Jencks

Page, M. L., Jencks, W. P. Entropic contributions to rate accelerations in enzymic and intramolecular interactions and the chelate effect. Proc. Natl. Acad. Sci. USA 68 (1971) 1678-1683... 

Jencks, W.P. Binding energy, specificity, and enzymatic catalysis the Circe effect. Adv. Enzymol. 43 219-410, 1975. 

W. P. Jencks, Catalysis in Chemistry and Eraymology, McGraw-Hill, New York, 1969. C. H. Rochester, Acidity Functions, Academic Press, New kbik, 1970. 

Jencks has discussed how the gradation from the 8fjl to the 8n2 mechanism is related to the stability and lifetime of the carbocation intermediate, as illustrated in Fig. 5.6. In the 8n 1 mechanism, the carbocation intermediate has a relatively long lifetime and is equilibrated with solvent prior to capture by a nucleophile. The reaction is clearly a stepwise one, and the energy minimxun in which the caibocation mtermediate resides is significant. As the stability of the carbocation decreases, its lifetime becomes shorter. The barrier to capture by a nucleophile becomes less and eventually disappears. This is described as the imcoupled mechanism. Ionization proceeds without nucleophilic... 

Fig. 5.7. Mote O Ferrall-Jencks diagtam showing concerted, ion-pair and stepwise mechanisms for nucleophilic substitution.

T. C. Bruice and S. I Benkovic, Bioorganic Mechanisms, Vol. 1, W. A. Benjamin, New brk, 1966, pp. 1-258 W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, New York, 1969 M. L. Bender, Mechanisms of Homogeneous Catalysis from Protons to Proteins, Wiley-Interscience, New York, 1971 C. Walsh, Enzymatic Reaction Mechanisms, W. H. Freeman, San Francisco, 1979 A. Fersht, Enzyme Structure and Mechanism, 2nd ed., W. H. Freeman, New York, 1985. 

W P Jencks, Catalysis in Chemistry and En2ymology, McGraw-Hill, New York, 1969. 

The overall reaction stoichiometry having been established by conventional methods, the first task of chemical kinetics is essentially the qualitative one of establishing the kinetic scheme in other words, the overall reaction is to be decomposed into its elementary reactions. This is not a trivial problem, nor is there a general solution to it. Much of Chapter 3 deals with this issue. At this point it is sufficient to note that evidence of the presence of an intermediate is often critical to an efficient solution. Modem analytical techniques have greatly assisted in the detection of reactive intermediates. A nice example is provided by a study of the pyridine-catalyzed hydrolysis of acetic anhydride. Other kinetic evidence supported the existence of an intermediate, presumably the acetylpyridinium ion, in this reaction, but it had not been detected directly. Fersht and Jencks observed (on a time scale of tenths of a second) the rise and then fall in absorbance of a solution of acetic anhydride upon treatment with pyridine. This requires that the overall reaction be composed of at least two steps, and the accepted kinetic scheme is as follows. 

Guthrie and Guthrie and Jencks have proposed an alternative mechanistic symbolism that is capable of more detailed description than the Ingold system, although at the expense of greater complexity. This system may be useful for the computer representation of reaction mechanisms. 

Jencks, W.P. "Catalysis in Chemistry and Enzymology McGraw-Hill New York, 1969. 

Jencks has emphasized a danger in the technique of reducing the reaction order by using an excess concentration of one reactant. If this reactant contains an impurity that itself is very reactive, the impurity concentration may be sufficiently high to lead to spurious results from the unsuspected reaction. 

When Jencks reacted hydroxylamine with p-nitrophenyl acetate, p-nitrophenolate ion was released at a rate faster than that at which acetohydroxamic acid was formed. This burst effect is evidence for a two-step reaction. In this case the intermediate is O-acetylhydroxylamine, which subsequently reacts with hydroxylamine to form the hydroxamic acid. 

Espenson gives examples from inorganic chemistry Jencks describes enzyme-catalyzed reactions in which the common intermediate is an acylated enzyme... 

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