One-stage mechanism

The lack of a uniform order of relative reactivity of the halogens in reactions of certain nucleophiles with nitro- and polynitro-phenyl halides led Parker and Read to propose a one-stage mechanism for some aromatic nucleophilic substitutions. An alternative explanation within the framework of the two-stage S Ar2 mechanism had been proposed earlier. A range of mechanisms has been considered in the past by Chapman, who properly points out that only in a limited number of examples is the evidence for the two-stage mechanism compelling even though the balance of evidence favors it. 

There is no clear-cut proof that a one-step Sn2 mechanism, so important at a saturated carbon, ever actually occurs with an aromatic substrate. The hypothetical aromatic Sn2 process is sometimes called the one-stage mechanism to distinguish it from the two-stage S jAr mechanism. A clean example of a Srn2 reaction has been reported, the conversion of 10 to 11 in methanol.Both the SrnI and Srn2 reactions have been reviewed. ... 

This is formally the reverse of the BA1,1 cleavage of an ester, and is the only one-stage mechanism for ester formation available for the ionized carboxyl group. Numerous methods are, of course, available which involve initial electrophilic attack on the carboxylate group, followed by a displacement at the carbonyl carbon atom of the intermediate formed, which is often an anhydride. An example134 is the esterification of carboxylic acids in the presence ofp-toluenesulphonyl chloride in pyridine, viz-... 

FIGURE 2.1.6 Reactions involved in parylene deposition process (top) sublimation of dimers at 100°C, splitting into monomers at 700°C, and polymerization at room temperature. Inexpensive system for parylene deposition (bottom) consists of a two-zone tube furnace and a 20-mm ID quartz tube containing parylene dimer powder and connected to a one-stage mechanical pump through a liquid nitrogen trap. A sample with prefabricated contacts and attached wire leads is placed in the tube at about 30 cm from the furnace. 

Enzyme inactivation has been frequently described by a very simple one-stage mechanism in which it is assumed that the enzyme suffers a highly cooperative conformational transition from a native active stiucture to an unfolded completely inactive form (Henley and Sadana 1985). This transition is represented by a hypothetical chemical reaction ... 

Melander s recognition of the non-existence of an isotope effect, however, is not an unequivocal proof for the SE2-mechanism. This was first pointed out by Hammond (1955). No isotope effect would be expected in the one-stage mechanism (3) if the transition state is reached before the energy of the carbon-hydrogen bond has decreased. Although it is not very probable that the transition state of the hydrogen ion release has this property, additional evidence for the two-step mechanism was desirable. 

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