Part 5: Acyl chlorides, esters and hydrolysis

Follow the acyl group through reactions with water, alcohols, ammonia and amines; compare reversible ester chemistry with alkaline hydrolysis.

Keep the carbonyl carbon attached to the right oxygen

An acyl chloride is R–C(=O)–Cl and an ester is R–C(=O)–O–R′. The group immediately attached to the single-bonded oxygen supplies the alkyl part of an ester name; the carbonyl-containing side supplies the alkanoate. CH₃CH₂COOCH₃ is methyl propanoate, whereas CH₃COOCH₂CH₃ is ethyl ethanoate.

Draw the two pieces before naming. The ester O is bonded to the carbonyl carbon on one side and the alcohol-derived carbon on the other. An ether R–O–R′ lacks C=O, and a carboxylic acid has H in place of R′.

Acid plus alcohol gives an equilibrium

Heat a carboxylic acid and an alcohol with an acid catalyst, commonly concentrated sulfuric acid. An ester and water form reversibly. Reflux allows heating without losing volatile material. An excess of one reactant or removal of a product can shift the equilibrium towards ester; a catalyst accelerates both directions and does not change the equilibrium yield.

For propanoic acid and ethanol, the organic product is CH₃CH₂COOCH₂CH₃, ethyl propanoate. Joining acid OH directly to alcohol H and removing H₂O is a useful overall atom-tracking device, not the full reaction mechanism. Acid-catalysed esterification mechanisms are beyond the explicit mechanism demand in this topic.

The theoretical mass of ester follows the limiting reagent. If 0.0800 mol propanoic acid reacts with 0.120 mol ethanol, the 1:1 ratio limits ester to 0.0800 mol. With Mᵣ = 102.0, theoretical mass = 8.16 g. Isolation of 5.71 g gives 5.71/8.16 × 100 = 70.0% to three significant figures. Equilibrium and work-up losses can both reduce yield.

Acyl chlorides react readily with nucleophiles

The acyl carbon is electron deficient and chloride can leave, so acyl chlorides readily undergo nucleophilic acyl substitution. At this level, know the reagents, products and conditions; the detailed addition–elimination mechanism is a helpful extension rather than the explicit Topic 17 mechanism requirement. Water must be excluded when an acyl chloride is being used to acylate another reagent.

Water gives a carboxylic acid and HCl, often a vigorous reaction with steamy acidic fumes in moist air. An alcohol gives an ester and HCl, usually readily at room temperature without the acid catalyst needed for carboxylic-acid esterification. HCl dissolves in aqueous mixtures, so visible fumes depend on conditions.

Concentrated ammonia gives a primary amide, RCONH₂. With excess NH₃, a second molecule accepts the released proton, so the full equation uses two NH₃ and produces NH₄Cl. A primary amine, R′NH₂, gives an N-substituted amide, RCONHR′; excess amine similarly forms an alkylammonium chloride. Keep N directly bonded to carbonyl C.

Acid and alkali give different final forms

Heat an ester under reflux with dilute acid and water: acid-catalysed hydrolysis gives the carboxylic acid and alcohol. It is the reverse of esterification and reaches an equilibrium. A large excess of water favours hydrolysis, so do not assign a universal low percentage yield to all acidic hydrolyses.

Heat with aqueous NaOH: alkaline hydrolysis gives a carboxylate salt and alcohol. Hydroxide is a reactant, not simply a catalyst. Formation of the carboxylate makes the overall reaction effectively irreversible under these conditions. Only after acidification should you draw the free carboxylic acid.

Worked split: CH₃CH₂COOCH₃ yields propanoic acid plus methanol in acid, or propanoate plus methanol in alkali. Mark the acyl C–O bond joining the two fragments, then restore OH to the acyl side and H to the alkoxy oxygen. Do not swap the two carbon chains.

Separate reaction yield from isolated purity

A typical liquid ester preparation involves heating the reaction mixture, cooling, separating layers or extracting, washing away water-soluble acid, drying the organic layer and redistilling. Each step solves a different problem. Washing is not drying, and drying is not a guarantee that dissolved organic impurities have disappeared.

A carbonate/hydrogencarbonate wash neutralises acid and releases CO₂, so a separating funnel must be vented frequently away from people. Identify the organic layer using supplied density information; it is not always the upper layer. Add a suitable anhydrous drying agent to the organic liquid, allow it to remove water, then decant/filter before distillation.

Choose heat sources and containment from actual hazard data: ester/alcohol/ether vapours may be flammable, and acyl chlorides or released HCl may require a fume cupboard. Avoid direct smelling as an identification method. A boiling range helps assess a purified liquid but cannot alone prove unique identity.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources