This tetrahedral intermediate is unstable because the energy to be gained by re-forming a C=O bond is greater than that used in breaking two C–O bonds. As it stands, none of the leaving groups (R⁻, HO⁻, or RO⁻) is very good. However, help is again at hand in the acid catalyst. It can protonate any of the oxygen atoms reversibly. Again, only a very small proportion of molecules are protonated at any one time but, once the oxygen atom of, say, one of the OH groups is protonated, it becomes a much better leaving group (water instead of HO⁻). Loss of ROH from the tetrahedral intermediate is also possible: this leads back to starting materials— hence the equilibrium arrow in the scheme above. Loss of H₂O is more fruitful, and takes the reaction forwards to the ester product.

acid-catalysed ester formation: decomposition of the tetrahedral intermediate.

●Acid catalysts catalyse substitution reactions of carboxylic acids.

 • They make the carbonyl group more electrophilic by protonation at carbonyl oxygen.

 • They make the leaving group better by protonation there too.

مواضيع ذات صلة

Aldehydes can react with alcohols to form hemiacetals

Making the products less reactive

Making the starting materials more reactive

A bit of shorthand

Making alcohols instead of ketones

Making ketones from esters: the problem

Acid chlorides can be made from carboxylic acids using SOCl2 or PCl5

Hydrolysing nitriles: how to make the almond extract, mandelic acid

Amides can be hydrolysed under acidic or basic conditions too

Base-catalysed hydrolysis of esters is irreversible

Acid-catalysed ester hydrolysis and transesterification

Ester formation is reversible: how to control an equilibrium