6.1.3(e)-(f) - Acyl chlorides in synthesis

The acyl chloride group

An acyl chloride contains the functional group $\text{–}\mathrm{COCl}$. The carbonyl carbon is bonded to oxygen by $\ce{C=O}$ and to chlorine by $C-\ce{Cl}$.

The formula detail matters. The group is $\ce{RCOCl}$, not $\ce{RCOOCl}$. Adding the extra oxygen changes the connectivity and is a common synthesis error.

Acyl chlorides are useful because the $C-\ce{Cl}$ bond is replaced readily by groups from water, alcohols, phenol, ammonia or primary amines. At A level, you do not need a full curly-arrow mechanism here; the key synthesis idea is that an acyl chloride lets you replace $\ce{Cl}$ with an oxygen- or nitrogen-containing group.

Making acyl chlorides

OCR specifies one route for this lesson: a carboxylic acid reacts with thionyl chloride, $\ce{SOCl2}$, to form the corresponding acyl chloride.

The $\text{–}\mathrm{OH}$ part of the carboxylic acid has effectively been replaced by $\ce{Cl}$. The carbon chain in $\ce{R}$ is unchanged.

Worked example: make propanoyl chloride from propanoic acid.

Propanoic acid is $\ce{CH3CH2COOH}$. The corresponding acyl chloride is propanoyl chloride, $\ce{CH3CH2COCl}$.

This equation is already balanced: one sulfur atom forms $\ce{SO2}$, and one hydrogen plus one chlorine form $\ce{HCl}$.

Both inorganic products are gases, $\ce{SO2}$ and $\ce{HCl}$, which leave the mixture and help separation from the acyl chloride. Acyl chlorides react with water, so dry apparatus and reagents are needed when preparing or using them in synthesis; adding aqueous reagent during their preparation would hydrolyse the product.

Water, alcohols and phenol

Acyl chlorides react with oxygen-containing molecules that have an $\ce{O-H}$ bond. In each reaction, $\ce{Cl}$ is replaced and $\ce{HCl}$ is formed.

With water, an acyl chloride is hydrolysed to a carboxylic acid:

With an alcohol, an acyl chloride forms an ester:

For example, ethanoyl chloride reacts with ethanol to form ethyl ethanoate:

Phenol is included explicitly in the OCR guidance. Phenol is not readily esterified by carboxylic acids, but it can react with an acyl chloride to form a phenyl ester.

Worked example: ethanoyl chloride with phenol forms phenyl ethanoate.

The $\mathrm{C_{6}H_{5}O}\text{–}$ part in the ester comes from phenol. The $\mathrm{CH_{3}CO}\text{–}$ acyl part comes from ethanoyl chloride.

Ammonia and primary amines

Acyl chlorides also react with nitrogen-containing molecules. The OCR products to know are primary amides and secondary amides.