br Ethylidene dibromide

Acetylene

H C = C H + NaNH2 Liquid NH3 H C C H + NaBr + NH3

H Br Vinyl bromide

Acetylene

H C C H + alc. KOH H C = C H + KBr + H2O Br H

H Br Ethylene dibromide

(1,2-Dibromoethane) Vinyl bromide

H Br

H C C H + alc. KOH H C = C H + KBr + H2O H Br

H Br Ethylidene dibromide(1,1-Dibromoethane)

Vinyl bromideH Br

H C = C H + NaNH2 Liquid NH3 H C C H + NaBr + NH3

H Br Vinyl bromide

Sodamide Acetylene

H C C H + 2Zn Alcohol H C C H + 2ZnBr2

Br Br

Br Br

Ethylene tetrabromide(1,1,2,2-Tetrabromoethane)

Acetylene or

Ethyne

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By electrolysis of aqueous solution of potassium salt of fumaric acid: Reactants : Potassium fumarate and water Process : Electrolysis Products : Acetylene, CO2, H2 and KOH Reaction : Acetylene is prepared by the electrolysis of aqueous solution of potassium salt of fumaric acid or maleic

acid. This reaction is called Kolbe’s electrolysis reaction. By dehalogenation of haloform: Reactants : Chloroform or iodoform and silver powder Products : Acetylene (alkyne) and silver halide Reaction : CHCl3 + 6Ag + CHCl3 Heat HC CH + 6AgCl CHI3 + 6Ag + CHI3 Heat HC CH + 6AgI Synthesis from carbon and hydrogen (Berthelot’s synthesis): Reactants : Carbon and hydrogen Product : Acetylene (alkyne) Reaction :

Acetylene can be prepared by passing a stream of hydrogen through electric arc struck between carbon electrodes at 3270 K. 2C + H2 Electric arc

3270 K HC CH

Note: Synthesis of higher alkynes: i. Higher alkynes can be prepared by treating lower alkyne with Grignard reagent to form a magnesium

complex followed by treatment with the required alkyl halide. ii. Terminal alkyne when treated with sodium metal or with sodamide in liquid ammonia forms sodium

acetylide which gives higher alkyne on reaction with alkyl halide.

Chloroform Acetylene

Iodoform Acetylene

Acetylene

Potassium maleate Acetylene

CHCOOK

CHCOOK+ 2H2O Electrolysis

CH

CH+ 2CO2 + H2 + 2KOH

General reaction

R C C H NaNH /liq.NH2 3 R C CNa+ R XNaX

R C C R Alkyne Alkyne

General reaction

(Lower alkyne) Grignard Reagent

C C – R

X Higher alkyne AlkaneR – C CH + RMgX dry

ether RH + Mg R X dry ether

R – C C – R + MgX2

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Properties Description

Nature i. ii. iii. iv. v. vi.

At room temperature, first three members (ethyne, propyne and butyne) are colourless gases. Next eight alkynes: liquids. Higher alkynes: solids. The alkynes are usually colourless. Ethyne has characteristic odour of garlic. Alkynes have linear structure. They are weakly polar in nature. Molecules are more closely packed in space as compared to alkanes and alkenes.

Solubility i. ii.

Lighter than water and hence immiscible with water. Soluble in organic solvents like petroleum ether, carbon tetrachloride, benzene, etc.

Melting point and Boiling point

i. ii.

Melting point and boiling points are slightly higher than alkanes and alkenes. Melting point and boiling point of alkynes increases with increase in molecularmass.

Note: Hydrocarbon Ethane Ethene Ethyne

Melting point (K) 101 104 191 Boiling point (K) 184.4 171 198

Acidic character of alkynes: i. An electron in p-orbital is at a large distance from the nucleus and is relatively held loose, whereas an

electron in s-orbital is closer to the nucleus and is held more tightly. ii. Thus, in hybridized state, the electronegativity of carbon increases with the increase in its percentage of

s-character. iii. Thus, sp-hybridized carbon atom in acetylene has highest electronegativity as compared to sp2-hybridized

carbon atom in ethylene and sp3-hybridized carbon atom in ethane. iv. The carbon atom of CH bond in acetylene attracts the shared electron pair towards itself to a great extent

and the hydrogen atom can be easily liberated as proton. v. Hence, the hydrogen atom attached to the triply bonded carbon atom in alkynes has appreciable acidic

character. a. Formation of alkali metal ethynides (acetylides): Eg.

Acetylene reacts with sodium metal to give sodium acetylide with the liberation of hydrogen gas.

H C C H + Na H C CNa+ + 12

H2

Physical properties of alkynes 15.5

Chemical properties of alkynes 15.6

Acetylene Monosodiumacetylide

(% of s character of carbon atom)

C = C

H H

H H sp2 -hybridized carbon atom

(Ethene) 33 %

H C C H

sp -hybridized carbon atom (Ethyne) 50 %

sp3 -hybridized carbon atom(Ethane) 25 %

H C C H

H H

H H

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H C CNa+ + Na Na+ C CNa+ + 12

H2

Similarly, methyl acetylene reacts with strong base such as sodamide to give sodium methyl

acetylide. CH3 C C H + Na+

2NH CH3 C C Na+ + NH3 These reactions are not shown by alkanes and alkenes. Note: Sodium ethynide is decomposed by water, regenerating ethyne, indicating that water is a

stronger acid than ethyne. b. Formation of heavy metal ethynides (acetylides): Ethyne (or any terminal alkynes) when treated with ammoniacal silver nitrate solution (Tollen’s

reagent), they form white precipitate of silver ethynide by replacing the acetylenic hydrogens. Similarly, with ammoniacal cuprous chloride solution, ethyne (or any terminal alkyne) forms a red

precipitate of copper ethynide. Note: Unlike alkali metal ethynides, silver and copper ethynides are not decomposed by water. They can be

decomposed with dilute mineral acids to regenerate ethyne. c. Formation of alkynyl Grignard reagents: Ethyne and other terminal alkynes react with Grignard

reagents to form corresponding alkynyl Grignard reagents. Eg.

Propyne (Methyl acetylene)

Sodium propynide (Sodium methyl acetylide)

HC ≡ CH + CH3 MgBr dryether HC ≡ CMgBr + CH3 – H

AcetylenicGrignard reagent

Ethyne Methyl magnesium

bromide

Methane

HC ≡ CNa+ + H2O HC ≡ CH + NaOH Monosodium

ethynide Ethyne

Ethyne HC ≡ CH + 2 [Cu (NH3)2]+ OH CuC ≡ CCu + 2H2O + 4NH3

Dicopper ethynide (Red ppt)

AgC ≡ CAg + 2HNO3 HC ≡ CH + 2AgNO3 EthyneDisilver

ethynide

CuC ≡ CCu + 2HCl HC ≡ CH + 2CuCl EthyneDicopper

ethynide

Disodiumacetylide

HC ≡ CH + 2 [Ag (NH3)2]+ OH AgC ≡ CAg + 2H2O + 4NH3 Ethyne Tollen’s reagent Disilver

ethynide (White ppt)

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

i. Addition reactions: Alkynes are less reactive than alkenes towards electrophilic addition reaction since the carbon atom in

alkynes has more percentage of s-character, hence attraction for electrons is strong. Alkynes undergo nucleophilic addition with electron rich reagents. This is due to the formation of vinylic carbanion, which is more stable than alkyl carbanion formed by alkene.

