And the Baker-Nathan effect

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... 

This came to be called the Baker-Nathan effect and has since been found in many processes. Baker and Nathan ° explained it by considering that hyperconjugative forms contribute to the actual structure of toluene ... 

While the effect on oxidation-reduction potentials of substituents on phenanthroline ligands is regular, studies of the oxidation of [Os(bipy)(terpy)X] + species, where X is an alkyl-substituted pyridine molecule (111), do not show a linear dependence of E° on the pA of X. These results have been explained in terms of the Baker-Nathan effect. However, taken in conjunction with the entropy data of Kratochvil and Knoeck (439) for substituted iron complexes, an explanation involving changes in solvation with substituents seems preferable. The potentials of various Os(II)/Os(III) couples (111) and Ru(II)/Ru(III) couples (220) have been used to study the effect of the overall charge on the... 

To summarize Baker and Nathan observed that hyperconjugation appeared to occur to a greater extent when H was the nonbonded fragment in the sacrificial valence bond form, compared with the case where CHJ was the fragment. This difference between the two (called the Baker-Nathan effect) was not very great in any case and was subsequently shown to be due to a solvation effect. This has led to the statement... 

The kinetics of the chlorination of some alkylbenzenes in a range of solvents has been studied by Stock and Himoe239, who again found second-order rate coefficients as given in Table 57. Although the range of rates varies by a factor of 104, there was no marked change in the toluene f-butylbenzene reactivity ratio, and it was, therefore, concluded that the Baker-Nathan order is produced by a polar rather. than a solvent effect. 

It was concluded that the variations in rate are due to variations in activation enthalpy rather than entropy, and since the rates of substitution rates at the para positions of toluene and /-butylbenzene varied by only 4 % for a change in reactivity of 6,430, it was concluded that the Baker-Nathan reactivity order does not arise from a solvent effect (c/. Table 57). 

Although the ground state solvation effects are quite small they have been shown to account for the Baker-Nathan order in the methanolysis of p-alkyl substituted benzyl chlorides (Clement and Naghizadeh, 1959 see also Hehre et al., 1974 Schubert and Sweeney, 1956). 

Baker-Nathan effect. Effect originally observed in the reaction of p-substituted benzyl bromides with pyridine and other processes in which the observed rates are opposite to those predicted by the electron-releasing inductive effect of alkyl groups, i.e., CH3 > CH3CH2 > (CH3)2CH > C(CH3)3. To explain it, a type of electron delocalization involving 2 electrons was proposed, termed hyperconjugation, which manifests itself in systems in which a saturated carbon atom attached to an unsaturated carbon or one with an empty orbital bears at least one hydrogen atom. 

In the low-absorptivity region, where the saturation effect is insignificant, the electronic and vibrational contributions to the absorptivity become comparable (Ballester et al., 1964b) and consequently acceptable correlations are obtained only in a very few instances, such as in alkylbenzenes. Actually, the square law has allowed the evaluation of the carbon-carbon and carbon-hydrogen bond hyperconjugation (Baker-Nathan effect) (Ballester and Riera, 1964a March, 1985, p. 65). 

These data permit the conclusion that according to their stabilizing effect on carbenium, and, in particular, on arenium, ions the alkyl groups are arranged in the Baker-Nathan order, the difference in the effect depending on the ion structure. However, the difficxxlty of estimating the relative role of electronic and steric effects requires much care, so one can hardly answer the question as to which of these effects actually determines this sequence (see... 

For a discussion of the role of polarizibility in donor abihty, see (a) Taft, R. W., Topsom, R. D. (1987). The Nature and Analysis of Substitutent Electronic Effects. Progress in Physical Organic Chemistry, 16, 1-83 (b) Exner, O., B6hm, S. (1997). Baker-Nathan effect, hyperconjugation and polarizability effects in isolated molecules. Journal of the Chemical Society, 6, 1235-1240. 

Subsequently it was shown that the order of reactivity found by Baker and Nathan does in fact result from differential solvation because in the gas phase this order is reversed. Hence, it was concluded that the Baker-Nathan results were only a solvation effect and not evidence for hyperconjugation. 

Theoretical Considerations.—Several papers have appeared on the application of 1-arylethyl ester pyrolyses to the study of electrophilic aromatic reactivities. By this means, accurate values have been determined for para-cyclohexyl and t-butyl substituents. The effect of mera-t-butyl substitution in the same reaction and in the protiodetritiation in TFA at 70 °C has been measured. The results indicate that the Baker-Nathan order in solvolysis reactions arises from steric hindrance to solvation of the transition state and that C-C is more important than C-H hyperconjugation. This approach has also been used to study non-additivity of methyl and chlorine" substituent effects. 

Arrhenius parameters for nitration of 4-aikylphenyltrimethyiammonium ions in nitric acid-sulphuric acid mixtures (Table 12). It was argued that the observed Baker-Nathan order of alkyl substituent effect was, in fact, the result of a steric effect superimposed upon an inductive order. However, a number of assumptions were involved in this deduction, and these render the conclusion less reliable than one would like it would be useful to have the thermodynamic parameters for nitration of the methyl substituted compound in particular, in order to compare with the data for the /-butyl compound, though experimental difficulties may preclude this. It would not be surprising if a true Baker-Nathan order were observed because it is observed for all other electrophilic substitutions in this medium1. 

However, Baker and Nathan ° observed that the rates of reaction with pyridine of p-substituted benzyl bromides (see Reaction 10-44) were about opposite that expected from electron release by the field effect. That is, the methyl-substituted compound reacted fastest and the tert-butyl-substituted compounded reacted slowest. 

Since the data for para nitration (Knowles et al., 1960) and acetylation are on a firm experimental basis these small deviations and trends are almost certainly real. Two interpretations are possible. The isotope effect observed for acetylation (Jensen, 1955) suggests that the trend may have its origin, in this case, in the competitive rates of formation and decomposition of the benzenium ions. The observation of a Baker-Nathan order, p-Me> p-t-Bu, is presumably a reflection of the greater stabihty of the l-acyl-4-methylbenzenium ion (27) compared to the... 

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