2003: Organic Problems Set 2

Question 1a

The modelling was done with the Chem3D program, using the MM energy function and the "dihedral driver" option. The MM2 mechanics field models the steric interactions and a simple Huckel calculation models the resonance effects. Note the "cusps" at around 0 and 180 degrees. Probably what needs to happen is the N-C=O amide bond must also rotate to some extent to avoid the worst of the steric repulsions. The "dihedral driver" geometry definition used to create this energy profile does not model this other bond rotation very well, and hence the occurance of "cusps". Nevertheless, it does appear from this profile that the steric effects quite overwhelm the pi resonance effects in this particular example. This balancing of two opposing effects is typical of many issues of selectivity in organic chemistry, and only quantitative models can predict the eventual outcome.
90 or -90 0 180

Question 1b

S-cis (-96.5 kcal/mol) S-trans (-91.2 kcal/mol)
S-cis (-46.9 kcal/mol) S-trans (-947.2 kcal/mol)

Question 2

This is an application of the "rabbit ear" way of looking at lone pairs on oxygen atoms. Where the possibility for adjacent stereoelectronic interaction is with a C-O sigma-* orbital, the most appropriate hybridisation of the two lone pairs is the "tetrahedral" or "rabbit ear" mode. This hybridisation ensures optimum overlap with the C-O sigma-* system. This contrasts with the previous example, where the oxygen was interacting with the more complex C=O system, which contains both pi and sigma components. Under these circumstances, the two oxygen lone pairs are best hydridised into a pure p lone pair (suitable for overlap with the C=O pi system) and into one sp2 lone pair (suitable for overlap with the C=O sigma-* system). The tricky decision of course is knowing when to use "rabbit ear" hybridisation and when to use the sigma/pi hybridisation. Ultimately, only the "variation principle" in quantum mechanics will allow a decision as to which leads to the lowest energy.


Tutorial Questions and Models

A set of tutorial questions and answers is available.

Qu 1

Qu 2

Qu 3