Butadiene Electron Delocalization

The carbon-carbon bond lengths of 1,3-butadiene have been determined and are shown here  

Double bonds are shorter than single bonds 

The Cl —C2 bond and the C3—C4 bond are (within experimental error) the same length as the carbon-carbon double bond of ethene. The central bond of 1,3-butadiene (1.47 A), however, is considerably shorter than the single bond of ethane (1.54 A). 

This should not be surprising. All of the carbon atoms of 1,3-butadiene are sp hybridized and, as a result, the central bond of butadiene results from overlapping sp orbitals. And, as we know, a sigma bond that is sp -sp is longer. There is, in fact, a steady decrease in bond length of carbon-carbon single bonds as the hybridization state of the bonded atoms changes from sp to sp (Table 13.1). 

Carbon-Carbon Single-Bond Lengths and Hybridization State 

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Butadiene Electron Delocalization 13.7A Bond Length of 1,3-Butadiene... 

Additional evidence for electron delocalization m 1 3 butadiene can be obtained by considering its conformations Overlap of the two rr electron systems is optimal when the four carbon atoms are coplanar Two conformations allow this coplananty they are called the s cis and s trans conformations... 

Dienes would be expected to adopt conformations in which the double bonds are coplanar, so as to permit effective orbital overlap and electron delocalization. The two alternative planar eonformations for 1,3-butadiene are referred to as s-trans and s-cis. In addition to the two planar conformations, there is a third conformation, referred to as the skew conformation, which is cisoid but not planar. Various types of studies have shown that the s-trans conformation is the most stable one for 1,3-butadiene. A small amount of one of the skew conformations is also present in equilibrium with the major conformer. The planar s-cis conformation incorporates a van der Waals repulsion between the hydrogens on C—1 and C—4. This is relieved in the skew conformation. 

FIGURE 10.6 Conformations and electron delocalization in 1,3-butadiene. The s-cis and the s-trans conformations permit the 2p orbitals to be aligned parallel to one another for maximum TT electron delocalization. The s-trans conformation is more stable than the s-cis. Stabilization resulting from tt electron de-localization is least in the perpendicular conformation, which is a transition state for rotation about the C-2—C-3 single bond. The green and yellow colors are meant to differentiate the orbitals and do not indicate their phases. 

The next cyclic alkadiene, 1,3-cyclopentadiene, has been experimentally studied by MW, GED and XR methods. The carbon skeleton is planar (C2v symmetry), and the small C=C—C angles compared to those in 1,3-butadiene (124.3°) or d.s-1-butene (126.40)58 do not seem to influence noticeably the lengths of the CC bonds, although other effects, such as 7r-electron delocalization, might have an opposite effect. The apparently normal structure parameters observed for 1,3-cyclopentadiene might therefore be a result of different forces having opposite effects on the structure parameters. 

Roth and coworkers23 reported NMR data of the orthogonal butadiene (Z,Z)-3,4-dimethylhexa-2,4-diene. (Z,Z)-13 having the planes of the double bonds at a dihedral angle not far from 90°. This diene serves as the model for conjugated diene lacking rr-electron delocalization and for the transition state for interconversion of antiperiplanar (trans) and synperiplanar (cis or gauche) butadiene. 

Homoconjugative electron delocalization leads to stabilization or destabilization of the molecule, which can be determined provided correct reference compounds with appropriate reference conformations are chosen. In the case of cyclopropyl homoconjugation, these should be vinylcyclopropane, 1,3-butadiene and their methyl derivatives. The discussion in Section III.G clearly shows that by the use of either correctly chosen homodesmotic... 

The basic catalyst in the isomerization of 1,2-butadienes to butynes acts by removing an alkenic proton from the hydrocarbon. Two different anions can be formed, each of which is stabilized by electron delocalization involving the adjacent multiple bond. Either anion can react with the solvent by proton transfer to form the starting material or an alkyne. At equilibrium the most... 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

Comparison of the calculated C—C bond distances (MP2) of 105 [C(1)=C(2) 1.357, C(2)—C(3) 1.439, C(3)=C(4) 1.371, C(l)—C(7) 1.497 A] with tran -butadiene [C(1)=C(2) 1.341, C(2)—C(3) 1.461 A] shows that a considerable degree of bond equal-ization is present in 105. Cremer and Kraka suggest that this is the consequence of homo-conjugative electron delocalization. This conclusion was confirmed by these authors from the values of the calculated bond orders and 7r-character indices. These results are contrary to predictions obtained at lower, less reliable levels of theory " ". ... 

In describing the 1,3-butadiene molecular orbitals, we say that the ir electrons are spread out, or delocalized, over the entire 77 framework rather than localized between two specific nuclei. Electron delocalization always leads to lower energy and greater stability of the molecule. 

The -cis and -trans conformations of 1,3-butadiene interconvert by rotation around the C-2—C-3 bond, as illustrated in Figure 10.5. The conformation at the midpoint of this rotation, the perpendicular conformation, has its 2p orbitals in a geometry that prevents extended conjugation. It has localized double bonds. The main contributor to the energy of activation for rotation about the single bond in 1,3-butadiene is the decrease in electron delocalization that attends conversion of the 5-cis or i-trans conformation to the perpendicular conformation. 

Only the Cs structure is a minimum on the 101 potential surface. The planar C2v structure is the transition state for pyramidal inversion between equivalent Cs structures, the inversion barrier being 16.2 kcalmol- 1 259 Note the large differences in the geometries of the C2v and Cs structures, in particular the much longer C-Si bond and the butadiene-like geometry of the carbon skeleton in the Cs structure. The fact that 101 is not planar indicates that electron delocalization is weak and relatively unimportant. A quantitative measure of the extent of electron delocalization in the Cs 101 is given by isodesmic equation 32. 

The TT electrons in 1,3-butadiene are delocalized over four sp carbons. In other words, there are four carbons in the rr system. A molecular orbital description of... 

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