Carboxylic acids have unusually high boiling points because in the liquid state their molecules
Correct answer: B. form dimers held together by two hydrogen bonds
- A. are ionised
- B. form dimers held together by two hydrogen bonds
- C. form covalent bonds with one another
- D. are non polar
Explanation
Two molecules pair up so that the hydroxyl hydrogen of each bonds to the carbonyl oxygen of the other, giving a cyclic dimer with double the effective molar mass, so ethanoic acid boils at 118 degrees Celsius, well above ethanol despite similar size. The same dimerisation occurs in non polar solvents and even in the vapour just above the boiling point. Only in water do the dimers break up, replaced by hydrogen bonds to the solvent.
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About Reactivity of Carboxylic Acids
Carboxylic acids show acidic behaviour because their carboxylate ions are resonance-stabilised, and electron-withdrawing groups can change their strength. Reactions include neutralisation, esterification, formation of acid chlorides and amides, reduction, decarboxylation and substitution at the acyl carbon, which differs from the reactions of phenols.
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