Last updated: 24 August 2026
Acyl chlorides sit at an important junction in Organic Chemistry. Their molecules resemble carboxylic acids, but replacing the –OH group of a carboxylic acid with –Cl changes the chemistry dramatically. The resulting compound becomes much more reactive towards nucleophiles.
Ethanoyl chloride, CH₃COCl, is therefore useful not because it introduces an entirely new type of carbon skeleton, but because it provides a very efficient way to attach the CH₃CO– group to other molecules. Once you see the common reaction pattern — nucleophile attacks the carbonyl carbon, followed by loss of chloride — its reactions with water, alcohols, phenols and nitrogen nucleophiles become much easier to connect.
Try these five questions before reading the chapter. Focus on the functional group, preparation and characteristic reactions of ethanoyl chloride.
- 5 diagnostic questions
- Structures and reaction ideas where useful
- Instant explanation for every answer option
What is an acyl chloride?
An acyl chloride is a derivative of a carboxylic acid in which the –OH group of the carboxyl group has been replaced by chlorine. Its general structural formula is RCOCl.
For ethanoyl chloride, R is CH₃, so its condensed structural formula is CH₃COCl. The chlorine atom is bonded directly to the carbonyl carbon. This is fundamentally different from a chloroalkane such as chloroethane, CH₃CH₂Cl, where chlorine is bonded to a saturated carbon.
Think of an acyl chloride as a carboxylic acid derivative: RCOOH → RCOCl. The carbonyl group remains, but –OH has been replaced by –Cl.
The carbonyl group remains the reactive centre
Oxygen is considerably more electronegative than carbon, so the C=O bond is polarised. Electron density is drawn towards oxygen, giving the carbonyl carbon a partial positive charge:
This electron-deficient carbon is an attractive target for nucleophiles, which are species that can donate an electron pair.
Acyl chloride: chlorine is attached directly to a carbonyl carbon, giving the characteristic –COCl functional group.
Alkyl chloride: chlorine is attached to a saturated carbon atom, as in CH₃CH₂Cl. Its substitution chemistry is therefore different.
Naming ethanoyl chloride
Acyl chlorides are named from the corresponding carboxylic acid. The ending –oic acid is replaced by –oyl chloride. Ethanoic acid therefore gives ethanoyl chloride.
Ethanoic acid, CH₃COOH → ethanoyl chloride, CH₃COCl. The two-carbon chain remains unchanged; only the group attached to the carbonyl carbon changes from –OH to –Cl.
Preparing ethanoyl chloride from ethanoic acid
Ethanoyl chloride can be prepared by reacting ethanoic acid with phosphorus(V) chloride, PCl₅. In effect, the –OH group of the carboxylic acid is replaced by –Cl.
Hydrogen chloride gas is formed during the reaction. In moist air this produces characteristic steamy white fumes, because HCl dissolves in tiny water droplets in the air.
Why should the apparatus be dry?
The product we are trying to make reacts readily with water. If appreciable moisture is present, some ethanoyl chloride will immediately hydrolyse back to ethanoic acid:
Dry conditions are therefore not merely a procedural detail. They prevent the product from being destroyed as soon as it forms.
| Feature | What to remember | Reason |
|---|---|---|
| Reagent | PCl₅ | Replaces the –OH group of the carboxylic acid by –Cl. |
| Observation | Steamy white fumes | HCl gas is produced and forms an acidic mist in moist air. |
| Apparatus | Keep dry | Acyl chlorides react readily with water. |
Why are acyl chlorides so reactive?
The reactivity of an acyl chloride comes from the combination of two structural features. First, the carbonyl carbon is electrophilic because the C=O bond is strongly polar. Second, chloride can leave from the molecule as Cl⁻ after a nucleophile has attacked.
This combination makes the –COCl group especially susceptible to nucleophilic acyl substitution.
High reactivity = an electrophilic carbonyl carbon + a group that can leave as Cl⁻.
The carbonyl carbon is electrophilic
A nucleophile contains an electron pair that it can donate. Because the carbonyl carbon is δ+, the nucleophile is attracted towards this carbon rather than towards the oxygen atom.
Chlorine also withdraws electron density inductively. More importantly for the reaction pattern, the chloride ion formed when the C–Cl bond breaks is sufficiently stable to act as a good leaving group.
Carbonyl group: polarises the molecule and makes the carbonyl carbon electrophilic.
Chloride group: provides a leaving group that can depart as Cl⁻ after nucleophilic attack.
The common reaction pattern: addition followed by elimination
Although different nucleophiles give different final products, the underlying pattern is similar.
First, the nucleophile attacks the carbonyl carbon and a tetrahedral intermediate is formed. The C=O bond then reforms while chloride leaves. Proton transfer completes the reaction when the nucleophile is a neutral molecule such as water, an alcohol, ammonia or an amine.
This is often described as an addition–elimination mechanism, or more generally as nucleophilic acyl substitution.
Reaction of ethanoyl chloride with water
Ethanoyl chloride reacts readily with water at room temperature. The chlorine-containing part of the molecule is effectively replaced by –OH, regenerating ethanoic acid.
The reaction can be vigorous, and steamy fumes may be observed because HCl is produced. This explains why ethanoyl chloride fumes in moist air: water vapour in the atmosphere is already enough to cause hydrolysis at the surface of the liquid.
Hydrolysis converts an acyl chloride back into the corresponding carboxylic acid.
Reaction of ethanoyl chloride with alcohols
Alcohols act as oxygen nucleophiles and react readily with ethanoyl chloride to form esters. For example, ethanol gives ethyl ethanoate.
Why is this different from esterification using a carboxylic acid?
Ethanoic acid and ethanol can also form ethyl ethanoate, but that reaction is reversible and normally requires concentrated sulfuric acid as a catalyst together with heating.
Ethanoyl chloride is considerably more reactive. Its reaction with an alcohol proceeds readily without the concentrated acid catalyst used in Fischer esterification.
| Method | Typical conditions | Important feature |
|---|---|---|
| Carboxylic acid + alcohol | Concentrated H₂SO₄, heat | Reversible equilibrium reaction |
| Acyl chloride + alcohol | Usually readily at room temperature | Much more reactive; HCl is formed |
| Product | Both routes can produce the same ester. | |
If a question gives CH₃COCl and ethanol, do not automatically write “concentrated H₂SO₄ and reflux” simply because an ester is being made. Those conditions belong to esterification using a carboxylic acid.
Reaction of ethanoyl chloride with phenol
Phenol can also be acylated to form an ester. Reaction with ethanoyl chloride gives phenyl ethanoate.
What is the role of NaOH?
Phenol is a weaker nucleophile than a typical aliphatic alcohol because one of the oxygen lone pairs is delocalised into the benzene ring. Sodium hydroxide converts some phenol into the phenoxide ion, C₆H₅O⁻, which is a stronger nucleophile.
The alkaline conditions also remove the acidic HCl formed during acylation. It is therefore better to understand NaOH as helping to generate the more reactive phenoxide nucleophile and neutralising acid, rather than simply describing it as a catalyst.
Phenol + NaOH gives phenoxide ion. Phenoxide is more nucleophilic than neutral phenol and attacks the acyl chloride readily.
Reaction with ammonia: forming a primary amide
Ammonia contains a lone pair on nitrogen and therefore acts as a nucleophile. Ethanoyl chloride reacts with excess ammonia to form ethanamide, CH₃CONH₂.
Why is excess ammonia used?
At first sight, it may seem that only one NH₃ molecule should be needed because ethanamide, CH₃CONH₂, contains only one nitrogen atom. However, ammonia has two different roles in the overall reaction.
One NH₃ molecule acts as the nucleophile. Its nitrogen lone pair attacks the electrophilic carbonyl carbon, and this nitrogen ultimately becomes part of the –CONH₂ group in ethanamide.
During the reaction, the nitrogen-containing species must also lose H⁺ to give the neutral amide. A second NH₃ molecule acts as a base and accepts this H⁺, forming NH₄⁺. The Cl⁻ that leaves from the acyl chloride is then present with NH₄⁺ as NH₄Cl.
First NH₃ molecule: acts as the nucleophile and supplies the nitrogen incorporated into ethanamide.
Additional NH₃: acts as a base, accepts H⁺ and forms NH₄Cl.
The extra ammonia is not used simply because NH₄Cl is a by-product we want to show in the equation. It removes H⁺ during the reaction so that the neutral amide can form efficiently.
