Depto. Química Orgánica

ADDITION OF ALCOHOLS  TO ALDEHYDES AND KETONES:
KETAL FORMATION

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15/07/2017
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Ketals
Provided there is an excess of alcohol, the hemiketal can keep on reacting with a second molecule yielding a "whole" ketal.
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The reaction is still an equilibrium that must be shifted getting rid of the by-produced water by means of a desiccant or a Dean-Stark apparatus. Ketals can be isolated as perfectly stable compounds.
In a ketal, the former carbon of the C=O loses the pi cloud and turns its hybridization into sp3.
Hence, the ketal is way less reactive than the initial aldehyde or ketone.
A ketal endures the attack of numerous nucleophiles that would easily react with a double C=O bond.
Ketal formation is thus used to "protect" a C=O group of aldehydes and ketones (NOT of carboxylic acid derivatives).
Look at the "Problems and exercises" to understand this "protection" business by means of a straightforward and enlightening example.