Topic 15 · page 18 of 28
Reactions of acyl chlorides
Ethanoyl chloride reacts quickly at room temperature with water, alcohols, concentrated ammonia and amines. Each reaction swaps the Cl for –OH, –OR, –NH₂ or –NHR and gives off hydrogen chloride or its salt.
Read first: Naming acyl chlorides and esters
Why acyl chlorides react so easily 15.14(i)
In an acyl chloride the carbonyl carbon is bonded to two electronegative atoms, oxygen and chlorine, and is strongly δ+. A nucleophile with a lone pair (water, an alcohol, ammonia or an amine) attacks that carbon very easily. The chlorine leaves as a chloride ion.
The overall result is that the Cl is replaced by a group from the nucleophile, and hydrogen chloride is formed. All the reactions are vigorous at room temperature, with no catalyst.
With water 15.14(i)
The products are ethanoic acid and hydrogen chloride. The reaction is fast and exothermic, and the HCl appears as steamy white fumes in moist air. This is why acyl chlorides must be kept dry.
With alcohols 15.14(ii)
The product is an ester, here ethyl ethanoate, together with HCl. Unlike esterification with an acid, this reaction is fast, not reversible and goes to completion, so it gives a better yield and needs no catalyst.
With concentrated ammonia 15.14(iii)
The products are an amide, ethanamide, and ammonium chloride, a white solid (seen as white smoke). Two ammonia molecules are needed: one forms the amide and the second takes up the HCl.
With amines 15.14(iv)
An amine is ammonia with one or more H replaced by an alkyl group. Methylamine, CH₃NH₂, reacts like ammonia and gives an N-substituted amide.
The organic product is N-methylethanamide. The "N" shows that the methyl group is bonded to the nitrogen. The by-product is methylammonium chloride, CH₃NH₃Cl.
| Nucleophile | Organic product | Other product |
|---|---|---|
| water, H₂O | carboxylic acid | HCl |
| alcohol, ROH | ester | HCl |
| ammonia, NH₃ (conc.) | amide | NH₄Cl |
| amine, RNH₂ | N-substituted amide | RNH₃Cl |
PracticalReactions of ethanoyl chloride
This is a demonstration, carried out by the teacher in a fume cupboard. Ethanoyl chloride is corrosive and flammable, and reacts violently with water.
- Add a few drops of ethanoyl chloride to water in a test tube. Observe steamy fumes and heat. Test the fumes with damp blue litmus paper, which turns red.
- Add a few drops to ethanol. Observe heat and fumes, then pour into sodium carbonate solution and smell the ester.
- Add a few drops to concentrated ammonia solution. Observe a violent reaction and white smoke of ammonium chloride.
Check yourself
- Write an equation for the reaction of propanoyl chloride with water, and state what is seen.
- Write an equation for propanoyl chloride with ethanol and name the organic product.
- Write an equation for ethanoyl chloride with concentrated ammonia. Explain why two moles of ammonia are used.
- Name the organic product of the reaction between ethanoyl chloride and ethylamine, CH₃CH₂NH₂, and give its structural formula.
- Give two reasons why an acyl chloride is a better reagent than a carboxylic acid for making an ester.
Answers cover these until you have tried the questions
- . Steamy white fumes of hydrogen chloride and the mixture warms up.
- . The ester is ethyl propanoate.
- . The first NH₃ forms the amide and the second reacts with the HCl to form ammonium chloride.
- N-ethylethanamide, CH₃CONHCH₂CH₃. The equation is .
- Any two: the reaction is fast at room temperature, it goes to completion (not an equilibrium) so the yield is higher, and it needs no catalyst or heating.