# Chemical bond: conjugated double bonds

## Chemical bond: torsion of the C2-C3 bond in butadiene

The different overlap of the orbitals at C2 and C3 suggests that the rotation about the bond axis differs from that of an alkane and an alkene. Therefore, the activation energy and the energy difference of the isomers for butane, 2-butene and butadiene are given in the following table.

Tab. 1
Torsional barriers of various types of bindings
 butane 2-butene Butadiene Mouseanti Mousetrans Mouses-trans Mousegauche Mousecis Mouses-cis $E.a$ = 19,7 $kJmol-1$ $E.a$ = 259,0 $kJmol-1$ $E.a$ = 28,0 $kJmol-1$ $ΔH°$ = 3,8 $kJmol-1$ $ΔH°$ = 3,6 $kJmol-1$ $ΔH°$ = 11,7 $kJmol-1$

The rotation barrier in butadiene is around 8 $kJmol-1$ bigger than those in butane. When rotating around the C2-C3 bond, only the partial π bond between these two centers is disturbed. When converting fromcis-Butene in trans-Butene, on the other hand, breaks a complete π bond. The barrier to rotation is much higher here.

The difference in energy between the two conformers is, however, exceptionally high in butadiene. This is due to steric hindrance. In thes-cis-Conformation two terminal hydrogens come very close to each other.