The barrier to C-C bond rotation in ethanol (CH3-CH2-OH) is 14 kJ/mol. What energy can you assign to an H-O eclipsing interaction?

6kJ/mol

Explanation:

The full eclipse stereoisomer of ethanol has two H-H overlapped and one H-OH overlapped. The energy cost for H-H eclipse is 4kJ/mol or 1kcal/mol.

Total energy= 14kJ/mol

2(H-H eclipsed) = 2(4kJ/mol) = 8kJ/mol

Total Energy = H-H eclipsed + H-OH eclipsed

14kJ/mol = 8kJ/mol + H-OH eclipsed

14kJ/mol - 8kJ/mol = H-OH eclipsed

Therefore H-OH eclipsed = 6kJ/mol