Thermodynamic constants for association of isomeric chlorobenzoic and toluic acids with 1,3-diphenylguanidine in benzene

Abstract

(J a nu a ry 16, 1961) . This p a per ~ep o rts values of !lF25, !lB, a nd fl S25 for t he associa tion of diphenylgua nidine wI t h t he IsomerIC m onochloroben zoi c acids a nd the isomeri c toluic a cids in be nzene from spect rophoto metric m easure men t s at 25 a nd 30 oe, using bromopht ha lein m age nt.a E (3', 5' , 3", 5"-tetrabromophenolpht halein ethyl ester) as t he indicator. The result s a re compared wit h a va ila ble d ata for other d onor-acce ptor a ssociations in aprotic solven ts which Jll clude t. he monomer-dimer equilibrium of benzoic acids, t he association of tert iar y a mines :vi t~ iod ine, a nd t he a sociation of certain ox. vgen bases wi t h p henols. The co mpari sons Jl1d lcate that t he valu e of t he ratio fll-l /298fl S is approxima tely con stant in the followin g as ociations in aprotic solven ts : (1) Association of phenolic or carboxyli c acids wi t h nitroge nollS bases to fo rm h ydrogen bo nded io n-p a irs; (2) h ydrogen bo nd in g of weakly acidic phe nols to nit rogeno us bases; (3) association of t ert ia ry a mines wi t h iod ine. A som ewh at s ma ll e r value fo r t hi s rat io seems to a ppl y t.o most assoc iation s of pheno ls wi th oxygen bases. P ossible appli cations of t hese findin gs in clude est imation of other t hermody na mi c constants whe n on e of t he constan ts flF, flfT, or t::. S is kn own , a nd clarificatio n of the relative importance of ionic a nd covalent con t ribu tions in h ydroge n bond formation.

Proton transfer rate‐equilibria in apolar aprotic solvents: a historical perspective

“…This led them to infer that with increasing strength of the bond between a proton donor and a proton acceptor, an increasing restraint on the motion of the component parts was exerted. These results supported the assertion that log K BHA values of acids measured in benzene were likely to be better indices of intrinsic strengths of acids and bases than their aqueous p K a values . A striking finding by the authors was whereas log K BHA values of Bromophthalein Magenta E and benzoic acid in reference to 1,3‐diphenylguanidine in benzene were nearly the same (5.34 and 5.30, respectively) paralleling similarity in their p K a values (4.15 and 4.20, respectively), log K BHA of benzoic acid as compared with Bromophthalein Magenta E in benzene was found much lower when the reference base was triethylamine (3.58 vs. 4.36).…”

“…These results supported the assertion that log K BHA values of acids measured in benzene were likely to be better indices of intrinsic strengths of acids and bases than their aqueous p K a values . A striking finding by the authors was whereas log K BHA values of Bromophthalein Magenta E and benzoic acid in reference to 1,3‐diphenylguanidine in benzene were nearly the same (5.34 and 5.30, respectively) paralleling similarity in their p K a values (4.15 and 4.20, respectively), log K BHA of benzoic acid as compared with Bromophthalein Magenta E in benzene was found much lower when the reference base was triethylamine (3.58 vs. 4.36). To explain this finding, the authors suggested that diphenylguanidinium ion, (PhNH) 2 C = can form two hydrogen bonds to a carboxylate anion, one to each oxygen, whereas triethylammonium ion, Et 3 NH + can form only one such bond .…”

“…Towards developing a scale of strengths of phenolic or carboxylic acids, Davis and her school made intensive studies on proton transfer equilibria in apolar aprotic solvents involving proton donors and acceptors of varying structure over a period of more than two decades. Their pioneering researches towards development of precision methods for determining strengths of acids and also bases in terms of some appropriate ion pair/associate formation constant, log K BHA for acid–base association remains a landmark in area of acid–base behavior in apolar aprotic solvents . To determine relative strengths of six isomeric (2,3‐, 2,4‐, 2,5‐, 2,6‐, 3,4‐, 3,5‐) dinitrophenols in benzene, they measured log K BHA of their association with triethylamine as the reference base and found 2,6‐dinitrophenol the most acidic and 2,5‐dinitrophenol the least acidic of the isomers.…”

Brønsted Acid—Base Behavior in “Inert” Organic Solvents

“…17, where the dotted, unlabeled line is the one for /><zra-substituted acids. 135, Table 3). 96 The striking reduction of the strength of o-methoxybenzoic acid in benzene is attributed to chelation.…”

“…135 This is not true for ionic dissociation of the acids in water. The ratio AH/TAS (Γ=298°Κ) is substantially constant for the reactions represented.…”