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12ChF322 Basics Concepts Basic Concepts in Organic Chemistry Organic chemistry is the chemistry of molecules, predominantly associated with living systems, which consist of carbon skeletons with FUNCTIONAL GROUPS attached (these are generally the bits which do the chemistry) Page 1 Candidates should be able to: (a) interpret and use the terms: (i) empirical formula as the simplest whole number ratio of atoms of each element present in a compound, (ii) molecular formula as the actual number of atoms of each element in a molecule, (iii) general formula as the simplest algebraic formula of a member of a homologous series, ie for an alkane: C n H 2n + 2 , (iv) structural formula as the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH 3 CH 2 CH 2 CH 3 or CH 3 (CH 2 ) 2 CH 3 , (v) displayed formula as the relative positioning of atoms and the bonds between them (vi) skeletal formula as the simplified organic formula, shown by removing hydrogen atoms from alkyl chains, leaving just a carbon skeleton and associated functional groups; (b) interpret, and use, the terms: (i) homologous series as a series of organic compounds having the same functional group but with each successive member differing by CH 2 , (ii) functional group as a group of atoms responsible for the characteristic reactions of a compound; (c) use the general formula of a homologous series to predict the formula of any member of the series; (d) state the names of the first ten members of the alkanes homologous series; (e) use IUPAC rules of nomenclature for systematically naming organic compounds; (f) describe and explain the terms: (i) structural isomers as compounds with the same molecular formula but different structural formulae, (ii) stereoisomers as compounds with the same structural formula but with a different arrangement in space, (iii) E/Z isomerism as an example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group, (iv) cis-trans isomerism as a special case of EIZ isomerism in which two of the substituent groups are the same; (g) determine the possible structural formulae and/or stereoisomers of an organic molecule, given its molecular formula; (h) describe the different types of covalent bond fission: (i) homolytic fission forming two radicals, (ii) heterolytic fission forming a cation and an anion; (i) describe a ‘curly arrow’ as the movement of an electron pair, showing either breaking or formation of a covalent bond; (j) outline reaction mechanisms, using diagrams, to show clearly the movement of an electron pair with ‘curly arrows’; (k) carry out calculations to determine the percentage yield of a reaction; (l) explain the atom economy of a reaction as: (molecular mass of the desired products sum of molecular masses of all products) × 100%; (m) explain that addition reactions have an atom economy of 100%, whereas substitution reactions are less efficient; (n) carry out calculations to determine the atom economy of a reaction; (o) describe the benefits of developing chemical processes with a high atom economy in terms of fewer waste materials; (p) explain that a reaction may have a high percentage yield but a low atom economy.

F322 Organic Molecules - Basics

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Page 1: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

Basic Concepts in Organic Chemistry

Organic chemistry is the chemistry of molecules, predominantly associated with living systems, which consist of carbon skeletons with FUNCTIONAL GROUPS attached (these are generally the bits which do the chemistry)

Because these molecules are based on non-metals, it is predominantly covalent bonding we need to consider.

We are not dealing with ions in solution or crystal lattices, but discrete molecules, so simple molecular structures, with low melting and boiling points.

Page 1

Candidates should be able to:(a) interpret and use the terms:

(i) empirical formula as the simplest whole number ratio of atoms of each element present in a compound,(ii) molecular formula as the actual number of atoms of each element in a molecule,(iii) general formula as the simplest algebraic formula of a member of a homologous series, ie for an alkane: CnH2n + 2,(iv) structural formula as the minimal detail that shows the arrangement of atoms in a molecule, eg for butane: CH3CH2CH2CH3 or CH3(CH2)2CH3,(v) displayed formula as the relative positioning of atoms and the bonds between them (vi) skeletal formula as the simplified organic formula, shown by removing hydrogenatoms from alkyl chains, leaving just a carbon skeleton and associated functional groups;

(b) interpret, and use, the terms:(i) homologous series as a series of organic compounds having the same functional group but with each successive member differing by CH2,(ii) functional group as a group of atoms responsible for the characteristic reactions of a compound;

(c) use the general formula of a homologous series to predict the formula of any member of the series;(d) state the names of the first ten members of the alkanes homologous series;(e) use IUPAC rules of nomenclature for systematically naming organic compounds;(f) describe and explain the terms:

