Ch17: C=O + 2 ROH = acetals

Chapter 17: Aldehydes and Ketones. Nucleophilic Addition to C=O

Reactions of Alcohols to give Acetals

reaction with alcohols giving an acetal

hemi-acetal acetal

Reaction type: Nucleophilic Addition then nucleophilic substitution

Summary

Typical reagents : excess ROH, catalytic p-toluenesulfonic acid (often written as TsOH) in refluxing benzene.
Aldehydes and ketones react with two moles of an alcohol to give 1,1-geminal diethers more commonly known as acetals.
The term "acetal" used to be restricted to systems derived from aldehydes and the term "ketal" applied to those from ketones, but chemists now use acetal to describe both.
Acetals are biologically important due to their role in the chemistry of carbohydrates.
Acetals are important chemically due to their role as "protecting groups"
The equilibrium is shifted towards the acetal by using an excess of the alcohol and/or removing water as it forms.
It is also possible to use 1,2- or 1,3-diols to form cyclic acetals, two common examples are shown below:

cyclic acetal from ethylene glycol

cyclic acetal from 1,3-propanediol

Acetals can be readily converted back to the aldehyde or ketone by heating with aqueous acid.
The mechanism for this is the reverse of that shown below for acetal formation.

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.
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.

© Dr. Ian Hunt, Department of Chemistry