Dienes electron delocalization

Section 10 7 Conjugated dienes are stabilized by electron delocalization to the extent of 12-16 kJ/mol (3 kcal/mol) Overlap of the p orbitals of four adja cent sp hybridized carbons in a conjugated diene gives an extended tt system through which the electrons are delocalized... 

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. 

The same mixture of H and I was obtained starting with either of the geometrically isomeric radical precursors E or F. A possible explanation is based on the assumption of a common radical conformer G, stabilized in the geometry shown by electron delocalization involving the radicaloid p-orbital, the p-peroxy oxygen and Jt of the diene unit. The structure of the compounds H and I were determined by H NMR spectra and the conversion of H to diol J, a known intermediate for the synthesis of prostaglandins. 

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. 

Recently, work has been reported on the 7-boranorbornene (89) and 7-boranorboma-diene (90) systems related to 84 and 85. Schleyer and colleagues have examined the parent molecules 87 and 88 theoretically [MP2(FU)/6-3 lG(d)] and concluded that they have very similar structures and electron delocalizations to the related cations (Table 8)203 204. X-ray structures of substituted derivatives of 89 and 90 have been determined and these have very similar distorted conformations to those of 84, 85 and 86205. Calculated (IGLO) NMR chemical shifts of 87 and 88 correspond well with those observed experimentally. 

The allylic cyclohex-2-enyl radical has its unpaired electron delocalized over two secondary carbon atoms, so it is even more stable than the unsubstituted allyl radical. The second propagation step may occur at either of the radical carbons, but in this symmetrical case, either position gives 3-bromocyclohexene as the product. Less symmetrical compounds often give mixtures of products resulting from an allylic shift In the product, the double bond can appear at either of the positions it occupies in the resonance forms of the allylic radical. An allylic shift in a radical reaction is similar to the 1,4-addition of an electrophilic reagent such as HBr to a diene (Section 15-5). 

Studies of C3H6 oxidation [38] under carefully controlled conditions have shown that the above difficulties can be reduced to a minor role essentially because the allyl radical produced in reaction (Ip) is stabilized by electron delocalization and is very unreactive towards O2. The allyl radicals are removed mainly by recombination to give hexa-1,5-diene (HDE) and over the temperature range 650-800 K, the initial products are accounted for by a simple mechanism. 

Dienes have two or more carbon-carbon double bonds which may be either isolated (R -C=C-R-C=C-R ), cumulated (R-C=C=C-R), or conjugated (R-C=C-C=C-R). Each state represents a different stability due to electron delocalization, and the conjugated form is generally favored in organic molecules. 

Isolated dienes typically show no "special" stability. Their two re-bonds interact independent of one another and they compete as intermolecular reaction sites depending on steric, kinetic, and thermodynamic factors. Often, tliese types of dienes can be treated merely as larger, more complex alkenes, because the two bonds cannot be said to have communication (proximal electron delocalization due to location). 

The assessment of aromatic character from structural criteria would appear to be a very valid approach. Aromatic -electron delocalization requires planarity of the aromatic molecule and leads to a typical carbon-carbon bond length intermediate between that of a single bond and a formal olefinic double bond. Indeed qualitative orders of the aromaticity of five-membered heteroaromatics have been derived from a consideration of the relative degree of diene character of the conjugated system as assessed from structural determinations of bond lengths (see, for example, Sections III,C, 1 and 5). 

AS you continue your study of organic chemistry, you will notice that the concept of having delocalized electrons is invoked frequently to explain the behavior of organic compounds. For example, in Chapter 8 you will see that having delocalized electrons causes certain dienes to form products that would not be expected on the basis of what you have learned about electrophilic addition reactions in Chapters 3-6. Electron delocalization is such an important concept that this entire chapter is devoted to it. 

Electron delocalization also causes a conjugated diene to be more stable than an isolated diene. The tt electrons in each of the double bonds of an isolated diene are localized between two carbons. In contrast, the rr electrons in a conjugated diene are delocalized. As you discovered in Section 7.6, electron delocalization stabilizes a molecule. Both the resonance hybrid and the molecular orbital diagram of 1,3-butadiene in Figure 7.9 show that the single bond in 1,3-butadiene is not a pure single bond, but has partial double-bond character as a result of electron delocalization. 

Until now, we have been concerned with the reactions of compounds that have only one functional group. Compounds with two or more functional groups exhibit reactions characteristic of the individual functional groups if the groups are sufficiently separated from each other. If they are close enough to allow electron delocalization, however, one functional group can affect the reactivity of the other. Therefore, we will see that the reactions of isolated dienes are the same as the reactions of alkenes, but the reactions of conjugated dienes are a little different because of electron delocalization. 

As we look at more examples, notice that the first step in all electrophilic additions to conjugated dienes is addition of the electrophile to one of the sp carbons at the end of the conjugated system. This is the only way to obtain a carbocation that is stabilized by resonance (i.e., by electron delocalization). If the electrophile were to add to one of the internal sp carbons, the resulting carbocation would not be stabilized by resonance. 

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