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#1 2011-05-11 13:03:51


Why do geminal diols or C=O hydrates favor the carbonyl group?

I know H2O is removed if 2 -OH groups on the same carbon, but why?

In order to understand this, we also have to understand that it is also not correct. Because carbon holds its electrons relatively loosely, then it is the more electron rich carbons that lose water and form a carbonyl group. For example, acetone, with two (weakly) electron donating CH3 groups far prefers being a ketone. If a CH3 group is replaced with the more electron withdrawing H, then a small fraction of acetaldehyde exists as its hydrate, CH3CH(OH)2. If both CH3 groups are replaced with a hydrogen, the carbonyl group will be entirely in the form of its hydrate, CH2(OH)2. This is the hydrate of formaldehyde.

So, the polar groups attached to a carbon will pull electrons away from it. If two OH groups are attached to the same carbon, then electron donating groups will enable this reaction to take place. Therefore simple alkyl groups in a ketone enable the loss of an OH group. Because the OH has non-bonded electrons or simultaneously, the non-bonded electrons of the oxygen are donated to the carbon to replace those lost with the OH group. Loss of the proton will return the compound to neutrality and the OH group to water.

This should not be confused with a steric effect. If the hydrogens of acetone are replaced with fluorine atoms, the carbonyl group will again prefer to exist as the hydrate form, CF3C(OH2)CF3. Similarly, if the hydrogens of acetaldehyde are replaced with chlorine atoms, the carbonyl group prefers to exist as the hydrate, in this case chloral hydrate.

The consistent properties of carbonyl groups is that their reactivity increases as more strongly electron withdrawing groups are attached. The pervasive and incorrect use of electronegativity has led to some of the complication, such as not recognizing that carbon is a better electron donor than hydrogen.


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