If ammonia were deliberately limited, some of it would be converted into NH₄⁺ instead of remaining available to attack further ethanoyl chloride. This would reduce the efficiency of converting the acyl chloride into the desired amide. For this reason, excess ammonia is normally used.
Reaction with primary amines: forming N-substituted amides
Primary amines, RNH₂, behave similarly to ammonia because the nitrogen atom also possesses a lone pair. They react with acyl chlorides to produce N-substituted amides.
For example, ethanoyl chloride reacts with ethanamine to form N-ethylethanamide.
The same two-role idea applies here. One ethanamine molecule acts as the nucleophile and becomes incorporated into the amide. Another ethanamine molecule acts as a base and accepts H⁺, producing an ethylammonium chloride salt.
| Nucleophile | Main organic product | What happens to the acidic product? |
|---|---|---|
| H₂O | Carboxylic acid | HCl is formed. |
| Alcohol, ROH | Ester | HCl is formed. |
| Phenol / phenoxide | Aryl ester | Base can neutralise the HCl formed. |
| NH₃ | Primary amide | Excess NH₃ accepts H⁺, giving NH₄Cl. |
| Primary amine, RNH₂ | N-substituted amide | Excess amine accepts H⁺, giving RNH₃Cl. |
The common pattern in ethanoyl chloride reactions
Water, alcohols, phenols, ammonia and primary amines may look like a collection of unrelated reagents. Structurally, however, they all contain an atom with a lone pair capable of acting as a nucleophile.
The product changes because the nucleophile changes, but the carbonyl carbon remains the electrophilic centre throughout.
| Reagent | Group introduced | Organic product type |
|---|---|---|
| H₂O | –OH | Carboxylic acid |
| ROH | –OR | Ester |
| C₆H₅OH / C₆H₅O⁻ | –OC₆H₅ | Phenyl ester |
| NH₃ | –NH₂ | Primary amide |
| RNH₂ | –NHR | N-substituted amide |
Do not memorise five completely separate reactions. Follow the nucleophile: the group supplied by the nucleophile replaces Cl at the acyl carbon.
Comparing the ease of hydrolysis of acyl, alkyl and aryl chlorides
Acyl chlorides, alkyl chlorides and aryl chlorides all contain chlorine, but the chlorine atom sits in very different electronic environments. Consequently, their behaviour towards hydrolysis is very different.
Under ordinary laboratory conditions, ethanoyl chloride hydrolyses very readily. Chloroalkanes are considerably slower and are usually hydrolysed using aqueous hydroxide ions with heating. Chlorobenzene is resistant to ordinary nucleophilic substitution and does not hydrolyse under the mild conditions used for these comparisons.
| Type of chloride | Hydrolysis behaviour | Structural explanation |
|---|---|---|
| Acyl chloride, e.g. CH₃COCl | Very rapid | The carbonyl carbon is strongly electrophilic and Cl⁻ can leave during nucleophilic acyl substitution. |
| Alkyl chloride, e.g. CH₃CH₂Cl | Much slower | Substitution occurs at a saturated carbon and normally requires a stronger nucleophile and/or heating. |
| Aryl chloride, e.g. chlorobenzene | Very resistant under ordinary conditions | Delocalisation gives the C–Cl bond partial double-bond character, and the bond to an sp² carbon is relatively short and strong. |
Why chlorobenzene is not simply an unusually slow chloroalkane
In chlorobenzene, one of chlorine's lone pairs can overlap with the delocalised π system of the benzene ring. This contributes partial double-bond character to the C–Cl bond. The bond is therefore more difficult to break than the corresponding C–Cl bond in a typical chloroalkane.
The carbon bonded to chlorine is also sp² hybridised. Together, these factors make chlorobenzene highly resistant to the ordinary nucleophilic substitution conditions that hydrolyse chloroalkanes.
