Nitroalkane anomalies

Interestingly, the fact that the trend in the rate constants is the opposite of that in the acidity constants translates into a negative Och value of about -0.5 which has been called nitroalkane anomaly. 51... 

W.J. Albery et al., Marcus-Grunwald Theory and the Nitroalkane Anomaly, J. Phys. Org. Chem., 1997,1, 29. 

Among the many topics not dealt with in this chapter are intrinsic barrier asymmetry [57], disparity/tightness and their relation to observed Bronsted coefficients [45a, 58], the nitroalkane anomaly and role of additional VB states dissimilar from reactants and products [59], and several other interesting models for FERs [60, 61]. This reviewer was surprised at the lack, as yet, of applications of hybrid quantum-mechanical/molecular-medianical methodology (other than EVB-based methods) for simulations of FERs for any chemical reactions, let alone PT. However, the subject is alive and well, and it is to be expected that many more theoretical simulations of FERs for PT will be performed in the relatively near future. 

An archetypal example of TS imbalance is the nitroalkane anomaly. The rate (kj) of deprotonation of arylnitroalkanes is more sensitive to substituents on the aryl ring than the equilibrium constant (ATj = fc j/kj), as reflected in an unusually large value of the Brpnsted parameter, ocn (equation 5). In addition, )3 > 1.0 for ki and )3 < 0 for k-i. These data have been interpreted as showing that... 

There is some question whether the unusual reactivity properties manifested in the nitroalkane anomaly would occur in the gas phase,which is a separate issue. 

An extensive kinetic study for the proton-transfer reactions of phenylnitromethanes under various reaction conditions revealed that, although ac/-nitro species may form, they are not on the main reaction pathways. " The Hammett plot for protonation of nitronate and a Brpnsted plot including P-NO2 showed that the nitroalkane anomaly exists for all substimted phenylnitromethanes except the P-NO2 derivative. This result is consistent with the notion that transition-state imbalance is the source of the anomaly. 

Example 3. Proton Transfer Reactions of Phenylnitroalkanes with Hydroxide ion in Water. The proton transfer reactions of phenyinitromethane (PNM) in water and in water—dimethylsulfoxide (DMSO) mixture have been studied extensively by Bemasconi and the results have played a prominent role in discussions of the principle of nonperfect synchronization. A recent paper on proton transfer reactions of PNM s was concerned with the implications on the nitroalkane anomaly. 

Table 1.21 Equilibrium and rate data often cited in support of the "Nitroalkane Anomaly"...

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