Acyl halides, also called acid halides, are highly reactive carboxylic acid derivatives with the general formula RCOX, where X is a halogen atom. In these compounds, the halogen is bonded directly to the carbonyl carbon of the acyl group. This makes them different from alkyl halides, in which the halogen is attached to a saturated carbon atom. ^[1–4]

Acyl halides are used in organic synthesis, particularly for the preparation of esters, amides, and anhydrides. They are also used in Friedel–Crafts acylation to introduce acyl groups into aromatic rings. As moisture-sensitive, corrosive compounds, they should be handled carefully under dry conditions with proper laboratory safety precautions.

Nomenclature

The names of acyl halides are derived from the names of the corresponding carboxylic acids. In the parent acid name, the ending -oic acid is replaced with -oyl halide. ^[1]

Some common examples are listed below:

• CH₃COCl → ethanoyl chloride
• C₆H₅COCl → benzoyl chloride
• C₂H₅COBr → propanoyl bromide

This naming pattern helps identify both the carbon chain and the halogen present in the compound.

Preparation

Acyl halides are commonly prepared from carboxylic acids by replacing the –OH group with a halogen atom. In practice, acyl chlorides are most often prepared using reagents such as thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus trichloride (PCl₃). ^[3–5]

The general reactions are:

RCOOH + SOCl₂ → RCOCl + SO₂ + HCl

RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

For example, ethanoic acid reacts with thionyl chloride to form ethanoyl chloride:

CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

Why Are Acyl Halides So Reactive

The high reactivity of acyl halides can be explained by two important factors. First, the carbonyl carbon is already partially positive because of the polarized C=O bond. Second, the halogen atom withdraws electron density through the inductive effect, making the carbonyl carbon electrophilic and even more susceptible to nucleophilic attack. ^[1]

Their reactivity is further increased because the halide ion can leave easily. For this reason, acyl halides usually react through nucleophilic acyl substitution, in which the halogen is replaced by another group.

A simplified general reaction pattern is:

RCOX + NuH → acyl-substituted product + HX

Here, NuH may be water, an alcohol, ammonia, or an amine.

Chemical Reactions

1. Reaction With Water ^[4,5]

Acyl halides react readily with water to form carboxylic acids:

RCOX + H₂O → RCOOH + HX

An example is the hydrolysis of ethanoyl chloride, producing ethanoic acid:

CH₃COCl + H₂O → CH₃COOH + HCl

2. Reaction With Alcohols

Acyl halides react with alcohols to form esters:

RCOX + R’OH → RCOOR’ + HX

For example, the reaction between ethanoyl chloride and ethanol produces ethyl ethanoate:

CH₃COCl + C₂H₅OH → CH₃COOC₂H₅ + HCl

3. Reaction With Ammonia and Amines

Acyl halides react with ammonia and amines to form amides and substituted amides, respectively. Usually, two molecules of ammonia are involved. One acts as the nucleophile, while the other neutralizes the hydrogen halide produced.

RCOX + 2 NH₃ → RCONH₂ + NH₄X

For example, ethanoyl chloride reacts with ammonia, forming acetamide or ethanamide:

CH₃COCl + 2 NH₃ → CH₃CONH₂ + NH₄Cl

With a primary amine:

RCOX + 2 R’NH₂ → RCONHR’ + R’NH₃X

With a secondary amine:

RCOX + 2 R’₂NH → RCONR’₂ + R’₂NH₂X

4. Reaction With Carboxylate Ions

Acyl halides react with carboxylate ions to produce acid anhydrides. This is a useful method for preparing both symmetrical and mixed anhydrides.

RCOX + R’COO⁻ → RCO–O–COR’ + X^–

This reaction highlights the importance of acyl halides as acylating agents.