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 Structure 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

, 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 edit

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

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.