Conformational isomers energy diagram

The effect of conformation on reactivity is intimately associated with the details of the mechanism of a reaction. The examples in Scheme 3.2 can illustrate some of the ways in which substituent orientation can affect reactivity. It has been shown that oxidation of ds-4-t-butylcyclohexanol is faster than oxidation of the trans-isomer, but the rates of acetylation are in the opposite direction. Let us consider acetylation first. The rate of the reaction will depend on the free energy of activation for the rate-determining step. For acetylation, this step presumably involves nucleophilic attack by the hydroxyl group on the acetic anhydride. An approximate energy diagram is given in Fig. 3.8. 

Problem 4.L Sketch a potential energy diagram, such as that in Figures 4.7 and 4.9, for propane (CH3CH2CH3). You may use the numbers for H-H and H-CFI3 interactions given in the text. Clearly indicate bond angles and, with sawhorse and Newman projections, show which rotational (or conformational) isomer is most stable and which is least stable. 

The stereochemistry of the y-isomer is shown in the diagram, and when converted into a conformational stereodrawing it can be seen that there are three axial chlorines and three equatorial ones. Ring flip produces an alternative conformation of equal energy, but it can be seen that this is identical to the first stracture rotation through 180° produces an identical and, therefore, superimposable structure. It can be seen that conformational change will not stop the compound interacting with the insect receptor site. 

Isomerization of the retinal Schiff s base can occur when the molecule is excited with light, because the C-l 1-C-12 bond loses much of its double-bond character in the excited state. The valence bond diagrams of figure S2.7 illustrate this point. In the ground state of rhodopsin, the potential energy barrier to rotation about the C-l 1-C-l2 bond is on the order of 30 kcal/mol. This barrier essentially vanishes in the excited state. In fact, the energy of the excited molecule probably is minimal when the C-11 -C-l2 bond is twisted by about 90° (fig. S2.8). The excited molecule oscillates briefly about this intermediate conformation, and when it decays back to a ground state it usually settles into the ail-trans isomer, bathorhodopsin. 

On the diagrams provided, fill in the appropriate atoms on the proper carbon and in the proper axial or equatorial position. These are two chair conformations for the same molecule, not isomers. Note the numbering scheme and assign the proper atoms to the properly numbered carbons in each chair. Which is lower in energy A or B ... 

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