Return to Contents  Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O  Ch 17 contents

Reactions of Alcohols to give Acetals

reaction with alcohols giving an acetal
                                                              hemi-acetal                                  acetal

Reaction type:  Nucleophilic Addition then nucleophilic substitution

Summary

cyclic acetal from ethylene glycol
cyclic acetal from 1,3-propanediol
Study Tip:
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.

Related Reactions

MECHANISM FOR THE ACID CATALYSED FORMATION OF ACETALS
Step 1:
An acid/base reaction. Since there is only a weak nucleophile we need to activate the carbonyl by protonating on O.
acetal formation
Step 2:
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.
Step 3:
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.
Step 4:
An acid/base reaction. In order for the -OH to leave we need to make it into a better leaving group by protonation.
Step 5:
Using the electrons from the other O, the leaving group departure is facilitated. 
Step 6:
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.
Step 7:
An acid/base reaction. Deprotonation of the alcoholic oxonium neutralises the charge and produces the acetal product and regenerates the acid catalyst.
 

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© Dr. Ian Hunt, Department of Chemistry, University of Calgary