JEE Main levelSingle correct optionDifficulty 2 of 3st139
Match the butane conformations (dihedral angle between the two methyl groups, viewed along C2-C3) in List I with their descriptions in List II.
List I: P = 0∘; Q = 60∘; R = 120∘; S = 180∘.
List II: 1 = anti (global minimum); 2 = gauche (local minimum); 3 = eclipsed with CH3/H and H/H and CH3/H pairs (local maximum); 4 = fully eclipsed (global maximum).
AP-4, Q-2, R-3, S-1
BP-1, Q-2, R-3, S-4
CP-4, Q-3, R-2, S-1
DP-2, Q-4, R-1, S-3
Answer: A
0∘ puts the methyls directly over each other: fully eclipsed, highest energy (4). 60∘ is the staggered gauche conformer (2). 120∘ is eclipsed with each methyl eclipsing an H, a local maximum (3). 180∘ is anti, the global minimum (1).