Acid catalysts can make bad leaving groups into good ones
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص208
2025-05-11
489
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.
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