3.4 Conformational analysis of butadiene

3.4 Conformational analysis of butadiene

Butadiene 27 is a molecule consisted in four successively bonded sp2carbon atoms. When the structure is presented, in a usual manner, as 27, it seems as if there are two independent double bonds.

Now we draw the structure of butadiene in which the overlap of p orbitals are emphasized. Since the maximum overlap, hence, the maximum stabilization, is obtained when adjacent p orbitals are parallel, there should be two stereoisomers of butadiene, 28 and 29. If p orbitals are omitted, 28 and 29 will be represented simply as 30 and 31, respectively. The two double bonds are placed on trans position for the central singles bond in 30, and cis in 31. Hence these are named s-trans and s-cis isomer, respectively. Prefixes s means the single bond. The s-cis isomer is rare except for a fixed ring structure like cyclohexadiene 32, since s-trans isomer has much less steric hindrance.