Hammett plots methylation

A second example is given in Eq. (10-12), which represents the denitrosation of A-methyl-Af-nitrosoanilines by acid.10 The Hammett plot shows upward curvature because the mechanism changes along the series. 

A Hammett plot (a) of the apparent first-order rate constants for semicarbazone formation at pH 3.9, where a change in the RCS occurs, and (b) of the denitrosation of A-methyl-A-nitrosoanilines, where the mechanism changes. Data are from Refs. 9 and 10. 

Waters61 have measured relative rates of p-toluenesulfonyl radical addition to substituted styrenes, deducing from the value of p + = — 0.50 in the Hammett plot that the sulfonyl radical has an electrophilic character (equation 21). Further indications that sulfonyl radicals are strongly electrophilic have been obtained by Takahara and coworkers62, who measured relative reactivities for the addition reactions of benzenesulfonyl radicals to various vinyl monomers and plotted rate constants versus Hammett s Alfrey-Price s e values these relative rates are spread over a wide range, for example, acrylonitrile (0.006), methyl methacrylate (0.08), styrene (1.00) and a-methylstyrene (3.21). The relative rates for the addition reaction of p-methylstyrene to styrene towards methane- and p-substituted benzenesulfonyl radicals are almost the same in accord with their type structure discussed earlier in this chapter. 

The one-electron reduction potentials, (E°) for the phenoxyl-phenolate and phenoxyl-phenol couples in water (pH 2-13.5) have been measured by kinetic [pulse radiolysis (41)] and electrochemical methods (cyclic voltammetry). Table I summarizes some important results (41-50). The effect of substituents in the para position relative to the OH group has been studied in some detail. Methyl, methoxy, and hydroxy substituents decrease the redox potentials making the phe-noxyls more easily accessible while acetyls and carboxyls increase these values (42). Merenyi and co-workers (49) found a linear Hammett plot of log K = E°l0.059 versus Op values of substituents (the inductive Hammett parameter) in the 4 position, where E° in volts is the one-electron reduction potential of 4-substituted phenoxyls. They also reported the bond dissociation energies, D(O-H) (and electron affinities), of these phenols that span the range 75.5 kcal mol 1 for 4-amino-... 

The kinetics of the alkaline hydrolysis of 5-ethyl-l-methyl-5-phenyl-2-thioxo-4,6-(7//,5//,5//)-pyrimidinedione (2-thioprominal) (370) have been reported. Hammett plots of pA a values were linear for a series of Al-2f-benzoyl N, Ai -diethylthioureas (371). 3... 

The rates of cycloaddition of methyl acrylate with six 1-substituted pyridinium-3-olates have been studied, A Hammett plot of the second-order rate constants against tr values of the substituents gave satisfactory correlation. 272.373... 

Fig. 2. Zucker-Hammett plot for the hydrolyses of methyl mesitoate and -gIycervI benzoate, at...

Fig. 5. Hammett plot for the hydrolysis of substituted methyl (O) and ethyl ( ) benzoates at 45°C in 99.9% sulphuric acid. Data from ref. 33.

One of the characteristics of the acid-catalyzed hydrolysis of esters, that is shared by ester formation also, is that substituent effects on the rate coefficients are small, and not simply related to a values (see below, p. 131). The data in Table 14 show that this is also true for the, sO-exchange reaction of substituted benzoic acids. This is borne out by the relative constancy of the ratio khyJkexch for the different substituted acids it was not possible to obtain a meaningful p value from the data of Table 14, because of the small number of points and the large amount of scatter evident on the Hammett plot. Mesitoic acid is highly unreactive, compared with the m- and p-substi-tuted esters used, as is its methyl ester towards alkaline hydrolysis138, and presumably reacts by the seriously hindered Aac2 route. 

While we can make substitutions on the aromatic ring of the nucleophile or the leaving group for methyl transfers, it is clearly impossible to do the same for the transferring group. The solvolysis of benzyl halides (Bennett and Jones, 1935) gives curved Hammett plots. The interaction of the aromatic ring with the reaction centre makes this a more complicated reaction. Hence we will not consider these reactions in detail. Thus the simplest system we can consider is... 

The aza substituent constants (vide supra) reflect the fact that electron-withdrawing annular nitrogens decrease the reactivity of any other ring nitrogen in the order ortho meta < para. For this reason, pyrazines should quaternize more readily than pyrimidines and pyridazines, and all three diazines should react faster than triazines. When the diazines are included in a Hammett plot for the methylation of substituted pyridines (p = —2.3), the positive deviations showed that they were all more reactive than indicated by their pK values. Relative rates compared with pyridine were pyridazine, 0.25 pyrimidine, 0.044 and pyrazine, 0.036 (72JA2765). Pyridazine in particular appears to be much more reactive than one would expect. (See Section III, A below). 

