Q59 · CSIR-NET Chemistry, June 2018
Paper: CSIR-NET June 2018 · Subject: Organic Chemistry · Chapter: Pericyclic Reactions · Topic: Cycloaddition Diels Alder · Marks: 2 · Difficulty: Hard
For the following thermal $[2+2]$ cycloaddition reaction, the correct statement about Transition State (TS) and preference for endo product formation is 
(a)TS: $\pi 2 \mathrm{s}+\pi 2 \mathrm{s}$; Me > i-Pr > t-Bu
(b)TS: $\pi 2s + \pi 2a$; t-Bu > i-Pr > Me
(c)TS: $\pi 2s + \pi 2a$; Me > i-Pr > t-Bu
(d)TS: $\pi 2 \mathrm{s}+\pi 2 \mathrm{s} ; \mathrm{t}-\mathrm{Bu}>\mathrm{i}-\mathrm{Pr}>\mathrm{Me}$
Answer
Answer: B ✓ checked by 4AB · confidence medium
The source book printed A; on checking, B is correct — see the explanation.
Explanation
Thermal ketene [2+2] is π2s + π2a; the larger ketene substituent prefers the endo position, so endo preference t-Bu > i-Pr > Me (b).
Study loop for Pericyclic Reactions