Nucleophile-substituted carbocation reactions, estimated rate constants

Figure 2. Estimated rate constants for reactions of nucleophiles with substituted 1-phenylethyl carbocations, plotted against the effective Hammett constant of the ring substituent with r+ = 2.1 (+) trifluoroethanol (A) methanol acetate anion ( ) trifluoroethoxide anion propanethiol (o) azide. (Reproduced from reference 17.

Rate constants and products of the nucleophilic substitution and elimination reactions of l-(4-metlioxyphenyl)-3-methyl-3-butyl derivatives 4-McOCf,H4CH2CMc2X (X = Cl, OH, O CCfiFs) have been determined in mostly aqueous solvents.153 The absolute rate constant for reaction of the corresponding tertiary carbocation in 50 50 (v/v) TFE-water was estimated as 3.5 x 1012 s 1. 

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. 

---