Kinetics and Mechanism of Proton Transfer between Disubstituted Benzoic Acids and Carbinol Base of Crystal Violet in Chlorobenzene

Abstract

An investigation on the detailed kinetics of proton transfer between a set of di- and monofluoro- and chloro- (2,3-, 2,5-, 2,6-, 3,5-, 2-, and 3-) benzoic acids (HA) and Crystal Violet carbinol base in chlorobenzene favors a mechanism in terms of fast equilibrium between HA and D to form a H-bonded complex, D...HA, followed by rate-limiting proton transfer along the H-bond to form the colored ion pair DH+A- under the combined influence of monomer HA catalyst, nonreactive cyclic dimer (HA)2 inhibitor, and hyperacidic homoconjugated complex H(HA2) catalyst through a transition state with nearly 60% charge separation.

Kinetics of Proton Transfer between Ortho Substituted Benzoic Acids and the Carbinol Base of Crystal Violet in Toluene. Ortho Effect on the Reactivity of Benzoic Acids in Apolar Aprotic Solvents

“…In fact, acids having one hydrogen bond donor center have their f-exponent within 2.0 ( 0.7 (Table 2). It seems that the presence of as many as three hydrogen bond donor centers in anthranilic acid causes extensive enough homoconjugation in its hyperacidic open chain complex, H(A 3 3 3 (HA) m ) 27 at the acid concertrations employed (10 À2 to 10 À3 M) and leads to such a high f-exponent (6.44). It may be noted that at concentrations below 10 À3 M, the reaction of anthranilic acid is too slow to monitor, whereas the reaction rate of salicylic acid is easily measured at concentrations as low as 10 À5 M. Due to the use of such low concentrations (∼10 À5 M), salicylic acid despite having two hydrogen bond donor centers has a normal f-exponent (1.34) ( Table 2).…”

“…This is exactly what one would expect in terms of the mechanism of the reaction eq 2, the rate controlling step of which is HA catalyzed formation of a benzoate anion as DH þ A À from a hydrogen bonded complex, D 3 3 3 HA. 27 This is favored not only by stronger electron-withdrawing substituents but also by bulkier ones apparently due to a greater relief in the steric strain in the more easily twistable acid anion during its formation from the acid molecule.…”

“…A mechanism for such abnormally slow proton transfers has been elucidated in terms of a fast equilibrium between an acid and a dye carbinol base to form an intermediate hydrogen bonded complex, followed by a rate limiting proton transfer along the hydrogen bond to form a colored ion-pair product under the combined influence of the monomeric acid catalyst, the nonreactive cyclic dimeric acid inhibitor, and the hyperacidic open chain dimeric and trimeric acid catalysts. 27 Another example of such a slow minute scale proton transfer is The Journal of Physical Chemistry A ARTICLE the rate determining step in the reactions of aliphatic and aromatic carboxylic acids with diazodiphenylmethane in different classes of solvents. 11,28À33 Further examples of slow proton transfers are the reactions between bromophenol blue and substituted pyridines in apolar aprotic solvents, time scales of which, however, are much shorter, in the range of milliseconds.…”

“…25,27,33 While the first two reports 25,27 concern a limited number of ortho substituted benzoic acids (five and two, respectively) but with due considerations to complex effects of acid concentrations on the rate constants of their reactions in apolar aprotic solvents, the third one 33 though involves as many as 27 ortho substituted benzoic acids and neglects the pronounced dependence of the rate constants of their reactions on acid concentrations.…”

General acid catalysis in benzoic acid–Crystal Violet carbinol base reactions in toluene

“…Among the different bases tried, a triarylmethane dye carbinol base, particularly Crystal Violet carbinol base, 4,4 ,4tris(dimethylamino) triphenyl carbinol, has been found to be an appropriate choice for the reference base. This colorless carbinol base reacts with a variety of acids (HA) ranging from simple carboxylic acids, phenols, nitro-/fluoroalcohols to complex hydrogen spiroborates in toluene and other apolar aprotic media, producing a colored ion associate [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19].…”