Substituent effects bromide/pyridin

Fig. 5.P23. The substituent effect in the Menschutkin reaction of 1-arylethyl bromides with pyridine in acetonitrile at 35°C. Circles represent kj for the bimolecular process and squares (for the uni-molecular process.

The Yukawa-Tsuno equation continues to find considerable application. 1-Arylethyl bromides react with pyridine in acetonitrile by unimolecular and bimolecular processes.These processes are distinct there is no intermediate mechanism. The SnI rate constants, k, for meta or j ara-substituted 1-arylethyl bromides conform well to the Yukawa-Tsuno equation, with p = — 5.0 and r = 1.15, but the correlation analysis of the 5 n2 rate constants k2 is more complicated. This is attributed to a change in the balance between bond formation and cleavage in the 5 n2 transition state as the substituent is varied. The rate constants of solvolysis in 1 1 (v/v) aqueous ethanol of a-t-butyl-a-neopentylbenzyl and a-t-butyl-a-isopropylbenzyl p-nitrobenzoates at 75 °C follow the Yukawa-Tsuno equation well, with p = —3.37, r = 0.78 and p = —3.09, r — 0.68, respectively. The considerable reduction in r from the value of 1.00 in the defining system for the scale is ascribed to steric inhibition of coplanarity in the transition state. Rates of solvolysis (80% aqueous ethanol, 25 °C) have been measured for 1-(substituted phenyl)-l-phenyl-2,2,2-trifluoroethyl and l,l-bis(substi-tuted phenyl)-2,2,2-trifluoroethyl tosylates. The former substrate shows a bilinear Yukawa-Tsuno plot the latter shows excellent conformity to the Yukawa-Tsuno equation over the whole range of substituents, with p =—8.3/2 and r— 1.19. Substituent effects on solvolysis of 2-aryl-2-(trifluoromethyl)ethyl m-nitrobenzene-sulfonates in acetic acid or in 80% aqueous TFE have been analyzed by the Yukawa-Tsuno equation to give p =—3.12, r = 0.77 (130 °C) and p = —4.22, r — 0.63 (100 °C), respectively. The r values are considered to indicate an enhanced resonance effect, compared with the standard aryl-assisted solvolysis, and this is attributed to the destabilization of the transition state by the electron-withdrawing CF3 group. 

Fig. 4.P18. Substituent effects on the rates of reaction of pyridine with 1-arylethyl bromides in acetonitrile at 35° C. Squares are the first-order rates and open circles are the second-order rates. Reproduced from Tetrahedron Lett. 38, 3243 (1997), by permission of Elsevier.

Few other reactions of series of substituted pyridines have been investigated extensively. Dondoni, Modena, and Todesco have measured the rate of N-oxidation of a limited series of pyridines and found a good correlation with normal u-values with a p-value of — 2.23. The A-alkylation of pyridines with alkyl iodides in nitrobenzene has been studied by Brown and Cahn and by Clarke and Rothwell. Unfortunately, the only data available are for the parent compound and for alkyl derivatives, and, since the a-values for the various alkyl groups in a given position are substantially constant, this leaves a correlation of only three independent points. However, the rates of A-alkylation of the j8- and y-alkyl derivatives are so nearly equal that it appears as if no correlation existed. Clarke and Rothwell have also studied the alkylation with allyl bromide in nitromethane at various temperatures, and in this case a more extensive series is available. The authors state that no overall Hammett correlation is obtained however, the j8-substituted derivatives fall on one straight line and the y-derivatives on another one with a different slope. The data are shown in Fig. 2. The line for the j8-compounds, p = — 2.53 0.31, r = 0.95, is seen not to be very good the line for the y-derivatives, p = — 1.42 0.06, r = 0.99, is much more satisfactory. It does not seem likely that the discrepancy is due to the intervention of resonance effects, since in this case one would expect the correlation for the y-derivatives to be poorer than that for the j8-analogs. More extensive studies with a wider variety of substituents would seem very desirable. 

Besides the effect of the position and identity of the substituent in the pyridinium groups on the transport number between anions, the water contents of the membranes differ according to the nature of pyridine derivatives reacted with the membranes. Figure 5.38 shows the relationship between the transport numbers of various anions relative to chloride ions and water contents of the membranes reacted with ethyl pyridines.103 The permeation of nitrate and bromide ions decreases and that of fluoride ions increases with increasing membrane water content when ethyl pyridines are reacted with the membranes. However, /cis°4 is independent of water content. This might be due to the low mobility of sulfate... 

Thiophenes with electron-deficient groups were more reactive compared with electron-donating substituents. ortfar-Substitution had no detrimental effect on reactivity and sterically demanding substrates could be accessed in excellent yields (80-91%). Heteroaryl bromides, such as pyridine, pyrimidine, quinoline, thiophene, and furan, were viable substrates. Notably, benzoth-iophene, benzofuran, furan, and pyrrole derivatives could also be directly arylated. 

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,... 

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