Electron-donating groups increase the

Chemical shift variations among N-methyl cations have been related to electron densities and, in the case of fV-methylpyridinium ions, to Hammett substituent constants.126 Upfield shifts of H,126-128 13C,126 and 14N nuclei126 are observed when electron-donating groups increase the electron density at the quaternized site. In instances where shift differences are large, application of these correlations to molecules giving isomeric products will enable conclusive identification of product structures to be made. 

This transformation has also been applied to other binaphthyl derivatives, and, curiously, it was observed that substitution at the C6 position of the naphthol precursors 68 influences the observed enantioselectivity (Table 36). Electron-withdrawing groups enhance the enan-tioselectivity but give lower yields, whereas electron-donating groups increase the yields but reduce the enantioselectivity. 

Electron donating groups increase the rate of addition, as do groups that can delocalise the positive charge. Carbonium ion intermediates that are more stable will have more time in which to undergo rotation, and so will give rise to a more even distribution of anti- and sy -addition products. 

The a values are always those determined from the ionization of substituted benzoic acids, but the p values are different for each substrate type and also for each reaction type because they reflect the sensitivity of the substrate to the electronic effect of the substituent. With p > 1, the ionization or reaction rate is more sensitive to substituent s electronic effects than is the ionization of benzoic acids. With p < 1, electron-withdrawing groups will increase the equilibrium constant or rate, but less than in benzoic acid dissociation. A negative p indicates that electron-donating groups increase the reaction constant. In consideration of reaction rates, a small r often indicates that the reaction involves either radical intermediates or some other pathway that requires little charge separation. 

Rate extremes with systematic variation of substituents are often considered to be evidence of changes in the molecularity of a reaction, but for hydrolyses of arenesulfonyl chlorides, rate extremes are more reasonably ascribed to variations in the extents of S-O bond making and S-Cl bond breaking. Variations in k+/k support this hypothesis (Table VI), and as for hydrolyses of acid chlorides (Table V), bond making seems to be important but introduction of electron-donating groups increases the importance of bond breaking in the transition state. 

Addition to a terminal alkene shows only a P effect. Both electron withdrawing and electron donating groups increase the rate of radical addition when in this position, because the orbital resulting after radical addition is lower in energy due to delocalization of the radical by either inductive, hyperconjugative, or resonance effects. The sensitivity to the Taft steric parameter is relatively small (S = 0.28), meaning that sterics play very little role in the p-position. 

Electron-donating groups increase the basicity of aniline by a small amount. Electron-withdrawing groups, such as nitro and cyano, decrease the basicity by substantially larger amounts because the lone pair electrons of the amine can be delocalized into the substituent group. 

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