Stability of substituted alkenes

Hyperconjugation, discussed in Section 7.5 in connection with the stability of substituted alkenes, is the stabilizing interaction between a p orbital and C-H cr bonds on neighboring carbons that are roughly parallel to the p orbital (Figure 7.9). The more alkyl groups there are on the carhocation, the more stable the carhocation. 

The simplest reaction of the double bond is its saturation with hydrogen. As discussed in Section 11-5, this reaction allows us to estimate the relative stability of substituted alkenes from their heats of hydrogenation. The process requires a catalyst, which may be either heterogeneous or homogeneous—that is, either insoluble or soluble in the reaction medium. 

In the original study by Peterson, the alkenation procedure was found to be compatible with sulfur and phosphorus substitution. The alkenation reaction has been tqrplied successfully to a variety of substituted alkenes. Because of the aiuon-stabilizing nature of the thiophenyl, the p-hydroxysiliuie is not isolated and the elimination to the alkene takes place directly to form a 1 1 mixture of ( )- and (Z)-isomers. Ager studied the reaction of the lithio anions of phenyl (trimethylsilyl)methyl sulfides (318) with a variety of carbonyl compounds (equation 72). Yields of this process were good, and addition occurred even with enolizable substrates. This reaction was extended to vinyl sulfones. In contrast to the sulfide case, the substituted sulfone silyl anion behaves as a base, leading to undesired enolization. The best yields were observed for the case where R is a hydrogen or phenyl. 

In the case of the El mechanism, where there is doubt over the regiochemistry of the resultant double bond, the orientation of the double bond in the final product may be deduced from the relative stabilities of the final unsaturated product. From heat of combustion data, it is found that the more highly substituted the double bond, then the greater is the stability of that alkene. Thus, in the case of 2-bromobutane, suggest what will be the resultant alkene after the elimination of HBr. 

Nucleophilic substitution and elimination are competitive processes. Which prevails depends on a variety of factors. One important consideration is the stability of the alkene that would result from elimination. Since tertiary halides form the more stable highly substitued alkenes, they are more likely to react by elimination than primary halides. 

The greater the substitution of the carbon-carbon double bond is, the lesser the heat of hydrogenation, and higher the stability of the alkene. 

The relative stability of isomeric alkenes with varying degrees of substitution. 

Related arguments involving Csps- Csp2 bonds were used in Section 8.2C to explain why the stability of an alkene Increases with Increasing alkyl substitution. 

The stability order of substituted alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tt bond and adjacent C-H cr bonds on substituents. In valence-bond language, the interaction is called hyperconjugation. In a molecular orbital description, there is a bonding MO that extends over the four-atom C=C-C-H grouping, as shown in Figure 7.5. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

The results of the preceding hydrogenation reactions may be generalized The relative stability of the alkenes increases with increasing substitution, and trans isomers are usually more stable than their cis counterparts. The first trend is due in part to hyperconjugation. Just as the stability of a radical increases with increasing alkyl substitution (Section 3-2), the p orbitals of a tt bond can be stabilized by alkyl substituents. 

The relative stability of isomeric alkenes can be established by comparing heats of hydrogenation. It decreases with decreasing substitution trans isomers are more stable than cis. 

An effect that results when two or more atoms or groups rnteract so as to alter the electron drstnbutron rn a system rs called an electronic effect The greater stability of more highly substituted alkenes is an example of an electronic effect... 

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