Substitution reactions, inhibition second-order

Reaction of an alkyl halide or tosylate with a nucleophile/base results either in substitution or in elimination. Nucleophilic substitutions are of two types Sn2 reactions and S l reactions. In the Sn2 reaction, the entering nucleophile attacks the halide from a direction 180° away from the leaving group, resulting in an umbrella-like Walden inversion of configuration at the carbon atom. The reaction shows second-order kinetics and is strongly inhibited by increasing steric bulk of the reactants. Thus, Sn2 reactions are favored for primary and secondary substrates. 

Radical decay kinetics have been shown to be 3/2 order, falling to first order, and also second order, falling with time deviations are apparently due to side reactions of 118. Radical half-lives are strongly influenced by the nature of the aryl substituents, being particularly short for ortho-substituted Ar because of inhibited delocalization. The corresponding compounds 39 have, accordingly, enhanced thermal stability, a factor useful in some commercial thermo- and photographic processes. 

Aqueous cationic micelles speed and anionic micelles inhibit bi-molecular reactions of anionic nucleophiles. Both cationic and anionic micelles speed reactions of nonionic nucleophiles. Second-order rate constants in the micelles can be calculated by estimating the concentration of each reactant in the micelles, which are treated as a distinct reaction medium, that is, as a pseudophase. These second-order rate constants are similar to those in water except for aromatic nucleophilic substitution by azide ion, which is much faster than predicted. Ionic micelles generally inhibit spontaneous hydrolyses. But a charge effect also occurs, and for hydrolyses of anhydrides, diaryl carbonates, chloroformates, and acyl and sulfonyl chlorides and SN hydrolyses, reactions are faster in cationic than in anionic micelles if bond making is dominant. This behavior is also observed in water addition to carbocations. If bond breaking is dominant, the reaction is faster in anionic micelles. Zwitterionic sulfobetaine and cationic micelles behave similarly. 

Ag+-catalysed isomerization of 4-substituted homocubanes (5) to norsnoutanes (6) also proceeds via pre-equilibrium complex formation and follows second-order kinetics. In the case R= Me adherence to Michaelis-Menten kinetics could also be demonstrated. C-4 Substituents capable of resonance interaction in the cationic transition state promote deviations in the rate of reaction relative to substituents which exhibit inductive effects only. With R=Bu bond-switching is reduced in rate, presumably because of steric inhibition of Ag+ attack on the homocubane to give an intermediate analogous to (4). Placement of deuterium or CDg at C-4 produces only a minor inverse kinetic deuterium isotope effect (kH/kD=0.97) which implies that a completely free carbonium ion intermediate is not involved and so argues in favour of a delocalized species analogous to (4). 

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