Out of 20 pts. Problem Set #3, 5.12 Spring 2003 Due Monday, March 3, 4pm 1. a) Using Newman projections, draw all of the staggered and eclipsed conformers of 2,3-dimethylpentane with respect to rotation about the C2-C3 bond. b) Assign the relative energies of each of the conformers in part a. Hint: Assume that an ethyl group behaves similarly to a methyl group. Newman Projections: Et H \ / C---C / \ H Me A H Et \ / C---C / \ H Me B Et Me \ / C---C / \ H H C Et H \ / C---C / \ Me H D H Et \ / C---C / \ Me H E Me Et \ / C---C / \ H H F Relative Energies: A: Erel = 4.0 + 0.9 + 1.0 = 5.9 kcal/mol (700 K) B: Erel = 0.9 + 0.9 + 0.9 = 2.7 kcal/mol C: Erel = 4.0 + 1.3 + 1.3 = 6.6 kcal/mol (560 K) D: Erel = 0.9 + 0.9 + 1.3 = 3.1 kcal/mol E: Erel = 4.0 + 1.3 + 1.3 = 6.6 kcal/mol (560 K) F: Erel = 0.9 + 0.9 + 0.9 = 2.7 kcal/mol c) Starting with the highest energy eclipsed conformer, draw a potential energy diagram for a 360° rotation around the C2-C3 bond of 2,3-dimethylpentane. d) What is the barrier to rotation around the C2-C3 bond of 2,3-dimethylpentane? Barrier = 9.0 - 1.8 = 7.2 kcal/mol (7.0 - 1.8 = 5.2 kcal/mol also ok) 2. You learned in lecture that cyclopentane prefers to adopt a low-energy envelope conformation. What you didn't learn is that mono-substituted cyclopentane derivatives favor the envelope conformation that puts the substituent (X) at the "flap" (A) rather than at the "base" (B). H H / \ / \ H X H H \ / \ / H H (A) (more stable) H H / \ / \ H H H X \ / \ / H H (B) (less stable) Draw Newman projections along the indicated bonds in A and B. Use these Newman projections to help you explain the energy difference between the two. Newman Projections: H H / \ / \ H X H H \ / \ / H H (A) H H / \ / \ H H H X \ / \ / H H (B) In (A), the substituent (X) is staggered relative to the H atoms on the adjacent carbons (no energy price to pay). In (B), the X is eclipsing a H atom on the adjacent carbon. It is reasonable to assume that an X/H eclipsing interaction is more destabilizing than an H/H eclipsing interaction. Therefore, (A) is lower in energy than (B). 4. As you would expect based on A-values, cis-1-tert-butyl-4-methylcyclohexane prefers the conformation in which the tert-butyl group is equatorial (eq 1). A related molecule, cis-5-tert-butyl-2-methyl-1,3-dioxane, prefers to put the tert-butyl group axial (eq 2). Use what you know about the conformational analysis of cycloalkanes to explain this apparent contradiction. H H \ / C---C---C / \ / \ H Bu Me H (eq 1) (preferred) H H \ / C---C---C / \ / \ Me H Bu H (eq 2) (severe 1,3-diaxial) H H \ / C---O---C / \ / \ Me O Bu H (moderate 1,3-diaxial) H H \ / C---O---C / \ / \ Bu O Me H (no 1,3-diaxial) (preferred) In equation 1, one of the major interactions that destabilizes the molecule on the right is 1,3-diaxial interactions between the t-butyl group and the axial Hs. Replacing the 1,3 carbons with oxygens results in a loss of 1,3-diaxial interactions with the axial methyl in the conformer on the left. The 1,3-diaxial interactions with the t-butyl group in the conformer on the right became more important, and the conformer on the right is favored.