the microscale laboratory Summary of Reagents Added Reagent Concentration Drops Voltage (M) (W (mv) 0.10M 15 0 AgNO3 1.0 M 4 117 NaHC03 1.0 M 4 227 NaOH 4 424 NaCl 1.0 M
[A$]
Keq
l.OE-O1 1.0E-03
8.1E-12
1.4E-05
1.5E-05
6.5E-09
1.8E-10
NH3
8.0 M
2030
435
4.2E-09
1.7E+07
KBr Na2S203
1.0 M
4
565
2.7E-I1
4.3E-13
1.0 M
4
679
3.1E-13
2.4Ef13
KI
1.0M
4
771
8.6E-15
8.3E-17
1.0M
6
1330 2.9E-24
6E-51
Recognizing that [AgtI,f., is effectively constant, rearranging, and solving for [Ag+l,ll : [Ag+l, = lo-['E~,, + 0.059~/0.0591 (The attachment ofmeter wires makes a sign difference. In the external circuit, electrons will travel from the reaction cell (where oxidation of the wire leads to an increase in [&+I) to the reference electrode. If the black lead of the meter (marked negative) is connected to the reaction cell electrode (where electrons enter the external metallic circuit), meter readings are positive. When connected in reverse, they are negative. Switching the electrode leads changes the sign but not the magnitude of the reading.) When only &NO3 has been added to the cell, both the cell and the reference electrode have essentially the same [MI,the cell voltage is 0 (or very nearly 01, and the expression evaluates to 0.1 M. In the silver one-pot reactions, successive additions of reactants lead to the formation of substances and complexes of increasing stability which, in turn, are in equilibrium with ever decreasing concentrations of Agf. In this experiment, successive additions of the reagents and amounts indicated in the table lead to the results shown. [We recommend removing the reference electrode after each reading, and placing it in a well containing the same concentration electrolyte solution (0.1 M KN03) that was used to swell the Soil Moist.] The semiquantitative nature of the experiment is apparent when the various silver ion concentrations observed are compared with those that might be expected on the basis of accepted values for the pertinent equilibrium constants. For example, after the various additions followed by the addition of 4 drops of 1M KI,the [I-] is expected to be approximately 0.036 M (2.5 drops/69 drops total volA164
Journal of Chemical Education
ume) x 1.0 MI. According to the K., for AgI, the [Ag+lin equilibrium with 0.036 M 1- is 2.33-15 (5).The observed value is 8.63-15, about four times larger than predicted. Indeed, equilibrium constants calculated from the silver ion concentration (determined from the cell millivoltage) and the excess last reactant added (corrected for amount reacted and for dilution to total final volume based on drop count) are within an order of magnitude of the literature values available to us. Perhaps the greatest insights come at those points in the experiment where a solid is dissolved in a solution forming a complex ion. At these points, the change in [Ag+lis predicted to be smaU until the dissolving reaction is complete. Chemically speaking, the silver atoms go from one "sink" (such a s AgCl or AgBr) into another (Ag(NH&+ or Ag(S203)2"). The results are rather dramatic - the cell voltage remains rather stable until the moment when all solid dissolves, and that moment is signaled by the clarification of the mixture and a jump in cell voltage. The jump is much more pronounced for Ag(SzO&& formation, since a much lower eauilibrium IAz+l is involved than is expected for Ag' in equilibrium with Br-. The ammonidAeCI reaction is auite slow: 8 M ammonia works better than-6 M, and concekated ammonia is better yet. (This choice is made on the basis of safety concerns.) All of the substances used are toxic. Adequate ventilation is needed when working with ammonia and sodium sulfide. The silver should be recovered bv an aoorooriate .. . procedure (7).We have not tested silver foil, but see no reasons whv it ought not tosubstitute for less widelv available silver wire. W: suspect that reusing the wire frbm year to year is easier than is reusing the foil. Literature Cited 1. Shalthsshiri, B. 2.;Dimen, G. E.: Juegens,P. J. C k m . Edae. 1880.37.813. 2. Shalthsshiri. B. Z. Chrmiml Dm~omtrofione.Universitv of Wlaeonain h a : Madison,wr im, pp 144-149. 3. Anderson. R. H. Papu T-DL15. 'Silver Ion Eqdhrium with N e d Equation aod ~
~~
~
~
~~
~
Nine Equilibrivm ConstantamChem Ed '93, Indianaplia, IN, 1993. (Abstracts, page 53). 4. Rmoks, D.W;Bmoka, H.B.J. Chpm Ed-. 1994.71, A62. 5. Clifford,A.F InorgonlcCkmis~ofQudifafi~~Andy~is,Renti-HaU: Englewod Cliffa, NJ, 1961, pp 448-468.(This is a good s o w e of eqdihfium mnstaots.) 6. Myera, R. J. J. C h .Edur 1988.63.687. 7. "It 11Silver Campaunds"Flinn Colalog, 1993,646.
