Reactions involving positively charged reactants

Relatively few comparative MW/A studies are presently available in this area. 

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The positively charged allyl cation would be expected to be the electron acceptor in any initial interaction with ethylene. Therefore, to consider this reaction in terms of frontier orbital theory, the question we need to answer is, do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another The orbitals that are involved are shown in Fig. 1.27. If we analyze a symmetrical approach, which would be necessary for the simultaneous formation of the two new bonds, we see that the symmetries of the two orbitals do not match. Any bonding interaction developing at one end would be canceled by an antibonding interaction at the other end. The conclusion that is drawn from this analysis is that this particular reaction process is not favorable. We would need to consider other modes of approach to analyze the problem more thoroughly, but this analysis indicates that simultaneous (concerted) bond formation between ethylene and an allyl cation to form a cyclopentyl cation is not possible. 

When positioned in hydrophobic microenvironments, anionic nucleophiles are partially desolvated. Whether this results in acceleration, however, depends on the type of the reaction involved. If the charge is more delocalized in the transition state than in the reactant, as in 13 and 14, desolvation of the nucleophile would selectively destabilize the reactant, leading to rate enhancement. On the other hand, if the charge is more localized in the transition state, the reaction would be retarded in hydrophobic microdomains. ... 

Steps 2 and 3, however, are new to us. Step 2 involves dissociation of an alky-loxonium ion to a molecule of water and a carbocation, a species that contains a positively charged carbon. In step 3, this carbocation reacts with chloride ion to yield tert-butyl chloride. Both the alkyloxonium ion and the carbocation are intermediates in the reaction. They are not isolated, but are formed in one step and consumed in another during the passage of reactants to products. If we add the equations for steps 1 through 3 together, the equation for the overall process results. A valid reaction mechanism must... 

Friedel-Crafts acylation usually involves the reaction of an acyl halide, a Lewis acid catalyst, and the aromatic reactant. Several species may function as the active electrophile, depending on the reactivity of the aromatic compound. For activated aromatics, the active electrophile can be a discrete positively charged acylium ion or a complex formed between the acyl halide and the Lewis acid catalyst. For benzene and less reactive aromatics, it is believed that the active electrophile is a protonated acylium ion or an acyiium ion complexed by a Lewis acid. Reactions using acylium salts are slow with toluene or benzene as the reactant and do not proceed with chlorobenzene. The addition of triflic acid accelerates the reactions with benzene and toluene and permits reaction with chlorobenzene. These results suggest that a protonation step must be involved. 

When reaction involves the approach of opposite charges, the result is a positive contribution to the entropy of activation, and an abnormally high frequency factor. The situation is represented in Figure 9.16b. The activated complex now has a lower charge than either of the reactants and there is less electrostriction in the activated state. There is some release of bound water molecules and a resulting increase in entropy. This simple theory leads to the result that this gain in entropy will be approximately... 

When nonsurfactant solutes (electrolytes, etc.) are added to the micellar reaction mixture, the results can be quite unpredictable. It is often found that the presence of excess surfactant counterions (common ions), when added to a system in which an ionic reactant is involved, retards the catalytic activity of the micelle, with larger ions being more effective in that respect. The effect can probably be attributed to an increase in ion pairing at the micelle surface and a reduction of its attractiveness to charged reactants. In contrast, the addition of neutral electrolyte has been found to enhance micellar catalysis in some instances. It has been proposed that the retardation effect of excess common counterions is due to a competition between the excess ions and the reactive substrate most closely associated with the micelle for the available positions or binding sites on or in the micelle. The enhancing effect, however, has been attributed to the more general effects of added electrolyte on... 

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