29.1 Formulas, functional groups and nomenclature

A functional group is the local arrangement of atoms that gives an organic family its characteristic reactions and many of its physical properties.

The carbon skeleton still affects boiling point, solubility and steric access, but the functional group supplies the main reaction pattern. Identify both before predicting behaviour.

Ethanol and ethanoic acid both contain two carbon atoms, yet –OH versus –CO₂H gives very different acidity and reactions. Propene and propane differ by C=C and therefore by addition chemistry.

A functional group is not the whole molecule and one atom alone is not enough to classify it; inspect the complete local connectivity.

General formulae show a family pattern; structural formulae show how atoms are connected; displayed formulae show every bond; skeletal formulae show the carbon framework with most carbon-bound hydrogens omitted.

Translate between styles without changing connectivity. In a skeletal formula, each vertex and line end is a carbon unless labelled otherwise, and hydrogens on heteroatoms are shown.

Ethanol may be written C₂H₆O, CH₃CH₂OH, a displayed C–C–O structure, or a two-carbon skeletal chain ending in –OH. Each representation describes the same molecule.

A general formula cannot distinguish isomers, and a skeletal corner is not an unlabelled oxygen or hydrogen.

Systematic naming selects the parent chain or ring, numbers it to give the principal functional group the lowest possible locant, then adds substituents and unsaturation prefixes.

For the syllabus range, use straight-chain esters and nitriles within the stated carbon limits, and cyclic compounds with one ring of up to six carbons. The suffix identifies the principal functional group.

CH₃CH₂CH₂OH is propan-1-ol, not propan-3-ol: numbering starts at the end nearest –OH. CH₃COOCH₂CH₃ is ethyl ethanoate, named as the alkyl group from the alcohol plus the carboxylate part.

The longest chain is not always numbered from the left, and the first number in an ester name does not describe the acid fragment.

For one benzene ring with simple substituents, name the ring parent and assign numbers that give the set of substituent locants the lowest possible value; list prefixes alphabetically when required.

The principal functional group can determine the parent name, such as benzoic acid or phenol. Start numbering at that group, then choose the direction that minimises the remaining locants.

A nitro group opposite the carboxylic acid is 4-nitrobenzoic acid. Three bromines on phenol at the 2, 4 and 6 positions give 2,4,6-tribromophenol.

Do not number a substituted benzene as if it were a straight chain, and do not choose the direction that merely gives the first substituent a small number while making the full locant set larger.