In the Newman projection along C2-C3 of butane, the anti conformer has front CH3 at 12 o'clock and rear CH3 at 6 o'clock. The rear carbon is rotated by 60 deg so that rear groups move to 8, 4 and 12 o'clock (the rear CH3 now at 8). Using eclipsing energies H/H 4.0, H/CH3 6.0, and gauche 3.8 kJ/mol above anti, the energy barrier for the gauche-to-anti conversion through this eclipsed conformer is

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In the Newman projection along C2-C3 of butane, the anti conformer has front $CH X_{3}$ at 12 o'clock and rear $CH X_{3}$ at 6 o'clock. The rear carbon is rotated by $6 0^{\circ}$ so that rear groups move to 8, 4 and 12 o'clock (the rear $CH X_{3}$ now at 8). Using eclipsing energies H/H 4.0, H/ $CH X_{3}$ 6.0, and gauche 3.8 kJ/mol above anti, the energy barrier for the gauche-to-anti conversion through this eclipsed conformer is

A $16.0 kJ mo l^{- 1}$
B $19.8 kJ mo l^{- 1}$
C $12.2 kJ mo l^{- 1}$
D $3.8 kJ mo l^{- 1}$

Answer: C

Rear groups are CH3 at 8, H at 4 and H at 12 (the originals at 6, 2 and 10 moved by two hours). Eclipsed pairs: front CH3 (12) with rear H (12) = 6.0; front H (4) with rear H (4) = 4.0; front H (8) with rear CH3 (8) = 6.0. Energy = 16.0 above anti. This is the transition state between anti and the gauche conformer (rear CH3 at 10), so the gauche-to-anti barrier = 16.0 - 3.8 = 12.2 kJ/mol; 16.0 is the anti-to-gauche barrier.

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Answer: C

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