Formation of hydrates from aldehydes/ketones and H2O

by James

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

Examples:

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.

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{ 2 comments… read them below or add one }

Carrie Douma

How would the mechanism change if this reaction were under basic or acidic conditions?

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James

Hi Carrie, the difference is one extra step.

Under basic conditions, HO- would attack the carbonyl carbon in an “addition” step. Then in the second step, the resulting O- would be protonated by water to give OH
[2 steps]

Under acidic conditions, you’d protonate the carbonyl oxygen, which makes it a better electrophile. Then, water would attack the carbonyl carbon. Then, there’s be a deprotonation of the oxygen that just attacked (since now it’s R-OH2 + ) by a molecule of solvent (H2O) to give the neutral hydrate.
[3 steps]

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