Carboxylic acids (A2)
Carboxylic acids (A2)
- Carboxylic acids can be made from alkylbenzenes and turned into acyl chlorides.
- Two of them can be oxidised further.
- Their acidity depends on how the charge is spread.
Making and oxidising
- an alkylbenzene (e.g. methylbenzene) is oxidised by hot alkaline $\text{KMnO}_4$ (then acid) → benzoic acid (the whole side-chain becomes –COOH).
- a carboxylic acid + $\text{PCl}_3$/$\text{PCl}_5$/$\text{SOCl}_2$ → an acyl chloride.
- two acids oxidise further: methanoic acid (HCOOH) and ethanedioic acid (HOOCCOOH) → carbon dioxide.
Practice
Oxidising methylbenzene with hot alkaline KMnO₄ gives:
The whole side-chain is oxidised to a –COOH group, giving benzoic acid.
Practice
Which carboxylic acid can be oxidised further?
Methanoic and ethanedioic acids can be oxidised further (e.g. to CO₂); most carboxylic acids cannot.
Relative acidities
$$\text{alcohol} < \text{phenol} < \text{carboxylic acid}$$
- A carboxylic acid is strongest because its anion spreads the charge over two oxygens.
- Chlorine atoms make it more acidic — they are electron-withdrawing (inductive effect), spreading the charge further.
Practice
Why is a carboxylic acid more acidic than a phenol or alcohol?
Spreading the charge over two oxygens stabilises the carboxylate ion most, so it is the strongest acid.
Practice
Adding chlorine atoms near the –COOH group makes the acid:
Electron-withdrawing chlorine pulls charge away, stabilising the anion, so the acid is stronger.
You've got it
Key idea
- alkylbenzene → benzoic acid (hot alkaline KMnO₄); acid → acyl chloride (PCl₅/SOCl₂)
- methanoic and ethanedioic acids can be oxidised further to CO₂
- acidity: alcohol < phenol < carboxylic acid (charge over two oxygens); Cl atoms make it stronger