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CHAPTER 4 129 
 
same for conformers A and B, then conformer B should 
be the lowest-energy staggered conformation for this 
isomer of 2,3-butanediol. 
 
4.76. 
(a) The all-equatorial chair conformation of compound 2 
experiences severe steric repulsion due to the nearby 
isopropyl groups as evident in the following drawing, in 
which we focus on the interactions between a pair of 
neighboring isopropyl groups. These interactions, 
highlighted below, occur for each pair of neighboring 
isopropyl groups. 
 
 
A Newman projection of 2, viewed down one of the C-C 
bonds connecting an equatorial isopropyl group to the 
cyclohexane ring, shows how crowded the equatorial 
groups are as the methyl groups are in constant contact 
with each other. These interactions, highlighted below, 
occur for each pair of neighboring isopropyl groups. 
 
 
 
(b) In the all axial chair conformation for 2, the C-H 
bond of each isopropyl group can all point towards each 
other (highlighted below) such that steric repulsion can 
be minimized. For clarity, this is shown only for the top 
face of the cyclohexane ring in the following structure. 
 
 
 
Furthermore, this results in a staggered conformation 
along each C-C bond between a ring carbon atom and an 
isopropyl carbon atom, which also helps to lower the 
energy of this conformation. For clarity, this is shown 
only on one face of the cyclohexane ring in the following 
structure. 
 
 
4.77. 
Each compound has three rings, labeled A, B, and C. 
 
 
 
In each compound, the A-B fusion represents a trans-
decalin system: 
 
 
 
The trans fusion imposes a severe restriction on the 
conformational flexibility of the system. Specifically, in 
a trans-decalin system, neither ring can undergo a ring-
flip to give a different chair conformation. For each ring, 
the chair conformation shown above is the only chair 
conformation that is achievable. However, both rings are 
still free to adopt a higher energy boat conformation. 
For example, the B ring of trans-decalin can adopt the 
following boat conformation: 
 
 
 
In compound 1, the C ring imposes a further 
conformational restriction, by locking the B ring into a 
chair conformation: 
 
 
 
In contrast, the C ring in compound 2 imposes the 
restriction of locking the B ring in a boat conformation: 
 
 
 
The boat conformation of compound 2 is expected to be 
higher in energy than the chair conformation of 
compound 1. Therefore, compound 2 is expected to 
have the higher heat of combustion. In fact, the 
investigators prepared compounds 1 and 2 for the 
purpose of measuring the difference in energy between 
chair and boat conformations. 
 
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