Rate constants derived from substituted aromatic

Table 21 Selected Rate and Equilibrium Constants for Dimerization of Radical Anions Derived From Substituted Aromatics... 

The relative rates of the various steps are a function of the pH of the solution and the basicity of the imine. In the alkaline range, the rate-determining step is usually nucleophilic attack of hydroxide ion on the protonated C=N bond. At intermediate pH values, water replaces hydroxide ion as the dominant nucleophile. In acidic solution, the rate-determining step becomes the breakdown of the tetrahedral intermediate. A mechanism of this sort, in which the overall rate is sensitive to pH, can be usefully studied by constructing a pH-rate profile, which is a plot of the observed rate constants versus pH. Figure 8.4 is an example of the pH-rate profiles for hydrolysis of a series of imines derived from substituted aromatic aldehydes and t-butylamine. The form of pH-rate profiles can be predicted on the basis of the detailed mechanism. The value of the observed rate can be calculated quantitatively as a function of pH, if a sufficient number of the individual rate constants and of the acid dissociation constants of the species involved are known or can be estimated reliably. Agreement between the calculated and observed pH-rate profile can then serve as a sensitive test of the adequacy of the postulated mechanism. Alternatively, one may begin with the experimental pH-rate profile and deduce details of the mechanism from it. 

Fleischmann et al s 34 report cyclic voltammetry data for the oxidation of a series of aromatic hydrocarbons in a molten salt electrolyte, AlCl3-NaCl-KCl at 150°. Electrooxidation in this medium occurs at unusually low oxidation potentials. Tris-(p-substituted phenyl)amines, with the exception of tri (p-nitrophenyl) amine, yield very stable radical cations by all electrochemical criteria 380>S42 Mono- and bis-p-substituted triphenylamines, however, dimerize with rate constants ranging from 101 to 10s M 1 sec 1 to benzidines 176 (Eq. (237)), which subsequently are oxidized to the radical cations 177, whose ESR-spectra are observed. Dimerization is fastest with the p-N02 andp-CN-derivative, in accordance with HMO calculations, which predict the highest spin sensity in the p-position of these compounds 542 ... 

A major breakthrough in theoretical organic chemistry resulted from the work of Hammett, around 1935. He derived Eqs. (3) and (4) to describe equilibrium constants K and rate constants k of various aromatic compounds by a certain reaction constant p, which depends on the reaction, and by substituent parameters a, which only depend on the nature of the substituents X of the corresponding aromatic compounds, based on hydrogen as a reference substituent p values are based on the ionization constants of substituted benzoic acids [9] ... 

Oxidation of diethyl a-benzylmalonate (25) by Mn(III) acetate in acetic acid at 70 °C in the presence of mono- or disubstituted alkynes leads to dihydronaphthalene derivatives (26) in moderate to good yields (equation 33). A mechanistic scheme involving the formation of the corresponding malonyl radical, its addition to a triple bond and intramolecular homolytic aromatic substitution of the vinyl radical adducts is discussed. Absolute rate constants, obtained from competitive studies, for the addition of a-benzylmalonyl radicals to a variety of alkynes cover few orders of magnitude e.g. the rate constants at 60 °C are 3x10 and 1 x 10 s for 4-octyne and phenylacetylene respectively. 

In the early 1960 s it was described 20,24,55-5 ) salt-like compounds of aromatic hydrocarbons are o-complexes, i.e. their cations AH possess the structure of arenium ions. This conclusion was first based on indirect arguments ensuing from the analysis of the AH -cation electronic absorption spectra (in particular, from the similarity of the spectra of anthracene and 1,1-diphenylethylene solutions in cone. HjSO j. It also results from the linear dependence of the logarithms of the. relative stability constants of A HF BF3 complexes on tho% of the rate constants of electrophilic substitution reaction of the hydrocarbons A Direct proof of this point of view was obtained from studies into the A HY mMY complexes and the solutions of aromatic hydrocarbons or their derivatives in various acids (HF, HF + BFj, HSO3F and others) by the nuclear magnetic resonance nKasurements of Dutch investigators... 

Hammen equation A correlation between the structure and reactivity in the side chain derivatives of aromatic compounds. Its derivation follows from many comparisons between rate constants for various reactions and the equilibrium constants for other reactions, or other functions of molecules which can be measured (e g. the i.r. carbonyl group stretching frequency). For example the dissociation constants of a series of para substituted (O2N —, MeO —, Cl —, etc.) benzoic acids correlate with the rate constant k for the alkaline hydrolysis of para substituted benzyl chlorides. If log Kq is plotted against log k, the data fall on a straight line. Similar results are obtained for meta substituted derivatives but not for orthosubstituted derivatives. 

A far more serious consideration is the adequacy of the solvolysis of phenyldimethylcarbinyl chlorides as a model reaction for electrophilic substitution. As will be shown, the cr -parameters derived from the phenyldimethylcarbinyl chloride studies are in good agreement with the a+-values deduced from the data for electrophilic substitution. Not all model reactions would have proved as satisfactory. As this research developed, it became clear that the influences of substituents on aromatic substitution reactions are quite accurately described by the other hand, the relative rates for electrophilic side-chain reactions of which the phenyldimethylcarbinyl chloride solvolysis is characteristic are not as adequately correlated by these constants. 

Table 10.11 shows the relative rates of nitration of a few benzene derivatives, and these demonstrate the electron donating (activating) and withdrawing (deactivating) effect of several substituents. In fact, most chemist s intuition as to what groups are electron donating and withdrawing is derived from rates of electrophilic aromatic substitution, as well as the (T constants associated with Hammett plots. 

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