1. K. Brice Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061
I I
Lewis Acid Base Equilibria An undergraduate laboratory experiment
Quantitative measurements involvine acid-base eauilibria have been part of the undergraduate laboratory-proeram in freshman. analvtical and ~ h v s i c a lchemistry for many years. he& experiments foi the most part have been limited to proton acids and cations in aqueous solution. since such measurements can be nerformed bv meam of relatively simple, well establishid procedures. Exoeriments desiened to illustrate the broader concepts of Lewis acidity in non-aqueous solutions have been fewer in number lareelv- because of the inherent difficulties of working in such systems where small traces of water may interfere with the measurements. The resent paper describes an experiment involving the s~ectrophot&etric determination of formation constants of Lewis adducts formed between metal halides and organic bases in diethyl ether as a solvent. B: MX, =# B:MX, (1) In contrast to most such systems, these reactions are relatively insensitive to small traces of moisture and work can be performed on an open bench rather than in a dry box
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A number of metal halides react with a variety of organic bases in aprotic solvents to form Lewis adducts (2). The halides are generally monomeric and not ionized in such solvents. The stoicbiometric ratio of the adduct in dilute solution is generally 1:1, though some MX,.2B adducts are known (I). The latter stoichiometry is generally preferred in the solid state by halides where the metal can coordinate two lieands (e.e.. ZnX3 and SnXn) -. (3). . . Adduct formation in &lute sol;tion can be detected by measurine the chance in absorbance (A = loetn(Zo/0 - - ~~, ) a t the wave'length maximum of the base or of the adduct. The absorbance maximum for the adduct is eenerallv found a t a wave length lower than that of the free%ase, an effect similar to the wave length shift produced by protonation of the base in water. In some cases, however, a new band appears a t higher wave lengths as a result of adduct formation. The latter bas been attributed to the formation of non-bonded structures involving relatively weak acidbase pairs (B:,MX,) rather than the electron pair bonded structures described above. The non-bonded structures give rise to longer wave length charge transfer spectra (4). In certain cases, the short wave length band may be hidden by the absorbance of the solvent or metal halide. The present experiment illustratesboth kinds of bonding. The Experiment
The equilibrium constant for reaction (1) can be obtaine.d as follows from absorbance measurements. It is easily shown that
where A, A. and A- are the absorbances of the equilibrium mixture of base and adduct, the pure base and the pure adduct respectively. Equation (2) assumes that the base, metal halide and adduct each obey Beer's law; i.e., that A = a$Bl + a,,.[BMX.I 430 /Journal of Chemical Education
where A' is the measured absorbance of the reaction mixture and the a's are the molar absorbancy indices of the absorbingspecies. Rearrangement of (2) yields
A plot of [MX,]/(Ao - A) versus [MX,] for a set of solutions for which [MX,] is varied, keeping [Blo constant, then yields l/(Ao - A = ) from the slope and 1/K(Ao A,) from the intercept, from which A,.. and K can be calculated. This method of analysis makes i t unnecessary to measure A,, the absorbance of the adduct, which may be difficult to obtain if the equilibrium (1) cannot be shifted sufficiently far to the right. If A, can be obtained independently, then K can be calculated. directly from eqn. (2). In those cases where multiple equilibria are involved, constant values of K are not obtained. Values of K for the reactions in dietbyl ether between ZnB- and meta- and para-nitroaniline can be determined as follows. Stock solutions needed are: 100 ml of 1 M ZnBrz, 50 ml of 2 X 10-3 M m-nitroaniline and 50 ml of 2 X 10-3 M p-nitroaniline, all in dry ether. Small amounts of moisture do not interfere, though tightly stoppered flasks should be used and precautions taken to minimize contact of the solutions with air. High purity grade ZnBrz, aniline base and dry ether can be used without further purification. ZnBrz should be dried a t 110°C for about an hour before use. The ZnBrz solutions can be standardized by determination of bromide by Fajan's method (5) using 0.1 M AgN03. (The standardization procedure can be omitted since reagent grade ZnBrz is sufficiently pure for the purposes