Carboxylic acids (A2)
Introduced| English | Meaning |
|---|---|
| alkylbenzene/ˈælkɪlbenziːn/ | A benzene molecule with one or more hydrogen atoms replaced by alkyl groups. |
| acyl chloride/əˈkɪl ˈklɔːraɪd/ | An organic compound containing a carbonyl carbon bonded to a chlorine atom and another carbon group or hydrogen. |
| benzoic acid/benˈzəʊɪk ˈæsɪd/ | A carboxylic acid with a COOH group attached to a benzene ring. |
| electron-withdrawing/ɪˈlektrɒn wɪθˈdrɔːɪŋ/ | Describing an atom or group that draws electron density towards itself from the rest of a molecule. |
| inductive effect/ɪnˈdʌktɪv ɪˈfekt/ | The shift of electron density through sigma bonds caused by differences in electronegativity or charge. |
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.
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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:
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The whole side-chain is oxidised to a –COOH group, giving benzoic acid.
Which carboxylic acid can be oxidised further?
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Methanoic and ethanedioic acids can be oxidised further (e.g. to CO₂); most carboxylic acids cannot.
Match each term to its meaning.
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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?
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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:
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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