Baker-Nathan order

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. 

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. 

This order is opposite to that expected from the field effect (p. 16). It is an example of the Baker-Nathan order (p. 71). 

However, the Baker-Nathan effect has now been shown not to be caused by hyperconjugation, but by differential solvation. This was demonstrated by the finding that in certain instances where the Baker-Nathan effect was found to apply in solution, the order was completely reversed in the gas phase. ° Since the molecular structures are unchanged in going from the gas phase into solution, it is evident that the Baker-Nathan order in these cases is not caused by a structural feature (hyperconjugation) but by the solvent. That is, each alkyl group is solvated to a different extent. 

For Z = alkyl, the Baker-Nathan order (p. 72) is usually observed both for SnI and Sn2 reactions. 

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... 

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). 

Most of the electrophilic substitutions follow the Baker-Nathan order (toluene more reactive than t-butylbenzene) for few substitutions (f. e. nitration) the opposite inductive order holds. According to the new explanation 28) the Baker-Nathan order would be associated with a transition state of the Wheland classical type (9),... 

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... 

Cooney, B. T., Happer, D. A. R. (1987). The Baker-Nathan order hyperconjugation or a solvent effect Australian Journal of Chemistry, 40, 1537-1544. 

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. 

In the experimental domain, criticisms of hypereonjugation have been of different kinds. The principal chemical evidence has been the Baker-Nathan order of alkyl substituent effects ... 

What seems to be emerging from this discussion is that the Baker-Nathan order appears when unidirectional electron demand is not very strong. It is thus favored by electron delocalization, but disfavored by too much development of positive charge, which brings into play the normally greater electropositivity of the alkyl groups. One indication of this is Schubert s (75) observation that the frequency of the longitudinal transition ... 

The stability of another kind of complex, the 1 1 w-complex between chloranil and alkylbenzenes, was investigated by Halevi and Nussim (100). In these complexes in which the two rings are some 3.5 A. apart, and there is strong overlap between the electron deficient 5T system of the acceptor and the relatively electron-rich system of the hydrocarbon donor, alkyl substituents in the donor stabilize the complex in the Baker-Nathan order. This order is not necessarily evidence for hyperconjugative stabilization of the complex, since it would also arise if steric interference to approach of the accep-... 

---