(i) structural isomers as compounds with the same molecular formula but different structural formulae,(ii) stereoisomers as compounds with the same structural formula but with a different arrangement in space,(iii) E/Z isomerism as an example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group,(iv) cis-trans isomerism as a special case of EIZ isomerism in which two of the substituent groups are the same;

(g) determine the possible structural formulae and/or stereoisomers of an organic molecule, given its molecular formula;(h) describe the different types of covalent bond fission:

(i) homolytic fission forming two radicals,(ii) heterolytic fission forming a cation and an anion;

(i) describe a ‘curly arrow’ as the movement of an electron pair, showing either breaking or formation of a covalent bond;(j) outline reaction mechanisms, using diagrams, to show clearly the movement of an electron pair with ‘curly arrows’;(k) carry out calculations to determine the percentage yield of a reaction;(l) explain the atom economy of a reaction as: (molecular mass of the desired products sum of molecular masses of all products) × 100%;(m) explain that addition reactions have an atom economy of 100%, whereas substitution reactions are less efficient;(n) carry out calculations to determine the atom economy of a reaction;(o) describe the benefits of developing chemical processes with a high atom economy in terms of fewer waste materials;(p) explain that a reaction may have a high percentage yield but a low atom economy.

Page 2: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

Functional Groups

"An atom or group of atoms responsible for the characteristic reactions of a compound"These gives organic chemistry its structures and systematic nature.

C=C the alkene functional group, a double bond between to C atoms e.g. ethene

Don't confuse this with benzene, represented as:It is an arene, not an alkene. Well study it at A2. or

-I, -Br or -Cl the halogenoalkane functional groupe.g. chloromethane

-OH the alcohol functional groupe.g. ethanol

The carbonyl group, C=O shows up in four different familiesof organic molecules:-CHO those with the aldehyde functional group

e.g. ethanal |

C=O those with the ketone functional groupe.g. propanone

-COOH those with the carboxylic acid functional groupe.g. ethanoic acid

-COO- and those with the ester functional group e.g.methyl ethanoate

and finally we have:

-NH2 the amine functional groupe.g. methylamine

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C C

H

H

H

H

C

H

ClH

H

C

H

H

H

C O H

H

H

C

H

H

H

C

H

O

H C

H

H

C

O

C

H

H

H

C

H

H

H

C

O

O H

C

H

H

H

C

O

O C

H

H

H

C

H

H

H

N

H

H

Page 3: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

Types of FormulaeDepending on the information we need to convey (or that we have available) we can represent organic compounds using a variety of formulae. In increasing order of information content:

Examples for HEXANE - a chain of 6 carbon atoms with 14 hydrogens bonded to it.

empirical formula : simplest whole-number ratio C3H7

molecular formula : number of each atom type in molecule C6H14

structural formula : the minimum info necessary necessary to show how the atoms are arranged in the molecule CH3CH2CH2CH2CH3

we can also use brackets in structural formulae e.g. CH3(CH2)3CH3

displayed formula : shows all bonds and all atoms(2-D representation of 3-D molecules)

skeletal formula : shows the carbon skeleton (but not C and -H)and emphasises functional groups.

Naming ConventionsIUPAC nomenclature is standardised to precisely describe the structure of molecules Basic rules apply: 1: stem of the name comes from the longest carbon chain (which may be bent !)

methane CH4 hexane C6H14

ethane C2H6 heptane C7H16

propane C3H8 octane C8H18

butane C4H10 nonane C9H20

pentane C5H12 decane C10H22

2: alkyl side chains have ‘’-yl’ suffix methyl-, ethyl-, propyl-

e.g. methylpropane

We need to introduce a number if it is ambiguous which carbon in the chain the alkyl group is connected to. We number from whichever end means using the smallest number in the name:e.g. 2-methylheptane, 3-methylheptane, 4-methylheptane

Page 3

Page 4: F322 Organic Molecules - Basics

H C

H

H

C

H

H

C

H

C

H

H

C

H

H

H

CH H

H

CH2

CH2

CH2

CH2

CH2CH2

12ChF322 Basics Concepts

Check your understanding:i) Name this molecule

ii) Why can't we have 5-methylheptane or 1-methylhexane ?

iii) What’s wrong with calling a molecule 2-ethylpentane ?