Do not compare hydrolysis using “C–Cl bond strength” alone. Acyl chlorides, alkyl chlorides and aryl chlorides undergo different substitution pathways because their carbon atoms have very different electronic environments.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Treating CH₃COCl as though it were simply another chloroalkane. | The chlorine is attached to a carbonyl carbon, so the molecule undergoes nucleophilic acyl substitution rather than ordinary substitution at a saturated carbon. | Identify the –COCl group first and focus on attack at the carbonyl carbon. |
| Assuming only one NH₃ molecule is needed because ethanamide contains only one nitrogen atom. | One NH₃ acts as the nucleophile, but another NH₃ is needed as a base to accept H⁺ during formation of the neutral amide. | For excess ammonia, write CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl and understand the two roles of NH₃. |
| Giving concentrated H₂SO₄ and reflux for ethanoyl chloride reacting with ethanol. | Those are typical conditions for reversible esterification between a carboxylic acid and an alcohol. | Remember that an acyl chloride is already highly reactive and reacts readily with alcohols without the acid catalyst. |
| Saying chlorobenzene is unreactive because the C–Cl bond is non-polar. | The important issue is not an absence of polarity. Delocalisation gives the C–Cl bond partial double-bond character and makes it relatively strong. | Explain the delocalisation of a chlorine lone pair into the benzene π system and the resulting partial double-bond character. |
What you should be able to do
Recognise an acyl chloride
Identify the –COCl functional group and recognise CH₃COCl as ethanoyl chloride.
Prepare ethanoyl chloride
Write the reaction of ethanoic acid with PCl₅ and state the relevant observation.
Explain its high reactivity
Relate the electrophilic carbonyl carbon and the chloride leaving group to nucleophilic acyl substitution.
Predict oxygen-nucleophile reactions
Predict the products formed with water, alcohols and phenol.
Write amide-forming equations
Write balanced equations for reactions with ammonia and primary amines, and explain why excess ammonia or amine is used.
Compare hydrolysis
Explain why acyl chlorides hydrolyse much more readily than alkyl and aryl chlorides.
These ten questions test whether you can apply the chapter to structures, reagents, products, mechanisms and comparisons of reactivity.
- 10 mastery questions
- Structures and reaction schemes where useful
- Instant explanation for every answer option
Frequently asked questions
Questions students often ask about acyl chlorides
Most difficulties with this topic disappear once you keep the electrophilic carbonyl carbon and the chloride leaving group in view. These questions connect some of the details that students commonly mix up.
Why is ethanoyl chloride more reactive than ethanoic acid?
Both compounds contain an electrophilic carbonyl carbon, but chloride is a much better leaving group than the –OH group of a carboxylic acid. This makes nucleophilic acyl substitution much easier for ethanoyl chloride.
Does ethanoyl chloride need heating to react with water or alcohols?
No strong heating is normally required. Acyl chlorides are very reactive and react readily with water and alcohols, often at room temperature. This contrasts with many reactions of chloroalkanes, which commonly require heating.
Why is excess ammonia used if ethanamide contains only one nitrogen atom?
The ammonia molecules have different roles. One NH₃ molecule acts as the nucleophile and supplies the nitrogen incorporated into ethanamide. Another NH₃ molecule acts as a base and accepts H⁺ during formation of the neutral amide. This gives NH₄⁺, which is present with Cl⁻ as NH₄Cl. Hence the overall equation is CH₃COCl + 2NH₃ → CH₃CONH₂ + NH₄Cl.
Why is NaOH used when ethanoyl chloride reacts with phenol?
NaOH deprotonates phenol to form phenoxide ions, C₆H₅O⁻. Phenoxide is a stronger nucleophile than neutral phenol and attacks the acyl chloride more readily. The alkaline conditions also help neutralise the acidic HCl formed.
Why does chlorobenzene not hydrolyse as readily as ethanoyl chloride?
They contain fundamentally different C–Cl environments. Ethanoyl chloride has an electrophilic carbonyl carbon that undergoes nucleophilic acyl substitution readily. In chlorobenzene, a chlorine lone pair can interact with the benzene π system, giving the C–Cl bond partial double-bond character and making ordinary nucleophilic substitution difficult.
Acyl chloride chemistry becomes much easier when the reactions are viewed as variations of one idea rather than as a list to memorise. Ethanoyl chloride contains an electrophilic carbonyl carbon and a chloride leaving group, so different nucleophiles can replace Cl to give carboxylic acids, esters or amides. For ammonia and amines, remember the additional acid–base step: one molecule acts as the nucleophile while another can remove H⁺. That same focus on structure also explains why acyl chlorides react far more readily than the alkyl and aryl chlorides encountered elsewhere in Organic Chemistry.