4. Alkanes and Cycloalkanes

The barrier to rotation for 1,2 -dibromopropane along the C1—C2 bond is 28 kJ/mol. Determine the energy cost associated with the eclipsing dibromine interaction.

Would you expect a CH₃/CH₃ eclipsing interaction to be larger or smaller than 1.3 kcal/mol?

Calculate the total strain in the eclipsed conformations shown.

(a)

The following table of strain energies is associated with a variety of 1,2-gauche interactions. Use this table to answer the questions (i)–(iv).

For each of the bond rotations shown, (i) identify which you believe to be more stable, (ii) calculate ∆G°, (iii) calculate the equilibrium constant, and (iv) draw the transition state.

(c)

Calculate the total strain in the eclipsed conformations shown.

(b)

How much more stable is the most stable staggered conformer than the least stable eclipsed conformer?

Draw a potential energy diagram for rotation about the C¬C bond of 1,2-dichloroethane through 360°, starting with the least stable conformer. The anti conformer is 1.2 kcal/mol more stable than a gauche conformer. A gauche conformer has two energy barriers, 5.2 kcal/mol and 9.3 kcal/mol.