Hyperconjugation primary

The importance of a primary steric effect in the nitration of alkyl-benzenes has been mentioned ( 9.1.1). The idea was first introduced by Le Fevre to account for the fact that -alkyltoluenes (alkyl = Et, -Pr,68a t-Bu ) are nitrated mainly adjacent to the methyl group. Without the rate data reported for the alkylbenzenes the effect might equally well have been accounted for by hyperconjugation. 

Draw a skeletal structure of the following carbocation. Identify it as primary, secondary, or tertiary, and identify the hydrogen atoms that have the proper orienta- tion for hyperconjugation in the conformation shown. 

The stability order can be explained by hyperconjugation and by the field effect. In the hyperconjugation explanation, we compare a primary carbocation with a tertiary. It is seen that many more canonical forms are possible for the latter ... 

In the examples shown, the primary ion has only two hyperconjugative forms while the tertiary has six. According to rule 6 (p. 41), the greater the number of equivalent forms, the greater the resonance stability. Evidence for the hyperconjugation explanation is that the equilibrium constant (K) for this reaction... 

As with carbocations, the stability order of free radicals is tertiary > secondary > primary, explainable by hyperconjugation, analogous to that in carbocations... 

As a result of the inductive and hyperconjugative effects it is to be expected that tertiary carbonium ions will be more stable than secondary carbonium ions, which in turn will be more stable than primary ions. The stabilization of the corresponding transition states for ionization should be in the same order, since the transition state will somewhat resemble the ion. Thus the first order rate constant for the solvolysis of tert-buty bromide in alkaline 80% aqueous ethanol at 55° is about 4000 times that of isopropyl bromide, while for ethyl and methyl bromides the first order contribution to the hydrolysis rate is imperceptible against the contribution from the bimolecular hydrolysis.217 Formic acid is such a good ionizing solvent that even primary alkyl bromides hydrolyze at a rate nearly independent of water concentration. The relative rates at 100° are tertiary butyl, 108 isopropyl, 44.7 ethyl, 1.71 and methyl, 1.00.218>212 One a-phenyl substituent is about as effective in accelerating the ionization as two a-alkyl groups.212 Thus the reactions of benzyl compounds, like those of secondary alkyl compounds, are of borderline mechanism, while benzhydryl compounds react by the unimolecular ionization mechanism. 

From comparison of Figs. 3.51 and 3.52 one can judge that Em,) includes contributions other than the six primary hyperconjugative interactions of Fig. 3.52. These omitted contributions are principally of secondary cr-a type, to be discussed in Section 3.4.2. However, these weaker hyperconjugative interactions do not significantly alter the preferred conformational angles established by the dominant six interactions of Fig. 3.52. 

Thus a tertiary carbocation like the above will give nine resonating structures while a primary will give only two hyperconjugative forms. This explains why tertiary carbocations are more stable than secondary which in turn is more stable than primary. This also explains why ethyl carbocation (CH3CIlf) is more stable than methyl carbocation (CH )-... 

Similar conclusions attend the insertions of CCI2 (from the thermolysis of ClsCCOONa at 120 °C) into a-deuteriocumene and cumene in which the primary fen/feo = 2.6, similar to Seyferth s finding with 32, and the p-secondary kinetic isotope effect is 1.20-1.25 for six deuteriums. Here, hyperconjugation at the p-CH (CD) bonds is thought to stabilize the partial cationic charge at the reaction center in transition state 33. 

Substituent effects Carbocations are formed in the S l reactions. The more stable the carbocation, the faster it is formed. Thus, the rate depends on carbocation stability, since alkyl groups are known to stabilize carbocations through inductive effects and hyperconjugation (see Section 5.2.1). The reactivities of SnI reachons decrease in the order of 3° carbocation > 2° carbocation > 1° carbocation > methyl cation. Primary carbocation and methyl cation are so unstable that primary alkyl halide and methyl halide do not undergo SnI reachons. This is the opposite of Sn2 reactivity. 

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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