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A-Level Carboxylic acids, acyl chlorides and esters
What the A-Level syllabus expects for Carboxylic acids, acyl chlorides and esters, and how to practise it.
What the syllabus expects
- Describe how carboxylic acids are made: from primary alcohols with acidified KMnO4 or acidified K2Cr2O7 under reflux; from aldehydes with the same oxidants under reflux; and from nitriles by heating with dilute acid, or with dilute alkali then acidification.
- Describe the reactions by which a carboxylic acid gives:
- salts, through reaction with metals, alkalis or carbonates
- esters, by condensing with an alcohol over a concentrated H2SO4 catalyst with heat, ethyl ethanoate being the example
- acyl chlorides, by reaction with PCl5, ethanoyl chloride being the example
- primary alcohols, by reduction with LiAlH4, ethanol being the example
- Explain, from their structures, why carboxylic acids and chlorine-substituted ethanoic acids are acidic.
- Describe how an acyl chloride hydrolyses in water.
- Describe the condensation reactions an acyl chloride undergoes with alcohols, phenols and primary amines.
- Explain why acyl chlorides, alkyl chlorides and aryl chlorides differ in how easily they hydrolyse.
- Describe how esters form when an acyl chloride condenses, taking phenyl benzoate as the example.
- Describe the hydrolysis of an ester under acid and under base, using aqueous acid or aqueous alkali with heat.
How it's examined
Questions on this topic most often ask you to explain, suggest. About 5% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Draw the structural formulae of the organic products formed when propanoyl chloride reacts with tryptophan (trp).
Show the worked answer
Tryptophan is (indol-3-yl)-CH2-CH(NH2)-COOH. Propanoyl chloride (CH3CH2COCl) is an acyl chloride: it acylates the primary amine -NH2 to give an amide, releasing HCl. The -COOH group and (under these conditions) the weakly nucleophilic indole N-H do not react. The organic product is the N-acyl amino acid.
Example 2 (4 marks)
Acetylcholine, [CH3COOCH2CH2N(CH3)3]+, is broken down by the enzyme acetylcholinesterase. At the active site a serine residue (treated as a surface-attached -OH) and a histidine residue (treated as a surface-attached imidazole ring) act as the binding points for acetylcholine. Choline, [(CH3)3NCH2CH2OH]+, is one of the products. Propose a mechanism for the breakdown of acetylcholine that shows how choline is formed.
Show the worked answer
The reaction is nucleophilic acyl substitution (transesterification) of the ester carbonyl of acetylcholine, CH3-CO-O-CH2CH2N(CH3)3+. 1. The histidine imidazole nitrogen acts as a base and removes the H from the serine -OH, making the serine oxygen a strong nucleophile. 2. The serine oxygen lone pair attacks the carbonyl carbon of the acetyl group (curly arrow O to C); the C=O pi electrons move onto the oxygen, giving a negatively charged tetrahedral intermediate. 3. The tetrahedral intermediate collapses: the C=O reforms and the C-O bond to the choline oxygen breaks (curly arrow), expelling the choline alkoxide -O-CH2CH2N(CH3)3+. 4. This alkoxide is protonated (by the protonated histidine) to give choline, [(CH3)3NCH2CH2OH]+, leaving the acetyl group esterified onto the serine (acetyl-enzyme).
Example 3 (2 marks)
Propose one procedure that could be performed during the preparation, and explain why it works, to eliminate salicylic acid that is contaminating the aspirin crystals.
Show the worked answer
Purify the crude aspirin by recrystallisation. Dissolve the impure aspirin crystals in the minimum volume of hot solvent (e.g. hot ethanol or a hot ethanol/water mixture) so that everything just dissolves, then allow the solution to cool slowly and filter off the crystals that form. This works because salicylic acid is present only as a small amount of impurity: on cooling, the aspirin (the major component) becomes saturated and crystallises out, while the small amount of more soluble salicylic acid remains dissolved in the cold mother liquor and is removed with the filtrate.
More worked questions on this topic
- Aspirin dissolves only slightly in water. Using the structure of aspirin shown in section 37 of (2 marks)
- Both methylamine (CH3NH2) and ethanamide (CH3C(O)NH2) possess a C-N bond. In methylamine, this (2 marks)
- Acetylcholine, [CH3COOCH2CH2N(CH3)3]+, is easy to prepare from cheaply available starting mater (2 marks)
- During the manufacture of zanamivir, step a (H+ with MeOH) turns a carboxylic acid group into t (2 marks)
- In 1897, Felix Hoffmann, a German chemist then employed by the pharmaceutical firm Bayer, disco (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry