Carboxylic acids (A2)
| English | Chinese | Pinyin |
|---|---|---|
| alkylbenzene | 烷基苯 | wán jī běn |
| acyl chloride | 酰氯 | xiān lǜ |
| benzoic acid | 苯甲酸 | běn jiǎ suān |
| electron-withdrawing | 吸电子 | xī diàn zi |
| inductive effect | 诱导效应 | yòu dǎo xiào yìng |
Acids, revisited
- Carboxylic acids can be made from alkylbenzenes 烷基苯 and turned into acyl chlorides 酰氯.
- Two of them can be oxidised further.
- Their acidity depends on how the charge is spread.
Carboxylic acid A2 map
Follow carboxylic acids through acyl derivatives and salts.
Carboxylic acids react with carbonates to release carbon dioxide.
A test for the acid group.
Making and oxidising
- an alkylbenzene (e.g. methylbenzene) is oxidised by hot alkaline $\text{KMnO}_4$ (then acid) → benzoic acid 苯甲酸 (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.
Acidity order: alcohol, phenol, carboxylic acid
Oxidising methylbenzene with hot alkaline KMnO₄ gives:
The whole side-chain is oxidised to a –COOH group, giving benzoic acid.
Which carboxylic acid can be oxidised further?
Methanoic and ethanedioic acids can be oxidised further (e.g. to CO₂); most carboxylic acids cannot.
Match each term to its meaning.
Each item links the term to its correct meaning.
Relative acidities
- A carboxylic acid is strongest because its anion spreads the charge over two oxygens.
- Chlorine atoms make it more acidic — they are electron-withdrawing 吸电子 (inductive effect 诱导效应), spreading the charge further.
Citrus fruits are sour because they contain citric acid, a carboxylic acid.
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.
Adding chlorine atoms near the –COOH group makes the acid:
Electron-withdrawing chlorine pulls charge away, stabilising the anion, so the acid is stronger.
Reactions to remember
- Carboxylic acids react with metals, bases and carbonates to form salts.
- The CO₂ test with carbonates confirms the acid group.
You've got it
- 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