Substituents alkene stability

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

On the other hand, if the single substituent can stabilize an adjacent carbocation center following protonation of the alkene, then reduction may occur. 

Since inactivated alkenes do not contain the substituent X stabilizing the positive charge in intermediate B (see Scheme 3.40), the latter can undergo a hydride shift to form a more stabilized cationic center (intermediate B in Scheme 3.42), which finally gives rise to an impurity of five-membered cyclic nitronate (24). 

Alkyl groups, on the other hand, inductively donate electrons. This is the same donating effect that causes alkyl substituents to stabilize alkenes (Section 6.7) and carbocations (Section 6.10). 

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. 

Donor substituents favor the 2,6-regioisomer, whereas EWGs favor the 2,4-orientation3 These results suggest that there is considerable polar character to the reaction between the excited aromatic and the alkene. Acceptor substituents at C(3) can stabilize the negative charge at C(3) and C(5), whereas donor substituents can stabilize the positive charge at C(l)3 ... 

One of the most important yet common trends in organic chemistry is the increase in carbocation stability with additional alkyl substituents. This stability relationship is fundamental to understanding many aspects of reactivity, especially of nucleophilic substitution. In recent years, it has become possible to put the stabilization effect on a quantitative basis. One approach incorporates gas phase measurements, which determine the proton affinity of alkenes leading to... 

Stabilized ylides are those which bear a carbanion-stabilizing substituent on the negatively charged carbon of the ylide. Stabilized ylides such as (carboethoxy-methylidene)triphenylphosphorane (entries 6 and 7) react with aldehydes to give exclusively trans- alkenes. Stabilized ylides react sluggishly or not at all with ketones. 

These substituent effects are presumably due to stabilization of the cation that is generated by protonation at the center carbon. Even if the allylic conjugation is not effective in the transition state, the aryl and alkyl substituents can stabilize the charge that develops at the terminal carbon. Allenes are not nearly as widely available as alkenes and acetylenes and the uses that have been made of these materials in synthesis, at least to date, have been restricted. 

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