Swap the OH of a carboxylic acid for Cl and you get an acyl chloride, which fumes in moist air. Its carbonyl carbon is so electron-poor that water, alcohols, phenols, ammonia and amines all attack it by addition-elimination at room temperature.
An acyl chloride, RCOCl, is made from a carboxylic acid with PCl5 (room temperature), PCl3 and heat, or SOCl2. Its carbonyl carbon is bonded to two electronegative atoms, O and Cl, so it is strongly δ+ and planar, open to attack by nucleophiles. Acyl chlorides react by nucleophilic addition-elimination: a lone pair adds to the C=O carbon, then the C=O reforms and Cl− leaves with H+ as HCl. With water they give the carboxylic acid (vigorous, steamy HCl fumes); with alcohols and phenols, esters; with ammonia and amines, amides. Ease of hydrolysis: acyl chloride >> chloroalkane > chlorobenzene.
Compare the hydrolysis of different chloro compounds in the simulator, watching how fast a precipitate forms with silver nitrate. Then match the results to the bond and structure in the table.
| Reagent | Conditions | Equation (ethanoic acid) |
|---|---|---|
| PCl5 | room temperature | CH3COOH + PCl5 → CH3COCl + POCl3 + HCl |
| PCl3 | heat | 3CH3COOH + PCl3 → 3CH3COCl + H3PO3 |
| SOCl2 | room temperature or warm | CH3COOH + SOCl2 → CH3COCl + SO2 + HCl |
| Compound | Hydrolysis | Reason |
|---|---|---|
| ethanoyl chloride, CH3COCl | cold water, instantly; steamy HCl fumes; AgNO3 gives white AgCl at once | C bonded to O and Cl: very δ+, and the planar C=O carbon is easy to attack |
| 1-chlorobutane | needs warm aqueous NaOH or AgNO3 in ethanol; precipitate slowly | C–Cl carbon only bonded to one electronegative atom |
| chlorobenzene | no hydrolysis under these conditions | Cl lone pair overlaps with the ring, so C–Cl is stronger; the ring repels nucleophiles |
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Which reagent converts ethanoic acid into ethanoyl chloride at room temperature with steamy fumes?
Which compound is hydrolysed most readily by water?
What is the organic product of ethanoyl chloride and excess ammonia?
What is the name of the mechanism for the reactions of acyl chlorides with nucleophiles?
Step through the mechanism in the simulator with water, ethanol and ammonia, then use the table to see that every nucleophile follows the same addition-elimination pattern as water in Fig. 1.
| Nucleophile | Product type | Equation |
|---|---|---|
| water | carboxylic acid | CH3COCl + H2O → CH3COOH + HCl |
| alcohol | ester | CH3COCl + CH3OH → CH3COOCH3 + HCl |
| phenol | ester | CH3COCl + C6H5OH → CH3COOC6H5 + HCl |
| ammonia (excess) | primary amide | CH3COCl + 2NH3 → CH3CONH2 + NH4Cl |
| primary amine (excess) | N-substituted amide | CH3COCl + 2CH3NH2 → CH3CONHCH3 + CH3NH3+Cl− |
Show the mechanism in two steps: addition (lone pair to C, C=O π electrons to O) then elimination (C=O reforms, Cl− leaves). Start every curly arrow from a lone pair or a bond, never from an atom or a charge sign. The by-product is always HCl (or its ammonium salt), not water. Chlorobenzene is the hardest to hydrolyse, not the easiest: its C–Cl bond has partial double-bond character.
Unlocks once the four checks above are done. Worth more XP, written to A Level Paper 4 standard.
Which order shows DECREASING ease of hydrolysis?
0.785 g of ethanoyl chloride (Mr 78.5) is added to water. What volume of 0.500 mol dm−3 NaOH neutralises the solution?
In the first step of the reaction of ethanoyl chloride with methylamine, where does the curly arrow start and end?
Which compound is formed when benzoyl chloride, C6H5COCl, reacts with ethylamine?
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