Neighboring-group model

Brown-Hammett correlation of 1-aryl-1-cyclobutylethyl p-nitrobenzoate gave a p+ value of -3.94 which is more positive than that of a model compound, l-aryl-l,2-dimethylpropyl p-nitrobenzoate (p+ = -4.65), which indicates substantial neighboring group participation of the cyclobutyl ring even in tertiary carbocationic systems.12 Solvent effects on the kinetics of the solvolysis... 

The first section, Chemical Reactions on Polymers, deals with aspects of chemical reactions occurring on polymers—aspects relating to polymer size, shape, and composition are described in detail. One of the timely fields of applications comprises the use of modified polymers as catalysts (such as the immobilization of centers for homogeneous catalysis). This topic is considered in detail in Chapters 2, 3, 8, 9, and 11 and dealt with to a lesser extent in other chapters. The use of models and neighboring group effect(s) is described in detail. The modification of polymers for chemical and physical change is also described in detail in Chapters 2 (polystyrene) 4 (polyvinyl chloride) 5 (polyacrylic acid, polyvinyl alcohol, polyethyleneimine, and polyacrylamide) 6 (polyimides) 7 (polyvinyl alcohol) 8 (polystyrene sulfonate and polyvinylphosphonate) 10 (polyacrylamide) and 12 (organotin carboxylates). 

The C02 yield considerably exceeds that of the acetylacetone-like product, and although C02 is certainly also formed in other reactions (cf. the low molecular weight model Ulanski et al. 1996b), this observation maybe taken as a hint that the H-abstraction reaction does not proceed with neighboring groups only (via a favorable six-membered transition state) but also with more distant sites. A chain-type autoxidation has also been observed with poly(vinyl methyl ether) and a related model compound (Janik et al. 2000a,b). 

The neighboring-group concept of Winstein has been used to explain the difference in reactivity exhibited by anomeric pairs of halides. This suggestion is by no means improbable, and would explain the facts that tri-0-benzoyl-/3-D-ribopyranosyl bromide is 19 times more reactive at 20° in 1 9 dioxane-methanol than is the a anomer, and that the rate for the trans-l3 chloride shows an 85-fold increase over that of the cis-a chloride. However, in view of the properties of the series XXVII to XXX, steric effects from the whole molecule may be equally important, although the disadvantage of this concept is that hindrance as assessed from models is difficult to define precisely. It is of interest to compare the rates of methanolysis of tri-O-benzoyl-a-D-xylopyranosyl bromide and of tri-O-acetyl-a-D-xylopyranosyl bromide [10 fc ° = 51(sec ) in 1 9 dioxane-methanol and = 139(sec 0 in 100% methanol, respectively]. Although the... 

Number of neighboring groups, exchange model also, velocity of light... 

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