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J. P. Byrne The New South Wales Institute of Technolow Broadway. N.S.W. 2007. ~ustralii
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Thermodynamic Data from the Thermochromic Effect An undergraduate physical chemistry experiment
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T h e oreoarations of a number of thermochromic and ohotochromic compounds have been reported recently in thisand other journals (1, 2). Because of t h e unusual color effects which can he induced in these compounds either by exposure to ultraviolet light, or more simply, by heating, these suhstances have been suggested a s interesting preparations for the undergraduate organic laboratory (3).Alternatively their use a s a chemical switch in'chemical gears' has also been described ( 4 ) . Apart from their interest a s chemical curiousities thermochromic compounds provide a very simple system for studying the thermodynamic effect of temperature on a chemical equilibrium. Hecause of the marked color changes induced hy temperature it is an easy task t o follow the change in equilibrium constant a s a function of temperature, using a spectrophotometric technique, and hence determine values of AHo,AGO, a n d ASo for t h e thermochromic reaction. The compound chosen for this study was 1,3,3-trimethylindoline2-spiro-6'-(2',3'-0-naphthopyran), a spiropyran which exhibits both thermochromic and photochromic behavior. This particular spiropyran was chosen because (1) it shows amarked thennochromic effect, (2) it is relatively easily synthesized from commercially available chemicals and (3) . . the reaction resoonsible for the color change - is a simple unimolecular rearrangement in which the colorless spiropyran (I) undergoes a simple ring 18peningprocess to iorm a tauwrneric pink quinone-metbide (11). ~
Figure 1. Specha of a 0.0320 Msolution of spiropyran in toluene as a funotion
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k ~ ,
I
Spiopyran (I) colorless
CH, Quinonemethide (11) pink
At room temperature, solutions of the spiropyran in non-polar solvents, such as hexane, are colorless, since the vast majority of the molecules are in form (I). As the temperature of the solution is increased, ring opening is induced and the solution turns pink as the equilibrium is shifted towards form (11). The colorless form (I) has no absorption peak in the visible region and the pink form (11) has a peak at 535 nm in non-polar solvents,hence the concentration of the ouinonemethide (11) can he determined over an aoorooriate temperature range us&a spectmphotametrictechnique. ~h;spiropyran concentration can then be calculated by difference from the original concentration of the solution, and the equilibrium constant for the rearrangementreaction, at each temperature, can then be determined
K = [Quinonemethide] [Spiropyran]
Experimental Preparation of the Spiropyran 1,3,3-trimethylindoline-2-spiro-6'-(2/.3n) is not available commercially, but can he easily prepared in s one-step synthesis from 1,3,3-trimethyl-Z-methyleneindoline(a FiseWer's base available from Fluka, Buchs, Switzerland) and Z-hydroxy-l-naphthaldehyde, using the method outlined by Bergmann, et al. (5). Since some conversion to the quinonemethide form is likely in polar
Figure 2. Variation of equilibrium constant with temperature
solvents. final recrystallization from n-herane is advisable. This interesting preparation can be incorporared as a student exercise in a prior organic laboratory session. Equilibrium Constant Measurement A solution of the spiropyran of known concentration (-100 mg/lO ml or 0.03 M )was prepared using A.R. grade toluene as solvent. This solvent was chosen because its relatively high hp allows measurement up to 80°C and because of the compound's high solubility in toluene. The solvent must be dry, since the equilibrium is affected by the presence of polar substances such as uater. Spectra were recorded in the range 4t1U-600 nm, using a Hitachi uvmisible recordinp: s~emrophotometerf~ttedwith an electrically heated. thermostaGd Eel1 comoartment. One-centimeter cells, fitted with Teflon stoppers, were used in order to prevent solvent evaporation at higher temperatures. Figure 1shows spectra recorded in the temperature range 3&10°C; thermal equilibrium is attained at each temperature in -10-15 min. The absorption peaks in this region are due solely to the pink quinonemethide, whose concentration at each temperature was determined using Beer's Law and the reported value
Volume 55, Number 4, April 1978 1 267
(6)of the molar extinction coefficient(e = 67,000 mole-' 1cm) for this hand. The concentrationof the colorless spiropyranwas then calculated by difference, using the original concentration of the spiropyran solution and the quinonemethide concentrationmeasured from the absorbance values at each temperature. The equilibrium, in fact, favors the spiropyran form to such an extent (K 10W) that even at the highest temperatures very little of the spiropyran is converted to quinonemethide so that the spiropyran concentration remains essentially constant. The equilibriumconstants were then determined at each temperature using eqn. (1).
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Results
The standard enthalpy change AHo for the reaction was determined from the Van't Hoff plot of log K versus (l/T) (Fig. 2). AGozss was obtained by interpolating a value of log K a t 298K from Figure 2 and applying the reaction isotherm AGO = -RT In K Values of AHo and A S o were obtained from the Gibb's equation
4G0 = 4H" - T4S"
(3)
The results from this analysis are as follows
imum of calculation, since the reaction is a simple unimolecular rearranaement. The values obtained for AHo and AGO are reprodu&le within 1 or 2%. hut A S o is prone to larger errors since it is derived from AHo and AGO whose values are rather close. A number of interesting points can be introduced into the students' discussion of their results. Why is the quinonemethide form pink hut the spiropyran form colorless? Can the signs of AH0 and ASo be correlated with the structures of the reactant and product molecules? What effect would solvent polarity have-on the position of equilibrium in view of the zwitterion structure of the quinonemethide form? This latter noint can..in fact. be elucidated bv further exoeriment on the student's part, if time permits. If the experiment is performed in a less polar solvent such as dodecane then the value of AGO is found to increase to 29.3 kJ mole-', compared with 24.4 kJ mole-' for the slightly more polar toluene. This shows that the formation of the polar quinonemethide is thermudynamically favored in more polar solvents. L~~~~~~
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Acknowledgment
The author wishes to thank Dr. K. L. Ormond for the synthesis of the compound used in this experiment.
AHozss = 30.1 kJ mole-' AGOzs8
= 24.4 kJ mole-'
4Som = 19 J mole-' K-'
Discussion
This experiment has been used in our basic physical chemistry undergraduate course for the past four semesters. I t is easily completed in one 3-hr laboratory session, and has the advantage that the equilibrium constants require a min-
268 / Journal of Chemical Education
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(5) Bergman", E. D., Welmann. A , and Fiseher. E., J Amer Chem Soe., '72. 5 W 9
(19501.
(6) nirsehberg.~., ~iacber.E.,J c h m Soa., 297 (19541.