Eg. Addition of water, cyanide, carboxylic acid and alcohols. Nucleophilic addition reaction: a. Addition of hydrogen (catalytic hydrogenation or reduction of alkynes): 1. When acetylene is heated with hydrogen gas in the presence of a catalyst like Raney nickel, ethane is

formed. 2. Hydrogenation may be carried out in presence of Pt or Pd at room temperature. 3. In this reaction, two hydrogen molecules are added across the triple bond in acetylene. Eg. b. Addition of halogens: 1. Action of chlorine: i. Acetylene reacts with chlorine gas to give acetylene tetrachloride. ii. Acetylene tetrachloride is known as ‘Westron’, which is non-inflammable and good solvent for

oils, fats and resins. Eg. Note: a. Acetylene reacts with chlorine or bromine even in dark. The reaction is catalyzed by light. b. A dihaloalkene is obtained in first step, which is then converted into tetrahaloalkane. c. Iodine adds to acetylene with difficulty. 2. Action of bromine:

i. With dilute bromine water: Acetylene reacts with dilute bromine water to give acetylene dibromide.

Eg.

Vinylic carbanion(more stable)

Alkyne C C + Nu C = C

Nu

Alkyl carbanion (less stable)

C = C + Nu C C

Nu

Alkene ;

Acetylene tetrachloride Acetylene dichloride

Acetylene H C C H Cl2 H C = C H Cl2 H C C H

ClCl Cl

Cl Cl

Cl(Ethyne)

(trans 1,2-Dichloroethene) (1,1,2,2-Tetrachloroethane)

Acetylene dibromide

Acetylene (Ethyne)

(trans-1,2-Dibromoethene)

H C C H Bromine water C = C

H

HBr

Br

EthaneEthene Ethyne

H C C H + H2 Raney NiΔ H C = C H Raney Ni/H2

Δ H C C H

H

H H H H

H

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ii. With liquid bromine: Acetylene reacts with liquid bromine to give acetylene tetrabromide. Eg.

3. Action of iodine: The reaction of alkynes with iodine occurs with difficulty. However, if the reaction is carried out in ethanolic solution, it stops after the addition of one molecule of iodine to form trans-1,2-diiodoalkene.

Eg. Note: i. Alkynes decolourise bromine water similar to alkenes. ii. The reactivity of halogens with alkynes follows the order, Cl2 > Br2 > I2. c. Addition of halogen acids (hydrogen halides): 1. Action of HCl: When acetylene is passed through dilute hydrochloric acid in presence of mercuric

chloride at 338 K, vinyl chloride is obtained. Only one molecule of hydrogen chloride is added. Eg. CH CH + HCl 2HgCl

338K CH2 = CH Cl Acetylene Vinyl chloride (Ethyne) (Chloroethene) 2. Action of HBr: Acetylene reacts with hydrogen bromide to give ethylidene dibromide. Eg. 3. Action of HI: Acetylene reacts with hydrogen iodide to give ethylidene diiodide or 1,1-diiodoethane. Eg. Note: Alkynes react with hydrogen halides to give first vinylic halides and finally geminal dihalides. The

order of reactivity of halogen acids (hydrogen halides) is in the order of HI > HBr > HCl > HF. d. Addition of water (hydration of alkynes): On passing alkynes through hot 40 % H2SO4, in the presence of 1 % mercuric sulphate, aldehydes or

ketones are formed. The enol formed being unstable, immediately isomerizes to a more stable keto form.

H C C H Br2CCl4

C = C Br2CCl4

H C C H

Acetylene(Ethyne)

(trans-1,2-Dibromoethene)Acetylene dibromide Acetylene tetrabromide

(1,1,2,2-Tetrabromoethane)

Br Br

Br Br

H

HBr

Br

Ethynetrans-1,2-diiodoethene

HC CH + I2 C H OH2 5 C = C

I

H

H

I

Acetylene (Ethyne)

Ethylidene dibromideVinyl bromide(Bromoethene) (1,1-Dibromoethane)

(Major Product)

HC CH HBr [CH2 = CH Br] HBr H3C CH Br

Br

Acetylene (Ethyne)

Ethylidene diiodide (1,1,-Diiodoethane) (Major product)

Vinyl iodideH C C H HI [CH2 = CH I] HI CH3 CH I

(Iodoethene)

I

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

Eg. 1. H C C H + H OH 2 4

4

40%H SO (dil.)

1%HgSO [CH2 = CH OH] isomerization CH3 CHO

Acetylene Vinyl alcohol Acetaldehyde (Terminal alkyne) (Ethenol) Enol form (Ethanal) keto form 2. The rearrangement of enol form into keto form is called tautomerism. ii. Oxidation reactions: a. Combustion : When alkynes are burnt in air or oxygen, CO2 and H2O are formed. The reaction is

highly exothermic and a large amount of heat is produced. 2HC CH + 5O2 4CO2 + 2H2O; H = – 1300 kJ. Ethyne b. Oxidation with alkaline potassium permanganate: Alkynes react with alkaline KMnO4 solution

(Baeyer’s reagent) and form diketones. When the reaction is carried out at high temperature, carboxylic acids and CO2 are formed due to the

complete cleavage at the site of the triple bond. Eg. 1. 2. Thus, in general, it can be noted that, CH (terminal alkynes) are oxidised to CO2 and H2O and CR (non-terminal alkynes) are oxidised to R – COOH. Note: i. In the reactions involving oxidation with alkaline potassium permanganate, the pink colour of

the alkaline KMnO4 (Baeyer’s reagent) gets discharged, therefore, this reaction is used as a test to detect unsaturation in alkynes.

ii. The reactions which involve the complete breaking of triple bond are called degradation or cleavage reactions, which are used to locate the position of the triple bond.

c. Oxidative hydroboration: Oxidative hydroboration of alkynes by treating it with BH3 followed by alkaline H2O2 gives an alkenol which tautomerises to give aldehyde or ketone.

Eg. Terminal alkynes on hydroboration-oxidation give aldehydes while non-terminal alkynes give ketones.

CH3 – C C – CH3 Alkaline KMnO4373 383 K CH3COOH + CH3COOH

But-2-yne Ethanoic acid

Ethanoic acid

3CH3 – C CH BH /THF3 (CH3 – CH = CH)3B H O2 2OH

CH2 – CH = CH – OH tautomerization CH3 – CH2 – CHO Propyne Propenol Propanal

CH3 – C CH Alkaline KMnO4373 383 K CH3 – COOH + CO2

Propyne Ethanoicacid

3CH – C CH + 3[O] Alkaline KMnO4298 303K CH3 – C – C – OH

Propyne

2-Oxopropanoic acid(Pyruvic acid)

O O

Butanone (keto form)

Isomerization CH3 CH2 C CH3

O But-2-en-2-ol

CH3 C C CH3 + H2O 2 4

4

40% H SO (dil.)

1%HgSO CH3 C = C CH3

2-Butyne (Non-terminal alkyne) H OH

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iii. Ozonolysis: Alkynes react with ozone in presence of CH2Cl2 at low temperatures to form ozonides which on decomposition with Zn/H2O gives 1,2-dicarbonyl compounds.

Eg. iv. Polymerization reactions: Alkynes have a larger tendency to polymerize than alkenes and they undergo

two types of polymerization. a. Linear polymerization: Acetylene undergoes linear polymerization under suitable conditions to

form a linear polymer polyacetylene or polyethyne which is a high molecular mass conjugated diene containing repeating units of (CH = CH – CH = CH) and it can be represented as .

b. Cyclic polymerization: Alkynes on passing through red hot iron tube at 873 K, undergo cyclic polymerization.

