Hyperconjugation with alkyl group

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. 

For the other alkyl groups, hyperconjugation is diminished because the number of C—H bonds is diminished and in ferf-butyl there are none hence, with respect to this effect, methyl is the strongest electron donor and rert-butyl the weakest. 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

In molecular orbital terms, alkyl groups can stabilize a carbocation by hyperconjugation. This is the overlap of the filled a orbitals of the C—H or C—C bonds adjacent to the carbocation with an empty p orbital on the positively charged carbon atom. As a result, the positive charge is delocalized onto more than one atom, and thus increases the stability of the system. The more alkyl groups there are attached to the carbocation, the more a bonds there are for hyperconjugation, and the more stable is the carbocation. 

The polarity alternation rule (PAR) considers two kinds of substituents. The donors are. those having unshared electronic pairs or -electrons, and +1 groups. These include OH, OR, OCOR, NH2, NRR, N(R)COR, SH, SR, halogens and alkyl groups. The donor properties of the alkyl groups may reflect the existence of hyperconjugation. On the other hand, the acceptors are electron sinks, i.e. polarizable it-bonds, atoms with empty orbitals, and —I groups. Examples of acceptors are C=0 (aldehydes, ketones, carboxylic acid derivatives), CN, S02, N02, SiRj. 

Here, the principal structure is the one on the far left, but a small contribution of the others is assumed to delocalize the C-H bonding electrons to the central carbon and thus stabilize the ion. Obviously, the number of such structures will decrease with decreasing number of alkyl groups attached to the cationic carbon and none can be written for CH3 . This explanation is known as the hyperconjugation theory. 

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

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