Kamlet reaction

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

More advanced scale was proposed by Kamlet and Taft [52], This phenomenological approach is very universal as may be successfully applied to the positions and intensities of maximal absorption in IR, NMR (nuclear magnetic resonance), ESR (electron spin resonance), and UV-VS absorption and fluorescence spectra, and to many other physical or chemical parameters (reaction rates, equilibrium constant, etc.). The scale is quite simple and may be presented as ... 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

In this respect, the solvatochromic approach developed by Kamlet, Taft and coworkers38 which defines four parameters n. a, ji and <5 (with the addition of others when the need arose), to evaluate the different solvent effects, was highly successful in describing the solvent effects on the rates of reactions, as well as in NMR chemical shifts, IR, UV and fluorescence spectra, sol vent-water partition coefficients etc.38. In addition to the polarity/polarizability of the solvent, measured by the solvatochromic parameter ir, the aptitude to donate a hydrogen atom to form a hydrogen bond, measured by a, or its tendency to provide a pair of electrons to such a bond, /, and the cavity effect (or Hildebrand solubility parameter), S, are integrated in a multi-parametric equation to rationalize the solvent effects. 

In 1976, Kamlet and Taft introduced their solvatochromic comparison method [25, 26], The hydrogen-bond donor acidity a and basicity /3 together with the solvent polarity and polarizability jv were employed to correlate the solvent effects on reaction rates, equilibria, and spectroscopic properties XYZ according to equations of the form... 

Correlation analysis of solvent effects on the heterolysis of p-methoxyneophyl tosyl-ate has been performed by using the Koppel-Palm and Kamlet-Taft equations. The reaction rate is satisfactorily described by the electrophilicity and polarity parameters of solvents, but a possible role for polarizability or nucleophilicity parameters was also examined. 

Studies of medium effects on hexacyanoferrate(II) reductions have included those of dioxygen,iodate, peroxodisulfate, - [Co(NH3)5(DMSO)] +, and [Co(en)2Br2]+. Rate constants for reaction with dioxygen depended strongly on the electron-donor properties of the organic cosolvent. Rate constants for reduction of peroxodisulfate in several binary aqueous media were analyzed into their ion association and subsequent electron transfer components. Rate constants for reduction of [Co(en)2Br2] in methanol water and dioxan water mixtures were analyzed by a variety of correlatory equations (dielectric constant Grunwald-Winstein Swain Kamlet-Taft). 

Equations containing a number of solvent parameters in linear or multiple linear regression and expressing the effect of the solvent on the rate of the reaction or the thermodynamic equilibrium constant. See Ej Values Kamlet-Taft Solvent Parameter Koppel-Palm Solvent Parameter Z Value... 

Equation (10) is only one example of several multiparametric equations17 that may be used to forecast the effect of variation of solvent on rates of reaction. One such equation18 that has been applied in organic chemistry is that of Kamlet and Taft (equation 11) ... 

The oxidation of meta- and para-substituted anilines with imidazolium fluorochro-mate (IFC)18 and nicotinium dichromate (NDC),19 in several organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in the oxidant and TsOH and is zero order with respect to substrate. A correlation of rate data in different solvents with Kamlet-Taft solvatochromic parameters suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation. The oxidation rates with NDC exhibited negative reaction constants, while the oxidation with IFC did not correlate well with any linear free energy relationships. 

The oxidation of substituted /3-benzoylpropionic acids by PFC follows the Hammett relation with a negative reaction constant. A possible mechanism for the oxidation has been discussed.5 The oxidation of maleic, fumaric, crotonic, and cinnamic acids by PCC is of first order with respect to PCC and the acid. The oxidation rate in 19 organic solvents has been analysed by Kamlet s and Swain s multiparametric equations. A mechanism involving a three-centre transition state has been postulated.6 The relative reactivity of bishomoallylic tertiary alcohols toward PCC, to yield substituted THF products via the tethered chromate ester, is dependent only on the number of alkyl groups. This observation suggests a symmetrical transition state in this intramolecular Cr(VI)-alkene reaction.7 Mechanisms have been proposed for the oxidation of 2-nitrobenzaldehyde with PBC8 and of crotonaldehyde with tetraethylammonium chlorochromate.9... 

The Kamlet-Taft u polarity/polarizability scale is based on a linear solvation energy relationship between the n it transition energy of the solute and the solvent polarity ( 1). The Onsager reaction field theory (11) is applicable to this type of relationship for nonpolar solvents, and successful correlations have previously been demonstrated using conventional liquid solvents ( 7 ). The Onsager theory attempts to describe the interactions between a polar solute molecule and the polarizable solvent in the cybotatic region. The theory predicts that the stabilization of the solute should be proportional to the polarizability of the solvent, which can be estimated from the index of refraction. Since carbon dioxide is a nonpolar fluid it would be expected that a linear relationship... 

This method assumes a different hierarchy of formation of product species from the detonation reaction of a CHNO explosive than the hierarchy used earlier, where CO is assumed to be formed preferentially prior to the formation of CO2. Here, with the Kamlet-Jacobs method, CO2 is assumed to be formed as the only oxidation product of carbon. As with the previous hierarchy assumptions, water is still formed first. The generalized reaction for an underoxidized explosive can be written as ... 

This equation has been used in several correlations of solvent effects on solute properties such as reaction rates and equilibrium constants of solvolyses, energy of electronic transitions, solvent-induced shifts in UV/visible, IR, and NMR spectroscopy, fluorescence lifetimes, and formation constants of hydrogen-bonded and Lewis acid/base complexes [Kamlet et al., 1986b]. 

The continuum model has been applied to an experimental study of the solvent effect on the 6-chloro-2-hydroxypyridine/6-chloro-2-pyridone equilibrium in a variety of essentially non-hydrogen-bonding solvents (Beak et al., 1980). In this study, a plot of log A nh/oh) versus (e - 1)/ (2e + 1), the solvent dielectric term, yielded a linear least-squares fit with a slope of 2.5 0.2, an intercept of -1.71, and a correlation coefficient of 0.9944. This result was used to estimate the gas phase free-energy difference of 9.2 kJ mole-1, which compares favorably with the observed value of 8.8 kJ mole-1 for this system. The authors also reported that alcohol solvents are correlated fairly well in this study but that other solvents seem to be divided into two classes, those that are electron-pair donors and those that are electron-pair acceptors in a hydrogen bond. The hydrogen bonding effect is assumed to be independent from the reaction field effect and is included in the continuum model by means of the Kamlet and Taft (1976) empirical parameters. The interested reader is referred to the original paper for a detailed discussion of the method and its application. 

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