Acyl Chlorides
A concise revision guide to acyl chlorides: naming and structure, why they are so reactive, their reactions with water, alcohols, ammonia and amines.
- 6.1.3(c)(ii)
- 6.1.3(f)-i
- 6.1.3(f)-ii
- 6.1.3(f)-iii
- 6.1.3(f)-iv
What these spec points say
- 6.1.3(c)(ii) esterification of acid anhydrides with alcohols
- 6.1.3(f)-i use of acyl chlorides in synthesis in formation of esters
- 6.1.3(f)-ii use of acyl chlorides in synthesis in formation of carboxylic acids
- 6.1.3(f)-iii use of acyl chlorides in synthesis in formation of primary amides
- 6.1.3(f)-iv use of acyl chlorides in synthesis in formation of secondary amides
Acyl Chlorides: Addition then Elimination
See why ethanoyl chloride is so reactive: a water molecule adds to its very δ+ carbon, the chloride ion is pushed out, and HCl is lost; then the same two steps with alcohols, ammonia and amines.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
Acyl chlorides are derivatives of carboxylic acids, so this page builds on Reactions of Carboxylic Acids. The esters they make are covered on Esters, Hydrolysis and Polyesters.
GCSE Recap: Carboxylic Acids and Esters
Three quick questions on the GCSE ideas this page builds on.
Structure and Naming of Acyl Chlorides
An acyl chloride has the functional group –COCl: the –OH of a carboxylic acid is replaced by a chlorine atom. The general formula is RCOCl.
Naming
Take the name of the parent carboxylic acid and change -oic acid to -oyl chloride. The –COCl carbon is carbon 1.
| Parent acid | Acyl chloride | Structural formula |
|---|---|---|
| ethanoic acid | ethanoyl chloride | CH₃COCl |
| propanoic acid | propanoyl chloride | CH₃CH₂COCl |
| 2-methylpropanoic acid | 2-methylpropanoyl chloride | (CH₃)₂CHCOCl |
| benzoic acid | benzoyl chloride | C₆H₅COCl |
Acyl chlorides are made from the carboxylic acid and SOCl₂, as shown on Reactions of Carboxylic Acids.
Why acyl chlorides are so reactive
- The carbonyl carbon is bonded to two very electronegative atoms, oxygen and chlorine.
- Both pull electron density away from it, so the carbon is strongly δ+.
- Nucleophiles, species with a lone pair to donate, are strongly attracted to this carbon.
- The chlorine leaves easily as a chloride ion, Cl⁻, so it is replaced by the nucleophile.
The reactions are fast at room temperature. Acyl chlorides even react with moisture in the air, giving misty fumes of hydrogen chloride, so they are stored and used dry.
Key idea: In every reaction on this page the Cl of –COCl is replaced by the nucleophile and HCl is formed. The acyl group, RCO–, stays intact.
Check: Naming Acyl Chlorides and Explaining Their Reactivity
Name acyl chlorides, work out their formulae and explain why the –COCl carbon is attacked by nucleophiles, using compounds not used on this page.
Reactions With Water, Alcohols, Ammonia and Amines
Water, alcohols, ammonia and amines are all nucleophiles: each has a lone pair on an oxygen or nitrogen atom. All four react vigorously with an acyl chloride at room temperature.
| Nucleophile | Equation | Organic product |
|---|---|---|
| water | CH₃COCl + H₂O → CH₃COOH + HCl | ethanoic acid (carboxylic acid) |
| ethanol | CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl | ethyl ethanoate (ester) |
| concentrated ammonia | CH₃COCl + NH₃ → CH₃CONH₂ + HCl | ethanamide (primary amide) |
| methylamine | CH₃COCl + CH₃NH₂ → CH₃CONHCH₃ + HCl | N-methylethanamide (N-substituted, or secondary, amide) |
Ethanoyl chloride and its four products. In each reaction the chlorine atom is replaced and hydrogen chloride is lost.
What is seen
- Each reaction is vigorous and exothermic, and gives misty (steamy) fumes of HCl.
- The reaction with water is a hydrolysis.
- With ammonia or an amine, white smoke or a white solid also forms.
Excess ammonia or amine
Ammonia and amines are bases, so they react with the HCl produced and form a salt. With an excess the overall equations are:
CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl
CH₃COCl + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃NH₃Cl
Naming the amides
- A primary amide, RCONH₂, has the ending -amide: ethanamide, propanamide.
- An N-substituted amide (a secondary amide), RCONHR′, names the group on nitrogen first, after the letter N: N-methylethanamide.
The four reactions of ethanoyl chloride at room temperature: equations, the type of organic product and the observations, with the safety precautions needed.
Worked example
Write the equation for the reaction of propanoyl chloride with methanol and name the organic product.
