Estabilidade de Conformações

Prévia do material em texto

For menthol, the groups are either all axial or all equatorial. Obviously the equilibrium is greatly in favor of the conformer with all the groups equatorial. The axial destabilization energy for this conformer is zero. For neomenthol, the conformer with both alkyl groups equatorial is more stable. (The axial strain energy for OH is 0.9 kcal/mol [3.8 kJ/mol], that for methyl is 1.7 kcal/mol [7.1 kJ/mol, and that for isopropyl is 2.2 kcal/mol [9.2 kJ/mol]). The axial destabilization energy of this conformation is 0.9 kcal/mol (3.8 kJ/mol) because it has an axial OH group. Overall, menthol is more stable than neomenthol by 0.9 kcal/mol (3.8 kJ/mol). 6.27 a) b) Cl 6.28 The tert-butyl group has a very large axial strain energy and, therefore, it is always equatorial. In the isomer on the left, the CO₂H group and the OCH₃ group are both axial when the tert-butyl group is equatorial. As a result, the H of the group is too far from the O of the OCH₃ to hydrogen bond to it. In contrast, in the case of the isomer on the right, the group and the OCH₃ group are both equatorial when the tert-butyl group is equatorial. As a result, the H of the CO₂H group is close enough to the of the OCH₃ to hydrogen bond to it. CH₃ CH₃ C H₃C C H₃C C CH₃ CH₃ OCH₃ CH₃ 92