Chemguide: Support for CIE A level Chemistry

Learning outcome 33: Carboxylic acids and derivatives

33.3: Acyl chlorides

Learning outcome 33.3.4

This statement is about the reactivity of a number of different organic chlorides: acyl chlorides, alkyl chlorides and aryl chlorides.

Before you go on, you should find and read the statement in your copy of the syllabus.

Let's be clear what we are talking about:

  • Acyl chlorides: These contain a COCl group, e.g. ethanoyl chloride, CH3COCl, or benzoyl chloride, C6H5COCl

  • Alkyl chlorides: These have a chlorine attached to a carbon chain, e.g. chloroethane, C2H5Cl

  • Aryl chlorides: These have a chlorine attached directly to a benzene ring, e.g. chlorobenzene, C6H5Cl

The syllabus talks about hydrolysis. This could be reaction with water, or reaction with hydroxide ions.

The facts

Any hydrolysis reaction which happens involving any of these chlorides can be thought of as nucleophilic substitution. (The reaction with acyl chlorides is usually described as nucleophilic addition / elimination, although the overall effect is nucleophilic substitution.)

Thinking about the reaction with water first:

Acyl chlorides generally react vigorously with cold water to produce a carboxylic acid and steamy fumes of hydrogen chloride gas. Where the COCl group is attached directly to a benzene ring, as in benzoyl chloride, the reactions are a lot slower.

Alkyl chlorides have virtually no reaction with cold water.

Aryl chlorides have no reaction with cold water.

And now the reaction with hydroxide ions:

Acyl chlorides generally react rapidly (even violently) with hydroxide ions from, say, sodium hydroxide solution. Again, if the COCl group is attached to a benzene ring, the reactions are slower. The reaction of ethanoyl chloride with sodium hydroxide solution would produce a solution containing sodium ethanoate and sodium chloride.

Alkyl chlorides will undergo a nucleophilic substitution reaction in which the chlorine is replaced by an -OH group if they are heated with an aqueous solution of sodium hydroxide (or a solution in a mixture of ethanol and water). An alcohol is produced.

Aryl chlorides such as chlorobenzene have no reaction with sodium hydroxide solution.

In summary

Acyl chlorides are much more reactive towards hydrolysis than alkyl chlorides. Aryl chlorides are resistant to hydrolysis.


The lack of reactivity of aryl chlorides

You have almost certainly met this before.

Read the page reactions of aryl halides which compares the reactivity of aryl halides with alkyl halides (halogenoalkanes).

The greater reactivity of acyl chlorides

Look at the section headed "Why are acyl chlorides attacked by nucleophiles?" on the page about nucleophilic addition / elimination reactions. Ignore the rest of the page.

Notice that the carbon atom being attacked by a nucleophile is made more positive by the fact that there are two electronegative atoms attached to it. In an alkyl chloride, all you have attached is one chlorine atom which is fairly, but not very, electronegative.

Note:  In addition, the substitution of a chlorine atom in an acyl chloride happens in two stages. First of all you get addition, followed by elimination. This two stage mechanism has a lower activation energy than the one step process that happens in the reaction of a primary halogenoalkane undergoing an SN2 reaction.

A lower activation energy leads to a faster reaction.

You have met this mechanism in statement 33.3.3.

It is probably unlikely that you will be asked why acyl chlorides with the COCl group attached to a benzene ring react more slowly than those where the group is attached to a hydrocarbon chain. However, it is possible, so I will just hint at the reason.

Remember that acyl chlorides are especially reactive because of the fair amount of positive charge on the carbon with the oxygen and chlorine attached. If the COCl group is attached to a benzene ring, that charge can get spread around the whole benzene ring by delocalisation. If it is more spread out, it is going to be less attractive to nucleophiles.

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© Jim Clark 2020