Selected topics in Chemistry

ORGANIC CHEMISTRYHydrocarbons: Alkanes, Alkenes, and Alkynes

Organic chemistry

The study of carbon compounds The first known organic compounds were

derived from animal and vegetable sources and were difficult to crystallize and purify.

Swedish chemist Torbern Bergman was the first person to express the difference between “organic” and “inorganic” substances, and the phrase organic chemistry soon to came to mean “the chemistry of compounds from living organisms”.

Organic Chemistry

To many chemist at that time, their only explanation for the difference in behavior between organic and inorganic compounds was that organic compounds contained a peculiar and “vital force” (vital force theory) as a result of their derivation from living sources.

Michel Chevreul (1816)Animal fat NaOH soap + Glycerine

Soap H3O+ “fatty acids”

Organic Chemistry

Friedrich Wohler (1828)- it was possible to convert the “inorganic”

salt, ammonium cyanate, into “organic” substance, urea.

O NH4

+-OCN heat H2N C NH2

Why is Carbon special? Why is it that sets Carbon apart from all other elements in the periodic table?

-The unique ability of carbon to bond together forming long chains and rings.

Carbon, alone of all elements, is able to form an immense diversity of compounds, from the

simplest to the staggeringly complex: from methane to DNA, which can contain hundreds

of billions.

Important Families of Organic Compounds

HydrocarbonsAlkanesAlkenes

Only C and H presentSingle bondDouble bond

Alkynes

Aromatic

Alcohols

Triple bond

R O H

Ethers

Aldehydes

R O R’ O

R C H

Ketones O

R C R

Esters O

R C OR

Carboxylic acids

O

R C OH

Amines

R’’(H)

R N R’(H)

Amides

O R

R C N R

HYDROCARBONS

Hydrocarbons are organic compounds that consist of only C and H atoms.

Hydrocarbons come in a variety of forms. They may be gases (methane and paropane), liquids (hexane and benzene), waxes (paraffin wax), or polymers (polyethylene and polystyrene).

Hydrocarbons can be processed to create plastics.

They are insoluble in water.

HYDROCARBONS

The simplest class of organic compounds. Nonpolar There are four main types of

hydrocarbons:- saturated hydrocarbons (alkanes)- unsaturated hydrocarbons (alkenes or alkynes)- cycloalkanes- aromatic hydrocarbons

SATURATED means that each carbon is bonded to four other atoms through single covalent bonds. Hydrogen atoms usually occupy all available bonding positions after the carbons have bonded to each other.

UNSATURATED hydrocarbons contain either double or triple bonds. Since the compound is unsaturated with respect to hydrogen atoms, the extra electrons are shared between 2 carbon atoms forming double or triple bonds.

HYDROCARBONS

ALKANES

made up of only carbon and hydrogen atoms

Also called “paraffins” single bonds only

between carbon atoms

general formula: CnH2n+2

(assuming non-cyclic structures)

name ends in "ane"

IUPACname

MolecularFormula

StructuralFormula

BoilingPoint (°C)

MeltingPoint (°C)

Density(g/ml, 20°C)

Methane CH4 CH4 -161.5 -182.5

Ethane C2H6 CH3CH3 -88.6 -183.3

Propane C3H8 CH3CH2CH3 -42.1 -189.7

Butane C4H10 CH3(CH2)2CH3 -0.5 -138.4

Pentane C5H12 CH3(CH2)3CH3 36.1 -129.7 0.626

Hexane C6H14 CH3(CH2)4CH3 68.7 -95.3 0.659

Heptane C7H16 CH3(CH2)5CH3 98.4 -90.6 0.684

Octane C8H18 CH3(CH2)6CH3 125.7 -56.8 0.703

Nonane C9H20 CH3(CH2)7CH3 150.8 -53.5 0.718

Decane C10H22 CH3(CH2)8CH3 174.1 -29.7 0.730

Alkanes

Many of these (alkanes) substances are familiar because of their widespread use.Example: Methane (CH4 ), a major component of natural gas

- propane (C3H8), major component of bottled gas

- butane (C4H10), used in disposable lighters and in fuel canisters for gas camping stoves and lanterns.

- alkanes with from 5 to 12 carbon atoms per molecule are found in gasoline.

