JEE Main 2022 · 29 June · evening · Numerical answer · +4 / −0
In the given reaction,
the number of sp² hybridised carbon(s) in compound 'X' is ________.
Answer and solution
Answer 8
Solution
Chemistry ma’am · 5 steps
Follow the four reagents one by one: ketone, Grignard alcohol, then lose water to make a C=C bond.
- Oxidise. K₂Cr₂O₇ oxidises the secondary alcohol cyclohexanol to the ketone, cyclohexanone.
- Grignard addition. C₆H₅MgBr attacks the carbonyl carbon. Hydrolysis with H₂O then gives the tertiary alcohol 1-phenylcyclohexan-1-ol.
- Dehydrate. H⁺ and heat remove water from the tertiary alcohol. The new C=C bond forms inside the ring, in conjugation with the benzene ring, so X is 1-phenylcyclohex-1-ene.
- Count sp2 carbons. All 6 carbons of the benzene ring are sp². The 2 carbons of the ring C=C bond are also sp². The other 4 ring carbons are sp³.
- So. X has 6 + 2 = 8 sp² hybridised carbons.
The common slip: 6. Stopping at the alcohol after step (iii) and missing the dehydration gives only the 6 benzene carbons, so 6, not 8.
The idea to keep. Grignard on a ketone gives a tertiary alcohol, and acid with heat turns it into an alkene. Every carbon in a double bond or a benzene ring counts as sp².
Answer from the NTA final answer key. Solution by PrepWiser.