Acyl Chlorides and Acid Anhydrides
A concise revision guide to acyl chlorides and acid anhydrides: naming and structure, why they are so reactive, their reactions with water, alcohols, ammonia and amines, and the nucleophilic addition–elimination mechanism.
- 3.3.9.2i-a
- 3.3.9.2i-b
- 3.3.9.2i-c
- 3.3.9.2ii-a
- 3.3.9.2ii-b
- 3.3.9.2ii-c
- 3.3.9.2ii-d
- 3.3.9.2ii-e
- 3.3.9.2ii-f
- 3.3.9.2ii-g
- 3.3.9.2ii-h
- 3.3.9.2iii
- 3.3.9.2iv-a
- 3.3.9.2iv-b
- 3.3.9.2iv-c
- 3.3.9.2iv-d
What these spec points say
- 3.3.9.2i-a know the structures of acid anhydrides
- 3.3.9.2i-b know the structures of acyl chlorides
- 3.3.9.2i-c know the structures of amides
- 3.3.9.2ii-a know the nucleophilic addition-elimination reaction of water with acyl chlorides
- 3.3.9.2ii-b know the nucleophilic addition-elimination reaction of alcohols with acyl chlorides
- 3.3.9.2ii-c know the nucleophilic addition-elimination reaction of ammonia with acyl chlorides
- 3.3.9.2ii-d know the nucleophilic addition-elimination reaction of primary amines with acyl chlorides
- 3.3.9.2ii-e know the nucleophilic addition-elimination reaction of water with acid anhydrides
- 3.3.9.2ii-f know the nucleophilic addition-elimination reaction of alcohols with acid anhydrides
- 3.3.9.2ii-g know the nucleophilic addition-elimination reaction of ammonia with acid anhydrides
- 3.3.9.2ii-h know the nucleophilic addition-elimination reaction of primary amines with acid anhydrides
- 3.3.9.2iii know the industrial advantages of ethanoic anhydride over ethanoyl chloride in the manufacture of aspirin
- 3.3.9.2iv-a outline the mechanism of nucleophilic addition-elimination reactions of acyl chlorides with water
- 3.3.9.2iv-b outline the mechanism of nucleophilic addition-elimination reactions of acyl chlorides with alcohols
- 3.3.9.2iv-c outline the mechanism of nucleophilic addition-elimination reactions of acyl chlorides with ammonia
- 3.3.9.2iv-d outline the mechanism of nucleophilic addition-elimination reactions of acyl chlorides with primary amines
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, Fats, Soap and Biodiesel.
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 |
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 synthesis uses the acyl chloride to turn an alcohol into an ester, or an amine into an 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.
The Nucleophilic Addition–Elimination Mechanism
All four reactions follow the same mechanism: nucleophilic addition–elimination. The nucleophile adds to the C=O carbon, then HCl is eliminated.
Addition
- A lone pair on the nucleophile (the O of water or an alcohol, the N of ammonia or an amine) attacks the δ+ carbon.
- At the same time the π pair of the C=O bond moves on to the oxygen.
- The intermediate has a negatively charged oxygen, O⁻, and a positively charged O or N where the nucleophile joined.
Elimination
- A lone pair on the O⁻ moves back to re-form the C=O double bond.
- The C–Cl bond breaks and both of its electrons leave with the chlorine as Cl⁻.
- The positively charged O or N loses H⁺, as the pair of electrons in an O–H or N–H bond moves on to it.
Nucleophilic addition–elimination of ethanoyl chloride with water and with ammonia: the lone pair attacks the δ+ carbon, then C=O re-forms as Cl⁻ and H⁺ are lost.
- With water the products are ethanoic acid and HCl.
- With ammonia the H⁺ is removed by a second ammonia molecule, giving NH₄⁺Cl⁻.
- Every curly arrow starts at a lone pair or at a bond, and each shows the movement of a pair of electrons.
Exam focus: Six things earn the marks: the lone pair on the nucleophile, the arrow to the δ+ carbon, the arrow from C=O to O, the charged intermediate, the arrows that re-form C=O and release Cl⁻, and the arrow showing loss of H⁺ from the O⁺ or N⁺.
Acid Anhydrides
An acid anhydride is formed, on paper, by removing one molecule of water from two carboxylic acid molecules. Ethanoic anhydride is (CH₃CO)₂O.
- The functional group is –CO–O–CO–: two acyl groups joined through one oxygen atom.
- The name comes from the parent acid: ethanoic acid gives ethanoic anhydride.
- Each carbonyl carbon is δ+, so anhydrides react with the same four nucleophiles as acyl chlorides.
- They react less vigorously, and the second product is a carboxylic acid in place of HCl.
| Nucleophile | Equation | Organic product |
|---|---|---|
| water | (CH₃CO)₂O + H₂O → 2CH₃COOH | ethanoic acid |
| ethanol | (CH₃CO)₂O + CH₃CH₂OH → CH₃COOCH₂CH₃ + CH₃COOH | ethyl ethanoate (ester) |
| ammonia | (CH₃CO)₂O + 2NH₃ → CH₃CONH₂ + CH₃COONH₄ | ethanamide (amide) |
| methylamine | (CH₃CO)₂O + 2CH₃NH₂ → CH₃CONHCH₃ + CH₃COO⁻CH₃NH₃⁺ | N-methylethanamide (N-substituted amide) |
Ethanoic anhydride and its four products. The organic products match those from ethanoyl chloride, but ethanoic acid is formed in place of hydrogen chloride.
With ammonia or an amine, the ethanoic acid formed reacts with a second molecule of the base, so the other product is a salt.
Quick Check: Three Routes to One Ester
Compare three routes to propyl pentanoate, an ester that is new to this page.
Making Aspirin: Anhydride or Acyl Chloride?
Aspirin is an ester. It is manufactured by reacting the –OH group of 2-hydroxybenzoic acid (salicylic acid) with ethanoic anhydride.
HOC₆H₄COOH + (CH₃CO)₂O → CH₃COOC₆H₄COOH + CH₃COOH
The manufacture of aspirin from 2-hydroxybenzoic acid and ethanoic anhydride. The ring –OH group is converted into an ester link, and ethanoic acid is the other product.
Ethanoyl chloride would give the same ester, but industry prefers ethanoic anhydride for these reasons:
- It is cheaper.
- It is less corrosive.
- It reacts less vigorously with water, so it is safer and easier to handle.
- It does not produce HCl, a toxic, corrosive gas; the other product is ethanoic acid.
Ethanoic anhydride and ethanoyl chloride compared for the manufacture of aspirin: cost, corrosiveness, reaction with water and the second product formed.
Exam wording: “Ethanoic anhydride is cheaper, less corrosive, less vigorously hydrolysed by water, and it does not give dangerous fumes of hydrogen chloride.”
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.
- Mechanisms: the name is nucleophilic addition–elimination, and every lone pair and charge must be shown.
- Anhydride equations: the second product is a carboxylic acid, never HCl.
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 acid anhydrides 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.
Why is ethanoic anhydride preferred for making aspirin?
Ethanoic anhydride is cheaper than ethanoyl chloride, less corrosive, reacts less vigorously with water and is safer to use. Its by-product is ethanoic acid rather than toxic, corrosive fumes of hydrogen chloride.
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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