Eg. 1. 2. v. Isomerization: When alkynes are heated with sodamide (NaNH2) in an inert solvent, they undergo isomerization

i.e., 2-alkynes isomerize to 1-alkynes. In the presence of alcoholic potash, reverse isomerization takes place. Uses of acetylene: Acetylene is used, i. in oxy-acetylene flame which is used for cutting and welding of metals. ii. as an illuminant in lamps. iii. for artificial ripening of fruits. iv. as a general anasthetic under the name narcylene. v. in the manufacture of westron (Cl2HC CHCl2) and westrosol (ClHC = CCl2). vi. in the manufacture of acetaldehyde, acetic acid, oxalic acid, etc. vii. in the manufacture of plastic, synthetic rubber, synthetic fibers (orlon), etc. viii. in the manufacture of Lewisite, a war gas, which was used in World War - II. It is obtained by the

action of arsenic trichloride on acetylene in presence of anhydrous AlCl3.

nCH CH CH CH

Benzene

Red hot tube873 K

HCHC

HCHC

CH

CH

HC

HC

CH

CH

CH

CH3 molecules of acetylene

CH3 – C CH O / CH Cl3 2 2196 200 K Zn/H O2

ZnO CH3 – C – CH Propyne

2-Oxopropanal O O

Propyne ozonide

CH

O

O O

CH3 C

CH3 – C C – CH3 NaNH in inert solvent2

alc.KOH CH3 – CH2 – C CH

But-2-yne But-1-yne

Red hot iron tube873 K

HC

CH3

H3C – CHC

CH

C – CH3

C

3 molecules of methylacetylene (Propyne)

HC

CH3 – C

CH

C – CH3

CH

CH

CH3

1,3,5-Trimethylbenzene(Mesitylene)

CH + AsCl2 Anhydrous AlCl3 CHAsCl2Ethyne Lewisite

CH Cl CHCl

Arsenic trichloride

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2H2OCa(OH)2

C2H2Br42 Zn, alcohol2ZnBr2

6AgX

Electric arc3270 K2C + H2

i. Alc.KOH/ii. NaNH2 /

C2H4Br2Ethylene

dibromide/Ethylidene dibromide

CH CH

H2/Raney Ni

2HBr

CH3 CH3Ethane

Ethyne

Cl2

2HI

40 % H2SO4 (dil)1 % HgSO4,

CH3CHO acetaldehyde

2HClAcetylene

tetrabromide

CH3CH I Ethylidene diiodide

I

5O2

4CO2 + 2H2O

Polymerization

Red hot tube873 K

Benzene

Polyethyne

CaC2Calcium carbide

Haloform2CHX3 + 6Ag

CH3 CH Cl Ethylidene dichloride

Ethyne: Preparation: Reactions: Reactions of propyne:

Quick Review

CH3 – C CH Propyne

KMnO4 , [O] 298 – 303 K CH3 – C – C – OH

Alkaline KMnO4, [O] 373 – 383 K CH3 – COOH + CO2

O O 2- Oxopropanoic acid

Ethanoic acid

Alkaline

CH3CH Br Ethylidene dibromide

Br

nCH CH CH CH

Cl

H C C H Acetylene tetrachloride

ClCl

Cl Cl

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Chapter 15: Alkynes 1. Which is represented by the formula CnH2n 2? [CPMT 1975, 76; EAMCET 1979;

MP PET 2003] (A) Alkane (B) Alkyne (C) Alkene (D) None of the above 2. Alkynes contain _______ hydrogen atoms less

than corresponding alkanes. (A) two (B) three (C) four (D) five 3. Alkynes are commonly called as _______. (A) paraffins (B) olefins (C) acetylenes (D) none of these 4. The names of alkynes are derived from the

corresponding _______ . (A) alkanes (B) alkynes (C) alkenes (D) alkyl 5. Aliphatic unsaturated hydrocarbons containing

two carbon-carbon triple bonds in their structure are called _______.

(A) alkadiynes (B) alkatriynes (C) alkynes (D) alkanes 6. The IUPAC name of dimethyl acetylene is

_______. (A) 3-methylbut-1-yne (B) 2-methylbut-1-yne (C) but-1-yne (D) but-2-yne 7. The IUPAC name of the compound is _______. (A) pent-1,4-diyne (B) dimethyl acetylene (C) 3-methylbut-1-yne (D) ethyl methyl acetylene 8. IUPAC name of the following compound is

_______. HC C CH2 CH2 C CH (A) hexa-1,5-diene (B) hexa-1,5-diyne (C) hexa-1,5-yne (D) hexa-1,5-ene

9. Which one is INCORRECT name? [BCECE 2015]

(A) Propyne (B) But-2-yne (C) Pent-3-yne (D) But-1-yne 10. Which one of the following structures

represent 3,3-dimethylbut-1-yne? (A) (B) (C) (D) 11. Give the IUPAC name of the following

compound: (A) 3-Ethyl-4-methylpent-1-diyne (B) 3-Ethyl-4-methylpent-1-ene (C) 2-Methyl-3-ethylpent-4-yne (D) 3-Ethyl-4-methylpent-1-yne 12. The IUPAC name of the compound with the

structural formula H3C CH2 CH = CH C CH is _______. (A) 1-hexyn-3-ene (B) 5-hexyn-3-ene (C) hex-3-en-1-yne (D) 3-hexen-5-yne 13. Name of the following compound is _______. (A) 2-chlorohex-5-yn-1-al (B) 5-chlorohexen-6-dial (C) 2-chlorohexen-5-al (D) 5-chlorohex-1-yn-5-al 14. The CORRECT IUPAC name of the following

structure is _______. (A) 2,6-dimethyloct-1-en-5-yne (B) 3,6-dimethynon-7-en-4-yne (C) 2,6-dimethyloct-7-en-4-yne (D) 3,7-dimethyloct-1-en-4-yne

Cl

HC C CH2 CH2 – CH – CHO

CH3

C2H5

H3C HC CH C CH

Multiple Choice Questions

Alkynes: Introduction 15.0

Nomenclature of alkynes 15.1

CH3 CH C CH

CH3

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15. Write the IUPAC name of H3C C C CH = CH – C C CH3 (A) Oct-4-en-2,6-diyne (B) Hept-4-en-2,6-yne (C) Hept-4-yn-2,6-ene (D) 2,6-Octen-4-yne 16. A carbon-carbon triple bond in ethyne

(C C) consists of _______. [AMU 2000] (A) all bonds (B) two bonds and one -bond (C) one bond and two -bonds (D) all bonds 17. The carbon atoms in propyne are arranged at

_______. (A) 90 to each other (B) 190 28 to each other (C) 120 to each other (D) 180 to each other 18. Carbon-carbon bond length is minimum in

_______. [CBSE PMT 1988, 91; MNR 1984; CPMT 1989; RPMT 1997; Pb. PMT 2001]

(A) ethane (B) ethene (C) ethyne (D) benzene 19. Maximum bond energy of CH bond is found

in the compound _______. [RPMT 1999] (A) ethane (B) ethene (C) ethyne (D) equal in all the three 20. Acetylene molecule has carbon in _______.

[Kerala (Engg.) 2002] (A) sp-hybridization (B) sp2-hybridization (C) sp3-hybridization (D) sp3d-hybridization 21. Acetylene has following bonds between the

carbon atoms _______. [BCECE 2015] (A) 1, 2 (B) 3 (C) 3 (D) 2, 1 22. Acetylene has _______. [BCECE 2014] (A) 5 bonds (B) 4 + 1 bonds (C) 3 + 2 bonds (D) 2 + 2 bonds 23. Number of and bonds present in

1-butene-3-yne respectively are _______. [RPMT 1999; MP PET 2000; DCE 2000]

(A) 7, 3 (B) 5, 2 (C) 8, 3 (D) 6, 2

24. How many bonds are there in CH C – CH = CH – CH3?

[BCECE (Stage 1) 2016] (A) 4 (B) 6 (C) 10 (D) 9 25. Which of the following molecules represent

the order of hybridisation sp2, sp2, sp, sp from left to right atoms? [NEET (UG) 2018]

(A) HC C – C CH (B) CH2 = CH – C CH (C) CH2 = CH – CH = CH2 (D) CH3 – CH = CH – CH3 26. In the compound, CH2 = CH CH2 CH2 C CH, the

C2 C3 bond is of the type _______. [IIT-1999] (A) sp – sp2 (B) sp3 sp3 (C) sp sp3 (D) sp2 sp3 27. _______ isomerism is due to the difference in

the type of carbon skeleton. (A) Position (B) Functional (C) Chain (D) Ring-chain 28. Which of the following alkynes exhibit

position isomerism? (A) Ethyne (B) Propyne (C) Butyne (D) All of these 29. Number of possible alkynes with formula

C5H8 is _______. [KCET 2015] (A) 2 (B) 3 (C) 4 (D) 5 30. The number of possible alkynes with

molecular formula C6H10 is _______. (A) 4 (B) 5 (C) 6 (D) 7 31. _______ isomerism is exhibited by alkynes

and dienes having the same molecular formula.

