Posted by: Mark Foreman | February 14, 2010

Conformations II

Cyclohexyl bromide drawn as a flat object.

OK we have seen something about conformations, now we need to consider how conformations have an effect on chemical reactions. A classic one will be the E2 elimination, I hope that you can all recall that the torsion angle for H-C-C-Br must be 180 degrees (or sometimes 0 degrees for the following alkyl halide if it is to do a E2 reaction.

Alkyl bromide for a E2 reaction

If the R groups are H then the most stable conformation will be the correct one for a E2, but if the R groups were the missing parts of a cyclohexane ring then the most stable conformation will be the wrong one for the E2.

While I have drawn the cyclohexyl bromide as a flat object it is better for you to think of it as a 3D object. What you will need to do is to convert the ring to either a boat or a chair and tip it through 90 degrees in your mind. Here are the first four different conformations which I imagined.

The boats will be high in energy becuase of the fact that two hydrogens (or worse still a bromine and a hydrogen) are forced close to each other. So we should discount the boats very quickly. We should concentrate on the chairs.

The top left chair has the right geometry for the E2 but it has the higher energy, the majority of the cyclohexyl bromide will be in the conformation shown on the top right (chair with equitorial bromine).

Conformations of bromocyclohexane

As a result cyclohexyl bromide is quite slow to react with a base in the E2 reaction. We can continue the ideas of how steric effects can either set the molecule up in the right conformation for a reaction or put it in the wrong conformation to help us a lot with organic chemistry.

Let us consider the effect of the tert-butyl group in the 4-tert-butylcyclohex-1-yl bromides. We start at random with the cis isomer. Start by drawing out the molecule as a flat object.

Next convert it to a chair, if you go through the different conformations you will find that the one shown below is the most stable.

Next imagine where the hydrogen is for the E2 reaction. You should find it where I have drawn one (or on the other carbon next to the CHCBr). You will see how the torsion angle is 180 degrees (ideal for a E2 !).

If you repeat the process with the trans isomer of 4-tert-butylcyclohex-1-yl bromide you should end up with a chair which looks like this. Here the torsion angles between the hydrogens and the bromides are all wrong for the E2 reaction.

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