Reaction rate enhancement proximity effects

Propinquity (proximity) effects are important in reaction rate enhancement. In the case of the following compounds, anhydrides (products formed on removal of water) form at different rates. Arrange the compounds in order of their rates of anhydride formation and explain the reasons for the ordering. 

The proximity effect. This is the simple idea that in an intramolecular reaction the substrate function may be exposed to a larger local concentration of the reagent than in an intermolecular reaction, because the two functions are covalently constrained to occupy adjacent space. This effect has been called the approximation or propinquity effect. The proximity effect certainly seems physically reasonable and is likely to make some contribution to intramolecular reactivity, but it cannot be a major contributor when EM is large, because EM is itself a measure of a presumed local concentration, and the observed large EM values are physically impossible concentrations. The magnitude of rate enhancement achievable by prox-... 

The unusual rate enhancement of nucleophiles in micelles is a function of two interdependent effects, the enhanced nucleophilicity of the bound anion and the concentration of the reactants. In bimolecular reactions, it is not always easy to estimate the true reactivity of the bound anion separately. Unimolecular reactions would be better probes of the environmental effect on the anionic reactivity than bimolecular reactions, since one need not take the proximity term into account. The decarboxylation of carboxylic acids would meet this requirement, for it is unimolecular, almost free from acid and base catalysis, and the rate constants are extremely solvent dependent (Straub and Bender, 1972). 

The effects of proximity are easier to comprehend than the chemical mechanisms of rate enhancement. In a reaction... 

In the intramolecular reactions studied by Bruice and Koshland and their co-workers, proximity effects (reduction in kinetic order and elimination of unfavourable ground state conformations) and orientation effects might give rate accelerations of 10 -10 . Hence, these effects can by themselves account for the enhancements seen in most intramolecular reactions. However, a factor of 10 -10 is less than the rate acceleration calculated for many enzyme reactions and certain intramolecular reactions, for example, hydrolysis of benzalde-hyde disalicyl acetal (3 X 10 ) (Anderson and Fife, 1973) and the lactonization reaction of[l] (10 ) where a trimethyl lock has been built into the system. If hydrolysis of tetramethylsuccinanilic acid (Higuchi et al., 1966) represents a steric compression effect (10 rate acceleration), then proximity, orientation, and steric compression... 

These PVP polymers provide a "proximal effect" without addition of free pyridine in the reaction mixture. Different studies have shown that only one pyridine per manganese catalyst is sufficient to enhance the rate of the catalytic oxygen atom transfer from the high-valent metal-oxo species to the organic substrate. The advantage of PVP polymer over a cationic Amberlite resin (see Scheme II for structures) have been recently illustrated in the modeling of ligninase (11). 

If MPT is solubilized in aqueous CTAB micelles together with a hydrophobic quencher, statistical and proximity effects will influence the efficiency and rate of the energy transfer in the same direction. Thus reaction 20 is enhanced in the confined space of a CTAB micelle by a factor of HO (19). 

It is interesting to speculate on the origins of the (stoichiometric) rate enhancement of these reactions inside the softball cavity. In more conventional forms of catalysis, such as reactions occurring at transition metal centres, the mode of catalysis often involves a significant statistical effect. The reactants are brought into dose proximity to one another by simultaneous coordination to the metal. This increases... 

There are several processes that account for the enhancement of reaction rates by enzymes. The major mechanisms are proximity-entropy effects, substrate strain, covalent catalysis, and acid-base catalysis. 

Binding of substrate(s) to an enzyme has two important effects, (i) The substrate is positioned or oriented properly for reaction, both with respect to functional groups in the active site and to other substrates, (ii) In cases where more than one substrate is involved (e.g. A + B C + DorA + B=f C), the enzyme can assemble or collect the substrates from solution and place them in close proximity. Studies on cleverly-designed synthetic systems have clearly demonstrated that proximity effects alone can result in tremendous rate enhancements, as shown in Figure 4.70 for selected systems. Formation of an eirzyme-substrate complex is, however, entropically unfavourable (AS < 0) and therefore costs energy. Typically, enzymes use the favourable enthalpic change (AH < 0) of substrate binding to overcome the unfavourable entropic term and to place the substrate in au environment in which it can be transformed, a phenomenon sometimes referred to as the "Circe effect" [66]. 

This is the facihtation of the proximity of reacting molecules. It implies that the rate of a reaction is enhanced by abstracting the molecnles from dilnte solntion and holding them in close proximity in the active site of the enzyme. This process raises the effective concentration of the reactants at their site of interaction. 

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