(A) Chain (B) Position (C) Functional (D) Ring-chain 32. _______ isomerism is exhibited by alkynes

and cycloalkenes. (A) Chain (B) Position (C) Functional (D) Ring-chain 33. The functional isomer of but-1-yne is

_______. (A) buta-1,3-diene (B) but-2-yne (C) propa-1,2-diene (D) penta-1,2-diene

Structure of triple bond15.2

Isomerism in alkynes15.3

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34. The ring-chain isomer of CH3 – C CH is _______.

(A) (B)

(C) (D) 35. Calcium carbide on reacting with water gives

_______. [CPMT 1991; MP PMT 1993, 94; RPMT 2002]

(A) methane (B) ethane (C) ethene (D) acetylene 36. Acetylene is prepared in the laboratory by the

action of water on _______. (A) calcium carbonate

(B) calcium carbide (C) calcium chloride (D) calcium oxide 37. Acetylene produced by the action of water on

calcium carbide generally has foul smell due to _______.

(A) its characteristic smell (B) impurities of PH3 and H2S (C) impurities of SO2 and SO3 (D) impurities of NH3 and SO2 38. Impurities of phosphine and hydrogen

sulphide in acetylene preparation are removed by passing acetylene through _______.

(A) alkaline KMnO4 (B) KOH (C) acidified CuSO4 (D) H2SO4 39. The reaction, CH3CHBr CH2Br + 2KOH

CH3 C CH + 2KBr + 2H2O is called _______. (A) dehalogenation (B) dehydrohalogenation (C) decarboxylation (D) dehydrogenation 40. 1,2-Dibromoethane when heated with

alcoholic potash gives _______. (A) ethane (B) acetylene (C) ethylene (D) methane 41. R CH2 CCl2 R Reagent R C C R The reagent is _______. (A) NaNH2/ (B) HCl and H2O (C) aq. KOH (D) Zn

42. The yield of acetylene formed by treatment of ethylene dibromide with alcoholic potash is poor because vinyl bromide is _______.

(A) less reactive (B) very reactive (C) an impurity (D) polar 43. Identify ‘X’ in the following reaction: X + 2Zn alcohol

H C C H + 2ZnBr2 (A) Ethylene dibromide (B) Ethylene tetrabromide (C) Ethylene chloride (D) Ethene 44. To prepare 2-butyne from

2,2,3,3-tetrabromobutane, ______ is used. (A) alc. KOH (B) sodamide (C) Zn in alcohol (D) conc. H2SO4 45. In the synthesis of acetylene by Kolbe's

process, the aqueous solution of _______ salt would be used.

(A) potassium acetate (B) potassium maleate (C) potassium formate (D) potassium succinate 46. The reaction of chloroform with Ag powder

gives_______. [BCECE (Stage 2) 2016] (A) CH4 (B) CH2 = CH2 (C) CH CH (D) HCOOAg 47. Acetylene is prepared industrially by passing

electric discharge through graphite electrodes in the atmosphere of _______. [CPMT 1985]

(A) air (B) N2 (C) H2 (D) CO2 48. Which of the following method is applicable

for the preparation of acetylene? (A) Double dehydrohalogenation of ethylene

dibromide. (B) Debromination of

1,1,2,2-tetrabromoethane in the presence of zinc in alcohol.

(C) Heating of vinyl chloride with sodamide in liquid ammonia.

(D) All of these. 49. Identify the product D in the following series

of reaction? CH3COOH LiAlH4A H

443K

B Br2

CCl4 C

2NaNH2

D

[CBSE PMT 1998] (A) Methane (B) Alcohol (C) Acetylene (D) Benzaldehyde

Methods of preparation of alkynes15.4

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Absolute Chemistry Vol - 1.2 (Med. and Engg.) 50. Acetylene has a characteristic _______ odour. (A) ginger (B) garlic (C) bitter almonds (D) phenolic 51. Highest boiling point will be of _______. (A) ethane (B) ethylene (C) acetylene (D) methane 52. Melting and boiling points of alkynes _______. (A) increase with increase in molecular mass (B) decrease with increase in molecular

mass (C) increase with decrease in molecular

mass (D) do not depend on molecular mass 53. Ethyne exhibits acidic nature because carbon

atom is in _____ hybridized state. (A) sp3 (B) sp (C) sp2 (D) None of the above 54. Acetylenic hydrogens are acidic because

_______. [CBSE PMT 1989; Pb. PMT 1999] (A) the sp-hybridized carbon atom in

acetylene has the highest electronegativity (B) acetylene has only one hydrogen on

each carbon (C) acetylene contains least number of

hydrogens among the possible hydrocarbons having two carbons

(D) acetylene belongs to the class of alkynes with molecular formula CnH2n2

55. Which of the following reacts with sodium eliminating hydrogen?

[BHU 1983, RPMT 2002] (A) CH4 (B) C2H6 (C) C2H4 (D) C2H2 56. Sodium acetylide is prepared by _______. (A) action of NaOH on acetylene (B) passing acetylene over heated Na metal (C) action of Na2CO3 on acetylene (D) action of NaCl on acetylene 57. What is the major product expected from the

following reaction? CH3 C C H NaNH2

NH3 (A)

(B) (C) CH3 C = CH2 (D) H2N CH2 C C H 58. This compound forms a sodium salt upon

reacting with NaNH2: [BCECE 2015] (A) C2H2 (B) CH3NH2 (C) C6H6 (D) C2H6 59. Acetylene readily undergoes ______ reaction. (A) addition (B) substitution (C) elimination (D) rearrangement 60. The CORRECT increasing order of acidity of

the following hydrocarbons is: Propyne Propane Ethene Ethyne (I) (II) (III) (IV) (A) III < I < II < IV (B) II < I < IV < III (C) II < III < I < IV (D) II < IV < I < III 61. Which of the following statements is

CORRECT? (A) Alkynes are more reactive than alkenes

in both electrophilic and nucleophilic addition reactions.

(B) Alkynes are more reactive than alkenes in electrophilic addition reactions but less reactive in nucleophilic addition reactions.

(C) Alkynes are less reactive than alkenes in electrophilic addition reactions but more reactive in nucleophilic addition reactions.

(D) Alkynes and alkenes both have low reactivity in electrophilic addition reactions but both have high reactivity in nucleophilic addition reactions.