3: If we have more than one of the same side group we use a ‘di-‘, ‘tri-‘ etc. prefix and position numbers in the form 2,3- where needed

e.g. 2,3-dimethylhexane 2,2,4-trimethylpentane

iv) What’s wrong with calling a molecule trimethylhexane ?

4: If different alkyl groups are present they are named in alphabetical ordere.g. 3-ethyl-2-methylpentane

5: If the carbon skeleton forms a ring rather than a chain we use a cyclo- prefix e.g. cyclopentane, cyclohexane

NOTE: when writing structural formulae for rings we need to show that they are not chains, so the formula becomes semi-displayed e.g. for cyclohexane we might write:

Check your understanding:v) How many different alkanes with C6H14 molecular formula can you make ?

Draw their skeletal formulae and name them.

Page 4

Page 5: F322 Organic Molecules - Basics

F C

F

F

C

H

H

H

12ChF322 Basics Concepts

6: Alkenes use the same stem, followed by an -ene ending. If the position of the C=C is ambiguous, an position number is used, referring to the carbon in the chain where the double bond starts

e.g. but-2-ene CH3CH=CHCH3 propene

if we have more than one C=C then the name ends with –diene or –triene etc. and we need numbers to show where the double bonds are.

e.g. buta-1,3-diene

7: Halogenoalkanes are named the same way as alkyl-substituted alkanes, with numbers if needed to show which carbon the halogen is bonded to, and –di or –tri prefixes if there are more than one of the same halogen atom.

e.g bromoethane, 1-chlorobutane, 1,1,1-trifluoroethane

Check your understanding:vi) Name the isomers of dibromopropane. (It may help to draw them first).

8: Alcohols are named in a similar way to alkenes, with an –ol endingand number, if needed, to show which carbon has the –OH group on.

e.g. propan-1-ol, propan-2-ol (note absence of 'e')

If there is more than one –OH group we have a –diol or –triol etc. and need numbers to show where these groups go. We also include the "e" from the alkane name, which we didn't do when only one –OH group was present.

e.g. ethane-1,1-diol propane-1,2,3-triol.

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Page 6: F322 Organic Molecules - Basics

H C

H

H

C

H

H

C

CH3

H

C

O

H

C C

HH

C

H

H

H

C

HH

HC

C C

C

HH

HH

HH

H

H

12ChF322 Basics Concepts

9: Ketones are named in a similar way to alkenes, with an –one endingand a number, if needed, to show which carbon is in the functional group.

e.g. propanone, pentan-3-one

10: Aldehyde (-al endings) and carboxylic acid (-anoic acid endings) functional groups can only occur on the end of a carbon chain, so they do not need a number. There is only one possible butanoic acid, or butanal. The number of carbon atoms for the stem- name includes the carbon atom in the functional group. Numbering starts from that C when needed to show the position of other functional groups.

e.g. butanoic acid, butanal 2-methylbutanal

11: Amines are named using the alkyl- prefix, followed by –amine, or as amino-…ane (use this form if you need a number to show where the amine group goes).

e.g. ethylamine or 2-aminopropane

ISOMERS

1) STRUCTURAL ISOMERSDefined as having the same molecular formula but different structural formulae. We are familiar with the straight chain alkanes, but alkanes can also be branched so there can be several different structures.

Check your understanding:vii) Which is the shortest chain-length alkane to have structural isomers ?

2) STEREOISOMERSDefined as having the same structural formulae but with a different arrangement in space (which can't just be rotated to make them the same)

e.g. but-2-ene has two stereoisomers

These are not the same because there is no rotation around the double bond.

