Question

Give an explanation for the trend in the dihedral angle of the trans-cycloalkene systems as a function of ring size. WHat is the smallest trans-cycloalkene that exists?

Answer 1

The C-C=C angle in alkenes normally is about 122^o, which is 10^o larger than the normal C-C-C angle in cycloalkanes. This means that we would expect about 20^o more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. The reason for this is not clear, but may be connected in part with the C-H bond strengths. Cyclohexene is too small of a ring to exhibit the trans isomer. It becomes really apparent if you try to make a model of this. Trying to force it to be trans would place a massive amount of ring strain on the molecule. This is true for all rings with an alkene up to cyclooctene.

Cyclooctene is the smallest ring that can show both cis and trans isomers because it is large enough to twist into the right conformation. An image of this is shown below.

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