Chapter 25: Carbohydrates

Acetals and Ketals applied to sugars

aldehyde                       hemi-acetal                                  acetal

ketone                          hemi-ketal                                    ketal

Reaction type:  Nucleophilic Addition then nucleophilic substitution

Summary

• Review of acetals and ketals in general ?
• 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.
• "Hemi" just means "half"- (think of hemisphere)

a hemi-acetal	an acetal

• The mechanism by which aldehydes or ketones are converted to acetals or ketals is catalysed with aqueous acid (review ?)
• In sugar chemistry, the equilibrium is important for:
• the interconversion of cyclic and acyclic forms of the sugar
• the interconversion of α- and β- anomers = mutarotation
• the formation and cleavage of glycosides

© Dr. Ian Hunt, Department of Chemistry