I was taught that for aldehydes and ketone to exhibit keto-enol tautomerism, it is essential that there must be at least one α-hydrogen.

For example, acetophenone and butan-2-one show keto-enol tautomerism, but benzaldehyde and benzophenone don't show keto-enol tautomerism, since they don't have any α-hydrogens.

Examples of carbonyl compounds that do or do not exhibit keto-enol tautomerism

However, I was subsequently told that the cyclic enone 6,6-dimethylcyclohex-2-en-1-one also exhibits tautomerism:


How is this possible? It does not have any α-hydrogens (the vinylic proton cannot be enolised as it is orthogonal to the C=O π* orbital).


If you have an α,β-double bond next to your carbonyl, a γ hydrogen can also take part in tautomerism. Take crotonaldehyde $\ce{H3C-CH=CH-CHO}$:

$$\ce{O=CH-CH=CH-CH3 <<=> HO-CH=CH-CH=CH2}$$

In general, a double bond is able to ‘extend’ the mesomeric properties of a carbonyl compound across two additional atoms. This is known as vinylogous; crotonaldehyde can form a vinylogous enol.

In the specific case of 6,6-dimethylcyclohex-2-en-1-one, these are the two tautomers:

Tautomeric forms of 6,6-dimethylcyclohex-2-en-1-one

| improve this answer | |

There should be possibility of an intramolecular acid-base reaction. Acid-base pair are usually Lewis acid-base but there may be others. Such molecules show tautomerism. There are a large number of tautomerisms that can be explained by this way.

Such as keto-enol (your problem too), oxime-nitroso tautomerism and many more.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.