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How do carboxylic acids and their derivatives interconvert, and what makes acyl chlorides so reactive?

The acidity and reactions of carboxylic acids, the derivatives (esters, acyl chlorides, acid anhydrides and amides), their interconversion, esterification and hydrolysis, and the reactivity of acyl chlorides.

An Eduqas A-Level Chemistry OA2.3 answer on the acidity and reactions of carboxylic acids, the derivatives (esters, acyl chlorides, anhydrides, amides), their interconversion, and the reactivity of acyl chlorides.

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  1. What this topic is asking
  2. Carboxylic acids and their acidity
  3. The acyl-group derivatives
  4. The reactivity of acyl chlorides
  5. Esterification and ester hydrolysis
  6. Examples in context
  7. Try this

What this topic is asking

Eduqas topic OA2.3 covers carboxylic acids and the family of derivatives based on the acyl group: esters, acyl chlorides, acid anhydrides and amides. It covers their acidity and reactions, how they interconvert, esterification and hydrolysis, and the high reactivity of acyl chlorides. The relative reactivity of the derivatives and the addition-elimination mechanism are the key ideas.

Carboxylic acids and their acidity

Carboxylic acids are weak acids, partially dissociating to the carboxylate ion and H+\text{H}^+. They react with reactive metals (salt and hydrogen), with carbonates and hydrogencarbonates (effervescence of CO2\text{CO}_2, the test for the group) and with bases (salt and water), and they form esters with alcohols.

The acyl-group derivatives

The reactivity of acyl chlorides

Because these reactions go essentially to completion, acyl chlorides are the preferred reagents for making esters and amides in synthesis, despite the corrosive HCl by-product.

Esterification and ester hydrolysis

Esters form reversibly from a carboxylic acid and an alcohol with an acid catalyst (as met at AS). Hydrolysis reverses this: acid hydrolysis (dilute acid, reflux) regenerates the acid and alcohol as a reversible equilibrium, while alkaline hydrolysis (aqueous NaOH, reflux) gives the carboxylate salt and the alcohol and goes to completion, because the salt cannot re-esterify (this is saponification, the basis of soap making).

Examples in context

Example 1. Aspirin synthesis. Aspirin is made by acylating salicylic acid with ethanoic anhydride, a milder, cheaper acylating agent than the acyl chloride; the choice of derivative balances reactivity, cost and safety.

Example 2. Soap from fats. Alkaline hydrolysis of the ester linkages in animal or vegetable fats gives the sodium salts of long-chain carboxylic acids (soap) and glycerol, a direct industrial use of saponification.

Try this

Q1. Write the equation for the reaction of ethanoyl chloride with water and state one observation. [2 marks]

  • Cue. CH3COCl+H2Oβ†’CH3COOH+HCl\text{CH}_3\text{COCl} + \text{H}_2\text{O} \rightarrow \text{CH}_3\text{COOH} + \text{HCl}; misty white fumes of hydrogen chloride are seen (a vigorous reaction).

Q2. State the products of refluxing ethyl ethanoate with aqueous sodium hydroxide. [2 marks]

  • Cue. Sodium ethanoate (CH3COONa\text{CH}_3\text{COONa}) and ethanol (alkaline hydrolysis goes to completion).

Exam-style practice questions

Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Eduqas 20194 marksEthanoyl chloride reacts with ethanol. (a) Name the organic product and the type of reaction. (b) Write the equation and name the other product, explaining why this is a useful way to make an ester.
Show worked answer β†’

(a) Ethyl ethanoate; the reaction is addition-elimination (acylation), a nucleophilic substitution at the acyl group (2).

(b) CH3COCl+C2H5OH→CH3COOC2H5+HCl\text{CH}_3\text{COCl} + \text{C}_2\text{H}_5\text{OH} \rightarrow \text{CH}_3\text{COOC}_2\text{H}_5 + \text{HCl} (1). The reaction goes essentially to completion (it is not reversible, unlike esterification with the acid), giving a high yield of ester quickly, with HCl released as misty fumes (1).

Eduqas 20214 marksAn ester, ethyl propanoate, is refluxed with aqueous sodium hydroxide. (a) Name this type of reaction. (b) Give the two organic products. (c) Explain why this hydrolysis goes to completion whereas acid hydrolysis does not.
Show worked answer β†’

(a) Alkaline hydrolysis (saponification) (1).

(b) The products are sodium propanoate (CH3CH2COONa\text{CH}_3\text{CH}_2\text{COONa}) and ethanol (2).

(c) The carboxylate salt formed does not react with the alcohol (it cannot re-esterify), so the reaction is not reversible and goes to completion; acid hydrolysis gives the free acid, which can re-form the ester, setting up an equilibrium (1).

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