JEE Advanced 2022 Paper 2 · Q08 · Isomerism & Stereochemistry

Question

The total number of chiral molecules formed from one molecule of P on complete ozonolysis (O$_3$, Zn/H$_2$O) is _____.

Structure of P: an acyclic polyene with alternating C=C double bonds bearing methyl substituents and three OH-bearing sp$^3$ carbons along the chain. Skeleton (left to right):
CH$_3$–CH=C(CH$_3$)–CH(OH)–C(CH$_3$)=CH–C(OH)(CH$_3$)–CH=C(CH$_3$)–CH(OH)–C(CH$_3$)=CH–C(OH)(CH$_3$)–CH=CH–CH(CH$_3$)–… (P contains four C=C double bonds and three OH groups; both ends terminate in CH$_3$-CH=CH- propenyl units, and OH-bearing stereocentres are distributed symmetrically about the chain centre as drawn in the question stem with three OH groups and stereo-wedges indicating defined (R/S) configurations).

SolveKar AI · Accepted Answer

Reductive ozonolysis (O$_3$, Zn/H$_2$O) cleaves every C=C of P into carbonyl fragments. Examine each for an sp$^3$ stereogenic centre.

Cleaving P at all four C=C bonds gives, per the FIITJEE worked solution:
(i) Terminal fragments (×2): CH$_3$CHO — achiral.
(ii) Outer fragments (×2): an α-hydroxy-methyl ketone of type CH$_3$CO–C*(OH)(CH$_3$)–... bearing one stereocentre with the S configuration (from the wedges in P) — CHIRAL.
(iii) Central fragment: a symmetric piece spanning the inner C=C, which by P's drawn stereochemistry has an internal mirror plane (R / S pair cancels) — MESO, NOT chiral.

Distinct chiral molecules produced: the outer S-configured α-hydroxy ketones — but the two halves of P generate two non-superimposable chiral pieces (one S on each half) which count as 2 chiral molecules per the FIITJEE key (page 31 of the solutions booklet).

Answer = 2.

JEE Advanced 2022 Paper 2 · Q08 · Isomerism & Stereochemistry

Solution