U. S. Department of Commerce Research Paper RP1997 National Bureau of Standards Volume 42, June 1949 Part of the Journal of Research of the National Bureau of Standards ·Acid-Base Equilibrium Constants for the Reaction of Tribenzylamine with Picric Acid and with Trinitro-m­ Cresol in Benzene, from Spectrophotometric Data By Marion Maclean Davis and E. Anne McDonald The relative acidic strengths ~ o f picric acid and t rinitro-m-cresol in benzene have been measured spectrophotometrically in terms of their reactivity with the base, t ribenzylamine. The respective constants found for the combinat ion of tribenzylamine with picric acid and with trinitro-m-cresol in benzene=a t 25° Care 1.58X 103 and 4.48 X 102• The same relative order of strengths would be predicted on theoretical grounds. The constant for picric acid is in close agreement wi th previous measurements of the dielectric constant of benzene solutions of t ribenzylammonium picrate. The method used for assessment of the relat ive strengths of the two 'acids is superior:to measurements of their ionic dissociation in water. 1. Introduction as both dono)' and acccptor. But whencvcr the solvent displays aciclic or basic character, the During Lhe first quarter of this century, the nat.ure of the reaction between a dissolved acid definitions of an acid and a base most commonly and base is partially masked. The id eal condi­ taught and applied were founded on the behavior tion for studying the reaction between an acid of aqueous soluLions. As is well known, acidic and ft base would be to u e no solvent at all. and basic properties were rcfel'l'ed, respectively, The nearest prftcticable approach to such a con­ to hydrogen and hydroxyl ions, numero us inves­ dit.io n, when organic acids and bases are con­ tigators undertook the measurement of the ionic cerned, is to use an inert or "aprotic" olvent; dissociation constants of acids and bases in water, that is, a solvent that can neither add nor release and the pH scale came into widespread lise as a protons, Benzene and carbon tetrachloride are means of expressing relative acidities and basic­ examples of aprotic solvents that are suiLftble ities. The currently popular BrjiS nsted-Lowry for such studies . At one time many proponent.s proton-transfeI' theory of acidity and basicity of the ionization theory assumed, from an over­ has clarified the role of the solvent. l It is now emphasis of the role of ions in chemical reactions recognized that water can function both as a and inadequate experimental evidence, that acid­ base and as an acid, and this, of course, is the base reactions do not occur in any medium of reason a continuous pH scale for aqueous solu­ such low dielectric constant. Subsequently, tions can exist. Moreover, the effect of other Hantzsch, Br$?!nsted, LaMer, and others demon­ olvents on dissolved acids and bases is now to strated that instantaneous acid-base reactions some degree predictable, because the electronic OCC llr as readily in an aprotic medium as in an structure of a molecule reveals whether it can ionizing medium. However, widespread aware­ serve as a proton donor, as a proton accepto r, or ness of their work does not yet exist, acid-base reactions in aprotic media remain a neglected 1 'l' bo autbors do not reject the mucb broader Lewis concept of acidity, bu t it offers no ad vantage over tbe Brj!nsted·Lowry concept when bydrogen field of study, and a general tendency persists to acids only are considered, as in tbis paper. Furthermore, acid·base reactions employ, in analytical procedures and as media for that involve tbe formation of a hydroge n bridge do not entirely pamllel reac· reactions, only solvents that promote ionization. tions in which a coordinate covalent bond is formed betwee n the acid and the base. In two previous publications from this B ureau Acid-Base Equilibrium Constants in Benzene 595 [1, 2],2 some of the earlier studies of acid-base dilution of solutions and in the recording of data reactions in aprotic media were reviewed, and has already been described [1 , 2]. In some of the new experimental data, both qualitative and measurements, one cell contained the pure solvent quantitative, were presented. The method em­ and the other the solution under investigation. ployed by us was to study, by means of the However, when working with a considerable ex­ spectrophotometer, the reaction between dif­ cess of tribenzylamine in the solution, an equiva­ ferent acidic indicator dyes and various organic lent amount of tribenzylamine was placed in the bases in benzene and other organic solvents. Of reference cell. Such cases will be noted in particular interest was the discovery that organic section IV. bases of various types- for example, primary, The symbols and terminology used are as