The “dimer nucleophile mechanism” for reactions with rate‐determining first step: Derivation of the whole kinetic law and further treatment of kinetic results

Abstract

Overwhelming evidence has been previously reported for the existence of the so-called "dimer nucleophile mechanism" in aromatic nucleophilic substitutions by amines in aprotic solvents, for which the most prominent feature is the fourth-order kinetics (third order in amine) that has been observed with many different substrate-nucleophile systems, especially those in which departure of the nucleofuge is the rate-determining step. The mechanism has been confirmed by several other features, although other alternative mechanisms were suggested to explain the fourth-order kinetics, no one has been able to explain the other above-mentioned features. The present paper affords additional experimental evidence and 736ALVARO AND NUDELMAN derivation of the kinetic expressions for reactions with good nucleofugues, where the first step is rate determining. The work involves studies of the reactions of 2,4-dinitrofluorobenzene and 2,4-dinitrochlorobenzene with aniline and with alkyldiamines in toluene. The novelty of this work lies in the selection of substrate-nucleophile systems exhibiting kinetic behavior that allows estimations of the different k s involved. The satisfactory agreement between the quotients of k s calculated from sets of data obtained under different reaction conditions hereby reported indicates that the assumptions made are correct and that the whole treatment applied to the kinetic data is justified. All together, the results fit well with the reaction scheme involving the dimer nucleophile mechanism, adding new evidence to this mechanism that it is well established in the current literature. C 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: [735][736][737][738][739][740][741][742] 2010

Aromatic nucleophilic substitution in aprotic solvents using hydrogen‐bonded biological amines. Kinetic studies and quantum chemical calculations

“…The obtained results are interpreted by the dimer nucleophile mechanism , which involves the attack of the amine hydrogen‐bonded dimer reacting with the substrate in the first step, superimposed to the classical attack by the monomer. This behaviour was fully confirmed by abundant evidence from several different approaches …”

“…For arginine and agmatine, the intercept in Fig. is null, indicating that these reactions proceed fully by the ‘dimer nucleophile’ mechanism . By the contrary, the intercept for lysine and dopamine is non‐zero (Figs and ), allowing to infer that classical and dimer mechanisms take place in the global reaction.…”

Mechanism and Linear Free Energy Relationships in Michael‐Type Addition of 4‐Substituted Anilines to Activated Olefin in Acetonitrile

“…It is interesting to note that this kind of mechanism involving dimer formation has been reported in the literature, especially for S N Ar reactions of nitroactivated substrates with amines in nonpolar aprotic solvents, for example, the reaction of 1‐halo‐2,4‐dinitrochlorobenzene with aniline in toluene and benzene/ n ‐hexane mixtures , the reaction of 2,4‐dinitrofluorobenzene with O‐anisidine in benzene [9b], and the reaction of 2,4‐ and 2,6‐dinitroanisole with cyclohexylamine in benzene [9a].…”

Practical Chemical Kinetics in Solution

“…As shown in Scheme 4, the reaction involves attack of the amine dimer on the aromatic compound to produce a zwitterionic intermediate. This pre-equilibrium is third order; decomposition of the latter intermediate is rate limiting and is catalyzed by an additional amine molecule, leading to a linear dependence of (k 3 ) on amine concentration [42]. Another example of third-order reaction is the acetylation of cellulose in a mixture of the ionic liquid (IL) 1-allyl-3-methylimidazolium chloride, AlMeImCl, and N,N-dimethylacetamide or acetonitrile (Figure 4).…”