Question

Rank the following carbonyl compounds in order of increasing equilibrium constant for hydration:

${\text{CH}}_{\text{3}}{\text{COCH}}_{\text{2}}{\text{Br BrCH}}_{\text{2}}\text{CHO} {\text{CH}}_{\text{2}}\text{O} {\text{CH}}_{\text{3}}{\text{COCH}}_{\text{3}} {\text{CH}}_{\text{3}}\text{CHO}$

Short answer

A ketone or an aldehyde in an aqueous solution is known to be in equilibrium with its hydrate, a geminal diol. The equilibrium favors the unhydrated keto form of the carbonyl with most ketones.

Step-by-step solution

Introduction

A ketone or an aldehyde in an aqueous solution is known to be in equilibrium with its hydrate, a geminal diol. The equilibrium favors the unhydrated keto form of the carbonyl with most ketones.

Reaction with water

The reactions of the given compounds with water are shown below:

The increasing amount of hydrate present at equilibrium

Stable hydrates are more likely to be formed by aldehydes than ketones.The electrophilic carbonyl group of a ketone is stabilized by the presence of its two electron-donating alkyl groups, whereas an aldehyde has one stabilized alkyl group. The partial positive charge of the aldehyde is also not stable. Therefore, aldehydes are more electrophilic and less stable than ketones. Formaldehyde without electron-donating groups is even less stable than other aldehydes. These stability effects are evident in the equilibrium constants of hydration of both ketones and aldehydes. The equilibrium constant value of a ketone is lies between ${10}^{-4}-{10}^{-2}$. The equilibrium constant for hydration of most aldehydes is close to $1$$1$. Formaldehyde on the otherhand, without an alkyl group attached to the carbonyl has an equilibrium constant of hydration to be about 40. The presence of strong electron-withdrawing substituents on the alkyl groups of ketones or aldehydes also destabilize the carbonyl group and prioritize the hydrates.Equilibrium favors the unhydrated keto form of the carbonyl and the order is as shown below: