A-level Chemistry/OCR (Salters)/Functional groups

Organic compounds are best thought of as relatively unreactive hydrocarbon skeletons decorated by functional groups — groups of atoms that undergo characteristic reactions. Compounds containing two or more different functional groups are described as polyfunctional.

Functional group	Family	Formula	Example
chloroformyl group	acyl chloride	RCOCl	ethanoyl chloride, CH₃COCl
hydroxyl group	alcohol	ROH	ethanol, C₂H₅OH
formyl group	aldehyde	RCHO	ethanal, CH₃CHO
alkenyl group	alkene	R₂C=CR₂	ethene, C₂H₄
alkynyl group	alkyne	RC≡CR	ethyne, C₂H₂
amide group	amide	RCONH₂	ethanamide, CH₃CONH₂
amino group	amine	1°: RNH₂ 2°: R₂NH 3°: R₃N	ethylamine, C₂H₅NH₂
	alpha amino acid	H₂CHRCOOH	glycine (aminoethanoic acid), (CH₃CO)₂O
	acid anhydride	(RCO)₂O	ethanoic anhydride, (CH₃CO)₂O
azo group	azo compound	RN₂R	azobenzene, C₆H₅N₂C₆H₅
carboxyl group	carboxylic acid	RCOOH	ethanoic acid, CH₃COOH
diazonium group	diazonium salt	[RN₂]⁺ X⁻	benzenediazonium chloride, C₆H₅N₂Cl
acyloxy group	ester	RCOOR	ethyl ethanoate, CH₃COOCH₃
alkoxy group	ether	ROR	ethoxyethane, CH₃OCH₃
halo group	haloalkane	RX	chloroethane, C₂H₅Cl
carbonyl group	ketone	RCOR	propanone, CH₃COCH₃
cyano group	nitrile	RCN	ethanenitrile, CH₃CN
nitro group	nitro compound	RNO₂	nitroethane, C₂H₅NO₂
phenyl group	phenyl compound	C₆H₅R
hydroxyl group	phenol	ArOH	phenol, C₆H₅OH
sulfo group	sulfonic acid	RSO₂OH / RSO₃H	ethanesulfonic acid, C₂H₅SO₂OH

Cyclic compounds and intramolecular reactions

Exam questions often ask you to identify a functional group that forms part of a ring. Here are some examples:

This compound contains an ester group that is part of a ring. Cyclic esters are called lactones.
Here are some simple cyclic amides. Cyclic amides are called lactams.
Amides in four-membered rings are called beta-lactams or β-lactams. All penicillins contain a β-lactam ring.

If a cyclic compound contains a ring with less than six members (i.e. three, four or five membered rings), the ring will be under strain and functional groups that make up part of the ring will often be much more reactive than they would in a straight-chain compound. For example, amides are normally very unreactive, but the amide group in a β-lactam ring is relatively easily hydrolysed because the four-membered ring is very strained and hydrolysis converts the ring to a straight chain, relieving the strain.

Cyclic compounds are often formed by intramolecular reactions, meaning reactions in which different parts of the same molecule react with each other.

For instance, if a molecule contains both an alcohol part (hydroxyl group) and a carboxylic acid part (carboxyl group), the two groups can react to form an ester. The reaction is identical in all respects to normal esterification (acid + alcohol → ester + water), except that the acid and alcohol are not two separate molecules but two different ends of the same molecule.

Intramolecular reactions are usually much faster than intermolecular reactions, because the reacting groups are tethered together and are thus much more likely to collide.