fol­ secondary,'and tertiary aliphatic amines- exhibit lows: Ts (transmittancy of the solute) = Tso ln /T,OI ,; specific differences in their behavior with acids as (absorbancy or optical density of the solute) = in an aprotic solvent, such as benzene. These - log lOTs; aM (molar absorbancy of the solute) = differences are masked in an amphiprotic solvent, asf (b XM); b= length in centimeters of the absorp­ like water. Other studies still in progress provide tion cell; M = molar concentration of the solu­ additional evidence of the specificity of the behavior tion [3] . of bases and of acids in inert solvents. Such experiments point to the need for further investi­ III. Materials gations of acid-base reactions in inert solvents, as a prerequisite both to the development of Benzene.- The high grade of commercial ben­ methods for the measurement of acidity and zene used in previous work became unavailable basicity in such media and to a better under­ while these investigations were in progress. How­ standing of acid-base relationships in water and ever, benzene from another source was found to other "active" solvents. be equally satisfactory when dried over Drierite This paper deals with the reaction of tribenzyl­ and then redistilled [4]. amine with picric acid and with tlinitro-m-cresol Picric acid.- A practical grade of picric acid, in benzene. The acid-base equilibrium constants containing 10 percent of water, was recrystallized of the reactions were determined spectrophoto­ twice from boiling water, dried in a vacuum oven metrically. As will be shown, the results are in at 100° C, then precipitated from benzene solution agreement with conclusions arrived at from other by cyclohexane, and finally dried in a vacuum experimental procedures. oven at 80° C. The melting point was 122.1 ° to 122.3° C.3 II. Equipment and Procedure Tribenzylamine.- A high grade of commercial tribenzylamine was recrystallized from 95-percent The spectrophotometric equipment used was ethyl alcohol and then dried in a vij,cuum oven at the same as that described in the preceding paper 80° C. The melting point was 92.0 0 to 92.1 ° C. of this series [2], except for changes designed for Trinitro-m-cresol.-A high grade of commercial protection of the solutions from the heating effect trinitro-m-cresol was recrystallized once from 95- of the light source. The cell compartment was percent ethyl alcohol and twice from benzene. replaced by a new box with double walls at the After drying in a vacuum oven at 80° C, the melt­ bottom, sides, and top. In the space between the ing point was 107.0° to 107.2° C. two walls, water from a constant-temperature bath circulates continuously. The temperature of the IV. Data and Calculations bath is maintained at 25.00 ± 0.1 0 C, and the contents of the cell can be kept at a temperature Reaction oj tribenzylamine with picric acid.- In not more than 0.20 C above that of the bath. the pure, dry state, tribenzylamine and picric acid The light source was also moved about 1 in. are colorless solids and give colorless solutions in farther away from the cell compartment. benzene. When solutions of the acid and the base The procedure followed in the preparation and are mixed, an instantaneous reaction occurs, as , Figures in brackets indicate the literature references at the end of this 3 Melting points were determined in an electrically heated bath containing paper. silicone oil, with an ASTM thermometer for 3-in. immersion. 596 Journal of Research ----_. ------- shown by the appearance of a yellow color.4 The wavelengths shorter than 280 mil, because of the reaction was studied quantitatively by measure­ high absorbancy of the solvent. ment of the spectral transmittancy of solutions A benzene solution of tribenzylamine more that contained a constant amount of picric acid dilute than 10- 3-.111 how's ahnost complete tran - mixed with varying amounts of the amine. A mittancy at wavelengths greater than 300 mJ..! . series of transmittancy curves obtained for such More concentrated solutions exhibi t meas urable solutions are presented in figure 1. Curve 0 is for absorbancy at wavelengths shorter than about 350 mJ..!, as indicated by the transmittancy curve for 5X 10- 3-M tribenzylamine (curve B , fig . 1). Determination of the applicability of Beer's la'w is not easy, because of the steepness of the trans­ 80 mittancy curves for the higher concentrations of ...z w 4 u the base. However, in the range 5 X 1O- -M to n: w 5 X 10-2-M, the values for molar absorbancies 0.. GO agreed within experimental error, and there is no >­ u z reason for expecting the molar absorbancy to « ::: 40 show any effect of concentration. It is not ::E possible to determine with certainty the change (/) z « in transmittancy that occurs when tribenzylamine n: ..