Nitromethane solvent effect

Evidence in favour of the molecular nitrating species is the relative insensitivity of rate to solvent effects, rates differing in nitromethane by only a factor of 6... 

The photodecomposition and thermodecomposition of nitromethane have been extensively studied as model systems in combustion, explosion and atmosphere pollution processes[l]. On another hand, nitromethane was selected as a model solvent in experiments aimed at examining non hydrogen-bonded solvent effects in a general acid-base theory of organic molecules [2.3]. This selection is based on the electronic and structural characteristics of nitromethane that has a high dielectric constant, and at the same time cannot form hydrogen bonds with solute molecules. 

Since these substitution reactions follow a two-term rate law, it is clear that solvent effects are very significant. Poorly coordinating solvents are benzene, carbon tetrachloride and sterically hindered alcohols and strongly coordinating solvents are water, lower alcohols, DMF, DMSO, acetonitrile and nitromethane. The first-order rate constants are greater in DMSO than in water. Since the majority of precursor platinum complexes used in synthetic and mechanistic studies are halo complexes, the replacement of halide ligands by solvent and the reversibility of this reaction are important features of platinum halide chemistry. 

Solvent effects on quantum yields have been studied to some extent. Yields for substitution fall precipitously and the nature of the reaction may change if the medium consists of a noncoordinating solvent. Thus tra s-[Cr(NCS)4(NH3)2] shows a 0 of about 0.3 in aqueous media (for thiocyanate aquation), but is photoinert in nitromethane. In a mixed solvent study, it was concluded that the photochemical behavior of this complex depended on the solvent composition of the solvation shell rather than on the stoichiometric composition.41 42... 

Roberts and his associates (1954) re-examined the nitration of the halobenzenes using fuming nitric acid as the reagent in both acetic anhydride and nitromethane. These workers detected a significant solvent effect on the relative rate. Iodobenzene was nitrated by nitric acid in acetic anhydride at a rate 0.13 times that of benzene, in nitromethane this reagent provided a rate ratio of 0.22. 

The role of solvent effects in quaternization is one of the first physical organic studies and this is due to Menschutkin (1879LA334). It shows an increase in relative rate from 1 to 742 on going from benzene to benzyl alcohol, which suggests no simple explanation. Typical ranges of solvent-dependent rate ratios are 15,700/1 (nitromethane/cyclohexane) in the alkylation of triethylamine by methyl iodide (68BSF2678), 1660/1 [dimethylsulfoxide (DMSO)/carbon tetrachloride] in the reaction of l,4-diazabicyclo[2,2,2]-octane (DABCO) (5) with (2-bromoethyl)benzene (75JA7433) (Scheme 5),... 

The scatter of the points in the two figures is not surprising in view of the simplifications made in deriving Eq. (5-81). Furthermore, the solvent effects observed in these reactions are very small with a comparatively large experimental error, and the solvents used include such dipolar and associated liquids as nitromethane and alcohols. 

The solvent effects on rates shown by these two reactions were determined employing the solvents chloroform, dichloromethane, acetonitrile, ethyl acetate, benzene, tetrahydrofuran and dioxane. Solvents which react with TCNE, such as nitromethane, dimethylformamide and protic solvents, as well as cyclohexane, carbon tetrachloride and tetrachloroethylene, in which the reactants have very low solubility, were deliberately excluded from the study. The observed solvent effects were virtually identical for both Diels-Alder and [2 + 2] cycloaddition processes. Statistical correlations of rate data using a multiparameter equation with dependencies based on acceptor properties, polarizability and inherent polarity of the solvents gave nearly identical coefficients through the regression analyses for each term for both reactions, and excellent linear fits to the rate data. 

S. Nagakura and M. Gouterman. J. Chem. Phys. 26, 881-6 (1957). UV solvent effect and spectral shifts, a-, -naphthols in ether, triethylaminc, nitromethane. 

Solvent effect. Because of its high dielectric constant (39), nitromethane is recommended as solvent for Koenig-Knorr condensation of glycosyl bromides with ethanol in the presence of silver carbonate to yield jS-glycosides. In less polar solvents the o-glycoside can be the major product. 

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. 

Nitration and nitratation. Nitronium triflate is generated in situ from Bu NNOj and TfjO. Adamantane gives 1-nitro- or 1-nitratoadamantane on exposure to this reagent in nitromethane or dichloromethane, respectively. Thus, there is a marked solvent effect. 

In another review on a related subject, Alkorta and Elguero have summarised the GIAO calculations of chemical shifts and absolute shieldings and their correlation with experimentally measured chemical shifts for aromatic hetero-cycles. Automatic assignment, conformational analysis, E/Z isomerism, and, in particular, tautomerism, have been discussed in detail. In addition, solid state and solvent effects, as well as the problem of heteroaromaticity have been examined. The review ends with the discussion of some methodological aspects with a special emphasis on the calculation of reference chemical shifts, such as those for TMS and nitromethane. 

Bribre. K.M. Detellier. C. Solvent effects in the metal interchange of crown ether-alkali metal cation complexes. Transition from an associative exchange in nitromethane to a dissociative exchange in acetonitrile studied by Na... 

Solvent effects on and for reaction of Co(NO)(CO)3 have been studied. Effects on k are fairly small and are consistent with the effect of varying dielectric constant on the bimolecular rate-determining step. Values of ki for these reactions conducted in the poor solvents cyclohexane, toluene, and nitromethane are similar. This similarity, and the determined activation parameters, are consistent with a dissociative mechanism for this reaction pathway. But values for reaction in tetrahydrofuran, acetonitrile, and dimethyl sulphoxide are different from the former k values and from each other, indicating an associative mechanism involving the solvent for the k term. The balance between the k and k pathways for substitution in Co(NO)(CO)2L depends both on the 7r-bonding and steric characteristics of L. This dependence on the bulk of L is perhaps surprising in view of the tetrahedral stereochemistry and consequent lack of crowding in the substrate. The reverse reaction of Co(NO)(CO)2L with carbon monoxide has also been studied, so that rates in both directions and equilibrium constants are known for ... 

N. S, Bayliss and E, G. McRae, / Phys, Chem., 58, 1006 (1954). Solvent Effects in Organic Spectra of Acetone, Crotonaldehyde, Nitromethane and Nitrobenzene. 

The diaryliodonium salts are highly stable in the absence of light, but decompose when irradiated at 313 or 365 nm with quantum yields of 0.2-0 3 (3). The rate of photodecomposition is independent of the structure of the counter anion, and is insensitive to temperature and to atmospheric oxygen (3). No thorough investigation of solvent effects has been reported, but decomposition rates appear to be identical in acetone and acetonitrile when quartz tubes are used and the irradiation source is a water-cooled Hanovia 450 W medium pressure mercury lamp (2). Decomposition in nitromethane is slower, probably as a result of the increased light absorption by this solvent. 

Nitration in organic solvents is strongly catalysed by small concentrations of strong acids typically a concentration of io mol 1 of sulphuric acid doubles the rate of reaction. Reaction under zeroth-order conditions is accelerated without disturbing the kinetic form, even under the influence of very strong catalysis. The effect of sulphuric acid on the nitration of benzene in nitromethane is tabulated in table 3.3. The catalysis is linear in the concentration of sulphuric acid. 

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