 IUPAC NOMENCLATURE

Key Concepts: - Name and number the longest carbon chain as for

a straight chain alkane Identify each of the branches (side-chains). Use the lowest number combinations for the branches

(side-chains). Name each branch or side-chain (alkyl group) :

methyl CH3

ethyl C2H5

propyl C3H7

Nomenclature

For more than 1 of the same alkyl group use:di = 2tri = 3tetra = 4

Use commas between numbers, eg, 1,2 or 2,3 Use hyphens between numbers and words, eg, 2-methyl

or 2,3-dimethyl If there is more than 1 type of branch or side-chain,

arrange their names alphabetically, ie, ethyl groups are named before methyl groups which are named before propyl groups

Structural Isomers

Isomers- compounds with the same molecular formula but

different in structures.

EXAMPLE: C4H10

H H H H H H H H - C – C – C - C- H H - C – C – C – H H H H H H H(CH3CH2CH2CH3 ) CH3

Butane Isobutene

Sample exercises

Name the following alkane:1. CH3 CH3

CH3CH – CHCH2CH2CH3

2. CH3

CH3 - CH2 – CH2 – CH2 – CH – CH2 – CH3

Sample Exercises:

Write the structural formula for?1. 3-ethyl,3-methylheptane2. 2-methylbutane3. 2,2-dimethylpropane

Reaction of Alkane

CombustionAlkane reacts with O2 to produce CO2 and Water

CH4 + 2 O2 → 2 H2O + CO2 + Energy C3H8 + 5 O2 → 4 H2O + 3 CO2 + Energy

ALKENES

containing a carbon-carbon double bond

Also called as “Olefins”

More reactive than alkanes

General formula: CnH2n

(where n is any integer)

Naming Alkenes and Alkynes

Name the longest carbon chain that contains the double or triple bond (see table). The name for the alkenes ends in ene instead of –

ane. The name for the alkynes ends in –yne instead of –

ane.  Number the main chain from the end nearest the double

or triple bond. Indicate the position of the double or triple bond with

the number of the first unsaturated carbon. Place the number and names of substituents in front of

the alkene or alkyne name.

Alkenes

The simplest alkenes are ethylene and propylene.

They are the two most important organic chemicals produced industrially.

Ethylene is a plant hormone. It plays important roles in seed germination and ripening of fruits.

For alkenes with four or more carbon atoms, several isomers (same formula but different structures) exist for each molecular formula.

AlkenesFor example: C4H8

H3C CH3 H3C HC=C C=C

H H H3C H cis-2-butene 2-methylpropene CH3 H2C H H3C H

C=C C=C H H H CH3

1-butene trans-2-butene

Geometric Isomers (cis and trans)- compounds that have the same

molecular formula and the same groups bonded to one another but differ in the spatial arrangement of these groups.

Sample Exercises:

Give the name of the ff. alkenes:1. CH3

CH3 - CH2 – CH2– CH – CH = CH - CH3

2. Br CH3 CH2 CH=CCH3

Sample Exercises

Give the structural formula for:

1. 1,4-pentadiene2. 3-ethyl-1-hexene3 . tra ns -2,3-dichloro-2-butene4. 1,3,5-hexatriene

ALKYNES

C C bonds More reactive than

alkanes and alkenes General formula:

CnH2n-2

Highly reactive molecules

Alkynes

C2H2, ethyne or acetylene is the most simplest alkyne and it is highly reactive molecule. When acetylene is burned in a stream of oxygen in an oxyacetylene torch, the flame reaches a very high temperature, about 3200 K.

Nomenclature of Alkynes

Sa m e with Alke ne s … . . However, when there is a choice in

numbering, d o uble bo nd s re c e ive lo we r num be rs tha n trip le bo nds …..

Example:

CH3CH CHC CCH2CH3

2-heptene-4-yne

Sample Exercises

Name the ff. compounds:1.

CH3 - CH2 – CH2– CH –C CH - CH2 CH3

CH2 CH3

5-ethyl-3-octyne

2 HC C - C(CH3)2CH2C CH

CH3

3. HC C – C CH3

CH3

Answer: 2. 3,3-dimethyl-1,5-hexediyne3. 3,3-dimethyl-1-butyne

4. CH3CH CHCH CHC CCH3

2,4-Octadiene-6-yne

Organic Chemistry

Reported by:Maria Crsitina d.L. Payofelin

Selected Topics in Chemistry