Rotation about a double bond

In principle cis 2 butene and trans 2 butene may be mterconverted by rotation about the C 2=C 3 double bond However unlike rotation about the C 2—C 3 single bond in butane which is quite fast mterconversion of the stereoisomeric 2 butenes does not occur under normal circumstances It is sometimes said that rotation about a carbon-carbon double bond is restricted but this is an understatement Conventional lab oratory sources of heat do not provide enough energy for rotation about the double bond m alkenes As shown m Figure 5 2 rotation about a double bond requires the p orbitals of C 2 and C 3 to be twisted from their stable parallel alignment—m effect the tt com ponent of the double bond must be broken at the transition state... 

S A discussion of rotation about a double bond on the basis of the quantum mechanics has been published by E. Hiickel, Z. Physik, 60, 423 (1930), which is, I feel, neither so straightforward nor so convincing as the above treatment, inasmuch as neither the phenomenon of concentration of the bond eigenfunctions nor that of change in quantization is taken into account. 

On account of this overlap there is considerable resistance to rotation about a double bond and it produces a rigid molecule, hi other words the disposition of groups attached to the carbon atom can be shown in different ways in space, giving rise to isomers. Therefore geometrical isomerism is a consequence of restricted rotation about double bonds. 

We know that the rotation about a double bond is impossible without disturbing the n bond, which requires a large amount of energy (about 60 K cals/mole). This restricted rotation gives rise to cis-trans isomerism in olefines and their derivatives. 

To get from one conformation to another, we can rotate about as many single bonds as we like. The one thing we can t do though is to break any bonds. This is why we can t rotate about a double bond— to do so we would need to break the k bond. Below arc some pairs of structures that can be intercon-verted by rotating about single bonds they are all different conformations of the same molecule. 

The last section has dealt with several cases of cis-trans isomerizations involving cyclic hydrocarbons the present one is concerned with those which involve rotation about a double bond, viz. 

In Secs. 4.20 and 5.6, we learned that stereoisomers can be classified not only as to whether or not they are mirror images, but also—and quite independently of the other classification—as to how they are interconverted. Altogether, we have (a) configurational isomers, interconverted by inversion (turning-inside-out) at a chiral center (b) geometric isomers, interconverted—in principle—by rotation about a double bond and (c) conformational isomers, interconverted by rotations about single bonds. 

W The process "rotation of a trans double bond should not be confused with "rotation about a double bond . The former, in fact, involves a co-operative rotation about the two carbon-carbon single bonds adjoining the double bond, which preserves the trans stereo-chemistry throughout the process. 

Because the two p orbitals that form the tt bond must be parallel to achieve maximum overlap, rotation about a double bond does not readily occur. If rotation were to occur, the two p orbitals would no longer overlap and the tt bond would break (Figure 3.1). The barrier to rotation about a double bond is 63 kcal/mol. Compare this to the barrier to rotation (2.9 kcal/mol) about a carbon-carbon single bond (Section 2.10). 

Because of the energy barrier to rotation about a double bond, cis and trans isomers caimot interconvert (except under conditions extreme enough to overcome the barrier and break the tt bond). This means that they can be separated from each other. In other words, the two isomers are different compounds with different physical properties, such as different boiling points and different dipole moments. Notice that trans-2- miem... 

The two isomers are not interconvertible at ordinary temperatures because there is no rotation about a double bond. 

Variable temperature n.m.r. studies on tetrabenzopentafulvalene show that the energy barrier to rotation about the interannular bond is very low for rotation about a double bond [242]. Rotation may be encouraged by steric strain in the ground state but also by a stabilised transition state, which is probably a biradical in which there is effective delocalisation of the unpaired electrons, and in which ground state strain is relieved [242],... 

In contrast to a single (free rotation is generally assumed, rotation about a double bond is not a low energy process. The presence of a double bond may therefore lead to stereoisomerism as is observed for N2F2. Each N atom carries a lone pair as well as forming one N—F single bond and an N=N double bond. Structures 2.43 and 2.44 show the trans- and cw-isomers respectively 0fN2F2. 

The orbital model explains the facts about double bonds listed in Table 3.1. Rotation about a double bond is restricted because, for rotation to occur, we would have to break the pi bond, as seen in Figure 3.6. For ethylene, it takes about 62 kcal/mol (259 kj/mol) to break the pi bond, much more thermal energy than is available at room temperature. With the pi bond intact, the sp orbitals on each carbon lie in a single plane. The 120° angle between those orbitals minimizes repulsion between the electrons in them. Finally, the carbon-carbon double bond is shorter than the carbon-carbon single bond because the two shared electron pairs draw the nuclei closer together than a single pair does. 

The presence of every double bond introduces the possibility of cis-trans isomerism. Free rotation about a double bond is not possible, so there are two stereoisomers... 

Figure 21.10 Restriction of rotation about a double bond. In general, the groups of atoms on either side of a single bond have the ability to rotate freely about the bond, but the presence of a double bond essentially locks the atoms in place, making geometric isomerism possible. 

To be considered isomers, molecules must maintain their identity when separated from each other. Rotation about a double bond has an activation energy (rotation barrier) of more dian 30 kcal/mole, and is therefore more restricted than rotation about a single bond. The reason for this difference is that rotation involving a w bond would destroy the lateral overlap of the p orbitals, as illustrated in Fig. 23.4. 

In our discussion of geometrical isomerism (page 485) it was noted that rotation about a double bond could not occur readily because it would necessitate destroying the tt bond formed by the overlap of p orbitals. However, on excitation, one electron undergoes an- w transition. The TT electron cancels the bonding effect of the ir electron. The IT bond is now broken and rotation can occur to give some of the other... 

Rotation about a double bond does not readily occur, because it can happen only if the tt bond breaks— that is, only if the p orbitals are no longer parallel (Figure 4.1). Consequently, the energy barrier to rotation about a carbon-carbon double bond is much greater (about 62 kcal/mol or 259 kJ/mol) than the energy barrier to rotation about a carbon-carbon single bond, which is only about 2.9 kcal/mol or 12 kJ/mol (Section 3.10). 

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