33. Carboxylic acids and derivatives

An alkylbenzene with at least one benzylic hydrogen can be oxidised by hot alkaline KMnO₄. After acidification, the side chain is converted to a carboxylic acid attached directly to the ring.

The carbon skeleton beyond the benzylic carbon is removed during vigorous oxidation, so different alkylbenzenes can give the same benzoic-acid ring product.

Methylbenzene + hot alkaline KMnO₄, then dilute acid → benzoic acid. The methyl carbon becomes the carboxyl carbon.

Do not oxidise the aromatic ring itself or stop at an alcohol; this is a strong side-chain oxidation followed by acidification.

Carboxylic acids react with chlorinating agents such as PCl₃ with heat, PCl₅ or SOCl₂ to replace the hydroxyl group by chlorine, forming an acyl chloride.

The acyl chloride is more reactive because chloride is a better leaving group than hydroxide. It can then be used to make esters, amides or other acyl derivatives.

Ethanoic acid + PCl₅ → ethanoyl chloride plus phosphorus-containing products and HCl. The exact by-products depend on the reagent.

Do not treat the reaction as simple substitution at a saturated carbon; the carbonyl carbon is the electrophilic centre.

Carboxylic acids are already highly oxidised at the carbonyl carbon, but some can undergo further oxidation under strong conditions, depending on the structure.

Do not assume every carboxylic acid has an ordinary oxidation product. Identify the carbon framework and the oxidant before predicting cleavage or further oxidation.

Methanoic acid is readily oxidised to carbon dioxide and water because it still has a hydrogen attached to the carbonyl carbon; ethanoic acid is much more resistant under typical syllabus conditions.

“Carboxylic acid cannot oxidise” is too absolute, while “all acids oxidise to CO₂” ignores structural and condition boundaries.

The relative acidity is carboxylic acid > phenol > alcohol. Carboxylate ions delocalise negative charge over two oxygens; phenoxide delocalises into the ring; alkoxides localise charge on one oxygen.

Compare conjugate-base stability and electron-donating or withdrawing substituents, not just the O–H bond itself.

A carboxylic acid reacts with aqueous carbonate to release CO₂; phenol is too weak for this under ordinary conditions, and ethanol is weaker still.

All three can donate H⁺, but “weak acid” is relative: a weak acid can still be substantially stronger than another weak acid.

Chlorine withdraws electron density through sigma bonds. This stabilises the negative charge of a carboxylate ion, so a chlorine-substituted carboxylic acid is more acidic than the unsubstituted acid.

The effect depends on distance: chlorine closer to the carboxyl group exerts a stronger inductive effect, and multiple chlorines reinforce it.

2-chloropropanoic acid is more acidic than propanoic acid; 3-chloropropanoic acid is also stronger than propanoic acid but less strongly affected by the greater separation.

Do not use chlorine’s aromatic directing effects here; acidity is controlled by its inductive withdrawal through the acid’s carbon chain.

An alcohol reacts with an acyl chloride to form an ester. The alcohol supplies the alkyl–O part and the acyl chloride supplies the acid-derived carbonyl part; HCl is released.

Name the two fragments before writing the product. The reaction is usually rapid at room temperature and is not the reversible equilibrium route used for direct acid/alcohol esterification.

Ethanol + ethanoyl chloride → ethyl ethanoate + HCl. Phenol + benzoyl chloride → phenyl benzoate + HCl.

Do not swap the two ester names or write water as the by-product; acyl-chloride esterification produces hydrogen chloride.

Carboxylic acids are converted to acyl chlorides by PCl₃ with heat, PCl₅ or SOCl₂. The hydroxyl group is replaced by chlorine at the acyl carbon.

Choose the reagent named in the question and remember that each route has different inorganic by-products. The product is a reactive derivative ready for further acyl substitution.

Ethanoic acid + SOCl₂ → ethanoyl chloride; the reagent is convenient because gaseous by-products can leave the mixture.

Do not confuse an acyl chloride RCOCl with an alkyl chloride RCl; the carbonyl group changes its reactivity and naming.

Acyl chlorides undergo nucleophilic acyl substitution. Water gives a carboxylic acid, an alcohol gives an ester, ammonia gives a primary amide, and a primary amine gives a substituted amide; HCl is formed.

The nucleophile attacks the carbonyl carbon, then chloride leaves. Match the nucleophile to the product family before writing the equation.

CH₃COCl + H₂O → CH₃CO₂H + HCl; CH₃COCl + NH₃ → CH₃CONH₂ + HCl.

Do not write an alkene addition product or omit HCl when balancing the overall reaction.

A nucleophile adds to the electrophilic carbonyl carbon of an acyl chloride, forming a tetrahedral intermediate. The intermediate then eliminates chloride and reforms the C=O bond.

This mechanism explains why several nucleophiles give different products while the acyl carbon remains the reaction centre.

In ethanolysis, ethanol attacks CH₃COCl, the tetrahedral intermediate collapses, and ethyl ethanoate forms with HCl as the by-product.

The mechanism is not direct displacement without an intermediate, and the carbonyl bond is temporarily changed rather than permanently lost.

Acyl chlorides hydrolyse rapidly because the carbonyl carbon is strongly electrophilic and chloride is a good leaving group. Alkyl chlorides need suitable substitution conditions, while aryl chlorides resist because the C–Cl bond has partial double-bond character on an sp² carbon.

Compare the carbon attached to chlorine and the stabilisation of the possible intermediate before deciding reactivity.

Ethanoyl chloride reacts vigorously with water at room temperature; chloroethane reacts much more slowly under ordinary aqueous conditions; chlorobenzene is least susceptible.

All three contain chlorine, but hydrolysis rate is governed by the local structure, not the element alone.