Medium effects groups

The rates increase up to a maximum at about 90 wt. % sulphuric acid (this point varies slightly according to the aromatic reactivity) and the increase with increasing acid concentration is consistent with the increase in the concentration of nitronium ions. The occurrence of a maximum indicates an opposing factor and is thought42 to be partly due to protonation of the aromatic (most of the measured compounds contain the group >X=0) but since it also occurs for PhNMe3, medium effects must be involved, i.e. the activities of the species present varies, whilst the concentrations remain the same. The kinetic equation for reaction of nitronium ion with an aromatic is... 

Phenyl vinyl sulfones reacted with cyclohexanone enamines 332 to afford adducts which, upon hydrolysis, gave 2-(2-phenylsulfonyl)alkylcyclohexanone 333a . However, in the reaction with phenyl styryl sulfone, two products 333b and 334 were obtained by the nucleophilic attack at the and a-carbon atoms . Steric effects, electrostatic interactions between the nitrogen atom of the enamine and the oxygen atoms of the sulfone group, and medium effects contribute to the regioselectivity of the reaction. ... 

Medium, Taxonomic Group, Organism, and Other Variables Cd, pg/L Exposure Period Effect Reference"... 

In his calculation of Es values, Taft made use of average values of log (krx/kr0). Evidence has been presented which suggests that this procedure is unjustified, as the dependence of log kvx/kvo0) on X is not independent of medium. Effective values, oef, of the steric parameter for some ICD groups have been obtained by means of a two step procedure, 24, 25 ... 

Kemp elimination was used as a probe of catalytic efficiency in antibodies, in non-specific catalysis by other proteins, and in catalysis by enzymes. Several simple reactions were found to be catalyzed by the serum albumins with Michaelis-Menten kinetics and could be shown to involve substrate binding and catalysis by local functional groups (Kirby, 2000). Known binding sites on the protein surface were found to be involved. In fact, formal general base catalysis seems to contribute only modestly to the efficiency of both the antibody and the non-specific albumin system, whereas antibody catalysis seems to be boosted by a non-specific medium effect. 

The second explanation for the solvent isotope effect arises from the dynamic medium effect . At 25 °C the rotational and translational diffusion of DjO molecules in D20 is some 20% slower than H20 molecules in H20 (Albery, 1975a) the viscosity of D20 is also 20% greater than H20. Hence any reaction which is diffusion controlled will be 20% slower in D20 than in H20. This effect would certainly apply to transition state D in Fig. 3 where in the transition state the leaving group is diffusing away. A similar effect may also apply to the classical SN1 and SN2 transition states, if the rotational diffusion of water molecules to form the solvation shell is part of the motion along the reaction co-ordinate in the transition state. Robertson (Laughton and Robertson, 1959 Heppolette and Robertson, 1961) has indeed correlated solvent isotope effects for both SN1 and SN2 reactions with the relative fluidities of H20 and D20. However, while the correlation shows that this is a possible explanation, it may also be that the temperature variation of the solvent isotope effect and of the relative fluidities just happen to be very similar (see below). 

It is interesting that for the different compounds in Table 21 AE/R has roughly the same value. We have also included the values for the solvent isotope effect for the solvolysis of isopropyl bromide. Heppolette and Robertson (1961) made an extensive study of the temperature variation of the solvent isotope effect for this compound. It can be seen that the values for isopropyl bromide match those of the other bromo compounds. The data for isopropyl bromide correlate well with the relative fluidities of HzO and D20 (Heppolette and Robertson, 1961) but this may arise because the AE/R values for the solvent isotope effect and for the relative fluidities happen by chance to be roughly equal. What is clear is that changing the group R has very little effect on the solvent isotope effect once one has allowed for the different temperatures at which the reactions are measured. This means that the solvent isotope effect for the solvolysis of halides is caused either by the LzO sites on the nucleophile having much the same fractionation factors or by the dynamic medium effect or by a combination of both. 

Since there is not much of conjugation between the P=0 group and the nitrogen in (2), 0 and N atoms can in principle behave as two independent basic centers. Medium effects upon the PMR shielding parameters of phosphoramidates demonstrated that in such a strong acid as trifluoromethanesulfonic amides (2) indeed exist, at least partly, as the 0 and N diprotonated species. 

In contrast to the dihydroxylations, the hydroboration of allylic alcohols takes place without an inside alkoxy effect in the sense 5.132 with the oxygen atom treated as the medium-sized group. It is probably significant that the inside alkoxy effect is most noticeable with reagents which are relatively electrophilic in nature, and not with boranes, which are only mildly electrophilic. 

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