An Efficient Microscale Procedure for the Preparation of 3,5-Dinitrobenzoates Richard F. smith1 and Gaetano M. Cristalli Hobart and William Smith Colleges Geneva, NY 14456 The oreoaration of 3.5-dinitrobenzoate derivatives is utilized friquently in the qualitative organic analysis laboratory for the identification of alcohols and phenols. The recommended procedures for the preparation of these derivatives employ 3,5-dinitrobenzoyl chloride as the acylab ing agent under reaction conditions that include: heating the reactants without solvent ( I ) ,heating the reactants in pyridine (Z),and heating the reactants under reflux in a solution of kmethylpyridine in cyclohexane (3). '~uthorto whom corres~ondenceshould be addressed. Current address: Department of chemistry,State University College, Geneseo, NY 14454.
Some problems that may arise from the above procedures are described below. The facile hydrolysis of 3.5dinitrohenzoyl chloride by a t m ~ s ~ h e r i c ~ m o i s tlimits ure the shelf life of this reagent. Recrystallization procedures for the ~ u r i f i c a t i o iof ~ a r t i a f l vhvdrolvzed samples of the reagent (3)and p;ocedur& &r the preparation of "fresh" reagent (2)are available. The ~ r e ~ a r a t i o n of the 3,5-dinitrocenzoates of tertiary alcohois dythe reaction of 3.5-dinitrobenzovl chloride in the Dresence of a pyridine base a t elevatedVtemperaturesfrequently fails due to the decom~ositionof the ester bv a n E7reaction. Wilcox (3)has deicribed a procedure fo; the acylation of tertiary alcohols in which the reaction is conducted under miid conditions in the presence of Cdimethylgminopyridine. \iie have found t h a t the microscale preparation of the 5,s-dinitrobenzoate derivatives of alcohols and phenols may be efficiently and simply accomplished by employing an acylating medium of 3,5-dinitrobenzoic acid and D-toluenesulfonvl chloride in ovridine. a n esterification procedure t h a t was describehAfirstby Brewster and Ciotti (4). The acvlatiue agent in this reaction is the anhydride t h a t is iormeiin-situ via the mixed carboxvlic-sulfonic acid anhvdride. Bv this Drocedure, whic6 employs stable reagents, wewere k c cessful in obtaining pure samples of the dinitrobenzoate derivatives of 13 alcohols including the tertiary alcohols: 2-methyl-2-propanol a n d 2-methyl-2-butanol. The average yield of the crude alcohol derivatives was 58%. The procedure also afforded pure dinitrobenzoate derivatives of nine diversely sibstituted phenols. The average yield of the crude phenol derivatives was 51%. ken necessary, purification of t h e crude alcohol a n d phenol derivatives was accomplished by a single recrystallization. Procedure
Add 95 mg (5 mmol) of tosyl chloride to a solution of 106 mg (5 mmol) of 2,Cdinitrobenzoic acid in 0.5 mL of dry pyridine. The reaction mixture is stirred vigorously and cooled in a n ice bath. Add 100 mg (or 0.1 mL) of the alcohol or phenol, stir vigorously and allow the reaction to oroceed while cooline in a n ice bath for 10 min. precipitate the alcohorderivatives by addition of 2 mL of water. The solid oroduct is collected bv vacuum filtration. washed with water and recrystallized, if necessary, from ethanol. Precipitate the phenol derivatives by the addition of 2 mL of 1 M sodium hydroxide to the reaction mixture. Isolate and wash the product as described above. If necessary, recrystallize the product by dropwise addition of N, N-dimethylformamide to a suspension of the solid in 1mL of boiling ethanol. After cooling the solution prepared by the above procedure, collect the crystals by Gatuum filtration and wash with 1mL of cold ethanol. Literature Cited 1. WilLammn. K L.Mocmscalp and Miemamle Oganie Experiments; Heath: Lea. "%on. MA, 1989:p 665. 2, Swner,R. L.; Fuaon, R. C.;Curtin, D . Y.;M o d l , T. C . T h Systematic Idsntificotion ofOgonic Compounds, 6th ed.: W h y : New York,1960:p 157. 6. Wdcm, C . F.. Jr Ezperimontol Organic Chemistry; Macmillan: New York,1984 pp 15&151. 4. Brew*, J. H.;Ciotti,C. J., Jr.J A m Chem. SW. 196.5 77,62144215.
Volume 72 Number 8 August 1995
A165
