Why is this the case? (Esters) (1 Viewer)

[may22]

I'm not sure if this goes beyond the scope of the syllabus but I'm curious.

Carboxylic acid + alcohol need to be heated in the presence of an acid catalyst to produce an ester + H2O.

Why wouldn't the carboxylic acid itself be considered the catalytic acid in this case?

 

[someth1ng]

In the mechanism of the reaction, a strong acid is needed to protonate the carboxylic acid (i.e. the carboxylic acid acts as a base). Generally, carboxylic acids are quite weak and are not strong enough to protonate another carboxylic acid (especially itself), so the reaction would be very slow. Therefore, you need an acid that's much stronger than a carboxylic acid (e.g. sulfuric acid).

There are other advantages of sulfuric acid such as the sulfate and hydrogen sulfate ions being very poor nucleophiles (i.e. unreactive), so they won't interfere with the reaction. Sulfuric acid is also a dehydrating agent because it's available at very high concentrations (98%).

 

[may22]

In the mechanism of the reaction, a strong acid is needed to protonate the carboxylic acid (i.e. the carboxylic acid acts as a base). Generally, carboxylic acids are quite weak and are not strong enough to protonate another carboxylic acid (especially itself), so the reaction would be very slow. Therefore, you need an acid that's much stronger than a carboxylic acid (e.g. sulfuric acid).

There are other advantages of sulfuric acid such as the sulfate and hydrogen sulfate ions being very poor nucleophiles (i.e. unreactive), so they won't interfere with the reaction. Sulfuric acid is also a dehydrating agent because it's available at very high concentrations (98%).

Thanks! Do you know if we would ever need to include this in trial/HSC responses?

 

[someth1ng]

Thanks! Do you know if we would ever need to include this in trial/HSC responses?

I don't think so, but I'm not that familiar with the HSC syllabus anymore.