62. Arrange the following carbanions in order of

their decreasing stability. (i) H3C – C C– (ii) H – C C– (iii) H3C – 2CH

[NCERT Exemplar] (A) (i) > (ii) > (iii) (B) (ii) > (i) > (iii) (C) (iii) > (ii) > (i) (D) (iii) > (i) > (ii) + CH3 C C Na

+H C C CH2Na

Chemical properties of alkynes

15.6

Physical properties of alkynes 15.5

NH2

SAMPLE C

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Chapter 15: Alkynes

63. When acetylene is heated with hydrogen gas in presence of a catalyst like Raney nickel, _____ is formed.

(A) methane (B) ethylene dichloride (C) ethane (D) ethylidene dichloride 64. The reagent ‘X’ used for the following

reaction is _______. [KCET 2018] (A) Ni (B) Pd/C (C) LiAlH4 (D) Na/liquid NH3 65. What is the major product of the following

reaction? 2 moles of Cl2

3 2 3CH C C CH CH [Kerala (Med.) 2003]

(A) (B) (C) (D) 66. Westron is formed by following reaction with

alkyne: (A) Addition of halogen (B) Substitution by halogen (C) Hydrolysis with acid (D) Dehydration of alcohol 67. An organic compound on treatment with

bromine water gives a dibromoalkene. The compound will be ______.

(A) CH3 – CH = CH2 (B) CH3 – CH = CH CH3 (C) HC CH (D) H2C = CH2

68. Liquid bromine reacts with acetylene to form _______.

(A) 1,2-dibromoethene (B) 1,1,2,2-tetrabromoethane (C) 1,1-dibromoethene (D) bromoethane 69. The reaction of CH CH with HX is an

example of a/an _______. (A) addition reaction (B) catalytic reaction (C) displacement reaction (D) substitution reaction 70. Acetylene on addition of haloacids (HX)

forms _______ as an intermediate product. (A) vinyl halide (B) dihaloethane (C) trihaloethane (D) haloethane 71. The reactivity of hydrogen halide with alkynes

is in the order of _______. (A) HI > HBr > HCl (B) HCl > HBr > HI (C) HI < HCl < HBr (D) HBr > HI > HCl 72. What is the major product expected from the

following reaction?

CH3 C C H HCl(g)excess

(A) (B) (C) (D) 73. When acetylene is reacted with HBr, we get

_______. [CPMT 1979; JIPMER 2002] (A) methyl bromide (B) ethyl bromide (C) ethylene bromide (D) ethylidene dibromide

H3C C = C

H

H Cl

C = CCl

CH3

Cl

CH2CH3

CH3 CH2 C CH2CH3

Cl

Cl

C = CCl

CH3 Cl

CH2CH3

CH3 C C CH2 CH3

Cl

Cl

Cl

Cl

CH3 C = CH2

Cl

CH3 CH2 C H

Cl

Cl

H3C C CH3

Cl

Cl

R – C C – R' + H2X

R R'

H H

H – C – C – H

SAMPLE C

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

74. The catalyst required for the reaction HC ≡ CH + dil. H2SO4 CH3CHO is _______.

(A) HgSO4 (B) Pd (C) Pt (D) AlCl3 75. When but-1-yne is treated with aqueous

H2SO4 in presence of HgSO4, the major product is _______. [Assam CEE 2015]

(A) CH3 – CH2 – CH2 – CHO (B) CH3 – CH2 – CO – CH3 (C) CH3 – CH2 – CH = CH2 (D) CH3 – CH2 – CH2 – CH2 – OH 76. Predict the CORRECT intermediate and

product in the following reaction.

[NEET (UG) 2017] (A) (B) (C) (D) 77. The product of CH CH i) KMnO ,OH4

ii) H

is _______. [BCECE (Stage 2) 2016] (A) CH3COOH (B) (C) CH3CHO (D) CH3CH2OH 78. 1-Butyne reacts with hot alkaline KMnO4 to

produce _______. (A) CH3CH2CH2OH (B) CH3CH2CH2COOH (C) CH3CH2COOH + CO2 (D) CH3CH2COOH + CH3COOH

79. Ethyne on ozonolysis gives _______. (A) ethylene glycol (B) glyoxal (C) glycollic acid (D) formic acid 80. Y O AlkalineKMnO4

373 383 K C3H4 O /CH Cl /196K3 2 2Zn H O2 Z

Identify Y and Z. (A) Y = Z = (B) Y = Z = (C) Y =

Z = (D) Y = Z = 81. The product formed when acetylene is passed

through red hot iron tube at 873 K is _______. [BHU 1989; RPMT 2003]

(A) benzene (B) cyclohexane (C) neoprene (D) ethane 82. Benzene is a polymer of _______. (A) ethene (B) ethane (C) ethyne (D) methane

A : H3C – C – CH3 B : H3C – C CH

O

A : H3C – C = CH2 B : H3C – C = CH2

SO4 OH

A : H3C – C = CH2 B : H3C – C – CH3

SO4 O

A : H3C – C = CH2 B : H3C – C – CH3

O OH

H3C – C CH H O,H SO2 2 4HgSO4

intermediate (A)

(B) product

COOH

COOH

O

O

OH

O

O

O

O

OH

O

H

O

OH

H

O

O

SAMPLE C

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83. Propyne on polymerization gives _______. [CPMT 1999, 2002]

(A) mesitylene (B) benzene (C) ethylbenzene (D) propylbenzene 84. But-2-yne on trimerisation forms _______ as

the final product. (A) mesitylene (B) xylene (C) hexamethylbenzene (D) 1,3,5-triethylbenzene 85. When 2-alkynes are heated with _______,

they isomerize to give 1-alkynes. (A) LiAlH4 (B) Br2/CCl4 (C) NaNH2 in an inert solvent (D) Alkaline KMnO4 86. The isomerisation of 1-butyne to 2-butyne can

be achieved by treatment with _______. [WB JEE 2017]

(A) hydrochloric acid (B) ammoniacal silver nitrate (C) ammoniacal cuprous chloride (D) ethanolic potassium hydroxide 87. 1,2-Dibromobutane 2NaNH2

P alc.KOHQ AlkalineKMnO4

373 383 KR Predict Q and R. (A) Q = But-1-yne R = 2-Oxobutanal (B) Q = 3-Methylbut-1-yne R = Butane-2,3-dione (C) Q = But-2-yne R = Ethanoic acid (D) Q = 2-Methylprop-1-ene R = Propanone 88. Acetylene is used in the manufacture of

______. (A) westrosol (B) thermocol (C) eastrol (D) glycol 89. Acetylene is used as an anaesthetic under the

name of _______. (A) narcylene (B) pyrene (C) neopyrene (D) pyroline 90. Which of the following gas is used for

welding? [CPMT 1996] (A) Methane (B) Ethane (C) Acetylene (D) Ethene

91. The poisonous gas formed by the reaction of ethyne and arsenic trichloride is _______.

(A) westron (B) lewisite (C) tear gas (D) westrosol 92. Number of bonds in CH2 = CH CH = CH C CH is _______.

[Kurukshetra CEE 1991; KCET 2000] (A) 2 (B) 3 (C) 4 (D) 5 93. Which of the following has a bond formed by

overlap of sp sp3 hybrid orbitals? [MNR 1993; UPSEAT 2001, 02]

(A) CH3 C C H (B) CH3 CH = CH CH3 (C) CH2 = CH CH = CH2 (D) HC CH 94. The IUPAC name of westron is _______.

[BHU (Mains) 2008] (A) 1,1,2,2-tetrachloroethane (B) 1,1,2,2-tetrachloroethene (C) 1,2-dichloroethyne (D) 1,3,3,3-tetrachloroprop-1-yne. 95. X Hydration Ethanal Ozonolysis Y Identify the reactants ‘X’ and ‘Y’. (A) X = Ethane Y = Propene (B) X = But-1-yne Y = Ethene (C) X = Propyne Y = But-2-yne (D) X = Ethyne Y = But-2-ene 96. The pair of electron in the given carbanion,

CH3C C is present in which of the following orbitals? [NEET P-I 2016]

(A) sp2 (B) sp (C) 2p (D) sp3 97. Which of the following is most acidic?

