Intermediate complex mechanism

On both experimental and theoretical grounds there is little doubt of the importance of polarizability as a major factor in determining the commonly encountered, though variable, high RS /RO ratios. Were thermodynamic carbon affinities mainly responsible for the usual reactivity order RS > RO, the peculiar behavior of chloroquinolines would be very difficult to understand. There is some indication, however, that carbon affinities roughly parallel basicities (hydrogen affinities), In the latter case, lower RS /RO ratios could be explained in terms of the intermediate complex mechanism, ... 

Sn2 nucleophilic aromatic substitution 2. The intermediate complex mechanism... 

In early rate studies the nucleophile was frequently an anion, and the intermediate complex mechanism was formulated as... 

The available experimental results are completely in accord with this formulation. Both of these limiting conditions have been observed experimentally, and plots of both k versus [B]0 and k versus [R2NH]0 have been shown to have characteristics consistent with this proposed mechanism. These observations thus constitute very convincing evidence for the intermediate complex mechanism in nucleophilic aromatic substitution. 

The mechanism has also been called by other names, including the Sn2At, the addition-elimination, and the intermediate complex mechanism. 

Although not strictly an association or recombination reaction the HO2 self reaction is best understood via an intermediate complex mechanism and the reaction shows some of the properties of an association reaction. 

The two-stage mechanism appears to be quite generally involved in nucleophilic substitution at acyclic unsaturated carbon as well as at carbocyclic and heterocyclic carbon (cf. Sections I,D,1 and II,A). In the case of carbonyl carbon, this intermediate-complex mechanism is supported by the form of base-catalysis of aminolysis of esters and by the exchange of during hydrolysis " of esters,... 

The relative orders in which the halogens are displaced in nucleophilic aromatic substitution reactions has been advanced as an u-ment both for and against the intermediate complex mechanism. In particular Fierens and Halleux (91), who found the order, F < Cl < Br < I, for the reactions of the l-halo-2,4-dinitrobenzenes and the l-halo-2,4-dinitro-6-methylbenzenes with potassium iodide in dry acetone, and Hammond and Parks (92), who obtained the same order for the reactions of the l-halo-2,4-dinitrobenzenes with N-methylaniline in both nitrobenzene and 99.8% ethanol, have offered these results as support for a one-step mechanism, since this is the sequence observed in non-activated nucleophilic aromatic substitution reactions and in typical aliphatic displacements. Hammond has, however, pointed out that these experiments do not permit a distinction between a one-step mechanism and a two-step mechanism in which the second step is rate-determining (90). 

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