JEE Advanced 2022 Paper 2 · Q06 · Isomerism & Stereochemistry

Question

The number of isomeric tetraenes (NOT containing sp-hybridized carbon atoms) that can be formed from the following reaction sequence is _____.

Substrate: 3-(prop-2-yn-1-yl)cyclohex-1-ene — a cyclohex-1-ene ring with a $-CH_2-C\equiv CH$ substituent on the allylic carbon.

Reagents (sequential):
1. Na, liq. NH$_3$
2. Br$_2$ (excess)
3. alc. KOH

SolveKar AI · Accepted Answer

Step 1 — Na/liq. NH$_3$ reduces the terminal alkyne $-CH_2-C\equiv CH$ to the terminal alkene $-CH_2-CH=CH_2$ (allyl). Ring C=C unchanged. Intermediate = 3-allylcyclohex-1-ene (two C=C: ring + chain).

Step 2 — Br$_2$ (excess) brominates both alkenes giving the tetrabromide (two $-CHBr-CHBr-$ units).

Step 3 — alc. KOH does double E2 on each $-CHBr-CHBr-$, generating two C=C bonds per centre. Per the constraint (no sp carbons, i.e. no alkynes/allenes), each $-CHBr-CHBr-$ must furnish a 1,3-diene with neighbouring CHs. Ring contributes cyclohexa-1,3-diene; chain contributes the 1,3-butadiene-like CH=CH-CH=CH$_2$ system. Total 4 C=C ⇒ tetraene.

Step 4 — count distinct isomers. Ring diene geometry is fixed (cis-only inside the 6-ring). The internal C=C of the chain diene can be cis or trans ⇒ 2 geometric isomers; the terminal CH=CH$_2$ has no geometry.

Number of isomeric tetraenes = 2.

JEE Advanced 2022 Paper 2 · Q06 · Isomerism & Stereochemistry

Solution