|Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O|
Reactions of Alcohols to give Acetals
Reaction type: Nucleophilic Addition then nucleophilic substitution
The important "piece" of an acetal is the central C which becomes the C of the carbonyl C=O.
It can be recognised by looking for the C that is attached to two O atoms by single bonds.
MECHANISM FOR THE ACID CATALYSED FORMATION OF ACETALS
An acid/base reaction. Since there is only a weak nucleophile we need to activate the carbonyl by protonating on O.
The nucleophilic O in the alcohol attacks the electrophilic C in the C=O, breaking the π bond and giving the electrons to the positive O.
An acid/base reaction. Deprotonation of the alcoholic oxonium ion neutralises the charge giving the hemi-acetal. Now we need to substitute the -OH by -OEt.
An acid/base reaction. In order for the -OH to leave we need to make it into a better leaving group by protonation.
Using the electrons from the other O, the leaving group departure is facilitated.
We now have what resembles a protonated ketone (compare with step 2). The nucleophilic O of the alcohol attacks the electrophilic C and the electrons of the π bond move to neutralise the charge on the positive O.
An acid/base reaction. Deprotonation of the alcoholic oxonium neutralises the charge and produces the acetal product and regenerates the acid catalyst.
|© Dr. Ian Hunt, Department of Chemistry, University of Calgary|