1- Butene, hyperconjugation

Thermolysis of the syn and anti /J-acetoxy stannanes, obtained by addition of Bu3SnLi to tram- or cis- 1,2-dimethyloxirane and subsequent acetylation, led to (Z)- or (E)-2-butene, respectively, by a stereospecific anti process (equation 34).73. It is postulated that a hyperconjugative interaction of the C—Sn bond facilitates departure of the acetate group in these acyclic systems. The trimethylstannyl and triphenylstannyl analogues likewise undergo anti elimination. 

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

The rate of H addition to 1-pentene is roughly equal to the addition to 1-butene, of the H addition to cis and trans isomers of 2-pentene as to cis-and frans-butenes. Cycloalkenes add a H atom in a similar way to simple alkenes of comparable structure. H attacks either the terminal or the internal C atom of 1,3-butadiene the first way predominates, probably due to allylic or hyperconjugative stabilization of the generated radical. 

In addition to the effect of hyperconjugation, bond strengths are also important in determining alkene stability. A bond between an sp carbon and an carbon is somewhat stronger than a bond between two spi carbons. Thus, in comparing 1-butene and 2-butene, we find that the mono-... 

The SE° and GSE° values of a series of alkyl-substituted alkenes are collected in Table XXVI. It is seen that the (stabilizing) hyperconjugation energy is approximately equal to 3.2 kcal mol 1, whatever the alkyl group attached to the double bond may be. Moreover, the global stabilization of cis-2-butene is smaller than that of trans-2-butene due to the steric repulsion between the methyl groups in cis position. 

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.)... 

The effect of hyperconjugative interactions can be evaluated from the difference between the heats of hydrogenation of ethylene and 1-butene that provides an estimate for stabilization of ethylene (in kcal/mol) by an ethyl substituent (2.4G3 2.2G3- (MP2) 2.7 experimental). Likewise, the hyperconjugative stabilization of acetylene by an ethyl group (4.9G3 4.8G3(MP2) 4.7 expt) is the difference between the heat of hydrogenation of acetylene and 1-butyne. Equivalently, the hyperconjugative stabilization can also be described by isodesmic reactions in Figure 6.130 that produces data consistent with the above evaluation ... 

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