4. Alkanes and Cycloalkanes

Choose the conformation in each pair that is most stable. If both are equally stable, then write 'no difference.'

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(••••) LOOKING AHEAD In Chapter 5, we introduce reaction coordinate diagrams as a plot of potential energy versus the progress of a reaction. Consider the reaction coordinate diagram drawn for the 'reaction' of conformation A becoming conformation B. Which structure is present at the top of the hill?

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5. Indicate which conformations are the most stable (lowest energy) and the least stable (highest energy).

Why is it incorrect to say that the dihedral angle shown is 120° or even 109°?

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Construct a graph, similar to [FIGURE 3-11] <IMAGE>, of the torsional energy of 3-methylpentane along the C2―C3

bond.

2. Place C2 in front, represented by three bonds coming together in a Y shape, and C3 in back, represented by a circle with three bonds pointing out from it.

3. Define the dihedral angle as the angle between the methyl group on the front carbon and the ethyl group on the back carbon.

Calculate the dihedral angle (θ) for the conformations shown.

(c) <IMAGE>