Rachel Ta-Shma

Solvent-induced Changes in the Selectivity of Solvolyses in Aqueous Alcohols and Related Mixtures

Publisher Summary This chapter deals with the selectivity of an electrophilic carbon centre carrying a nucleofuge (RX) or of a solvolytically generated carbocation species (R + ) toward the two nucleophilic solvent components. The chapter explains the solvent-induced changes in the selectivity values k A /k B in A–B mixtures (A and B being water (W), methanol (M), ethanol (E) or trifluoroethanol (TFE)). The chapter explains the selectivity of a single electrophilic species usually varies when the solvent composition is changed, owing to different responses of the individual rate constants k A and k B to the composition change. Solvent sorting around the electrophile does not explain the trends found for the variations of the individual rate constants. A proper analysis of changes in k A /k B must be in terms of the changes in the individual rate constants. Because the selectivities are derived parameters, the importance of their solvent-induced changes as mechanistic probes will decrease if the values of the individual rate constants at various solvent compositions are known.

On the reactivity-selectivity relationship in the solvolysis of several reactive alkyl halides

Abstract In contrast to previous reports, the selectivities of four carbenium ions R + towards N 3 − and H 2 O (log(K N 3 − /K H 2 O )) change more than the solvolytic reactivities (log K solv ) of their rather reactive precursors RCl.

Nucleophilic attacks on carbon–nitrogen double bonds. Part 4. Substitution of N-arylbenzimidoyl cyanides by amines in acetonitrile and by alkoxides in alcohols

The substitution of the cyano-group of N-arylbenzimidoyl cyanides PhC(CN)NC6H4Y by nucleophiles was studied. With amines in acetonitrile several reactions had showed first-(k′) and second-order (k″) terms in the amine. With EtO– in EtOH, both zero-(k0) and first-order (k′) terms in the nucleophile were found, while with ButO– in ButOH the reaction is of first order in the base. It is suggested that a common initial step in these reactions is nucleophilic attack of the amine, an alkoxide ion, or ethanol on the imidoyl cyanide. The subsequent expulsion of the leaving group may be uncatalysed or amine-catalysed in acetonitrile, and solvent-assisted in the alcohols. The reactiois were compared with those of the corresponding N-arylbenzimidoyl chlorides.

Nucleophilic attacks on carbon–carbon double bonds. Part XIV. Low-element effects and amine catalysis in the substitution of 1,1-dicyano-2-p-dimethylaminophenyl-2-halogenoethylenes by anilines in alcohols

The substitution of 1,1-dicyano-2-p-dimethylaminophenyl-2-chloro- and -2-fluoro-ethylenes by substituted anilines in methanol, propan-2-ol, and t-butyl alcohol is of the first order in the amine for the chloride, and of higher than first order in the amine for the fluoride. The reactions have low ΔH‡ and high negative ΔS‡ and (especially for the chloride), high negative Hammetts ρ values. A mechanism is suggested in which a reversibly formed zwitterion Ar(Ar[graphic omitted;]H2)CX–(CN)2 reacts further by either (a) solvent-assisted expulsion of the halide ion followed by N–H bond cleavage, or by (b) amine-catalysed N–H bond cleavage followed by expulsion of the halide ion. The chloro-ethylene reacts via(a) and the fluoro-ethylene via both (a) and (b). The element effects (kF/kCl) for route (a) are low (3·05–0·71) and decrease with increasing temperature, on increasing the basicity of the amine, and on decreasing the dielectric constant of the solvent.

Nucleophilic attacks on carbon–carbon double bonds. Part XIII. Vinylic substitution of 1,1-dicyano-2-p-dimethylaminophenyl-2-halogenoethylenes by aromatic amines in acetonitrile

The substitution of 1,1-dicyano-2-p-dimethylaminophenyl-2-chloro- and -2-fluoro-ethylenes by substituted anilines in acetonitrile is of the second order in the amine for the fluoro-compound and of increasing first order in the amine for the chloro-compound. The reactions are slower with sterically hindered amines, have low ΔH‡ and high negative ΔS‡, and show high negative Hammetts ρ values. The reaction of the fluoro-ethylene with p-toluidine is catalysed by pyridine. The order in the amine is higher than two in benzene and in cyclohexane. The results are discussed in terms of initial nucleophilic attack to form the zwitterion Ar(ArN+H2)CX–C–(CN)2 followed by competition between (i) expulsion of the halide ion followed by N–H bond cleavage, and (ii) amine-catalysed N–H bond cleavage followed by a carbon–halogen bond cleavage. Process (i) governs the behaviour of the chloro-compound and (ii) the behaviour of the fluoro-compound. The differences are due to the much easier C–Cl bond cleavage.

A kinetic study of competing fragmentation and hydrolyses of phenyl hydrogen α-hydroxyiminobenzylphosphonate—a case of acid mediated inhibition of acid catalysis

The behavior of phenyl hydrogen α-hydroxyiminobenzylphosphonate (E)-2 in aqueous hydrochloric acid solution was examined by 31P NMR spectroscopy and by HPLC. Compound (E)-2 was found to undergo two competing acid-catalyzed reactions. 1) Fragmentation to phenyl phosphate (6) and benzonitrile, similar to the fragmentation of other hydroxyiminophosphonates to metaphosphate examined previously. The fragmentation of (E)-2 was found to be slower by a factor of 4 than that of hydrogen methyl α-hydroxyiminobenzylphosphonate ((E)-1). This phenomenon is interpreted in terms of inductive effects on the suggested metaphosphate intermediate. 2) Compound (E)-2 was found to undergo hydrolytic cleavage of the oxime group giving NH2OH and hydrogen phenyl benzoylphosphonate (4), which was found to hydrolyze to phenol and benzoylphosphonic acid (5). The latter reacted with the NH2OH liberated in the previous step to give α-hydroxyiminobenzylphosphonic acid ((E)-3), which fragmented to benzonitrile and phosphoric acid. The rate of a possible hydrolysis of the phenol group in oxime (E)-2 was shown to be slower by two orders of magnitude than that from ketone 4. This phenomenon is interpreted in terms of acid mediated retardation of acid catalyzed hydrolysis of phenol due to initial protonation of the oxime nitrogen in (E)-2.