Q10 · CSIR NET Chemistry, December 2016
Paper: CSIR NET December 2016 · Subject: Organic Chemistry · Chapter: General Organic Chemistry · Topic: Conformational Analysis · Marks: 2 · Difficulty: Medium
Among the structures given below, the most stable conformation for the following compound is 
(a)
(b)
(c)
(d)
Answer
Answer: C ✓ checked by 4AB · confidence high
Answer: The chair with the C-1 methyl axial and the C-2 and C-4 methyls equatorial.
Explanation
The compound is all-cis-1,2,4-trimethylcyclohexane. In a chair, cis-1,2 needs axial/equatorial, cis-1,4 needs axial/equatorial and cis-2,4 (a 1,3-relationship) needs equatorial/equatorial. Only the chair with the C-1 methyl axial and the C-2 and C-4 methyls equatorial meets all three, giving just one axial methyl. Its ring-flipped chair (a) has the C-2 and C-4 methyls both axial, a severe 1,3-diaxial Me/Me clash. The most stable conformation is therefore (c).
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