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Formation of hydrates from aldehydes/ketones and H2O

Description: Addition of water to an aldehyde or ketone results in the hydrate [otherwise known as a “geminal diol”]

Notes: This is an equilbrium reaction. For most aldehydes and ketones the hydrate is not favored by equilibrium (hydrates are less stable than the parent aldehyde/ketone). However if an electron-withdrawing group is present adjacent to the aldehyde or ketone the hydrate will be more stable.


Notes: In the bottom two examples the hydrate is more stable due to the presence of the electron withdrawing groups.

Mechanism: Addition of water to the carbonyl of the ketone in an addition reaction (Step 1, arrows A and B) leads to the formation of a doubly-charged intermediate (“zwitterion”) which then undergoes proton transfer to give the neutral hydrate.

Notes: It’s also reasonable to show the proton transfer occurring in two steps rather than the one step depicted here.