Organic Chemistry Guide

A functional group is a specific group of atoms within a molecule that is responsible for the characteristic chemical reactions of that molecule. For example, the hydroxyl group (–OH) makes alcohols, the carbonyl group (C=O) is present in aldehydes and ketones, and the carboxyl group (–COOH) defines carboxylic acids. The functional group largely determines the physical and chemical properties of the compound.

The main IUPAC rules are: (1) Find the longest carbon chain containing the principal functional group — this is the parent chain. (2) Number carbons to give the principal group the lowest possible number. (3) Name substituents as prefixes (alphabetically) with their position numbers. (4) Add the suffix for the functional group (-ol, -al, -one, -oic acid, etc.). (5) Use multiplying prefixes (di, tri, tetra) for identical substituents.

Both contain the carbonyl group (C=O), but differ in position. Aldehydes (–CHO) have the carbonyl at the end of a carbon chain (C-1), so one bond goes to H. Ketones have the carbonyl in the middle of a chain, bonded to two carbon atoms. Aldehydes are easily oxidised to carboxylic acids; ketones resist oxidation. Tollens test and Fehling's test distinguish them — aldehydes give positive results, ketones do not.

A homologous series is a family of organic compounds with the same functional group and general formula, differing by a CH₂ unit. They show a regular gradation in physical properties: boiling point, melting point, and density increase with increasing chain length. Each member can be prepared by similar methods and shows similar chemical properties. Examples: alkanes (CₙH₂ₙ₊₂), alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH).

Markovnikov's rule states that in the addition of HX (HCl, HBr, HI, H₂O) to an unsymmetrical alkene, the hydrogen atom adds to the carbon that already has more hydrogen atoms (the less substituted carbon), and the X adds to the more substituted carbon. For example, CH₂=CH–CH₃ + HBr → CH₃–CHBr–CH₃ (not CH₂Br–CH₂–CH₃). The rule is explained by the stability of carbocation intermediates: more substituted carbocations are more stable.