[J & K 2005] (A) Methane (B) Acetylene (C) 1-Butene (D) Neopentane 98. Which is the most reactive hydrocarbon in the

following? [MP PAT 1993; JIPMER 2002] (A) Ethane (B) Ethyne (C) Ethene (D) Methane

Miscellaneous

SAMPLE C

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

99. Predict the set of reagents required for the following reaction.

(A) x = conc. H2SO4/443 K y = HBr/peroxide z = NaNH2/ w = Raney Ni/ (B) x = conc. H3PO4/623 K y = Liquid bromine z = alc. KOH/NaNH2 - w = Alkaline KMnO4/ (C) x = conc. H2SO4/443 K y = Br2/CS2 z = NaNH2/ w = alc. KOH (D) x = Al2O3/623 K y = 2Br2/CCl4 z = Zn/alcohol w = NaNH2/liq.NH3 100. In the reaction,

H C C H i.NaNH /liq.NH2 3ii.CH CH Br3 2

X

i.NaNH /liq.NH2 3ii.CH CH Br3 2

Y

X and Y are _______. [NEET P-I 2016]

(A) X = But-2-yne ; Y = Hex-2-yne (B) X = But-1-yne ; Y = Hex-2-yne (C) X = But-1-yne ; Y = Hex-3-yne (D) X = But-2-yne ; Y = Hex-3-yne 101. CH3CH(Cl)CH2(Cl) Zn/alcohol

A Br /CCl2 4 B 2NaNH2

C H O,40% H SO ,2 2 421% Hg

D

Predict D. (A) (B)

(C) (D) 102. Which of the following decolourizes KMnO4

(neutral or slightly alkaline)? [NCERT 1974, 77; CPMT 1974, 77, 78;

MP PMT 1996; MP PET 1996, 99] (A) Ethane (B) Ethene (C) Acetylene (D) Both (B) and (C) 103. This has one C – C bond, one C = C bond and

one C C bond. [BCECE 2014]

(A) C4H6 (B) C4H4 (C) C4H3 (D) C4H5 104. CH3 – C CMgBr can be prepared by the

reaction of _______. [WB JEE 2018]

(A) CH3 – C C – Br with MgBr2

(B) CH3 – C CH with MgBr2

(C) CH3 – C CH with KBr and Mg metal

(D) CH3 – C CH with CH3MgBr 105. Which one is the CORRECT order of acidity?

[NEET (UG) 2017]

(A) CH2 = CH2 CH3 – CH = CH2

CH3 – C CH CH CH

(B) CH CH CH3 – C CH CH2 = CH2 > CH3 – CH3

(C) CH CH CH2 = CH2 CH3 – C CH > CH3 – CH3

(D) CH3 – CH3 CH2 = CH2 CH3 – C CH

> CH CH

O

OHOH

O

O

O H

z w

OH

x y

Br Br

Br Br

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Chapter 15: Alkynes

1. (B) 2. (C) 3. (C) 4. (A) 5. (A) 6. (D) 7. (C) 8. (B) 9. (C) 10. (C) 11. (D) 12. (C) 13. (A) 14. (D) 15. (A) 16. (C) 17. (D) 18. (C) 19. (C) 20. (A) 21. (A) 22. (C) 23. (A) 24. (C) 25. (B) 26. (D) 27. (C) 28. (C) 29. (B) 30. (D) 31. (C) 32. (D) 33. (A) 34. (A) 35. (D) 36. (B) 37. (B) 38. (C) 39. (B) 40. (B) 41. (A) 42. (A) 43. (B) 44. (C) 45. (B) 46. (C) 47. (C) 48. (D) 49. (C) 50. (B) 51. (C) 52. (A) 53. (B) 54. (A) 55. (D) 56. (B) 57. (A) 58. (A) 59. (A) 60. (C) 61. (C) 62. (B) 63. (C) 64. (B) 65. (D) 66. (A) 67. (C) 68. (B) 69. (A) 70. (A) 71. (A) 72. (D) 73. (D) 74. (A) 75. (B) 76. (D) 77. (B) 78. (C) 79. (B) 80. (D) 81. (A) 82. (C) 83. (A) 84. (C) 85. (C) 86. (D) 87. (C) 88. (A) 89. (A) 90. (C) 91. (B) 92. (C) 93. (A) 94. (A) 95. (D) 96. (B) 97. (B) 98. (B) 99. (D) 100. (C) 101. (C) 102. (D) 103. (B) 104. (D) 105. (B)

3. Alkynes contain triple bond, which is also called acetylene linkage and the first member of the alkyne series is

acetylene. Hence, alkynes are commonly called acetylenes. Alkanes are called paraffins and alkenes are called olefins. 6. CH3 – C C – CH3 Dimethyl acetylene (But-2-yne) 8. 9. Correct IUPAC name: Pent-2-yne 10. 11. 12. 13. 14.

Answers to MCQs

Hints to MCQs

HC C – CH2 CH2 – C CH Hexa-1,5-diyne

1 2 4 5 6 3

H3C – CH2 – C C – CH3 1 2 3 4 5

3,3-Dimethylbut-1-yne

1

2 4

3

Hex-3-en-1-yne

6 5 4 3 2 1

H3C CH2 CH = CH C CH

4

1

3,7-Dimethyloct-1-en-4-yne

2 3

5

67 8

3-Ethyl-4-methylpent-1-yne

CH3

C2H5

1 2 3 4 5

H3C CH CH C CH

2-Chlorohex-5-yn-1-al

1 2 3 4 5

Cl

6

HC C CH2 CH2 – CH – CHO

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Absolute Chemistry Vol - 1.2 (Med. and Engg.) 15. The priority is first given to double bond and then to the triple bond. 18. The carbon-carbon bond length is minimum in ethyne (120 pm) as compared to ethene (134 pm) and ethane (154 pm). 19. Bond energy is maximum for the triple bond. 20.

sp spCH CH

21. In acetylene/ethyne molecule, the sp hybrid orbital of one carbon atom overlaps axially with sp hybrid

orbital of other carbon atom to form one sigma bond whereas the unhybridized 2py and 2pz orbitals of each of these carbon atoms overlap laterally to form two pi bonds. Thus, the C C bond in ethyne consists of one sigma bond and two pi bonds.

22. 23. 2

7 ,3CH CH C CH

But-1-en-3-yne (1-Butene-3-yne) 24.

25. 26. CH2 = CH CH2 CH2 C CH 28. Position isomerism is exhibited by alkynes having four or more than four carbon atoms. 29. The possible alkynes are: 30.

Oct-4-en-2,6-diyne

7 6 5 4 3 2 1 8

H3C C C CH = CH C C CH3

H C C H

3 and 2 bonds

10 and 3 bonds

H – C C – C = C – C – H

H H H

H 1 2 3 4 5

CH2 = CH– C CH sp2 sp2 sp sp

sp3

1 2 3 4 5 6

sp2

CH3 – CH2 – CH2 – C CH CH3 – CH2 – C C – CH3 CH3 – CH – C CH

CH3

3-Methylbut-1-ynePent-2-ynePent-1-yne

CH3 – CH2 – CH2 – C C CH3

Hex-2-yneCH3 – CH2 – C C – CH2 CH3

Hex-3-yne

CH3 – CH – CH2 C CH

4-Methylpent-1-yne

CH3

CH3 – CH2 – CH C CH3-Methylpent-1-yne

CH3

CH3 – CH C C CH3

CH3

4-Methylpent-2-yne

CH3 – C C CH

CH3

3,3-Dimethylbut-1-yneCH3

CH3 – CH2 – CH2 – CH2 – C CHHex-1-yne

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Chapter 15: Alkynes

33. CH3 – CH2 – C CH H2C = CH – CH = CH2 But-1-yne Buta-1,3-diene 34. 35. 38. Impurities such as phosphine (PH3) and hydrogen sulphide (H2S) are removed by passing acetylene through

acidified copper sulphate solution. 41. R CH2 CCl2 R 2NaNH2

R C C R + 2NaCl + 2NH3

This reaction is an example of double dehydrohalogenation. 43. 44. 45. 46. Chloroform (or iodoform) undergoes dehalogenation on heating with silver powder to form acetylene.

