edited bv
tested demonstrations Visual Demonstration of Solvent Polarities Submitted by
Dawn A. Johnson. Roosevelt Shaw. and Ernest F. ~ilversmith' Morgan State University Baltimore, MD 21 239 Checked by
James Ealy The Peddie School Hightstown, NJ 08520 Solvent polaritv is a fundamental and often-used wncept in &d organic chemistry. Anumber of experimental methods for assessiw solvent polarity have been developed ( 1 ) . One of these,-introduced by ~osower,involves the measurement of W-visible absorption maxima of solutions of I-alkvl-4-carhomethoxMvridinium iodide in different solvents (5).Later, ~ i m r o t i & dReichardt found that. when 4-(2.4.6-trinhenvlnvridinio)-2.6-dinhenvlnhe... . nolate (1, also known us Dimruth's or keichardtk dye, is used. the maxima fall in the visible rehon and chance considerably with solvent polarity (3-5). Ls a result, th; solutions show visible color differences as solvent polarity is ~~~
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changed. ~ h t dye ? costs about $60/g; however, if an overhead proiector is used. the amount needed is so small that the cost 'bf the demonstration is less than $1.
GEORGE L. GILBER; Denison University Granville. OH 43023
Dlscusslon The colors in beakers A, B, and C show that the relative polarities are methanol z ethanol > 2-propanol. This is related to the relative sizes of the polar (OH) and nonpolar (Rgroups) parts of these molecules. The changing colors in beaker D show that the polarity of solvent-water mixtures increases with the ~ercentazeof water. The explanation bf the coibr changes is based on the assumption that light absomtion bv the dve results in the transfer of an ecctron from the negative to the positive region (2-6). Thus, the excited state is less polar than the ground state. As a result, polar solvents stabilize the ground state more than the excited state, so the energy difference (a) between these states is increased. Hence, the more polar the solvent the shorter will be the wavelength absorbed. (Wavelength is inversely proportional to a.) AS solvent polarity increases, the light absorbed should change fmm red (long wavelength) to orange to yellow to green to violet (short wavelength). The color of the solution is the complement of the color absorbed, so solution wlor should change from green to blue to violet to red to orange to yellow. This is consistent with the results of the demonstration. Hazards and Dis~osal Methanol, ethanol, 2-propanol, and acetone a r e volatile and flammable. The beakers should not be left on the overhead projector longer than necessary. Methanol is poisonous. The other three solvents a r e not considered t o b e toxic, though hallucinations, headaches, a n d other discomforts c a n result from long exposure. T h e d e m o n s t r a t o r s h o u l d minimize breathing of the vapors and should dispose of the solutions a s soon a s possible. They may be washed down the sink with water. Literature Cited 1.Mareh, JAduondOlgonle Chemistry,4th ed. John Wiley: New Yorh, 1992,pp 36& x 9
Materials Needed Four 30-mLbeakers (A, B, C, and Dl,each containing 2 mg of Reichardt's dye Overhead projector Samples of methanol, ethanol (absolute),2-propanol,acetone, and water Pasteur pipets A chart showing that, as solvent polarity increases, solution color changes from green to blue to violet t o red to orange
2 K c.& E M. J.Am Chem .% 1868.80 32533260 3 Duoroth. K.Rerhnnll, C % rM Chom 19M.215.344350. 4.D1mroth.K:Retchrrdr.C .S~~pmdnn.T.Bonlma~.F LtebwsAnn Chem 1988.661. ,-,7
5.Rei;hrddt.
C.anPanp. Chem, ht Ed. En& 1979,18,9&110. C. J.J.Og.Chem. 1992,57,35783582.
6. Coleman, C.A.;M-y,
Extraction of Organic Compounds from Aqueous Solutions Submitted by
Brian Love Auburn University Auburn, AL36849
Demonstration Place the beakers on the projector and add 4-mLportions of methanol, ethanol, 2-propanol, and acetone to beakers A, B, C and D, respectively. Shake the beakers. The solutions will be red-violet. violet. blue and blue-ereen.,resnectively. Using a pastem pipet, add water,-dropwise, to beaker D. Shake evew few drops. The color will chanee to blue-violet, violet, redkolet, &d, and finally orange.
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'Correspondence may be addressed to this author.
Checked by
Wayne C. Wolsey Maca aster Co lege St. Pa~l,Mh 55105-1899 Of all the sewration techniaues available to the oreanic chemist, extraction is one of the most com&only employed. During a "survey" organic chemistry course (which had no associated lab), the need for illustrating this Volume 71 Number 6 June 1994
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technique to a class of mughly 100 students arose, so the following demonstration was developed. Procedure Two solutions are DreDared . . (see "Materials"section) and am shown to the class. Solution A is a yellow solution of 2.4-dinitroohenvlhvdrazine t2.4-DNP, in 334 aaueous methanol, and shution B is a reddish-Pink solution of cobalt(I1) chloride in the same solvent. Solubility properties of the two solutes can be discussed at this stage, as well as the solvating ability of the solvent system. (It is "organic enough" to dissolve the 2,4-DNP, while "aqueous enough" to dissolve the CoClz). Solution A and Solution B are then mixed to give an orange solution, and the students are asked to consider the difficulty of separation of the orange solution back into its red and yellow components once the two have been mixed. (Apaint mixing analogy often is useful here. It is one that the students can appreciate readily.) Because the "pigments" in this mixture have distinctly different solubility properties; however, the two can be separated simply by extraction with an organic solvent (in this case, diethyl ether). To illustrate this, the orange solution prepared by mixing Solutions A and B is poured into a 500-mL separatory funnel, followed by 125 mL of diethyl ether. The funnel is shaken (with careful venting!) to produce two layers, the upper of which contains the 2,4DNP and is bright yellow, the lower is the pink color of the original CoClz solution. Thus, the students are given a striking demonstration of the separation of two components of a mixture via extraction. (If desired, small por-
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tions of the original solutions can be retained for compariUT~~RPR). snn ..-D r-r----,. Note: If a sellowkreenhlue color scheme is referr red. a solution of cippertli~ chloride 1"Solution C") can be sub& tuted for the CoCL solution, althoueh the color differences in this demonstration are not quite-as distinct. Materials SolutionA. 75 mL of approximately 2 x 104M 2,4-DNP in 33% aqueous methanol. Solution A is best prepared by dissolving 0.003 g of 2,4-DW in 25 mL of methanol, followed by dilution with 50 mL of water. Solution B. 75 mL of approximately 0.135 M CoClz in 33%aqueous methanol. Solution B is best prepared by dissolving 2.4 g ofCoC1~6HzOin 50 mL of water, followed by dilution with 25 mL of methanol. Solution C. 75 mL of appmximately 0.135 M CuClz in 33%aqueous methanol. Solution C is best prepared by dissolving 1.7 g of CuClz-2Hz0 in 50 mL of water, followed by dilution with 25 mL of methanol. Safety and Disposal Considerations Both methanol and diethyl ether are highly flammable (especially the latter); therefore, there should b e n o open flames present. Gloves and protective eyewear should be worn during the demonstration. Because the organic phase of the extraction contains diethyl ether, it should be collected as waste solvent and disposed of as described in Prudent Practices for Disposal of Chemicals from Laboratories;National Academic Press: Washington, DC, 1983.
