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In organic chemistry, a hydrate is a compound formed by the addition of water or its elements to another molecule.
Order the following molecules with respect to their ability to form hydrates:
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Correct Answer: D. B > A > D > C
When in aqueous solution, aldehyde and ketones groups can form hydrates: this is simply the result of addition of water to the carbonyl (the partial negative charge of the oxygen in water attacking the partial positive charge of the carbonyl group).
The ability to form a hydrate (addition of water across the C=O bond) can be seen as measure of the stability of a carbonyl group.
Factors affecting this are the electronic and steric effects of substitutents attached to the electrophilic carbonyl carbon. Hydrogen bond acceptors also aid hydrate formation.
Strained cyclopropanone B is the most ready to form a hydrate as the ring bond angle is far from the ideal trigonal planar angle. This strain is relieved somewhat upon formation of a near tetrahedral hydrate.
Formaldehyde A is more reactive (unstable) than aldehyde D which in turn is more reactive than ketone C. Across this sequence more +I methyl groups are being added to the electrophilic C=O carbon atom. In addition, the less hydrophobic the carbonyl compound (and the corresponding hydrate), the better the solvation through intermolecular hydrogen bonding (i.e., A > D > C).
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