of the experiment.) Concentrations of the aniline base solutions need be only approximate since these are not needed for the determination of K. Stock solutions are fairly stable, but should not be used more than a week after orenaration. The absorption spectra of the reactants are determined over the ranee 340-440 m e usine the stock ZnBrz solution and solutions of the aniline bases diluted from their stock M for M for the meta and 4 X solutions to 4 X the para isomer. The adherence to Beer's law may be determined a t the wave length maxima for the bases by measuring the absorbances of several additional more dilute solutions of the reactants. Satisfactory results can be obtained with a Bausch and Lomb S~ectronic20 e . a u.. i~~ed with a Roto-cell. Next. solutions in 10 ml volumetric flasks containine both base and ZnBrz are prepared in which the base COG centration is fixed (4 X M is convenient for the meta isomer, 4 X 10-5 M for the para) and the ZnBrz concentration is varied (roughly between 0.02 and 0.5 M for the meta isomer, 0.1 and 1.0 M for the para). The absorbances of the mixtures change slowly with time (possibly resulting from precipitario" of the addurt or abiorption of water). The absorbance of earh mixture therefore should be measured immediately after its preparation..For two or three of the equilibrium mixtures, the spectrum in the neighborhood of the absorption maximum (340-440 mp) may be determined. Temperature need be controlled only to within a few degrees. A
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Figure 2. Absorption spectra of (a) rneta- and (b) para-nitroaniline and their solutions with ZnBr2 in diethyl ether at 25%. Curve (1) in each case is that of the ether solution containing aniline base only.
Figure 1. Determination of K tor reaction (1) at 25% in diethyi ether from absorbance data (see eqn. (4)). (a) B = rn-nitmaniiine. MX, = ZnBr2: ( b ) B = p-nitroaniline, MX, = ZnBr,.
The equilibrium constant, K, is calculated by the method described above using eqn. (4). (The adduct for the para isomer has a weak absorbance at the wave length maximum of the base; for the adduct of the meta isomer, A, is nearly zero above 340 mp.) The results of student determinations are shown in Figures 1 and 2. Values of K a t 25°C calculated from the data in Figure 1are 10.0 M - 1 for the reaction in dry diethyl ether of the meta isomer with ZnBr2 and 1.15 M - I for that of the para isomer with ZnBr2, in good agreement with values obtained by Satchell(1) (10.0 and 1.2, respectively a t 22'12). The absorption spectra of the two bases and their mixtures with ZnBrz in dry diethyl ether are shown in Figure 2. In the case of the D-nitroaniline-ZnBr? mixtures. another absorption maximum appears at a linger wave length indicating the formation of a weakly bonded adduct, B : , ~ n ~ r z , - w h i cproduces h a charge transfer hand amund 425 mp. The isobestic point a t 375 mp indicates only two absorbing species are present. I t will also he noted that there is a small medium effect on the wave
length of the absorbance maximum for the para isomer. The effect is small enough, however, so that corrections are unnecessary 16). Mixtures of ZnBrz and m-nitroaniline (a stronger base than the para isomer) show no new absorption bands a t longer wavelengths. (Measurements a t lower wave lengths show the appearance of a new band in the uv, corresponding to the formation of a more strongly bound "anilinium" type adduct (I).) The relative basicities observed for subsituted anilines in water toward protons is preserved in aprotic solvents toward Lewis acids (2), as illustrated by the results of the present experiment. (The values of Kb at 25'C for metaand para-nitroaniline are 3.2 X 10-12 and 0.1 x 10-12, respectively (7).) Further, the results illustrate the generalization that Lewis acids in aprotic solvents are much more acidic than protons in water toward organic bases (21, a fact that is taken advantage of in the use of Lewis acids as catalysts in non-aqueous media. The present experiment shows that ZnBrz in diethyl ether is of the order of 1013 times more acidic than is H30+ in water toward the two aniline bases studied. Literature Cited
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". ston.NewYork. 1965. p.220
(61 Cahen. S . G.. Streitwieser. A,. and Taft, R. W., "Bogress in Physical O ~ a n i e Chemisfry,"vol. 1,John Wiley,Neu.York. 1963, p. 2 4 4 (7) hng,F.A.andPaul, M.A., C h m . R e u , 51. 1(19571.
Volume 50.Number 6, June 1973 /
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