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Page 7: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

E/Z ISOMERISMThis is a form of stereoisomerism which we get with alkenes. Here the C=C bond cannot twist round (we say it has restricted rotation) – so when we have different groups attached to each carbon atom there are two ways in which they can be attached.

e.g. Z-but-2-ene E-but-2-ene

(The Z-isomer has the functional groups on the same side)

CIS-TRANS ISOMERISMThis is a special case of E/Z isomerism in which one of the groups on each C in the C=C is the same. The cis- isomer has these on the same side, and the trans- isomer has them on opposite sides across the double bond

e.g. cis-1,2-dichloroethene trans-1,2-dichloroethene

In order to have E/Z or cis-trans isomerism we have to have 2 conditions fulfilled: we have to have a C=C neither of these C atoms can have two identical atoms/groups bonded to it

Check your understanding:viii) Draw all the isomers of C4H8 you can think of. Which can have cis-trans isomers ?

NOTE: when we draw skeletal formulae or displayed formulae we need to show whether the molecule is E- or trans- (opposite sides); or Z- or cis- (same side).

Check your understanding:ix) Draw skeletal formulae of cis-hex-3-ene and trans-hex-3-ene

Page 7

Page 8: F322 Organic Molecules - Basics

H BrH

x

x x

x x

x x

H Br

H x

x x

x x

x x

H

BrH x

x x

x x

x x

+ -+

H3C - Br CH3+ + :Br

-

12ChF322 Basics Concepts

Bond BreakingA covalent bond consists of a pair of electrons lying between the nuclei of two atoms. The negatively charged electrons attract both nuclei, binding them together. We can break this type of bond in two ways:

Consider a molecule of hydrogen bromide:

Homolytic fission:When we break the H-Br bond each element can take one of the electrons from the bond. Each element now has a single unpaired electron. Atoms or groups of atoms with unpaired electrons are known as RADICALS.

Breaking in this way is known as homolytic fission.If a bond is broken homolytically, the energy to do this is usually provided by ultraviolet light or high temperature

Unpaired electrons are represented by a single dot. As an example, the homolytic fission of chloromethane to form a methyl free radical and a chlorine free radical may be represented as follows:

CH3-Cl CH3 + Cl

Heterolytic Fission:Alternately the bond may be broken so that one element takes both the covalent bond electrons, hence forming ions. This is heterolytic fission. The more electronegative element is the one which takes both the electrons.

The movement of two electrons from the bond to the same atom is shown using a curly arrow. For example, the heterolytic fission of bromomethane to form a positive methyl ion and a bromide ion may be represented as:

Note: Care with curly arrows. Must come from the bond and go to the atom.Note: Two dots may be used to show an unbonded (lone) electron pair

The CH3+ ion is an example of a CARBOCATION (CH3

- would be a CARBOANION).

Page 8

Page 9: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

Free radicals, carbocations and carboanions are all highly reactive and will react with molecules causing covalent bonds to break and new covalent bonds to form.

Industrial process chemistryIn industry when chemical reactions processes are carried out, two concepts are important to consider for process efficiency. These are the %yield and the atom economy.

Atom economyThe atom economy tells us how much waste there will be – for sustainable chemistry we try to minimise this. We use atom economy to help select between different possible reactions to create the product we want.

For example: Hydrogen can be manufactured by reacting methane (natural gas) with steam. Calculate the atom economy of this reaction.

CH4(g) + 2 H2O(g) CO2(g) + 4 H2(g)

% atom economy = 4 x 2 x 100 (12 + 16 + 16) + (4 x 2)

= 15.4% which is not very good at all !

Check your understanding:x) Hydrogen can also be produced by cracking ethane. Calculate the atom economy of this reaction.

C2H6(g) C2H4(g) + H2(g)

xi) Suggest a reason why the latter reaction may be chosen rather than the first despite the worse atom economy

Remember: Any reaction which only produces one product has 100% atom economy !

% yieldAnother consideration is how much of the product is likely to be made. There are a number of reasons why all the reactants may not be turned into products.

- impure reactants- losses when purifying products and removing from reaction vessels- other side reactions making unwanted products- incomplete reaction of products e.g. when equilibria are set up

The % yield is used to compare how much product we actually made (the yield) to how much we could have made in theory (a mole calculation).

Page 9

% atom economy = mass of the atoms in the product(s) we want x 100mass of all the atoms in the products (or reactants)

Page 10: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

% yield = mass of product obtained x 100 or moles of product obtained x 100 theoretical mass of product theoretical moles of product

For example: Methane can react with bromine when exposed to uv light, forming bromomethane. If 1.6g of methane reacted with excess bromine and 6.0g of bromomethane were formed, what was the % yield ?

