Benzonitrile, proton transfer

Eight generalizations are given arising from world-wide studies of proton transfer reactions in aqueous media carried out over the past twenty-five years. Future directions of research on proton transfer kinetics are predicted, and recent kinetic studies by the authors on proton transfer in nonaqueous media (methanol, acetonitrile, and benzonitrile) are reviewed. 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

The mechanism for acidic hydrolysis of a nitrile resembles the basic hydrolysis, except that the nitrile is first protonated, activating it toward attack by a weak nucleophile (water). Under acidic conditions, the proton transfer (tautomerism) involves protonation on nitrogen followed by deprotonation on oxygen. Propose a mechanism for the acid-catalyzed hydrolysis of benzonitrile to benzamide. 

A benzenoid ylid has been recently generated in the gas phase by collisional reduction of an ylid isomer of benzonitrile cation-radical [154]. Dissociative ionization of o-cyanobenzaldehyde produced a cation-radical (36+ ) that was distinguished by ion-molecule reactions from its conventional [benzonitrile]+ isomer. In particular, 36+ reacts with tert-butylnitrite by NO transfer to form 2-nitrosobenzonitrile, while [benzonitrile] reacts by proton transfer. Collisional reduction of 36 provided a +NR+ mass spectrum that was distinctly different from that of benzonitrile (Fig. 7). 

These data show that unlike calix[6] and calix[8] (negative ApCfi), the proton transfer reaction for p-rerf-butylcalix[4]arene and these amines in benzonitrile is much less favoured (positive ApG ) and these findings explain the low conductivities observed for these systems. Unfortunately, some experimental difficulties are found in measuring the pKd values for p-ferr-butylcalix[4]arene in nitrobenzene and we were unable to obtain quantitative data on this system. 

TABLE III. Equilibria data for the proton-transfer reaction from p-rerr-butylcalix[4]arene and amines in benzonitrile at 298.15 K. Derived Gibbs energies (molar scale). 

Proton transfer from H3O+ to amines is strongly exothermic and reactions are fast. Non-dissociative proton transfer dominates and for some reactions, such as for dimethy-lamine and aniline, this is the sole product channel [40]. Nitriles are similarly sturdy, with acetonitrile and benzonitrile yielding 100% protonated parent species when reacting with H3O+ [40]. 

The selective two-electron reduction of C60 to C60H2 has been achieved by photo-induced electron transfer in benzonitrile-TFA solution with 10-methyl-9,10-dihydroa-cridine (AcrHy.65 The proposed mechanism begins with electron transfer from AcrIU to C60 to afford the radical ion pair (C60 AcrHj ). The strongly acidic AcrHj species protonates Q,(l to afford the C6qH radical which is rapidly converted into the dihydrofiillerene (C60H2) by electron transfer from AcrH in the presence of TFA. The... 

Controlled potential electrolysis at a Hg pool cathode in the absence of proton donors at potentials corresponding to the first electron transfer consumed le/molecule, yielding the radical anion with no C—CN bond breaking. When proton donors were added under otherwise similar conditions, 1,2-dicyanobenzene gave benzonitrile and cyanide ion (2e/... 

More recently, Morris reported the hydrogenation of benzonitrile catalyzed by a ruthenium complex containing a P-NH-NH-P tetradentate ligand (Equation 15.121). The presence of an amine N-H and metal hydride make it likely that the reaction occurs by an outer-sphere mechanism involving two sequential simultaneous transfers of the hydride and the N-H proton, first to the nitrile and second to the imine (Scheme 15.27). This proposal was supported by DPT calculations. The hydrogenation of aryl and heteroaryl nitriles catalyzed by a combination of [Ru(COD)(2-methylallyl) J and DPPF has also been reported to occur in high yields. 

In conclusion, the electron transfer to the protonated anthraquinone moiety occms from the cyclopentadienyl ring ( orbital) of the ferrocene moiety when 1-FcAq is protonated in benzonitrile and from the iron center a g or eig orbital) when 2-FcAq is protonated. 

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