Question

What is the 'true boat' conformation?

Answer 1

See this site.

Explanation:

A cyclohexane (`C_6H_12`) ring can have several possible conformations. The MOST stable is the so-called chair conformation, for which each carbon atom in the ring has an axial and equatorial hydrogen. Axially disposed hydrogens on the same face of the ring (which means that are substituted `1,3` with respect to the carbons) undergo so-called transannular steric interaction.

An alternative conformation is the BOAT conformation (I am afraid my poor google-fu prevents me posting a nice picture, but look at the given website!). Here the cyclohexane ring assumes the shape of a boat. In this conformation, the two, alternate `1,4`-substituted hydrogens, occupy the same physical space, which is of course a very unfavourable steric outcome, so the boat conformer is a very high energy species, with a very short lifetime. Twist boat conformations, which separate these hydrogens are a little lower in energy!

How to remember all this? Get a set of molecular models, and look at conformers of the cyclohexane ring, and relate them to the website. Not only do you have to be able to use the model, you have to be able to represent the conformation on the printed page - a non-trivial exercise that takes a bit of practice.

For substituted cyclohexanes, e.g. methylcyclohexane, and more so `Bu^tC_6H_11`, there is a great steric preference for the methyl to be in an equatorial conformation.

If I have been unclear, please voice your objections, and I (or someone else) will give it another go.