Alkanes and alkenes

Unbranched and branched acyclic alkanes follow CnH2n+2. Alkenes with one C=C and no ring follow CnH2n. For n = 4, the formulae are C4H10 and C4H8 respectively.

Alcohols

The required alcohol formula is CnH2n+1OH. This notation highlights the hydroxyl group. Counting all atoms gives CnH2n+2O, but the functional-group form is more useful for classification.

Carboxylic acids

The required formula is CnH2n+1COOH, where n counts the carbon atoms in the alkyl part before COOH. Methanoic acid is a special first member written HCOOH; ethanoic acid is CH3COOH.

Using and checking formulae

Substitute the carbon number carefully and remember what n counts. Check valency and compare with a structural formula. General formulae describe a family but do not show the position of a double bond or hydroxyl group.

High-value recall and connections

Essential recall: What is the general formula of alkenes? CnH2n. What is the formula of a four-carbon alkane? C4H10. Write the structural formula of ethanoic acid. CH3COOH. Practical connection: Create a table for n = 1 to 4, generate molecular formulae and match each to a correct structural formula. Examination connection: For CnH2n+1COOH, total carbon atoms equal n + 1.

Review questions and suggested answers