O Level / IGCSE Chemistry · Organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Organic compounds are sorted into families called homologous series. Members of a series have the same functional group (the atom or group of atoms that decides the chemical properties), the same general formula and similar chemical properties. Each member differs from the next by a –CH2– unit, and physical properties such as boiling point show a trend.
The molecular formula gives only the numbers of atoms. The structural formula shows how the atoms are arranged, e.g. CH3CH2OH. The displayed formula shows every atom and every bond.
Structural isomers have the same molecular formula but different structural formulae. Saturated compounds have only single carbon–carbon bonds; unsaturated compounds have at least one carbon–carbon bond that is not single.
In the carboxylic acid formula CnH2n+1COOH, n does not count the carbon atom of the COOH group. Ethanoic acid, CH3COOH, has n = 1 but two carbon atoms.
Answer: CH3CH2COOH (propanoic acid), molecular formula C3H6O2, Mr = 74.
Try each one, then open the answer.
Answer: C. Five C–H bonds, one C–C bond, one C–O bond and one O–H bond: 5 + 1 + 1 + 1 = 8.
Answer: B. Alkanes fit CnH2n+2. If 2n + 2 = 12 then n = 5, so the molecule is C5H12 (pentane).
Answer: D. Carboxylic acids contain the carboxyl group, –COOH. C=C is the alkene group, –COO– is the ester linkage found in esters, and –NH2 is the amine group.
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