Carboxylic acids and derivatives

📚The chemistry

33.1 Carboxylic acids

Making benzoic acid

Heat an alkylbenzene such as methylbenzene with hot alkaline KMnO₄, then acidify with dilute acid (topic 30): C₆H₅CH₃ + 3[O] → C₆H₅COOH + H₂O.

Making acyl chlorides

The OH of the carboxyl group is replaced by Cl — the same three reagents that convert alcohols to chloroalkanes:

Acids that can be oxidised further

Most carboxylic acids resist oxidation, but two do not:

• Methanoic acid, HCOOH, contains an H–C=O group, like an aldehyde. It is oxidised to CO₂ and H₂O by Fehling’s or Tollens’ reagent (giving a brick-red precipitate or silver mirror, the only acid that does), or by acidified KMnO₄ or K₂Cr₂O₇: HCOOH + [O] → CO₂ + H₂O.
• Ethanedioic acid, HOOCCOOH, is oxidised by warm acidified KMnO₄ to carbon dioxide: HOOCCOOH + [O] → 2CO₂ + H₂O. This is the basis of the manganate(VII)–ethanedioate titration (topic 28).

Acidity: acids, phenols and alcohols

Each is judged by the stability of its anion:

• Carboxylate ion, RCOO⁻: the negative charge is delocalised over two oxygen atoms (both C–O bonds become identical). Two electronegative atoms share the charge, so the ion is very stable, and carboxylic acids are the strongest of the group — strong enough to react with carbonates.
• Phenoxide ion: the charge is delocalised into the ring, but the ring’s carbon atoms are much less electronegative than oxygen, so this spreads the charge less effectively. Phenol is weaker than carboxylic acids (topic 32).
• Alkoxide ion: no delocalisation, and the alkyl group’s electron-donating effect intensifies the charge. Alcohols are the weakest.

Chlorine-substituted acids

Replacing H atoms in the alkyl group with chlorine makes the acid stronger (approximate pK_a values; sources differ slightly, and a question will give its own):

Chlorine is electronegative: it withdraws electron density from the carboxylate group through the chain (a negative inductive effect). This spreads out the negative charge on the carboxylate ion and stabilises it, so the acid dissociates more. More chlorine atoms mean more withdrawal and a stronger acid. The effect also weakens with distance: a Cl on the carbon next to COOH strengthens the acid more than one further along the chain (2-chlorobutanoic acid is stronger than 4-chlorobutanoic acid).

33.2–33.3 Acyl chlorides and esters

An acyl chloride, RCOCl, is the most reactive derivative of a carboxylic acid. The carbonyl carbon is bonded to two electronegative atoms, O and Cl, which both pull electron density away from it, so it is strongly δ+. Acyl chlorides react rapidly at room temperature with nucleophiles, always releasing steamy fumes of HCl:

• Water — hydrolysis to the carboxylic acid (violent, fuming): \[ \mathrm{CH_3COCl + H_2O \rightarrow CH_3COOH + HCl} \]
• Alcohol — an ester: \[ \mathrm{CH_3COCl + C_2H_5OH \rightarrow CH_3COOC_2H_5 + HCl} \quad \text{ethyl ethanoate} \]
• Phenol — an ester. Phenols do not form esters well with carboxylic acids, so an acyl chloride is the way to make them: \[ \mathrm{C_6H_5COCl + C_6H_5OH \rightarrow C_6H_5COOC_6H_5 + HCl} \quad \text{phenyl benzoate} \]
• Ammonia — a primary amide (the HCl then reacts with excess ammonia to form NH₄Cl): \[ \mathrm{CH_3COCl + NH_3 \rightarrow CH_3CONH_2 + HCl} \quad \text{ethanamide} \]
• Primary or secondary amine — a substituted amide: \[ \mathrm{CH_3COCl + CH_3NH_2 \rightarrow CH_3CONHCH_3 + HCl} \quad \text{N-methylethanamide} \]

The addition–elimination mechanism

All five reactions follow the same two stages. For ethanoyl chloride with water:

1. Addition: curly arrow from a lone pair on the O of water to the δ+ carbonyl carbon; curly arrow from the C=O π bond to the oxygen. This gives a tetrahedral intermediate, with O⁻ and a positively charged OH₂⁺ group.
2. Elimination: curly arrow from a lone pair on O⁻ back to reform the C=O bond; curly arrow from the C–Cl bond to Cl, which leaves as Cl⁻. Then H⁺ is lost from the OH₂⁺, and it combines with Cl⁻ to form HCl.

With an alcohol, ammonia or an amine the nucleophile is ROH, NH₃ or RNH₂ instead; the steps are identical.

Ease of hydrolysis

• Acyl chlorides are hydrolysed by cold water, instantly. The carbonyl carbon is very δ+ (bonded to both O and Cl), and it is planar and unhindered, so the nucleophile can easily attack by addition.
• Alkyl chlorides need warm aqueous alkali (or are hydrolysed slowly by water, as in the silver nitrate test). Their carbon is less δ+ (only one electronegative atom), and attack requires breaking a C–Cl bond at the same time (topic 15).
• Aryl chlorides are not hydrolysed under normal conditions: the C–Cl bond is strengthened by overlap with the ring’s π system, and the π electrons repel nucleophiles (topic 31).