Synthesis and Properties of an Optically Active Complex
Harold R. Hunt, Jr. Georgia Institute of Technology Atlanta 30332
A polarimeter experiment for general chemistry
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Ootical isomerism in oreanic and coordination comoounds is a topic which is commonly discussed ingeneral chemistry lectures, b u t rarely demonstrated in the laboratory because of the relatively high cost of precision polarimeters and the lack of suitable experiments. T h e appearance of inexpensive polarimeters such-= the I2R and the home-madepol&imeter described by Kapauan' has removed the first of these obstacles, and in this article we wish to describe the synthesis of d-tris(1,lO-phenanthro1'me)cohdt(nI)perchlorate d i h ~ d r a t e , ~ which we have successfully adapted for use in the general chemistry laboratory. I n order t o conserve time and chemicals, stoichiometric amounts of C o C 1 ~ 6 H z 0and 1,lO-phenanthroline are used in the synthesis, transferring operations are kept to a minimum, and we recommend t h a t students work in pairs. We have found t h a t students can oerform the exneriment in one three-hour lahoratory period except for the'final weighingof the oroduct, which is a r r o m ~ l i s h e din the followinn period. hec cost of chemicals is nomfnd, amounting to onlyibout 60 cents per student pair.
generator
sample
safety trap
NaOH trap
Fwe 1. m b a tor me chiwine oxldation of me Wit)wmplex. CcmpMwmts: A, rubber hlbing: 8. pinch clamp: C. 125-ml flask; D. 50-ml flask:E, connection
to a~piratw.
The Experiment The synthesis involves two separate steps. The first step is the synthesis of tris(1,lO-pbenanthroline)cobalt(II) ion in aqueous solution, followed by the precioitation of the com~lexas the antimony d-tartrate salt.
Under the given experimental conditionsboth reactions are essentially quantitative. Because of the lability of the Co(I1) complex and the preferential precipitation of one stereoisomer by the antimony dtartrate ion, it is probable that the Co(phen)a2+is present in the product in only one stereoisomeric form. The second step is the oxidation of an aqueous suspension of the Co(I1) compound to form a clear yellow solution of the d - C ~ ( p h e n ) ~ ~ + complex.
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2Co(phen)3[(CdH066)Sb]z. nHzO + Clz 2Co(phen)~~+ + 4Sh(C4HzOs)- + 2C1The reaction proceeds smmthly at room temperature and is complete in about 15 min. Although gaseous chlorine from a cylinder could be used far this purpose, we have found that the chlorine generator illustrated in Figure 1works very well, and also serves to demonstrate the synthesis of Clz by the reaction of HCI with KMnOa.
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8HC1+ 2KMn04 2MnOz + 3C12 4H20 + 2KCl Following the synthesis of d-C0(phen)3~+in solution, the optical rotation of the solution is measured using an inexpensive polarimeter. The comolex is then . ~reeinitated as d-Co(~hen)~lCIO&. 2H10 so . that the &Ad and s~ecifirrotntinn of the okdurtv&m be&lcuiated. and cell volume. Observations are made using a yellow plastic filter,3 which we have found to give results in excellent agreement with those obtained using a Nao interference fdter. Optical rotations of 6' to 12' are normally observed.
Procedure Add30.0mlof1.11 X 1W2M CoCIr6H~Osc~lution toa50-mleon. ical flask containing 0.200 of 1.10-phenanthtoline.H*O.Heat the 710 / Jouml of Chemical Education
Figure 2. Poiarimeter cell. mixture with continuow swirling in a bath of hot water until all of the phenanthroline is in solution. Add 0.33 g of potassium antimony dtartrate to [he hot solurion,swirl to dissolw the solid, and allow the solution to cool slowly for about 20 min to precipitate the greenishyellow Co(I1) product. Next, cool the flask in an ice water bath for an additional 15 min to ensure complete precipitation. While waitingfor the Co(I1) complex to precipitate, assemble the chlorine generator as shown in Figure 1,and place the apparatus in a fume hood for use. Add about 3 g of KMnOa and 15 ml of water to the generator, and 60 ml of 1M NaOH solution to the trap to absorb excess chlorine. When chlorine is needed for the oxidation of Co(II), the reaction is initiated by inserting a medicine dropper filled with concentrated HCI into the rubber addition tube, and then opening the pinch clamp temporarily while squeezing the bulb of the medicine dropper. Add about seven droppers full of HCI in this manner, and then swirl the generator to mix the reactants. Additional increments of concentrated HCI may he added as needed. After the Co(I1) complex bas precipitated for the specified time, decant the supernatant liquid into a waste beaker, retaining the solid product in the conical flask. Then add 16 ml of distilled water to the Presented at the American Chemical Society Southeast Southwest Regional Meeting, Memphis, Tennessee, October 1975. 'Kapauan, A. F., J. CHEM. EDUC., 50,376 (1973). %Lee.C. S.. Carton. E. M.. Neumann. H. M.. and Hunt.. H. R...lr.. Znorg. ~ h e r n :5,1397 . (1966). 3MediumLemon color filter sheet, catalog no. 82,004,available from Edmund Scientific Co., Barrington, N.J. 08007.
