Alkene stability

Analogous data for a host of alkenes tell us that the most important factors gov emmg alkene stability are... 

Section 5 6 Electron release from alkyl substituents stabilizes a double bond In gen eral the order of alkene stability is... 

The pattern of alkene stability determined from heats of hydrogenation parallels exactly the pattern deduced from heats of combustion... 

Part B of Table 1.5 gives heats of formation for the C4, C5, and some of the Cg alkenes. A general relationship is also observed for the alkenes. The more highly substituted the double bond, the more stable is the compound. There are also other factors that enter into alkene stability. trans-Alkenes are usually more stable than cis-alkenes, probably largely because of increased nonbonded repulsion in the cis isomer. ... 

A second factor that contributes to alkene stability involves bond strengths. A bond between an sp2 carbon and an sp3 carbon is somewhat stronger than a bond between two sp3 carbons. Thus, in comparing 1-butene and 2-butene, the monosubstituted isomer has one sp -.sp3 bond and one sp3-sp2 bond, while... 

A particularly interesting piece of evidence concerning the nature of this sequence has been presented by Barluenga et al.21 When the vinylcarbene complex 14 was heated, decarbonylation afforded the alkene-stabilized complex 15. Upon treatment with dimethyl acetylenedicarboxylate, the alkyne-insertion product 16 was isolated. This complex proved to be unstable in solution at room temperature and decomposed readily to 17, the expected product of a Dotz reaction with an aminovinylcarbene. 

Electron donor on radical electron acceptor on alkene— stabilizing interaction between SOMO and alkene LUMO. 

Figure 3 also highlights some curious data which complicates our understanding of alkene stability. That is, although the enthalpies of formation of the fraws-2-n-alkenes are... 

Apply the two general rules for alkene stability to rank these compounds. First, more highly substituted double bonds are more stable than less substituted ones. Second, when two double bonds are similarly constituted, the trans stereoisomer is more stable than the cis. The predicted order of decreasing stability is therefore ... 

The stereochemistry of the alkene product in Wittig reactions is thought to be influenced by the reversibility of formation of the isomeric threo and erythro oxaphosphetanes (or betaines) which undergo stereospecific loss of triphenyl-phosphine oxide to give the trans (E) and cis (Z) alkenes, respectively (Scheme 4). Factors that enhance the reversibility of this initial step favour the threo intermediate and hence the (E) alkene. Stabilized phosphoranes give a predominance of the (E) alkene while non-stabilized phosphoranes give the (Z) alkene. In general, stabilized phosphoranes react readily with aldehydes (see Protocol 4) while non-stabilized phosphoranes will react with aldehydes, hemiacetals (see Protocol 5) and ketones.2,3... 

Problem 8.6 Several factors can affect alkene stability. Explain why alkene A is more stable than alkene B even though both contain disubstituted carbon-carbon double bonds. 

In Section 12.3 we learned that hydrogen adds to alkenes to form alkanes, and that the heat released in this reaction, the heat of hydrogenation, can be used as a measure of alkene stability. 

Alkene stability not only determines orientation of dehydrohalogenation, but also is an important factor in determining the reactivity of an alkyl halide toward elimination, as shown at the top of the next page. 

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