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

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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 Acyl-chloride.svg
Acetyl-chloride-2D-flat.png Acetyl-chloride.png Acetyl-chloride-3D-balls.png

ethanoyl chloride,
hydroxyl group alcohol ROH Alcohol-(general)-skeletal.png
Ethanol-2D-flat.png Ethanol-2D-skeletal.png Ethanol-3D-balls.png

ethanol, C2H5OH
formyl group aldehyde RCHO Aldehyde2.png
Acetaldehyde-2D-flat.svg Acetaldehyde-tall-2D-skeletal.png Acetaldehyde-3D-balls.png

ethanal, CH3CHO
alkenyl group alkene R2C=CR2 Alkene-2D-skeletal.svg
Ethene-2D-flat.png Ethylene-2D-skeletal.png Ethylene-3D-balls.png

ethene, C2H4
alkynyl group alkyne RC≡CR Alkyne-2D-flat.png
Ethyne-2D-flat.png Acetylene-2D-skeletal.svg Acetylene-3D-balls.png

ethyne, C2H2
amide group amide RCONH2 Amide-general.png
Acetamide-2D-flat.png Acetamide skeletal.svg Acetamide-3D-balls.png

ethanamide, CH3CONH2
amino group amine 1°: RNH2
2°: R2NH
3°: R3N
Ethylamine-2D-flat.png Ethylamine-2D-skeletal.png Ethylamine-3D-balls.png

ethylamine, C2H5NH2
alpha amino acid H2CHRCOOH Alpha-amino-acid-2D-flat.png
Glycine-2D-flat.png Glycine-2D-skeletal.png Glycine-3D-balls.png

glycine (aminoethanoic acid), (CH3CO)2O
acid anhydride (RCO)2O Carboxylic-acid-anhydride.png
Acetic-anhydride-2D-flat.png Acetic anhydride-2D-skeletal.png Acetic-anhydride-3D-balls.png

ethanoic anhydride, (CH3CO)2O
azo group azo compound RN2R Azo-group-2D-flat.png
Azobenzene-circles-2D-skeletal.png Azobenzene-trans-3D-balls.png

azobenzene, C6H5N2C6H5
carboxyl group carboxylic acid RCOOH Carboxylic-acid.svg
Acetic-acid-2D-flat.png Acetic-acid-2D-skeletal.svg Acetic-acid-3D-balls.png

ethanoic acid, CH3COOH
diazonium group diazonium salt [RN2]+ X Diazonium-salt-2D.png

benzenediazonium chloride, C6H5N2Cl
acyloxy group ester RCOOR Ester-general.png
Ethyl-acetate-2D-flat.png Ethyl-acetate-2D-skeletal.png Ethyl-acetate-3D-balls.png

ethyl ethanoate, CH3COOCH3
alkoxy group ether ROR Ether-(general).png
Diethyl-ether-2D-flat.png Diethyl-ether-2D-skeletal.png Diethyl-ether-3D-balls.png

ethoxyethane, CH3OCH3
halo group haloalkane RX Haloalkane-2D-flat.png
Chloroethane-2D-flat.png Chloroethane-2D-skeletal.png Chloroethane-3D-balls.png

chloroethane, C2H5Cl
carbonyl group ketone RCOR Ketone-general.svg
Acetone-2D-flat.png Acetone-2D-skeletal.svg Acetone-3D-balls.png

propanone, CH3COCH3
cyano group nitrile RCN Nitrile-group-2D.png
Acetonitrile-2D-flat.png Acetonitrile-2D-skeletal.png Acetonitrile-3D-balls.png

ethanenitrile, CH3CN
nitro group nitro compound RNO2 Nitro-group.png
Nitroethane-2D-flat.png Nitroethane-2D-skeletal.png Nitroethane-3D-balls.png

nitroethane, C2H5NO2
phenyl group phenyl compound C6H5R Phenyl-group-circle-2D-skeletal.png
hydroxyl group phenol ArOH Alcohol-(general)-skeletal.png
Phenol-2D-flat.png Phenol-2D-skeletal.png Phenol-3D-balls.png

phenol, C6H5OH
sulfo group sulfonic acid RSO2OH / RSO3H Sulfonic-acid.png
Ethanesulfonic-acid-2D-flat.png Ethanesulfonic-acid-2D-skeletal.png Ethanesulfonic-acid-3D-balls.png

ethanesulfonic acid, C2H5SO2OH

Cyclic compounds and intramolecular reactions[edit | edit source]

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.