A-Substituent effects

The reaction of MeO /MeOH with 2-Cl-5(4)-X-thiazoles (122) follows a second-order kinetic law, first order with respect to each reactant (Scheme 62) (297, 301). A remark can be made about the reactivity of the dichloro derivatives it has been pointed out that for reactions with sodium methoxide, the sequence 5>2>4 was observed for monochlorothiazole compounds (302), For 2.5-dichlorothiazole, on the contrary, the experimental data show that the 2-methoxy dehalogenation is always favored. This fact has been related to the different activation due to a substituent effect, less important from position 2 to 5 than from... 

The second possible cause of nonlinearity is a change in mechanism. Within a reaction series any change in mechanism must be such as to provide a smaller free energy of activation for the reaction (otherwise the mechanism would not change). If a substituent effect can produce a change in mechanism, the result must therefore be curvature that is concave upward. Figure 7-2 is a per plot for the S l solvolyses... 

For the exchanges carried out in liquid ammonia, kinetic isotope effects kD kT of 2.3-2.5 have been obtained for reaction of benzene, toluene, and naphthalene and for the reactions of the 2 positions of furan and thiophene with -butoxide in dimethyl sulphoxide somewhat lower values, 1.5 and 1.3, respectively, were obtained591, but whether this was a solvent or a substituent effect is not apparent from the data. 

A substituent effect on the rate and stereoselectivity of INOC reaction has been observed (Eq. 1) [13]. Thus, gem-dicarboalkoxy and gem-dithioalkoxy groups were found to have profound accelerating effect on the cyclization (Entries g and h, Table 1). When C-3 in 2 was monosubstituted, good diastereoselectivity was observed depending on the relative size of the substituents (Ph > Me > C02Me). 

It is of interest to consider the form of the Bronsted plot or Eigen plot to be expected for reaction of a series of related intramolecularly hydrogen-bonded acids with hydroxide ion by the mechanisms in Schemes 5 and 6. The effect of a substituent on the value of the dissociation constant of an intramolecularly hydrogen-bonded acid (8 log K) will be two-fold. The stability of the undissociated acid will be modified because of a substituent effect on the... 

These micellar charge effects also seem to be present for reactions at heteroatoms. For example, in the spontaneous hydrolysis of a series of benzenesulfonyl chlorides (12), values of k+/k for reactions in CTAC1 and SDS increase from 0.85 for X = OMe to 22 for X = NOz, suggesting that there is a substituent effect upon the relative extents of bond-making and breaking (Bunton et al., 1985). 

Replacement of a substituent X by its second row counterpart should lead to better a-a interactions. Once more, a substituent effect exerted principally on the minimum will give rise to the orders Cl > F, SR > OR, PR2 > NR2, SiR3 > CR3. Here, unfortunately, the same orders are expected if the principal effect of the substituent is exerted on the maximum. The experimental results collected in Table 51 clearly show that the predicted unequivocal trend is observed. 

V. NMR DETERMINATIONS OF STRUCTURE IN SOLUTION A. Substituent Effects on One-bond 15N-13C Couplings... 

Radical recomination of X and "CCl3 occurs but dimerization of X to form a B-B bonded product is not observed. The quantum efficiency of reaction (44) increases with a substituent effect in the direction R = CH2 > iC3H7 > 3115. 

A related effect is observed on quatemization, but in this case the operation of a / -substituent effect results in the overall change at the a. carbon atom normally being small (Table 5). A further important general trend in the azines arises on A-oxidation, which... 

Fig. 23 Energy diagram that illustrates (a) the generation of a simplified SN2 reaction profile from reactant, N R- -X, and product, N- -R X , configurations. Dotted lines denote avoided crossing (i.e. the reaction profile after configuration mixing). (b) The effect of stabilization of N- -R X (e.g. by a substituent effect) is indicated by dotted lines. Arrows indicate the positions of the transition states with and without the stabilization...

If one or two half-wave potentials show a deviation from the linear Ei—a plot, it is first necessary to verify that for the compounds in question, the electrolysis products, the value of an and the number of protons consumed in the given supporting electrolyte are the same for the compounds which show the deviations, as they are for the rest of the reaction series. If all these quantities show no differences for the deviating compounds, it can be concluded that either a change in the mechanism (i.e. a change in the value of Qn, r) or a substituent effect... 

The Jen-values have been measured for all the heterobenzenes. All values are extremely close to that of benzene. Indeed, the larger variation for the 4-substituted stiba- and bismabenzenes may be a substituent effect. Jq, values are a sensitive function of hybridization 59). Thus, the near identity of all the 1JCH values indicates a hybridization identical to that of benzene. 

The magnitude of a substituent effect depends on its distance from the carboxyl group. Substituents on the a carbon atom are most effective in increasing acid strength. More distant substituents have smaller effects on acidity, showing that inductive effects decrease rapidly with distance. 

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