Acyl chlorides

In this new post, I’m discussing Acyl Chlorides.

You’ll probably have come across them as Ethanoyl Chloride or Benzoyl Chloride

What you might not know however is that molecules like these are major constituents of tear gas. 

Protestor catches Tear Gas canister in Tahir Square, Cairo

Exploding tear gas canister on the fly!!

CS and CN gas (or Mace) contain the following molecules:

CS gas

Tear gas was first used when the French fired canisters into German trenches at the start of WW1

Tear gas and acyl chlorides are examples of molecules called lachrimators

Molecules in tear gas and molecules like them causes burning of the eyes and skin, tears, and a sensation of suffocation.

Their use is aimed at driving people from their cover and disrupting their protest.

The gas also provokes fear and insecurity playing on the human survival instinct. 

It has been claimed that it is easier for men to face bullets than invisible debilitating gas.

You can read more about tear gas here.

Preparation of acyl chlorides

Acyl chlorides can be prepared from the corresponding carboxylic acid using a suitable chlorinating agent e.g. PCl₅ phosphorus pentachloride or SOCl₂ thionyl chloride

Or

The formation of this intermediate then enables the formation of many different carboxylic derivatives in very high yields and atom economies. 

So for example:

Ethanoyl chloride and ammonia produce acetamide (ethanamide) a quick reaction at room temperature

Ethanoyl chloride and phenylamine produce N-phenylethanamide, an anti-coagulant.

Ethanoyl chloride and ethanol (or any carboxylic acid and any alcohol) form esters at high yield.

Ethanoyl chloride is easily hydrolysed using water to form the corresponding carboxylic acid.

Summary of carboxylic acid derivative chemistry

“Unsaturated carbon in a suitable environment undergoes substitution  via a two step process  consisting of addition followed by elimination.” ROC Norman

Substitution at a carbonyl group involves a nucleophile. 

Carboxylic acid derivatives where the carbonyl group is attached and adjacent to an electronegative atom are substituted via an addition–elimination mechanism.

Among the common carboxylic derivatives the order of reactivity is acid chloride > acid anhydride > ester > amide.

Acid chlorides react rapidly with water, anhydrides slowly and esters and amides not at all.

Here is a typical nucleophilic substitution mechanism: