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

Formation of Hydrates

hydrate

Summary

• Aldehydes and ketones react with water to give 1,1-geminal diols known as hydrates.
• In general, hydrates are not stable enough to be isolated as the equilibrium shifts back to starting materials.
• However, hydrates are the reactive species in the oxidation of aldehydes to acids.
• Understanding the mechanism is useful before looking at the very closely related reactions of alcohols.

• Formation of Acetals
• Hydrolysis of Esters

MECHANISM FOR THE ACID CATALYSED FORMATION OF HYDRATES
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 water attacks the electrophilic C in the C=O, breaking the p bond and giving the electrons to the positive O.
Step 3: An acid/base reaction. Deprotonation of the oxonium ion neutralises the charge giving the hydrate.