The relative rate data of Table II were subjected to Hammett plots of log kx/kh versus o and o+. In the nitrobenzene oxidations, a better correlation was found with +(p+ = -0.48, r = -0.994). The preferred dependence on <7+ was also found in the copper(II) oxidations when the kinetic point for the methyl... 

Fig. 9.5 Hammett plot for migratory insertion of alkenes into palladium(ll)-methyl bond.

The effects of substituents R on the methyl loss (24-+25) from ionized trimethylaryl-oxysilanes 24 was also studied by determining the corresponding IE and AE values516. Hammett plots as a function of as we as ionization energies IE of 24 decrease... 

On the other hand, the reaction of aryl-phenylfulvenes with maleic anhydride in benzene at 20.6°C seems to exhibit a maximum, although not well pronounced, in the Hammett plot the unsubstituted diphenylfulvene has the highest rate coefficient, while p-Me, p-OMe and p-Cl substituents all decrease the rate . However, in the reaction with methyl acetylene dicarboxylate in heptane at 60-80°C, (p-chlorophenyl)-phenylfulvene was found to be slightly more reactive than diphenylfulvene . It is, in any case, a matter of small differences. 

Hammett plots. For phenylacetic acid ionization constants and for benzoic acid in ethanol. Data to generate these plots were taken from Bright W. L., and Briscoe, H. T. "The Acidity of Organic Acids in Methyl and Ethyl Alcohols." J. Phys. Chem., 37,787 (1933), and Dippy, J. F., and Williams, F. R. "Chemical Composition and Dissociation Constants of Mono-Carboxylic Acids. Part I. Some Substituted Phenylacetic Acids." /. Chem. Soc., 161 (1934). 

HjO Hammett plot of N vs. c m-methyl isomer gives very similar spectrum. Hammett plot of N vs. (T. ) Related spin adduct reported. ... 

The rates of hydrolysis of a series of substituted anisoles have also been reported. The reactions are shown in Fig. 9.21, and the rate constants, along with the a values, are summarized in Table 9.2. A Hammett plot produces a p value of 2.3 (Fig. 9.22), showing that the mechanism of reaction involves a simple Sn2 displacement by water on the methyl group of the substituted anisole (Fig. 9.23). Finally, the rates of hydrolysis of anisole are found to be substantially faster than that of the more sterically hindered phenetole (Fig. 9.24), consistent with the postulated Sn2 mechanism. 

Hammett plots have been constructed for the acid- and base-catalysed decomposition of methyl hemiacetals of benzaldehydes in aqueous solution. The data are analysed in terms of three-dimensional More O Ferrall-Jencks diagrams and of Cordes interaction effects. 

The Hammett plot for substituted anilines gave p -= —1.73. The negative values of p indicate the reaction as being electrophilic. The rate determining step was suggested to be the attack of methyl cation to anilino group (step III) in the following scheme. 

The quaternization of pyridine in non-polar solvents has been studied as a means of clarifying the much debated nature of the displacement reaction under such circumstances . Swain and Eddy i deduced evidence for their theory of specific solvation from the reaction of pyridine and methyl bromide in benzene containing various hydroxylic solutes. Swain and Langs-dorf45ii> found the Hammett plot for the reaction of substituted benzyl bromides with pyridine in acetone to be markedly concave, and indeed to fall into two separate lines for meta- and / zm-substituents. The curvature and the division illustrate the effects of substituents upon reactions of intermediate character. Ingold and his co-workers from reactions in sulphur dioxide,... 

Suitable mechanisms have been proposed following determination of the kinetic and activation parameters for oxidation of 2-naphthol and cyclic ketones by nicotinium dichromate some a-amino acids by tripropylammonium fluorochromate " distyryl ketone by quinaldinium fluorochromate methanol by benzyltriethylammonium chlorochromate catalysed by 1,10-phenanthroline substituted benzyl alcohols by tetraethylammonium bromochromate L-cysteine by pyridinium bromochromate lactic acid and 3,5-dimethyl-2,6-diaryl piperidin-4-one oximes by pyridinium chlorochromate allyl alcohol by IDC benzophenoxime by bispyridine silver(I) dichromate and alkyl phenyl sulfides by cetyltrimethylammonium dichromate. A non-linear Hammett plot obtained for the oxidation of substituted benzyl alcohols by IDC has been attributed to the operation of substituent effect on two steps of the proposed mechanism. " Kinetic and activation parameters for oxidation of o-toluidine and of A-methyl-2,6-diphenyl piperidin-4-one oxime and its 3-alkyl derivatives by sodium dichromate have been determined and suitable mechanisms have been suggested. Micellar catalysis in the 1,10-phenanthroline-promoted chromic acid oxidation of propanol... 

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

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