(E)-2-Butene (larger view)

(Z)-2-Butene (larger view)

(How to manipulate JSmol structures)

 

(E)- and (Z)-2-Butene

 

(E)-and (Z)-2-Butene, aka trans- and cis-butene, respectively, are stereoisomers of one another. The presence of a π-bond (~ 63 kcal/mol) prohibits the interconversion of the two isomers. (Z)-2-Butene is 1.0 kcal/mol less stable than (E)-2-butene. This difference is reflected in their respective heats of formation, -1.7 kcal/mol and -2.7 kcal/mol, respectively. Since both isomers afford n-butane upon hydrogenation and 4 moles of CO₂ and 4 moles of water upon combustion, the heats of hydrogenation and heats of combustion also differ by 1.0 kcal/mol. In both instances (Z)-2-butene liberates more heat (difference = 1.0 kcal/mol) than does (E)-2-butene. The increased instability of (Z)-2-butene is caused by the steric interaction of the two methyl groups.

Fig. 1

The van der Waal's radius of a methyl group is 2Å. Twice this distance is the minimal distance for non-interacting methyl groups. Measure this distance [(How to manipulate JSmol structures)] or see Fig. 2. The distance is 3.17Å, less than the sum of the van der Waal's radii. The C-C-C bond angle in the (Z)-isomer is larger than in the (E)-isomer (Fig. 1), 127.2o vs. 124.0o, owing to the greater strain in the (Z)-isomer. This increased bond angle in the (Z)-isomer causes the C(sp3)-C(sp2)-H bond angle to be smaller (115.1o) than in the (E)-isomer (116.6o).

Fig. 2