Question

Correct order of percentage of enol content is?

A. b > d > c > a
B. d > b > c > a
C. b > d > a > c
D. a > b > d > c

Answer 1

Hint: To comment on enol content, we have to check the stability of the carbanion formed during the mechanism process and also look at factors like hydrogen bonding, extended conjugation, aromaticity formation etc.
- Enol is an organic compound that contains a hydroxyl group bonded to a carbon having a double bond and that is usually characterised by the grouping \[C=C\left( OH \right)\]

Complete Solution :
- The stability of the state determines the amount of enol content. This is because enols exist in a constant tautomeric state with the ketones. A stable state of the compound is maintained by factors like resonance, aromaticity, hyper conjugation, polarization etc.
Order of the enol content is obtained according to the following logic:
Carbon containing electron withdrawing groups > carbonyl carbons > number of alpha hydrogens.
- Alternatively, we can say that larger the number of alpha hydrogens on a ketone, more is enol content. This is because the \[=O\] can be broken and \[-OH\] is formed in the presence of more alpha hydrogen.
So, the correct answer is “Option B”.

Note: Keto-enol transformation is possible between a ketone and an aldehyde. The keto and enol forms are tautomer of each other. Usually the keto form is stable when there is hydrogen bonding. It is stable when there is a carbon – carbon double bond.
Also, the more substituted alkene makes the enol bond stable.

- For example:
Other factors affecting enol content mainly include:
1. Gain in aromatic character
2. Extension in conjugation
3. Less steric hindrance about the migratory H atom.
4. Increase in acidic nature of \[\alpha H\] due to the presence of groups that promote its acidic nature.