The functional groups

What a functional group is

• A functional group is the atom or group of atoms in a molecule that determines how it reacts.

• The rest of the molecule — the carbon chain — is largely a spectator.

• Two compounds with the same functional group undergo the same reactions, whatever the length of their chains.

• This is why the whole standard is organised by functional group rather than by compound.

The ten groups you need

Family	Group	Suffix	Example
alkene	$\text{C}=\text{C}$	-ene	propene
haloalkane	$-\text{X}$ (F, Cl, Br, I)	halo- (prefix)	2-bromopropane
alcohol	$-\text{OH}$	-ol	propan-1-ol
amine (primary)	$-\text{NH}_2$	-amine	propan-1-amine
aldehyde	$-\text{CHO}$	-al	propanal
ketone	$-\text{CO}-$	-one	propanone
carboxylic acid	$-\text{COOH}$	-oic acid	propanoic acid
ester	$-\text{COO}-$	alkyl ...-oate	methyl propanoate
acyl chloride	$-\text{COCl}$	-oyl chloride	propanoyl chloride
amide	$-\text{CONH}_2$	-amide	propanamide

The carbonyl family

• Six of the ten contain a carbonyl group, $\text{C}=\text{O}$. What is attached to that carbon is what distinguishes them:

• aldehyde — carbonyl carbon has an H and is at the end of the chain

• ketone — carbonyl carbon has two carbons attached, so it is inside the chain

• carboxylic acid — carbonyl carbon also carries an –OH

• ester — carbonyl carbon also carries an –OR

• acyl chloride — carbonyl carbon also carries a –Cl

• amide — carbonyl carbon also carries an –NH₂ or –NHR

• Learning the family this way means you only have to look at one carbon to identify six of the ten groups.

Aldehyde or ketone?

• This distinction is asked constantly, and it is purely about position:
  • An aldehyde has the carbonyl at the end of the chain, so the carbonyl carbon carries a hydrogen.
  • A ketone has the carbonyl within the chain, so the carbonyl carbon carries no hydrogen.
• That missing hydrogen is what makes a ketone resistant to oxidation, so this is not a naming technicality — it drives the chemistry.

Classifying alcohols and haloalkanes

• Primary (1°) — the functional group carbon is attached to one other carbon.

• Secondary (2°) — attached to two other carbons.

• Tertiary (3°) — attached to three other carbons.

• The class matters enormously for alcohols, because it determines what they oxidise to. Always classify before predicting an oxidation.

Compounds with more than one group

• A molecule may contain several functional groups, and each one reacts independently.
• An amino acid, $\text{H}_2\text{N}-\text{CHR}-\text{COOH}$, has both an amine and a carboxylic acid, and shows the reactions of both.
• When a question gives you a compound with two groups, work through them one at a time.