Bimolecular Electrophilic Substitution at Saturated Carbon

In analogy to the traditional terms SW1 and Sw2, which refer to the extreme aliphatic substitution mechanisms, workers in the field of electrophilic substitution refer to S 1 (for substitution-electrophilic-unimolecular) and SE2 (substitution-electrophilic-bimolecular) mechanisms. Equations 4.40 and 4.41 

81 For recent reviews of electrophilic substitution see (a) F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials, McGraw-Hill, New York, 1968 (b) O. A. Reutov, Pure and Appl. Chem., 17, 79 (1968) (c) D. S. Matteson, Organometal. Chem. Rev. A., 4, 263 (1969) (d) C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd ed., Cornell University Press, Ithaca, N.Y., 1969, pp. 563-584. 

Mercury compounds are often used as electrophiles in displacements on organomercurials. There are five possible combinations of reactants in such mercury exchange reactions. A dialkylmercurial substrate may be attacked either by a mercury salt (Reaction 4.43) or by a monoalkylmercurial (Reaction 4.44) and likewise a monoalkylmercurial may react either with a mercury salt or with another monoalkylmercurial (Reactions 4.45 and 4.46). Finally, a dialkylmercurial may react with another dialkylmercurial (Reaction 4.47). (In Reactions 4.43-4.47 and in other reactions in this section, one of the reactants, and the fragments derived from that reactant in the products, are written in italics. This is done to make it easier to follow the course of the reaction.) 

82 In the gas phase, divalent mercury has been shown to be linear and therefore to be sp hybridized. However, in solution the X—R—X, R—Hg—X, or R—Hg—R bond angle in divalent mercury compounds varies from 130 to 180°. The variation in geometry is not yet entirely understood, so we shall follow Jensen s example and assume that, even in solution, divalent mercury is sp hybridized and that if a divalent mercury compound donates one empty orbital to coordinate with a Lewis base it rehybridizes to sp2 (F. R. Jensen and B. Rickborn, Electrophilic Substitution of Organomercurials, pp. 35, 36). 

We shall only discuss experiments that shed light on the first three types (Reactions 4.43-4.45), but the characteristics of all five types of mercury exchange reactions seem to be very similar. 

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ENTROPIES OF ACTIVATION (cal.deg" .mole- ) FOR SOME BIMOLECULAR ELECTROPHILIC SUBSTITUTIONS AT SATURATED CARBON... 

These results are consistent with the HSAB model if we consider that bimolecular elimination is a hard-hard interaction resulting from attack of the hard nucleophile, OH, at the proton, a hard electrophilic center. Whereas, nucleophilic substitution occurs by attack of the soft nucleophile, HS, at saturated carbon, a soft electrophilic center. The softness of H2O is intermediate between that of OH and HS, resulting in a mixture of substitution and elimination products. 

The spectrum of mechanisms of the electrophilic substitution at a saturated carbon atom is very broad [1,72-74]. The bimolecular Se2 exchange reactions in the series of organo-mercury compounds have been studied in the most detailed manner. For these reactions as well as for the Se2 reactions of other organometallic compounds, the derivatives of Li, Mg, B, Sn, Ge, a four-center transition state of XXIV type is assumed ... 

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