Check: Products of Acyl Chloride Reactions
Predict and name the organic product, and write the equation, for acyl chlorides reacting with water, alcohols, ammonia and amines not used on this page.
Acyl Chlorides in Synthesis
An ester can be made from an alcohol in two ways. The acyl chloride route is usually the better laboratory method.
| Carboxylic acid + alcohol | Acyl chloride + alcohol | |
|---|---|---|
| Equation | CH₃COOH + CH₃CH₂OH ⇌ CH₃COOCH₂CH₃ + H₂O | CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl |
| Reversible? | yes: an equilibrium | no: goes to completion |
| Rate | slow | fast |
| Conditions | concentrated H₂SO₄ catalyst and heat | room temperature, no catalyst |
| Yield | limited by the equilibrium | high |
| Drawback | low yield | HCl fumes are corrosive; must be kept dry |
Two routes to ethyl ethanoate compared: the slow, reversible reaction of ethanoic acid with ethanol, and the fast, complete reaction of ethanoyl chloride with ethanol.
- Acyl chlorides are also the standard way to make amides. A carboxylic acid and ammonia only form a salt at room temperature.
- A typical two-step synthesis: carboxylic acid → acyl chloride → ester or amide.
- Work in a fume cupboard with dry apparatus.
Exam wording: “The reaction of an acyl chloride with an alcohol is not reversible and is faster, so it gives a higher yield of ester than the reaction of a carboxylic acid with an alcohol.”
Quick Check: Amide Product and Mass of Amine
Name the product, then work out how much amine the reaction really uses.
Esters From Acid Anhydrides
An acid anhydride has two acyl groups joined through an oxygen atom. Ethanoic anhydride is (CH₃CO)₂O. It reacts with an alcohol on gentle warming to give an ester and a carboxylic acid.
(CH₃CO)₂O + CH₃CH₂OH → CH₃COOCH₂CH₃ + CH₃COOH
- The reaction is not reversible, so the yield is better than from a carboxylic acid and an alcohol.
- It is less vigorous than the acyl chloride reaction, and no HCl fumes are produced.
- The second product is a carboxylic acid, here ethanoic acid.
| Route to an ester | Second product | Goes to completion? |
|---|---|---|
| carboxylic acid + alcohol | H₂O | no |
| acid anhydride + alcohol | carboxylic acid | yes |
| acyl chloride + alcohol | HCl | yes |
Quick Check: Three Routes to One Ester
Compare three routes to propyl pentanoate, an ester that is new to this page.
Common Exam Points
Say
- “Misty fumes of hydrogen chloride are given off.”
- “The carbon atom is strongly δ+ because it is bonded to two electronegative atoms, O and Cl.”
- “With excess ammonia the products are the amide and ammonium chloride.”
Do not say
- “Chlorine gas is given off” (the gas is hydrogen chloride, HCl).
- “The reaction with an alcohol needs an acid catalyst” (no catalyst and no heating are needed).
- “Ethanoyl chloride dissolves in water” (it reacts with water: it is hydrolysed).
Watch for
- Amide formulae: –CONH₂ for a primary amide, –CONH– for an N-substituted amide.
- Balancing with excess ammonia or amine: two moles react with each mole of acyl chloride.
- Naming: -oyl chloride, counted from the –COCl carbon.
Check: Using Acyl Chlorides in Synthesis
Choose between routes to esters and amides, justify the choice, and describe the observations and precautions, for target compounds not used on this page.
FAQs
Use these quick answers to check acyl chlorides and their reactions.
Why are acyl chlorides so reactive?
The carbonyl carbon is bonded to two electronegative atoms, oxygen and chlorine, so it is strongly δ+ and readily attacked by nucleophiles. Chloride is also a good leaving group. Reactions are vigorous at room temperature and give misty fumes of HCl.
How are acyl chlorides named?
Replace -oic acid in the name of the parent acid with -oyl chloride. CH₃COCl is ethanoyl chloride and CH₃CH₂COCl is propanoyl chloride. The COCl carbon counts as carbon 1 of the chain.
What do acyl chlorides give with water, alcohols, ammonia and amines?
Water gives a carboxylic acid, an alcohol gives an ester, ammonia gives a primary amide and a primary amine gives an N-substituted (secondary) amide. Each reaction also forms HCl, which reacts with excess ammonia or amine to form a salt.
Why make an ester from an acyl chloride instead of a carboxylic acid?
The reaction with an alcohol is fast at room temperature, needs no catalyst and is not reversible, so it goes to completion and gives a better yield: CH₃COCl + CH₃CH₂OH → CH₃COOCH₂CH₃ + HCl.
What does an acid anhydride give with an alcohol?
An ester and a carboxylic acid: (CH₃CO)₂O + CH₃CH₂OH → CH₃COOCH₂CH₃ + CH₃COOH. The reaction is less vigorous than with an acyl chloride and no fumes of HCl form.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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