Alkene hyperconjugation

Alkenes Nevins 1996a Alkenes (hyperconjugated) Allinger 1996c Alkyl chlorides (mono-, di-, polyhalogen)... 

For larger alkenes, hyperconjugation with an alkyl group a to an olefinic carbon atom eliminates the need for rotation, so the radical cations of almost all alkenes other than ethene are planar. One-electron oxidation of alkanes leads to a-radical cations. Such ionization removes an electron from an orbital associated with o bonding among carbon atoms (Scheme 2.44). 

BLW estimates by Hiberty and coworkers suggest that for both C Hg and C Hio alkenes, hyperconjugation effects stabilize the most snbstituted prodnct by about bkcal/mol. NBO analysis of such interactions will be discussed below along with the discnssion of the role of hyperconjugative effects in conformational behavior of alkenes. 

Hydrophilic (Section 19 5) Literally water loving a term applied to substances that are soluble in water usually be cause of their ability to form hydrogen bonds with water Hydrophobic (Section 19 5) Literally water hating a term applied to substances that are not soluble in water but are soluble in nonpolar hydrocarbon like media Hydroxylation (Section 15 5) Reaction or sequence of reac tions in which an alkene is converted to a vicinal diol Hyperconjugation (Section 4 10) Delocalization of a electrons... 

The stability order of alkenes is due to a combination of two factors. One is a stabilizing interaction between the C=C tr bond and adjacent C-H a 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—< -H grouping, as shown in Figure 6.6. The more substituents that are present on the double bond, the more hyperconjugation there is and the more stable the alkene. 

Figure 6.6 Hyperconjugation is a stabilizing interaction between an unfilled w orbital and a neighboring filled C-H <7 bond on a substituent. The more substituents there are, the greater the stabilization of the alkene.

The following carbocation is an intermediate in the electrophilic addition reaction of HCl with two different alkenes. Identify both, and tell which C-H bonds in the carbocation are aligned for hyperconjugation with the vacant p orbital on the positively charged carbon. 

Hyperconjugation (Sections 6.6, 6.9) An interaction that results from overlap of a vacant p orbital on one atom with a neighboring C-H a bond. Hyperconjugation is important in stabilizing carbocations and in stabilizing substituted alkenes. 

A more complete discussion of the mechanism of addition of hydrogen halides to alkenes is given in Chapter 6 of Part A. In particular, the question of whether or not discrete carbocations are involved is considered there. Even when a carbocation is not involved, the regioselectivity of electrophilic addition is the result of attack of the electrophile at the more electron-rich carbon of the double bond. Alkyl substituents increase the electron density of the terminal carbon by hyperconjugation (see Part A, Section 1.1.8). 

Hyperconjugation has also been invoked to account for the greater thermodynamic stability of alkenes in which the double bond is not terminal, e.g. (30), compared with isomeric compounds in which it is, e.g. (31) in (30) there are nine hyperconjugable a-hydrogen atoms, compared with only five in (31) ... 

Hyperconjugation does not seem to have much effect on alkene reactivity towards bromine, since (16) applies whatever the number of alkyl groups on the double bond. However, only cis-olefins are involved in this correlation. To include geminally substituted olefins, an additional term is necessary, as in (19) where d is unity for the pem-disubstituted compounds and zero for... 

The 29Si-NMR chemical shift of 12 (8 = 66.34 ppm) is considerably deshielded as compared to the progenitor alkene 13 (8 = 0.14 ppm), which is in accord with substantial P-Si-C-bond hyperconjugative delocalization of positive charge to silicon. 

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. 

An alkyne is less reactive than an alkene. A vinyl cation is less able to accommodate a positive charge, as the hyperconjugation is less effective in stabilizing the positive charge on a vinyl cation than on an alkyl cation. The vinyl cation is more stable with positive charge on the more substituted carbon. Electrophilic addition reactions allow the conversion of alkenes and alkynes into a variety of other functional groups. 

As with alkenes, in general, anti-addition is often the course of reaction, especially when halonium ions are involved109-112. However, as mentioned earlier, syn addition can take place in the bromination of /Tsilylslyrenes. This stereochemistry is explained by stabilization of the open-chain carbocation by the aromatic group, compared to the cyclic bromonium ion. In this case the conformer 83 has the maximum hyperconjugative stabilization, and is formed by the least motion rotation about the carbon-carbon bond. 

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