Study of the dissociation of the products of some proton transfer reactions in acetonitrile solvent

Abstract

WLODZIMIERZ GALEZOWSKI and ARNOLD JARCZEWSKI. Can. J. Chem. 70, 935 (1992) The conductometric study of the products of the proton transfer reactions of C-acids (nitriles, nitroalkanes, and 2,4,6-trinitrotoluene) with the strong amine bases (I ,l,3,3-tetramethylguanidine (TMG), 1,8-diazabicyclo[5.4.O]undec-7-ene (DBU), 1,8-bis(dimethy1amino)naphthalene (DMAN), and piperidine) in acetonitrile shows their large degree of dissociation into free ions. The dissociation constant values have been estimated at 25'C to be larger than I X lo-' M. This weakens the formalism commonly accepted in spectrophotometric kinetic studies of these systems of reactions, based on the assumption that the product is an ion pair. Spectrophotometric equilibrium and kinetic measurements provided evidence that reverse reaction is a second-order process (pseudo-first order because cation concentration is controlled by side reactions). The influence of the common cation (TMGH+) on the equilibria of the proton abstraction from 2-methyl-1-(4-nitropheny1)-1-nitropropane and 4-nitrophenylcyanomethane with TMG base in acetonitrile at 25'C was examined and was found to be compatible with the assumption of large dissociation of the reaction product for free ions. "Equilibrium constants" estimated by the Benesi and Hildebrand method (which assumes an ion-pair product) decreased with increasing concentration of added TMGH+ cation, but these "equilibrium constants" multiplied by [TMGH+] are constant. The observed pseudo-first-order rate constants of the proton transfer reaction, measured at large excess of the base over C-acid, grow with the cation concentration due to the increase of the backward reaction rate. The concentration of added common cation shows a negligible influence on the observed rate constants of deuteron transfer reaction. Thus, as a result of side reactions, in which extra amounts of cation are formed, some second-order rate constants kfH and also kinetic isotope effects (KIEs) (k:'/kF) that have been measured in acetonitrile can be substantially overestimated.Key words: ion-pair dissociation, proton transfer reactions, kinetic isotope effects. .O]undCc-7-t n e (DBU), 1,8-bis(dim6thylamino)naphtaltne (DMAN) et pipkridine) dans I'acCtonitrile montre qu'ils sont fortement dissociCs en ions libres.A 25"C, on a CvaluC que les constantes de dissociation sont plus grandes que 1 x M. Ces rksultats affaiblissent le formalisme gCnCralement accept6 dans les Ctudes cinktiques spectrophotomCtriques de ces systemes de rkactions, qui est bask sur I'hypothtse que le produit est une paire d'ions. L'Cquilibre spectrophotomCtrique et les mesures cinCtiques fournissent des donnCes suggCrant que la reaction inverse est du deuxitme ordre (pseudo-premier ordre parce que la concentration des cations est contr61Ce par des rCactions parasites). OpCrant B 2572, dans 17acCtonitrile, on a examin6 I'influence du cation commun (TMGH+) sur 1'Cquilibre de la rCactions d'enltvement du proton du 2-methyl-I-(4-nitrophCny1)-I-nitropropane et du 4-nitrophCnylcyanom...

The influence of common cation BH+on the products of reactions between C-acids and strong guanidine bases in acetonitrile solvent

“…The absorption band of the product of the reaction of acetonitrile to 33000 in toluene, while the reduced molar absorptivities of the ion pairs as compared to free anions in acetonitrile are conspicuous (Figs. [1][2][3]. In this paper the molar extinction coefficient of the ion I in acetonitrile is 28 500, which is in accord with Rogne and co-workers" and the value obtained in the reaction of 1 with DBU.'…”

Kinetic and equilibrium study of the deprotonation of 4-nitrophenyl[bis(ethylsulphonyl)]methane by organic bases in acetonitrile in the presence of common cation BH+ and tetrabutylammonium perchlorate

“…Electronic spectra (Fig.1) indicate that the product of reaction of this carbon acid with TBD and MTBD is largely dissociated into free carbanion which has an intense visible absorption at λ max = 446 nm. Similar large dissociation has already been found for other reaction of carbon acids with strong organic bases where the dissociation into free ions reached at least 87 %, in acetonitrile [28]. For these reactions the equilibrium constants are large (Table 1), provided the concentration of the base is reasonably high.…”

Physical Organic Chemistry of Transition Metal Carbene Complexes. 14.1 Thermodynamic Acidity Measurements of Fischer Carbene Complexes in Acetonitrile

“…Even though, as stated above, our results are consistent with the free ion model of eq 9, the question whether ion pairing might be significant, e.g., eq 11, needs to be addressed In a series of papers dealing with the ionization of various carbon acids by amine bases, spectrophotometric equilibrium measurements appeared to be consistent mainly with ion-pair formation (small or zero K d in eq 11) . However, on the basis of conductivity measurements, Galezowski and Jarczewski could show that the spectrophotometric results were misleading and that in most cases C - and BH + exist as free ions rather than ion pairs (large K d ), particularly with B = TMG and DBU.…”