Polymers
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Why is an acyl chloride, rather than a carboxylic acid, preferred in the condensation reaction to form a polyamide? Surely the carboxylic acid would be safer, as no HCl vapour is produced.
Igloo writes
You’re absolutely correct in realizing that the use of the carboxylic acid would be safer. There are two obvious dangers using the acyl chloride. One is that the acyl chloride itself is a far more hazardous and volatile material to employ than the carboxylic acid, and, as you rightly point out, its use produces toxic HCl vapour.
The reason is all to do with the relative reactivities of the acyl chloride and the carboxylic acid. As you should know from the Nuffield course (Topic 15), acyl chlorides are far more reactive electrophiles than carboxylic acids, and their reactions can always be carried out under less vigorous conditions, often at room temperature. Providing that the acyl chloride is handled carefully and that the ensuing reaction is carried out in a fume cupboard, it is the preferred choice of reagent for the reasons given. Another consideration is that since HCl is gaseous, its escape from the reaction mixture causes the position of equilibrium to swing to the side of the products, so that larger yields are obtained.
Look at the Nuffield Chemistry Students’ Book (Topic 15) to remind yourself of the reasons why acyl chlorides are such reactive electrophiles.
Note that this discussion applies to laboratory chemistry and not to processes carried out on an industrial scale where a wider range of conditions of temperature and pressure is possible.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 02 December 2003
