Nitromethane proton transfer

Abstract—The equilibrium diagrams of the binary systems of sulphuric add with nitromethane and with o-y m- and p-nitrotoluene have been investigated. It has been shown that addition compounds of the type 1 1 are formed in these systems, analogous to the compound sulphuric acid-nitrobenzene (Chebbuuez Helv, Chim, Acta 1923 6 281 and Masson J. Chem. Soc. 1931 3201)t The formation of these addition compounds is due to hydrogen bonding between the components, rather than to proton transfer. Their stability in the crystalline phase seems to be contradictory to the known basicities of mononitrocompounds (Gillespie and Solomons J. Chem. Soc, 1957 1796), because of the effect of temperature on the equilibria in the liquid phase. 

All Pi values obtained for oxygen bases A- are between 0.94 and 1.09 (Table 7), with the exception of two reactions involving nitromethane, with lower p2 values. This indicates that proton transfer between these oxygen bases must be fast as expected. (The best value is probably a little larger than 1.) It follows that the assumption of the constancy of the p, values is essentially correct for these examples. Furthermore, the predicted relationship between p value (of a CH bond) and resonance effect in the carbanion appears to be fulfilled (Table 7). For some reactions of the type RH + A-, the values calculated for n2max = 1 — nmax are of approximately the same magnitude as the experimental Bronsted (5 values [132],... 

The proton transfer process described above is uniquely associated with the condensed fluid phase of nitromethane. This bond specificity is remarkable, since in the gas phase the C-N bond is the weakest in the molecule (Do = 60.1 kcal/mol) [46], and is therefore expected to be the dominant dissociation channel and the initial step in the decomposition of nitromethane, even at high temperature. In contrast, the C-H bond is the strongest in the nitromethane molecule. 

The second event in simulation C is a second proton transfer, observed at 60 fs, and is due to an intramolecular 0-H attraction. The fast vibrations of the methyl group induce an umbrella inversion type motion that, along with a stretch in a N-O bond, leads to the hydrogen transfer from the methyl to the nitro group. This reaction yields the aci acid form of nitromethane. The process of bond rearrangement is illustrated as ... 

Parrinello ab initio/MD study of Decker et al. [39] that showed a proton transfer reaction of liquid nitromethane when compressed by a factor of 3.0 at 150 K. 

The observed increases in k0 upon addition of (CH3)2SO are consistent with similar increases in proton-transfers (13, 15, 16, 25-27) and are easily understood in terms of a reduced 8 log K C-) in the (CH3)2SO-containing medium (equation 6, (5nucn instead of (5). The magnitude of the effects is smaller than in the proton-transfer reactions, though. For example, 8 log k0 = 0.45 for nucleophilic addition to (5-nitrostyrene contrasts with 8 log k0 = 1.35 for the same solvent change in the deprotonation of nitromethane anion by the same amines (C. F. Bernasconi, A. Mullin, and D. Kliner, unpublished results). This attenuation of the solvent effect is another manifestation of the smaller imbalances in the olefin reactions. 

In our study of the proton transfer reactions of three simple nitroal-kanes (nitromethane, nitroethane and 2-nitropropane), absorbance—time profiles due to the formation of reactive intermediates were observed at the isosbestic points where changes in reactant and product absorbance... 

Solvolysis. Solvolysis of the [BHJ anion has been studied in liquid ammonia and in DMSO. In both cases determined activation parameters are compared with those for hydrolysis. The comparison of rates in DMSO and in water underlines the importance of easy proton transfer in the latter solvent in determining reactivity. Kinetics of solvolysis of [SnCl4(py)2] and of related compounds in acetonitrile and in nitromethane have large, positive values of SS, betokening 5 nI mechanisms. ... 

Figure 10. Transition state characteristics for methyl acetyl acetone (A, B) and nitromethane (C) in terms of the degree of proton transfer (a, P) and the solvation scale (c). The bases are...

For the reaction with a stabilized phosphoniutn ylide, the betaine intermediate undergoes proton transfer and even extrudes a sulfonamide group to give a vinyl phosphonium salt that can be trapped with water, a stabilized phosphonium yUde [223], or nitromethane (solvent) [225]. These findings suggest that the conversion of the betaine to the azaphosphetane is much slower than the interconversion between the two betaine diastereomers (Scheme 49). Thus, the Z/E ratio for the alkene product does not correspond to the diastereoselectivity for the initial imine/ ylide addition. Instead, the Z/E selectivity is decided by the different rates for the transformation of the two betaine diastereomers into their corresponding azaphosphetanes. 

Since Corey s group first reported 0(9)-allyl-N-(9-anthracenylmethyl) cinchonidi-nium bromide as a new phase-transfer catalyst [13], its application to various asymmetric reactions has been investigated. In particular, this catalyst represents a powerful tool in various conjugated additions using chalcone derivatives (Scheme 3.2). For example, nitromethane [14], acetophenone [15], and silyl eno-lates [16] produce the corresponding adducts in high enantioselectivity. When p-alkyl substrates are used under PTC conditions, asymmetric dimerization triggered by the abstraction of a y-proton proceeds smoothly, with up to 98% ee [17]. 

All these studies on bimolecular processes indirectly indicate the involvement of acz -nitromethane, but more detailed studies will be required to estimate the energy requirements for its formation. Our own preliminary analysis for the bimolecular transfer of hydrogen between carbon and oxygen shows this process to be spontaneous for the transfer from acz-nitromethane to the nitromethide anion, whereas that from nitromethane is 10 kcal/mol endothermic (Scheme VI). Thus, under gas phase conditions H-transfer from acz -nitromethane to the nitromethide anion should occur exclusively, but the reverse process may well occur in liquid and solid phases as the endothermicity is only modest. This is further supported by the 12 kcal/mol stabilization that results on the spontaneous carbon to oxygen transfer of hydrogen from protonated nitromethane to nitromethane [28]. 

Evidently the a-H is likely to be involved in the early stages of the decomposition process [16], perhaps moving to an adjoining nitro group to form a nitronic acid tautomer, 4 [35,45,46]. These are known to be reactive and unstable [47], The transfer or loss of a proton to yield a nitronic acid or a nitronate (aci) anion has also been invoked as the initial step in the decompositions of other energetic molecules, e.g. picric acid [35,48] and amine-sensitized nitromethane [49-51]. 

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