Chemistry A2 · Carboxylic acids and derivatives. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Benzoic acid is made by oxidising the side chain of an alkylbenzene such as methylbenzene with hot alkaline KMnO4, then adding dilute acid. Carboxylic acids are converted into acyl chlorides by PCl3 with heat, PCl5 or SOCl2.
Most carboxylic acids cannot be oxidised further, but two can. Methanoic acid has an H atom on its C=O carbon, like an aldehyde, and ethanedioic acid is also oxidised. Both give carbon dioxide.
Acid strength depends on how stable the anion is. In a carboxylate ion the negative charge is shared between two oxygen atoms, so carboxylic acids are stronger than phenols and alcohols. An electron-withdrawing chlorine atom spreads the charge further and makes the acid stronger still.
Reading pKa the wrong way. A smaller pKa means a stronger acid, so the acid with the most chlorine atoms near –COOH has the lowest pKa.
Answer: 6.88 g of PCl3 (3 significant figures)
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