SolutionsWater Addition
1. Acid and base catalysis
Write the mechanism of the hydration of acetaldehyde catalyzed by acid and by base.
In acid the carbonyl oxygen is protonated, the carbon becomes more electrophilic and a neutral water molecule attacks it; loss of a proton gives the hydrate (1,1-ethanediol). In base hydroxide attacks, being a better nucleophile, and the alkoxide formed takes a proton from water.
Catalysts speed up the reaction but do not change the position of the equilibrium.
2. Labeled oxygen
If acetone is dissolved in water enriched with 18O, after a while the recovered acetone contains 18O. Why, if hardly any hydrate forms?
Although the equilibrium lies towards acetone, hydration and dehydration take place continuously. In the hydrate the two OH groups are equivalent, so on dehydration the original oxygen is lost half of the times and acetone with 18O forms.