Solvolytic nucleophilic displacements

It should be mentioned that the ionization step in Eq. (2-13) is analogous to that involved in SnI and Sn2 reactions of aliphatic substrates. For example, in solvolytic reactions of haloalkanes, the process of going from a covalently bonded initial state to a dipolar or ionic activated complex (transition state) is similar to the ionization step in Eq. (2-13). Therefore, those solvent properties that promote ionization are also important in the estimation of solvent effects on nucleophilic displacement reactions [161] (cf. Section 5.4.1). 

The nucleophilic displacement reaction taking place at a saturated carbon atom is either a bimolecular substitution, Sv2 type, with complete Walden inversion, or a solvolytic reaction, characterized as a nucleophilic substitution of the first order, S l type, with predominant inversion. 

In the work from my own laboratory over the last ten years or so we have used a combination of experimental measurements on selected reactants and model calculations relating to a-deu-terium effects on the rates of solvolytic nucleophilic displacement on carbon to try to correlate all known results on such reactions in terms of Scheme I. 

Comprehensive reviews of S 2 substitution can be found in (a) S. R. Hartshorn, Aliphatic Nucleophilic Substitution, Cambridge University Press, London, 1973 (b) A. Streitwieser, Solvolytic Displacement Reactions, McGraw-Hill, New York, 1962 (c) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969. 

Solvolytic displacement reactions can be affected by solvents in several ways, including nucleophilic solvent assistance (NSA) and electrophilic solvent assistance (ESA). NSA can be defined as electron donation from solvent to the developing positive dipole of a reacting C-X bond, and ESA can be defined as electron acceptance by the solvent from the leaving group, I. 

Quantitative Scales of Solvent Nucleophilicity. Solvolytic studies in solvents of low nucleophilicity led to renewed interest in quantitative measures of solvent nucleophilicity. Peterson and Waller (44) derived a scale of solvent nucleophilicity (Npw) from the rates of displacement by solvent of tetramethylenehalonium ions (VI) in liquid sulfur dioxide. The reaction is approximately half-order in carboxylic acid, possibly because dimer-monomer preequilibrium occurs (44). More recently, hydrolysis of the iodonium salt (VIII) in competition with anionic or solvent nucleophiles was studied. A scale of nucleophilicity relative to water was obtained by quan-... 

The above SnI reaction is facilitated by the presence of the para methoxy substituent, which allows for extra stabilization of the positive charge. In the absence of this substituent, Sn2 processes may dominate. Thus, the parent phenylmethyl (benzyl) halides and sulfonates undergo preferential and unusually rapid Sn2 displacements, even under solvolytic conditions, and particularly in the presence of good nucleophiles. As in allylic Sn2 reactions (Section 14-3), two factors contribute to this acceleration. One is that the benzylic carbon is made relatively more electrophilic by the neighboring sp -hybridized phenyl carbon... 

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