Posted by: Mark Foreman | February 13, 2010

Conformations

Dear All,

We have seen how steric effects control the energy of the transition state for the SN2 reaction, now we need to consider how steric effects control the arrangement of atoms in a molecule. The first molecule we will start with is ethane (C2H6, or H3CCH3).

This molecule can be thought of as two methyl groups bonded together by a single (sigma) bond. The molecule can be arranged in two different ways, the hydrogens can be staggered as shown below.

Staggered ethane

Or lined up with each other, this arrangement (as shown below) is the higher energy conformation.

Higher energy conformation

This is due to a combination of steric effects (please see my thoughts on electrons and atoms) and an interaction between the filled bonding orbital and an empty antibonding orbital.

If you consider biphenyl (H5C6-C6H5) for a moment you will see that if the C-C single bond is turned through 90 degrees then the energy changes greatly.

Biphenyl

If the molecule is altered to increase the steric bulk of the aryl group (the group made up of sp2 carbons) and if a couple of groups are added then a diphosphine can be made which is both chiral and unable to change from one stereoisomer to the other at normal tempertures.

2,2'-bis-(dimethylphosphino)-1,1'-binapthyl

Chiral diphosphines are very useful as they allow asymmetric cataylsts to be made which can do useful things, do not worry too much about them yet. In the Masters level organic chemistry classes at Chalmers some of their chemistry will be discussed.

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