6.1.3 (e,f) Acyl Chlorides

(e) the formation of Acyl Chlorides from Carboxylic Acids using SOCl₂

(f) use of Acyl Chlorides in synthesis in formation of Esters, Carboxylic Acids and primary and secondary Amides.

{Including esterification of Phenol, which is not readily esterified by Carboxylic Acids.}

What does this mean?

There's generally more than one way to make anything but the syllabus only covers the reaction of carboxylic acids with Thionyl Chloride (SOCl₂).

Clearly, producing HCl and acidic SO₂ gas means that this would be a dangerous reaction to carry out unless using a fume-cupboard.

The electronegativity of the O and Cl atoms bonded to Carbon 1 make it very d+, and so prone to attack by any substance with a lone pair to donate (nucleophile).

Water is one such nucleophile.

The lone pair on the water molecule's O atom forms a bond to the d+ Carbon atom.

Carbon atoms can make only 4 bonds at a time so the weakest bond breaks (the pi bond).

An unstable intermediate forms.

The pi bond reforms, and a chloride ion is created as the C-Cl bond breaks.

One of the two H atoms can then form HCl with the Chloride ion, making a Carboxylic acid in the process.

This reaction looks like a substitution reaction - which, in a sense, it is - as the Cl is replaced by an OH group.

However, the examiner wants you to call it an addition-elimination reaction because water is added in the first step before HCl is eliminated.

Examiners are like that.

We've already said that Acyl Chlorides react in similar ways to Carboxylic acids.

So it shouldn't be a surprised that they react with Ammonia (a base)

It seems obvious that HCl would form and that the remaining NH₂ group would replace the Cl.

This is not an amine, because the presence of the C=O bond changes the properties somewhat.

This is an amide.

Ethanamide rather than Ethanamine.

Ethanamide is a primary amide (the N atom is bonded to only one Carbon)

This is a secondary amide, the N atom is bonded to two Carbon atoms.

It would be N-methyl ethanamide (telling us that the methyl group is bonded to the N atom).

To form a secondary amide we react the acyl chloride with a primary amine instead of ammonia.

Tertiray amides aren't on the syllabus but have to start with N,N to show where, in this case, the two methyl groups are bonded.