Acyl chlorides: formation and reactions

In a nutshell 

Acyl chlorides have the functional group ($$-COCl$$). They are formed by reacting carboxylic acids with $$SOCl_2$$ and are able to undergo multiple reactions.

Equations

These are general chemical equations. You will need to be able to use these when you write equations for specific substances. 

​$$acyl \space chloride \,+\,water \rightarrow carboxylic \space acid \,+ \, hydrogen\space chloride \\ \ \\acyl \space chloride \,+\,alcohol \rightarrow ester \,+ \, hydrogen\space chloride \\ \ \\acyl \space chloride \,+\,ammonia \rightarrow amide \,+ \,hydrogen\space chloride \\ \ \\acyl \space chloride \,+\,primary \space amine \rightarrow N-substituted \space amide \,+ \,hydrogen \space chloride \\ \ \\acyl \space chloride\,+\,phenol \rightleftharpoons ester\,+ \,hydrogen \space chloride \\ \ \\acid \space anhydride\, + \, alcohol \rightarrow ester \,+\, carboxylic \space acid$$​​

Formation of acyl chlorides

Acyl chlorides are formed by reacting carboxylic acids with $$SOCl_2$$. They have the suffix '-oyl chloride.' The $$-OH$$ from the carboxylic acid is replaced by $$Cl$$ from the $$SOCl_2$$.

Example

Reactions of acyl chlorides 

When acyl chlorides react with cold water or alcohol, the reaction is vigorous. When they react with ammonia or primary amines, the reaction is violent. For each reaction misty fumes of hydrogen chloride is given off, which makes it a driving force for the reaction to lie to the right. 

​​

Reaction with water

​$$acyl \space chloride \,+\,water \rightarrow carboxylic \space acid \,+ \,hydrogen\space chloride$$

Example

​$$methanoyl \space chloride\,+\,water \rightarrow methanoic\space acid \,+\,hydrogen\space chloride \\HCOCl \,+\,H_2O \rightarrow HCOOH \, + \, HCl$$

​

Reaction with alcohol

​$$acyl \space chloride \,+\,alcohol \rightarrow ester \,+ \,hydrogen\space chloride$$

Example

​$$methanoyl \space chloride\,+\,ethanol \rightarrow ethyl\space methanoate \,+\, hydrogen\space chloride \\HCOCl \,+\,CH_3CH_2OH \rightarrow HCOOCH_2CH_3 \, + \, HCl$$​​

​

Reaction with ammonia  

​$$acyl \space chloride \,+\,ammonia \rightarrow amide \,+ \, hydrogen\space chloride$$

Example

​$$methanoyl \space chloride\,+\,ammonia \rightarrow methanamide \,+\,hydrogen\space chloride \\HCOCl \,+\,NH_3 \rightarrow HCONH_2 \, + \, HCl$$​​

​

Reaction with primary amines

​

​$$acyl \space chloride \,+\,primary \space amine \rightarrow N-substituted\space amide \,+ \, hydrogen\space chloride$$

​​

Example

​$$methanoyl \space chloride\,+\,ethyl \space amine \rightarrow N-ethylmethanamide\space \,+\, hydrogen\space chloride \\HCOCl \,+\,CH_3CH_2NH_2 \rightarrow HCONH(CH_2CH_3) \,+ \,HCl$$​​

​

Reaction with phenols

Phenols react very slowly with carboxylic acids. Therefore a faster way to make esters is by using an acyl chloride. 

​$$acyl \space chloride\,+\,phenol \rightleftharpoons ester\,+\,hydrogen\space chloride \\$$​​

Example

​$$methanoyl \space chloride\,+\,phenol \rightleftharpoons phenyl \space methanoate\,+\,hydrogen\space chloride \\ HCOCl \,+\,C_6H_5OH \rightleftharpoons HCOOC_6H_5 \, + \, HCl$$​​

​

Reaction mechanism

Reactions with acyl chlorides are nucleophilic addition-elimination reactions. 

​

Acid anhydrides

Acid anhydrides are made from two identical carboxylic acids. They have the suffix '-anhydride.' 

Example

Acid anhydrides react in the same way as acyl chlorides, except the reactions are less vigorous and instead of forming hydrogen chloride, a carboxylic acid is formed instead.

​$$acid \space anhydride\, + \, alcohol \rightarrow ester \,+\, carboxylic\space acid$$​​

Example  

​$$methanoic \space anhydride \,+\, ethanol \rightarrow \,ethyl\space methanoate\,+\, methanoic \space acid\\(HCO)_2O \,+ \, CH_3CH_2OH \rightarrow HCOOCH_2CH_3 \,+ \, HCOOH$$

​

Aspirin

Aspirin is manufactured using ethanoic anhydride instead of ethanoyl chloride because it is safer and cheaper.