Acid catalysts increase the reactivity of a carbonyl group
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص207-208
2025-05-11
631
We saw in Chapter 6 that the lone pairs of a carbonyl group may be protonated by acid. Only strong acids are powerful enough to protonate carbonyl groups: the pKa of protonated acetone is –7 so, for example, even 1M HCl (pH 0) would protonate only 1 in 107 molecules of acetone. However, even proportions as low as this are sufficient to increase the rate of substitution react ions at carbonyl groups enormously because those carbonyl groups that are protonated become extremely powerful electrophiles.
It is for this reason that alcohols will react with carboxylic acids under acid catalysis. The acid (usually HCl or H2SO4) reversibly protonates a small percentage of the carboxylic acid molecules, and the protonated carboxylic acids are extremely susceptible to attack by even a weak nucleophile such as an alcohol. This is the fi rst half of the reaction:
acid-catalysed ester formation: forming the tetrahedral intermediate
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