Dehydration of Cyclohexanones using Conc. H2SO4

When cyclohexanones are treated with concentrated sulfuric acid, they undergo dehydration to form α,β-unsaturated ketones.

The dehydration reaction occurs via the E1 mechanism, where the carbonyl group is protonated by the sulfuric acid, and the leaving group (water molecule) is eliminated from the molecule.

The degree of dehydration depends on the position of the hydroxyl group on the cyclohexanone molecule. The maximum dehydration occurs when the hydroxyl group is located at the β-position relative to the carbonyl group.

Comparison of Cyclohexanones

A) 2-hydroxycyclohexanone
- Hydroxyl group is located at alpha position
- Dehydration leads to the formation of enol, which is unstable
- Further dehydration leads to the formation of a diene, which is not favored

B) 3-hydroxycyclohexanone
- Hydroxyl group is located at beta position
- Dehydration leads to the formation of an α,β-unsaturated ketone

C) 4-hydroxycyclohexanone
- Hydroxyl group is located at gamma position
- Dehydration leads to the formation of a conjugated enone, which is more stable than the α,β-unsaturated ketone formed from 3-hydroxycyclohexanone

D) 2-methyl-3-hydroxycyclohexanone
- The presence of the methyl group on the cyclohexanone ring does not affect the position of the hydroxyl group
- Dehydration leads to the formation of an α,β-unsaturated ketone

Conclusion

Therefore, the 3-hydroxycyclohexanone is dehydrated to the maximum extent using concentrated sulfuric acid, as it forms the most stable α,β-unsaturated ketone.

In 4th optionafter formation of alkenes it is stable because of more substituded alkene is more stable so 4th option is ans