Propene hyperconjugation

Figure 3.49 Orbital contours for hyperconjugative interactions in propene (a)

The interaction between a and -it electrons of single and multiple bonds (or the -tt electrons in the aromatic ring) is usually interpreted (Baker, 1952) as a, -conjugation (hyperconjugation). In this process the hydrocarbons become stronger protolytes. For example, the acidic and basic behaviour of propene and toluene are more pronounced than the corresponding properties of ethylene and benzene. 

Referring to the discussions presented in Chapter 5 regarding the relative stabilities of carbocations (and hyperconjugation), we are reminded that tertiary carbocations are more stable than secondary carbocations, which, in turn, are more stable than primary carbocations. Since, as shown in Scheme 7.9, protonation of propene results in cationic character at both a secondary carbon and a primary carbon, a greater presence of cationic character on the secondary site is expected compared to the primary. This allows... 

The difference in AhyiH between 1-butene and methyl propene is often called a hyperconjugation energy , in this case, 2.0 kcal mol 1 = 8.2 kJ mol 1. In general, a methyl group alpha to a double bond is said to stabilize the reactant (See, however, e. g., Rogers, D. W. Tetrahedron Letters 1987,28,1967.)... 

Hyperconjugation is a stabilizing interaction between a C-H bond and an adjacent a-bond. For example, propene may be regarded as a resonance hybrid of two canonical forms ... 

All the examples of resonance cited in the previous section dealt with conjugation through TT bonds. VB theory also incorporates the concept of hyperconjugation, which is the idea that there can be electronic interactions between ct and a bonds and between ct and tt bonds. In alkenes such as propene or 2-methylpropene, the electronreleasing effect of the methyl substituents can be represented by hyperconjugated... 

The conformation of propene is influenced by hyperconjugation. The methyl substiment has an overall stabilizing effect (2.7kcal) on the double bond, as can be concluded from the less negative heat of hydrogenation compared to ethene (see Section 3.1.1). This stabilization arises from a-ir interactions. The major effect is a transfer of electron density from the methyl a C-H bonds to the empty tt orbital. 

A similar situation arises in the case of hyperconjugation, in comparing, say, propene with ethylene. In this case we may write... 

According to NBO analysis, hyperconjugative interactions are the main reason for the greater stability of the eclipsed structure of propene (Figure 6.29). The most important hyperconjugative interactions observed between the methyl and vinyl groups are divided into three components the interaction, the cH interaction, and the vicinal interaction between the in-plane orbitals of the methyl group and the o -orbital of the antiperiplanar vinyl C-H bond. 

But, if we look at the hyperconjugative structure carefully in the case of propene, we note that the hydrogen becomes positive and the bond from that hydrogen to carbon weakens as that resonance occurs. But notice also that the other carbon at the end of the chain becomes negative. Suppose that instead of carbon, we substitute another atom there, one that will better accommodate negative charge Should not the effect then be magnified ... 

Evidently, the C-H bond should hyperconjugate more strongly in acetaldehyde than it does in propene, and the C-H bond length difference as we go from 0° to 90° in torsion angle should increase more. And that s what we find. The calculations show that in this case the C-H bond length increase in acetaldehyde is 0.0047 A, vs. 0.0030 A in propene. This particular type of bond stretching has also been referred to in the literature as the carbonyl effect ... 

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