Introduction to AS Level Organic Chemistry

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Cambridge International AS & A Level Chemistry (9701)

Introduction to AS Level Organic Chemistry

The big idea: An organic molecule is a carbon skeleton carrying a functional group, and the functional group decides how it reacts. Learn to read every type of formula, name the compound, say what kind of reaction it is doing, and spot every isomer it can have.

Summary — What This Chapter Covers

A hydrocarbon contains C and H atoms only; alkanes are hydrocarbons with no functional group.
General, structural, displayed and skeletal formulas, and systematic names up to six carbon atoms.
Reaction vocabulary: homologous series, homolytic and heterolytic fission, radicals, nucleophiles, electrophiles, addition, substitution, elimination, hydrolysis, condensation, [O] and [H].
sp³, sp² and sp carbon: shapes, bond angles, σ and π bonds; ethene is planar.
Structural isomerism (chain, positional, functional group) and stereoisomerism (cis/trans, optical).

1. Formulas, Functional Groups and Naming

13.1

Hydrocarbons and functional groups

A hydrocarbon is a compound made up of carbon and hydrogen atoms only. Alkanes are the simplest hydrocarbons: single C–C and C–H bonds and no functional group. A functional group is the atom or group of atoms that gives a homologous series its characteristic chemical (and much of its physical) behaviour.

Homologous series	Functional group	Example
alkene	C=C	CH₃CH=CH₂, propene
halogenoalkane	–X (halogen)	CH₃CH₂CH₂Cl, 1-chloropropane
alcohol (1°, 2°, 3°)	–OH (hydroxyl)	CH₃CH₂CH₂OH, propan-1-ol
aldehyde	–CHO (carbonyl at chain end)	CH₃CH₂CHO, propanal
ketone	C=O within the chain	CH₃COCH₃, propanone
carboxylic acid	–COOH (carboxyl)	CH₃CH₂COOH, propanoic acid
ester	–COO–	CH₃COOCH₂CH₃, ethyl ethanoate
amine (primary)	–NH₂	CH₃CH₂CH₂NH₂, propylamine
nitrile	–C≡N	CH₃CH₂CN, propanenitrile

Four kinds of formula

General formula: the formula for any member of the series, e.g. alkanes C_nH_2n+2, alkenes C_nH_2n, alcohols C_nH_2n+1OH.
Structural formula: the minimum detail that shows the arrangement unambiguously, e.g. CH₃CH(OH)CH₂CH₃.
Displayed formula: every atom and every bond drawn.
Skeletal formula: carbon chain as a zig-zag line; each end and each corner is a carbon; H atoms on carbon are left out; functional groups drawn in full.

Skeletal formula of butan-2-ol, CH₃CH(OH)CH₂CH₃.

Naming

Find the longest carbon chain that contains the functional group: meth-, eth-, prop-, but-, pent-, hex- (1–6 C).
Number from the end that gives the functional group the lowest number. The carbon of –CHO, –COOH and –CN is always C1.
Name branches (methyl, ethyl) and halogens (fluoro, chloro, bromo, iodo) as prefixes in alphabetical order, with di-, tri- for repeats.
Suffixes: -ane, -ene, -ol, -al, -one, -oic acid, -nitrile, -amine. Esters are named alkyl alkanoate: the part from the alcohol first, then the part from the acid.

Worked Example

Name (a) (CH₃)₂CHCH₂OH, (b) CH₃CH₂COCH₃, (c) CH₃COOCH₂CH₂CH₃.

Worked Example

The skeletal formula of ethyl butanoate is a three-bond zig-zag ending in C(=O)–O–, followed by a two-carbon chain. Give its molecular and empirical formulas.

2. Characteristic Organic Reactions

13.2

The vocabulary examiners expect

Term	Meaning
homologous series	₂
saturated / unsaturated
homolytic fission

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Hybridisation sets the shape
Same molecular formula, different molecule

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Introduction to AS Level Organic Chemistry