Substituent effects bimolecular nucleophilic substitution

The mechanism involves nucleophilic addition to a Z-substituted olefin followed by an intramolecular bimolecular nucleophilic substitution. Several side reactions also occur. Discuss the chemistry involved in this reaction, pointing out substituent effects at each stage. 

The substituent effect of a methyl group on the rate coefficient of the basic hydrolysis of ethylene oxide is small. The kOH values for ethylene oxide, propylene oxide, and isobutylene oxide are almost the same (Table 9). It has been demonstrated by Long and Pritchard [150] with the aid of experiments in oxygen-18 labeled water that hydroxide ion attacks propylene oxide and isobutylene oxide predominantly at the primary carbon. Consequently, the base catalyzed hydrolysis of epoxides is a simple bimolecular nucleophilic substitution. 

Much of the discussion which follows is related to Sjf2 reactions and more specifically to bimolecular nucleophilic substitution at a saturated carbon atom, (Ingold, 1953 Bunton, 1963). Many branches of chemistry have profited from the detailed studies made on this deceptively simple reaction (2), which has attracted the attention of physical organic chemists for many years. Especially notable contributions have been made by Hughes and Ingold (Ingold, 1953). These have led to important advances in our understanding of mechanisms, steric effects, polar substituent effects, salt effects and solvent effects. 

It is generally agreed that both hydrolysis and condensation occur by adder base-catalyzed bimolecular nucleophilic substitution reactions involving, e.g., Sf Z-Si, S 2 -Si, or S 2 -Si transition states or intermediates. The acid-catalyzed mechanisms are preceded by rapid protonation of the OR or OH substituents bonded to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Statistical and steric effects are probably most important in influencing the kinetics however. Inductive effects are certainly evident in the hydrolysis of organoalkoxysilanes. 

Kinetic and substituent effects indicate that the majority of nucleophilic substitution reactions of sulfonyl chlorides follow the bimolecular SN2-type mechanism415,6 involving the linear transition state (72) (Scheme 40) (see Chapter 3, p. 29). 

Rate constants for three distinct competing processes were separately determined solvolysis k, unimolecular k, and bimolecular k2. The Hammett equation with cross-terms was applied to the effects of substituents X in the nucleophile and Z in the leaving group on the analysed rate constants, but in most cases the pxz term was negligible. The Hammett equation with cross-terms has also been applied to the reactions of Z-substituted benzyl X-benzenesulfonates with Y-substituted thiobenzamides in acetone at 45 The findings pz < 0 and pyz > Pxz indicate that this reaction proceeds by a dissociative 5ivr2 mechanism. 

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