Nitroalkanes thermodynamics

Nitroalkanes show a related relationship between kinetic acidity and thermodynamic acidity. Additional alkyl substituents on nitromethane retard the rate of proton removal although the equilibrium is more favorable for the more highly substituted derivatives. The alkyl groups have a strong stabilizing effect on the nitronate ion, but unfavorable steric effects are dominant at the transition state for proton removal. As a result, kinetic and thermodynamic acidity show opposite responses to alkyl substitution. 

The condensation of nitro compounds and imines, the so-called aza-Henry or nitro-Mannich reaction, has recently emerged as a powerful tool for the enantioselective synthesis of 1,2-diamines through the intermediate /3-amino nitro compounds. The method is based on the addition of a nitronate ion (a-nitro carbanion), generated from nitroalkanes, to an imine. The addition of a nitronate ion to an imine is thermodynamically disfavored, so that the presence of a protic species or a Lewis acid is required, to activate the imine and/or to quench the adduct. The acidic medium is compatible with the existence of the nitronate anion, as acetic acid and nitromethane have comparable acidities. Moreover, the products are often unstable, either for the reversibility of the addition or for the possible /3-elimination of the nitro group, and the crude products are generally reduced, avoiding purification to give the desired 1,2-diamines. Hence, the nitronate ion is an equivalent of an a-amino carbanion. 

TABLE 18. Thermodynamic properties for ionization of nitroalkanes in DMSO at 25 °C... 

Since then, groups have been derived for heats of formation of solid nitroaromatics (Ref 20) and for solid and liquid nitroalkanes (Ref 22) Group Additivity. The basic idea behind Group Additivity is that chemical thermodynamic properties of molecules consist of contributions from the individual groups that make up the molecule. Group Additivity is therefore an extension of the series atom additivity, bond additivity,. . . , and turns out to be an excellent compromise between simplicity and accuracy... 

An unsuccessful attempt has been made to determine the separate electronic and steric effects of alkyl groups on the acidities of hydrocarbons, acetophenone derivatives, and acetone derivatives CH3COCHR1R2 (at either site) by multivariational analyses of experimental and theoretical acidities for each set.15 A thermodynamic cycle has been used to estimate the aqueous phase p/C, = 22.7 1.0 for the methyl group of acetic acid and p/C, = 3.3 1.0 for the corresponding enol.16 Equilibrium acidities have been determined for several nitroaryl substituted nitroalkanes and cyanomethanes, 2,4,6-TNT, and 9-cyanofluorene17 in acetonitrile the influence of common cation BH+ on the electronic spectra of the anions obtained in the presence of strong guanidine bases (B) has been attributed to formation of two types of ion pair.18... 

The 1,4-addition (or conjugate addition) of resonance-stabilized carbanions. The Michael Addition is thermodynamically controlled the reaction donors are active methylenes such as malonates and nitroalkanes, and the acceptors are activated olefins such as a,P-unsaturated carbonyl compounds. 

In the preparation of dynamic nitroaldol systems, different aldehydes and nitroalkanes were first evaluated for reversible nitroaldol reactions in the presence of base to avoid any side- or competitive reactions, and to investigate the rate of the reactions. 1H-NMR spectroscopy was used to follow the reactions by comparison of the ratios of aldehyde and the nitroalcohols. Among various bases, triethylamine was chosen as catalyst because its reactions provided the fastest exchange reaction and proved compatible with the enzymatic reactions. Then, five benzaldehydes 18A-E and 2-nitropropane 19 (Scheme 9) were chosen to study dynamic nitroaldol system (CDS-2) generation, because of their similar individual reactivity and product stabilities in the nitroaldol reaction. Ten nitroaldol adducts ( )-20A-E were generated under basic conditions under thermodynamic control, showing... 

Nitrite ions generally give nitroalkanes 4.29 as the major products, which are also thermodynamically more stable than alkyl nitrites 4.28. Again, with carbocationic electrophiles, it is only when the reaction rate has reached the diffusion-controlled limit that alkyl nitrites can be detected or even be the... 

Nitrite ion generally gives nitroalkane 80 as the major product. Since a nitroalkane is thermodynamically more stable than the corresponding alkyl nitrite, the alkyl nitrite isomerizes to the corresponding nitroalkane in a manner as shown below. 

The observation of the unusual inverse order of kinetic and thermodynamic acidities of the simple nitroalkanes (nitromethane = NM, nitroethane = NE, and 2-nitropropane = 2-NP) during their reactions... 

Solubility, thermodynamic and kinetic acidities of nitroalkanes are of great importance in the course of the reaction, but the hydrophobic effect (Lubineau et al., 1994a) could be involved, since additives such as glucose or saccharose accelerate the reaction even more (Lubineau and Aug6, 1992). Amphiphilic molecules can also influence the hydrophobic interactions. In fact, cetyltrimethylammonium chloride (CTACl) as cationic surfactant promoted the Michael reaction of various nitroalkanes with conjugated enones in dilute aqueous solutions of sodium hydroxide (Ballini and Bosica, 1996) ... 

---