Barrier, to bond rotation

Although essentially free rotation is possible around single bonds (Section 3.6), the same is not true of double bonds. For rotation to occur around a double bond, the -rrbond must break and re-form (Figure 6.2). Thus, the barrier to double-bond rotation must be at least as great as the strength of the 7r bond itself, an estimated 350 kj/mol (84 kcal/mol). Recall that the barrier to bond rotation in ethane is only 12 kj/mol. 

Free-Energy Barriers to Bond Rotation (kcal/mol) in nr j... 

In allylic systems, favorable overlap of the p orbitals of the n system should require a coplanar arrangement of the three sp2 carbons and their five substituent atoms evidence that such a structure is indeed preferred comes, for example, from proton magnetic resonance observations that demonstrate barriers to bond rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and barriers to rotation about the partial double bonds are sufficiently high to prevent interconversion at low temperature. At — 10°C, 21, the least stable isomer,... 

Conformational energies were estimated using methods described previously, 3). In the present calculations, however, terms were also introduced as indicated below to account for inherent barriers to bond rotation and for the ancmeric effect, following methods proposed by Pbe and Mark (15). Thus, threefold torsional... 

The factors involved in barriers to bond rotation are together called torsional strain and include the repulsive interactions called steric hindrance between electron clouds of the bonded groups. 

---