CH4(g) + Br2(g) CH3Br(g) + HBr(g)

Step 1: do a mole calculation to find out how many moles of CH3Br would be made if all the methane formed bromomethane.

Moles of CH4 = mass RFM = 1.6 16 = 0.1 moles Mole ratio from equation is 1:1 so 0.1 moles of methane makes 0.1 moles of CH3Br Mass of CH3Br = moles x RFM = 0.1 x (12 + 3 + 79.9) = 9.49g

Step 2: work out % yield

% yield = 6.0 x 100 = 63.22% 9.49

Check your understanding:xii) Methane can also react with chlorine when exposed to uv light, forming chloromethane. If 1.6g of methane reacted with excess chlorine and 4.0g of chloromethane were formed, what was the % yield ?

Page 10

Page 11: F322 Organic Molecules - Basics

H C

H

H

C

H

H

C

H

C

H

H

C

H

H

H

CH H

H

12ChF322 Basics Concepts

Answers to Check your Understanding questions:

i) Name this molecule

- the longest carbon chain is 5 long, so it’s a pentane- there is a methyl group on the 3rd carbon, so it 3-methylpentane

ii) Why can't we have 5-methylheptane ?- because if we number from the other end of the chain we'd get a smaller number in the name. Its correctly named 2-methylheptane

or 1-methylhexane ?- because a methyl group on the first carbon of a chain just means the chain is one carbon longer than you thought ! It would be heptane.

iii) What’s wrong with calling a molecule 2-ethylpentane ?- an ethyl group on the second carbon means you haven't found the longest chain.In fact the ethyl group is part of the main chain and you have a methyl group on the second carbon, meaning the molecule is actually 2-methylhexane.

iv) What’s wrong with calling a molecule trimethylhexane ?- you don't know where the three methyl groups go. There are lots of possible molecules all of which could be called trimethylhexane, such as 2,2,3-trimethylhexane, or 2,3,4-trimethylhexane etc.

v) How many different alkanes with C6H14 molecular formula can you make ? Draw their skeletal formulae and name them.

hexane, 2-methylpentane, 3-methylpentane,

2,2-dimethylbutane, 2,3- dimethylbutane

vi) What are the isomers of dibromopropane ? 1,1-dibromopropane 1,2-dibromopropane1,3-dibromopropane2,2-dibromopropane

vii) Which is the shortest chain-length alkane to have structural isomers ?butane – which has the isomer methylpropane

Page 11

Page 12: F322 Organic Molecules - Basics

12ChF322 Basics Concepts

viii) Draw all the isomers of C4H8 you can think of. Which can have cis-trans isomers ?but-1-ene; no cis- trans- isomers becuase 2 x H on the first Cbut-2-ene; yes, has cis- and trans- isomersmethylpropene no, both C have identical groups on themcyclobutane no, doesn't have a C=C double bond

ix) Draw skeletal formulae of:

cis-hex-3-ene trans-hex-3-ene

x) Hydrogen can also be produced by cracking ethane. C2H6(g) C2H4(g) + H2(g) Calculate the atom economy of this reaction.

% atom economy = 2 x 100 = 6.7% 28 + 2

xi) Suggest a reason why the latter reaction may be chosen rather than the first despite the worse atom economy

- ethene is itself a useful product, if used rather than wasted the %Atom Economy would be 100%

- the energy costs, or the raw materials may be cheaper/more readily available

xii) Methane can also react with chlorine when exposed to uv light, forming chloromethane. If 1.6g of methane reacted with excess chlorine and 4.0g of chloromethane were formed, what was the % yield ?

Moles of CH4 = mass RFM = 1.6 16 = 0.1 moles Mole ratio from equation is 1:1 so 0.1 moles of methane makes 0.1 moles of CH3Cl Mass of CH3Br = moles x RFM = 0.1 x (12 + 3 + 35.5) = 5.05g

% yield = (4.0 / 5.05) x 100 = 79.2%

Page 12