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flask, connect i t to the chlorine generator, and begin the generation of chlorine. Swirl the sample flask to keep the CoiII) compound in suspension,and continue the addition of chlorine until the solid has all reacted and a clear yellow solution is obtained. Then remove the rubber tubes connecting the sample flask and safety trap to the generator and NaOH trap, respectively, and connect glass tubeE of the safety trap to a rubber hose running to an aspirator. Using the aspirator, draw a gentle steam of air through the sample solution for about two or three minutes to remove excess chlorine. Before measuring the optical rotation of the Co(II1) solution, quench the reaction in the chlorine generator by flushing the content8 of the generator down the sink in the hood with a generous amount of water. Pour the CoiIII) solution into a clean polarimeter cell until the meniscus extends slightly above the top of the cell. Slide the cover glass into place, and then remove any spilled solution from theoutside of the cell using an absorbant tissue. Place the filled cell in the polarimeter, measure the optical rotation several times, and compute the averaee. I'our the conwnts of the polarimrter cell into a cbnn 30.ml flask, add about 2 g of NaCIOuH7O, and mix well. After a few ~econdu, yellow crystals of d-Co\phen,,nCI041:I.2Hz0will hegin to prrripiute. Cool the mixture in ice for about 30 min, and then collect the product on a previously weighed fdter paper. If necessary, use one or two small portions of the filtrate to wash the last traces of product out of the flask onto the filter. Return all of the filtrate to the polarimeter cell, and add a few drops of water to fill the cell completely before replacing the cover glass. Measure the optical rotation of the filtrate several times, and calculate the average. Wash the product and fdter paper using a few small squirts of water from a wash bottle, followed by three 5-ml volumes of ethanol. Let each portion drain well before adding the next. When the last portion of ethanol has drained completely, open up the filter paper and place it on a clean piece of filter paper to dry to constant weight. Calculate the corrected rotation of the CoiIII) complex by suh-
Per Cent Yield and Specific Rotation of d.Co(phenl,iC10,),~2H,O % Yield
101 , ( g l r n i ) "
dm-'
Studentsa Author Literature6 a A v e r a 9 e leIUit6 Of 29 paill. b~*rlearuredusing yellow plastic filter. C M e a 6 ~ l e dusing Nag interference filter. dsee footnote 2.
trading the average rotation of the filtrate from the average rotation of the Co(II1) solution. Calculate the concentration of the CoiIII) solution using the weight of product and the cell volume. Finally, [a], using calculate the specific rotation of d-Co(phen)3(ClO~)s2HzO, the relation [u]= (corrected rotation, deg)/
iconcentration, glm1)ipoth length, dm)
Results The average results obtained by 29 pairs of students are shown in the table. Although all students were successful in synthesizing an optically active pmduct, good experimental technique is necessary to obtain a specific rotation close to the literature value. Low results are most likelv due to incom~leteoxidation of the Co(I1) . . comdex. . . or to incomple~dryingof the product. The experiment may he extended by measuring the rate of thermal racemization of the CoilII) solution, by observing the catalyticeffect on the racemization of d-Coiphen)a3+,or by measuring of C0iphen)3~+ the specific rotation using a series of color filters to show the dependence of the specific rotation on the wavelength of transmitted light.
Volume 54. Number 11, November 1977 1 711