47. 2C + H2 Electric arc3270K HC CH

CH3 – C CHPropyne

Cyclopropene

HO H + C C + H OH H C C H + Ca(OH)2

Calcium carbide

Ca Acetylene Water Water

2PH3 + 3CuSO4 Cu3P2 + 3H2SO4Cupric

phosphide (Black precipitate)

H2S + CuSO4 CuS + H2SO4 Black

precipitate

Ethylene tetrabromide

(1,1,2,2-Tetrabromoethane)

Acetylene(Ethyne)

H C C H + 2Zn Alcohol H C C H + 2ZnBr2

Br Br

Br Br

(2,2,3,3-Tetrabromobutane)

2-ButyneCH3 C C CH3 + 2Zn Alcohol

CH3 C C CH3 + 2ZnBr2

Br Br

Br Br

CHCOOK + 2H2O Electrolysis CH + 2CO2 + H2 + 2KOHPotassium

maleate Acetylene

CHCOOK CH

CHCl3 + 6Ag + CHCl3 CH CH + 6AgCl Chloroform AcetyleneChloroform

Acetylene

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Absolute Chemistry Vol - 1.2 (Med. and Engg.) 49. 51. The boiling point of alkynes is comparatively higher than that of the corresponding alkanes and alkenes. 55. 2CH CH + 2Na 2CH CNa+ + H2 Acetylene Monosodium acetylide 56. Alkanes and alkenes do not show this type of reaction. 2H C C H + 2 Na 2HC C– Na+ + H2 2H C C–Na+ + 2Na 2Na+ –C C– Na+ 57. CH3 C C H 2

3

NaNHNH CH3 C C Na+

Methyl acetylene Sodium methyl acetylide 58. The hydrogen atoms attached to the triply bonded carbon atoms in alkynes are acidic in nature. Acetylene (H – C C – H) reacts with strong base such as sodamide (NaNH2) to give sodium acetylide. 60. Ethyne contains two terminal hydrogen atoms bonded to sp-hybridized carbon, which is highly

electronegative and hence, it is more acidic as compared to others. As the number of alkyl groups on sp-hybridised carbon increases, acidity decreases due to +I effect. Hence, the correct increasing order of acidity for the given compounds is,

Propane < Ethene < Propyne < Ethyne (II) (III) (I) (IV) 62. +I-effect decreases the stability of carbanions. The presence of a methyl group destabilizes the carbanion.

Therefore, carbanion (ii) is more stable than carbanion (i). An sp3 hybridised carbon atom is less electronegative than an sp hybridised carbon atom. Therefore, the carbanion (iii) is less stable than both carbanions (i) and (ii). Hence, the correct order of decreasing stability of carbanion can be given as:

63.

Disodium acetylide

Monosodiumacetylide

H – C C – H + Na+2NH H – C C– Na+ + NH3

H – C C–Na+ + Na+2NH Na+C– C– Na+ + NH3

Acetylene Sodamide Monosodium acetylide

Ammonia

AmmoniaDisodiumacetylide

Monosodium acetylide

H – C C > H3C – C C > CH3 – CH2– – –

(ii) (i) (iii)

Ethyne Ethene

Ethane

H – C C – H + H2 Raney Ni H – C = C – H Raney Ni/H2

H – C – C – H

H H H H

H H

CH CH + 2NaBr + 2NH3

Br Br

Acetylene

EtheneAcetic acid EthanolCH3COOH LiAlH4 CH3 CH2 OH H

443K

CH2 = CH2 Br2

CCl4 CH2 – CH2 2NaNH2

‘A’ ‘B’

‘D’

1,2-Dibromoethane‘C’

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Chapter 15: Alkynes 65. 66. Acetylene reacts with 2 moles of chlorine gas to give acetylene tetrachloride which is known as Westron. 67. 68. 72. The reaction takes place according to Markownikoff’s rule, 73. 75. 76.

CH3 – C C – CH2 – CH3 Cl2 CH3 – C = C – CH2 – CH3 Cl2 CH3 – C – C – CH2 – CH3 Pent-2-yne

Cl Cl Cl

Cl

Cl

Cl2,3-Dichloropent-2-ene

2,2,3,3-Tetrachloropentane

AcetyleneAcetylene dibromide

(trans-1,2-Dibromoethene)

H – C C – H Bromin e water H

C = CBr

Br H

Acetylene

1,1,2,2-TetrabromoethaneTrans-1,2-Dibromoethene

H – C C – H Br2CCl4

H

C = CBr

Br H Br2

CCl4 H – C – C – H

Br Br

Br Br

2,2-Dichloropropane

CH3 C C H (g)HClexcessCH3 C = CH2 HCl CH3 C CH3

Cl Cl

Cl

Propyne

2-Chloropropene

H3C – C CH H O, H SO2 2 4HgSO4

H3C – C = CH2 Intermediate(enol form)

(A)

Product (Propanone)

(B)

(Markownikoff addition)

Propyne

O

OH

H3C – C – CH3

Acetylene Vinyl bromide

Ethylidene dibromide

H C C H HBr [CH2 = CH Br] HBr CH3 CH Br

Br

Tautomerism

CH3 – CH2 – C CH + H – OH 40% H SO2 41% HgSO4

[CH3 – CH2 – C = CH2] But-1-yne (Markownikoff

addition) OH

CH3 – CH2 – C – CH32-Butanone

O

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Absolute Chemistry Vol - 1.2 (Med. and Engg.) 77. 78. 79. 80. i. ii.

81.

82. Ethyne undergoes cyclic polymerisation to form benzene in a red hot iron tube at 873 K. 83.

1-Butyne Propanoic acid CH3 CH2 C CH alk. KMnO4

373 383 K CH3CH2COOH + CO2

CH3 C CH O AlkalineKMnO4373 383 K CH3 COOH + CO2

(C3H4) Propyne

Ethanoic acid ‘Y’

Benzene Acetylene

+ red hot iron tube873K

CH CH

CH CH

CH

CH

Red hot iron

3 molecules of methylacetylene (Propyne)

HC

CH3

H3C – CHC

CH

C – CH3

C HC

CH3 – C

CH

C – CH3

C

CH

CH3

1,3,5-Trimethylbenzene (Mesitylene)

O

CH3 C CH O /CH Cl3 2 2196K CH3 C CH Zn H O2 CH3 C C H + ZnO

OO

O

Propyneozonide

2-Oxopropanal‘Z’

O(C3H4)

Propyne

CH CH + 2[O] KMnO /OH4

CH = O COOH

Acetylene Glyoxal Oxalic

acid

CH = O O

H COOH

CH CH O /CH Cl3 2 2196 200K H C CH Zn/H O2 H C C H + ZnO

EthyneO O

O

Ethyne ozonide

Glyoxal (1,2-Ethanedial)

O O

O

OH O AlkalineKMnO4

373 383 K CH3 C CH O /CH Cl /196K3 2 2Zn H O2

‘Y’‘Z’

(C3H4)

+ ZnOH

O

O

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Chapter 15: Alkynes

84. 86. Terminal alkynes (1-alkynes) can be converted to non-terminal alkynes (2-alkynes) on treating terminal

alkynes with ethanolic potassium hydroxide (alcoholic potash). 87. 90. 2CH CH + 5O2 4CO2 + 2H2O, H = 1300 kJ The combustion of acetylene is highly exothermic and the heat produced during the combustion can be used

for welding purposes in the form of oxyacetylene flame. 91. 93. 95. i. ii. 97. The s-character of C H bond in acetylene is higher in comparison to that of C H bond in ethene and

ethane. The electrons of the C H bond in acetylene are strongly held by carbon nuclei thus facilitating the removal of hydrogen as proton.

