C The Continuum between Sn1 and Sn2 Mechanisms

For solution-phase nucleophilic substitution reactions, the American physical organic chemist Sol Winstein treated Sn1 and Sn2 mechanisms as limiting cases that lie along a continuum of possible interactions (Scheme 9.21).

covalent

tight-ion pair

loose-ion pair

dissociated

reactant

reactant

reactant

reactant

Scheme 9.21

In this scheme, [R-X] is the covalently bonded form, [R+X—] is a so-called tight ion pair, [R+| |X"] is a loosely associated ion pair, and [R+ + X"] represents the fully dissociated ions. Note also that —N: is the nucleophile, and RN and NR are the two optical isomers formed by attack on either face of the fully dissociated electrophilic center.

Winstein's dictum was that intermediate species of slightly different reaction geometries lie on an undefined continuum between structures predicted for purely SN1 and SN2 mechanisms, and depending on the nature of the particular reaction mechanism, one may anticipate intermediate values for secondary kinetic isotope effects arising from these hybrid mechanisms. Other factors (e.g., nucleo-philic strength, solvent nucleophilicity, other solvation effects, the detailed nature of the leaving group, and temperature) can greatly influence the mechanistic course of a reaction. Even examples from physical organic chemistry

FIGURE 9.20 Atomic structure ofglycosyl-enzyme intermediate for hen egg white lysozyme. Shown are the electron density contours and inferred structure of the reaction intermediate. Image from Vocadlo et al. (2001) is reproduced with permission of the authors and publisher (Nature).

TABLE 9.8 Secondary Kinetic Isotope Effects for Acid-, Base-, and Lysozyme-Catalyzed Hydrolysis of Glucoside

Reaction Conditions Observed kH/kD

TABLE 9.8 Secondary Kinetic Isotope Effects for Acid-, Base-, and Lysozyme-Catalyzed Hydrolysis of Glucoside

Reaction Conditions Observed kH/kD

Acid (2 M HCl)

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