AS organic chemistry conventions and functional group reference

In the syllabus, X represents a halogen atom, while R and R′ represent an alkyl group or hydrogen where appropriate. These symbols show the reaction pattern without committing to one named molecule.

Read the notation together with the functional group: R–X is a halogenoalkane, R–OH an alcohol and R–COOH a carboxylic acid. R and R′ need not be identical, and X is not a variable element chosen after the equation.

The substitution pattern R–Br + OH⁻ → R–OH + Br⁻ describes many bromoalkanes. A specific example such as CH₃CH₂Br is one member of the general family.

Do not treat R as “any atom” or replace X by a whole molecule. The symbols have fixed structural meanings.

A functional group is the atom or bond arrangement that gives an organic molecule its characteristic reactions. At AS level, recognise alkenes, halogenoalkanes, alcohols, aldehydes, ketones, carboxylic acids, esters and amines from their structures.

Identify the group before choosing a reagent or mechanism. For example, C=C suggests addition chemistry, C–X suggests substitution or elimination, and C=O distinguishes carbonyl compounds from hydrocarbons.

CH₃CH₂OH is an alcohol because it contains –OH; CH₃CHO is an aldehyde because the carbonyl carbon is bonded to H. The names follow the structure, not the other way round.

A functional group is not just a substring in a name. Check connectivity and distinguish an aldehyde, ketone, acid and ester carbonyl.