Red hot iron tube873 K

3 molecules of but-2-yne

C

CH3

C C

C CH3

C – CH3

C

CH3

CH3

CH3

Hexamethylbenzene

H3C

H3C

CH3

CH3H3C

H3C

CH3 – CH2 – C CH alcoholic KOH CH3 – C C – CH3

But-1-yne (Terminal alkyne)

But-2-yne (Non-terminal alkyne)

AlkalineKMnO ,373 383 K4

Oxidation 2CH3COOH

Ethanoic acid ‘R’

CH3 CH2 CH CH2 Dehydrohalogenation2NaNH ,2 CH3 CH2 C CH

alc.KOHIsomerisation

1,2-Dibromobutane

Br Br

But-1-yne ‘P’

But-2-yne‘Q’

CH3 C C CH3

CH

CH+ Anhydrous

AlCl3

Cl

AsCl2

CHCl

CHAsCl2

Ethyne Arsenic trichloride

Lewisite

CH3 – C C – H sp3 sp sp

CH CH H O,40%,H SO2 2 421% Hg ,

CH = CH2 OHC CH3

Ethyne ‘X’

Vinyl alcohol

Ethanal

OH

CH3 CH = CH CH3 O /CH Cl3 2 2196 K CH3 – CH

O O

OCH CH3 Zn/H O2 2CH3CHO + ZnO

But-2-ene‘Y’

But-2-ene ozonide

Ethanal

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

99. 100. 101. 102. Ethane does not decolourize KMnO4 whereas acetylene and ethene decolourize KMnO4 solution as both are

unsaturated hydrocarbons. 103. 104. The terminal H atom attached to sp-hybridized C-atom of propyne is highly acidic in nature and hence can

be easily approached by Grignard reagents. 105. Alkynes are more acidic than alkenes, which are in turn more acidic than alkanes. This is due to difference

in electronegativity of sp, sp2 and sp3 hybridized carbon atoms. Order of electronegativity is sp 2 3sp spC C C .

In addition, the +I effect decreases the acidic strength of alkynes and hence the correct order of acidic strength is,

CH CH CH3 – C CH CH2 = CH2 CH3 – CH3.

OH

Al O2 3623 K 2Br2

CCl4 Zn

alcohol

Br Br

Br Br Pentan-2-ol Pent-2-ene

2,2,3,3-Tetrabromopentane

Pent-2-yne

NaNH /liq.NH2 3 Pent-1-yne

(x) (y) (z)

(w)

H C C H NaNH /liq.NH2 3 H C CNa+

CH CH Br3 2 H C C CH2 CH3 + NaBr Ethyne But-1-yne

(X)

NaNH2/liq.NH3

NaBr + CH3 CH2 C C CH2 CH3 CH CH Br3 2 +NaC C CH2 CH3

Hex-3-yne(Y)

HC C – C = CH2

H(C4H4)

CH3 CH CH2Zn/alcohol

CH3 CH = CH2 Br /CCl2 4 CH3 CH CH2 2NaNH2

CH3 C CH

Cl Cl

Br Br

1,2-Dichloropropane

Propene‘A’

1,2-Dibromopropane ‘B’

Propyne‘C’

H O,40% H SO ,2 2 421% Hg

CH3 C = CH2

Isomerization CH3 C CH3

OH O

Propanone‘D’

CH3 – C CH + CH3MgBr dryether CH3 – C CMgBr + CH4

Propyne Propynemagnesium bromide

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Chapter 15: Alkynes

1. A straight chain hydrocarbon has the

molecular formula C8H10. The hybridization for the carbon atoms from one end of the chain to the other are respectively sp3, sp2, sp2, sp3, sp2, sp2, sp and sp. The structural formula of the hydrocarbon would be _______.

(A) CH3 C CCH2 CH= CH CH = CH2 (B) CH3CH2CH=CHCH2CCCH= CH2 (C) CH3CH=CHCH2 C C CH = CH2 (D) CH3CH=CHCH2CH=CH C CH 2. Which of the following reactions will give an

alkyne? (A) CH3CBr2CHBr2

Zn/alc/

(B) CH3CH2CHBr2 2

i. alc.KOH/ii.NaNH /

(C) CH3CHBrCH2Br 22NaNH / (D) All of these 3. Which of the following is an isomer of

but-2-yne? (A) (B) (C) (D) 4. What is the product obtained when large

excess of HCl(g) is reacted with 2-butyne? (A) 2,2-Dichlorobutane (B) 2,3-Dichlorobutane (C) 2-Chloro-2-butene (D) 3-Chloro-2-butane 5. Write the IUPAC name for the following structure. (A) 3-Methylpent-1-yne (B) 3-Ethylbut-1-yne (C) 3-Methylpent-4-yne (D) 2-Ethylbut-3-yne

6. Identify the reagents required for the conversion of:

Propyne i. xii. y Pent-2-yne

(A) x = Alcoholic KOH y = Alkaline KMnO4/ (B) x = H2O/dil H2SO4/1% Hg2+ y = methyl chloride (C) x = Sodamide y = ethyl bromide (D) x = 2Br2/CCl4 y = Zn/alcohol 7. When propyne is treated with aqueous H2SO4

in presence of HgSO4, the major product is _______.

(A) propanone (B) 2-propanol (C) ethanal (D) ethyne 8. 2,3-Dibromobutane on treatment with

alcoholic KOH gives an intermediate B, which on treatment with sodamide gives a hydrocarbon C with sp hybridised carbon atoms. Identify C. (A) CH3 – C C – CH3

(B) CH3 – C CH (C) CH3 – CH2 – CH2 – COOH (D) 9. Ethyne, on passing through red hot iron tube

at 873 K undergoes _______. (A) halogenation (B) oxidation (C) cyclic polymerisation (D) linear polymerisation 10. _______ is the other name for acetylene

tetrachloride. (A) Westron (B) Cicutoxin (C) Capillin (D) Citronella

Topic Test

HC C – CH – CH2 – CH3

CH3

CH3 – CH – CH2 CH2Br

OH

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Absolute Chemistry Vol - 1.2 (Med. and Engg.)

11. Acetylene on treatment with liquid bromine yields _______.

(A) acetylene monobromide (B) 1,1,2,2-tetrabromoethane (C) 1,2-dibromoethene (D) acetylene tribromide 12. _______ isomerism is observed in alkynes. (A) Optical (B) Functional (C) Conformational (D) Geometrical 13. An alkyne on ozonolysis forms 2-oxobutanal

as its final product. The alkyne is _______. (A) but-2-yne (B) pent-2-yne (C) but-1-yne (D) pent-1-yne 14. H /Raney Ni 773K,10 20 atm2

V O2 5P Q

Identify Q. (A) 2-Methylpentane (B) Benzene (C) 2,2-Dimethylbutane (D) 3-Methylpent-1-ene 15. Ethyne adds to HCl in presence of Hg2+ to

give _______. (A) ethyl chloride (B) methyl chloride (C) vinyl chloride (D) ethane

1. (D) 2. (D) 3. (A) 4. (A) 5. (A) 6. (C) 7. (A) 8. (A) 9. (C) 10. (A) 11. (B) 12. (B) 13. (C) 14. (B) 15. (C)

Answers to Topic Test

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