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Properties of Acyl Halides

This page defines acyl halides and discusses their simple physical properties, introducing chemical reactivity in a general way.

Acyl halides as "acid derivatives"

A carboxylic acid such as ethanoic acid has the following structure:

 

 

There are a number of related compounds in which the -OH group in the acid is replaced by something else that leaves the acyl carbon in a +3 oxidation state. Compounds like this are described as acid derivatives. Acyl halides (also known as acid halides) are one example of an acid derivative. In this example, the -OH group has been replaced by a chlorine atom; chlorine is the most commonly used acid halide.

 

The acyl group

The acyl group is a hydrocarbon group attached to a carbon-oxygen double bond:

 

 

The "R" group is normally restricted to an alkyl group. It could, however, be a group based on a benzene ring.

Naming acyl halides

The easiest way to name an acyl halide is to consider the relationship with the corresponding carboxylic acid:

 

carboxylic acid name acyl halide name acyl halide formula
ethanoic acid ethanoyl chloride CH3COCl
propanoic acid propanoyl chloride CH3CH2COCl
butanoic acid butanoyl chloride CH3CH2CH2COCl
butanoic acid butanoyl bromide CH3CH2CH2COBr
butanoic acid butanoyl iodide CH3CH2CH2COI

 

If something is substituted into the hydrocarbon chain, the carbon in the -COX (X = halide) group is the number 1 carbon.

For example, 2-methylbutanoyl chloride is named as follows:

 

Physical properties of acyl halides

An acyl halide such as ethanoyl chloride is a colorless, fuming liquid. The strong smell of ethanoyl chloride is a mixture of the smell of vinegar (ethanoic acid) and the acrid smell of hydrogen chloride gas. The smell and the fumes originate from the reactions between ethanoyl chloride and water vapor in the air.

Solubility in water

Acyl halides do not dissolve in water because they react (often violently) with it to produce carboxylic acids and hydrogen halides (e.g. HCl). The strong reaction makes it impossible to obtain a simple aqueous solution of an acyl halide.

Boiling points

Taking ethanoyl chloride as a typical example: ethanoyl chloride boils at 51°C and is a polar molecule. Therefore, it has dipole-dipole attractions between its molecules as well as van der Waals dispersion forces. However, it does not form hydrogen bonds. Its boiling point is, therefore, higher than an alkane of similar size (which has no permanent dipoles) such as ethane, which boils at -88.5°C, but not as high as a similarly sized alcohol (which forms hydrogen bonds in addition to everything else) such as ethanol, which boils at 78.37°C.

Reactivity

Acyl halides are extremely reactive, and in each of their reactions the halogen atom is replaced by another functional group. In each case, in the first step, hydrogen halide (normally hydrogen chloride) gas is produced as steamy acidic fumes. However, in some cases the hydrogen halide goes on to react with one of the substances in the reaction mixture. Taking ethanoyl chloride as typical, the initial reaction is the following:

 

 

These reactions involve water, alcohols and phenols, or ammonia and amines as nucleophiles. Each of these particular cases contains a very electronegative element with an active lone pair of electrons—either oxygen or nitrogen. The most commonly performed reaction with acyl halides and the example reaction is known as nucleophilic acyl substitution

Contributors

Jim Clark (Chemguide.co.uk)

  • Mark Tye (Diablo Valley College)