Mlchael R. F. Bazley
and Gordon R. Woolley University of St. Andrews Fife, Scotland
Photochemical Kinetics An integrated laboratory experience Reaction Mixture
I n searching for a single system t o illustrate t h e general principles of photochemical kinetics a n d actinometry, along with t h e determination of quantum yields, we have reexamined t h e photohromination of cinnamic acid.
This chain reaction between hromine and cinnamic acid i n tetrachloromethane solution has been t h e subject of several studies (1-3)and i t has been found t h a t a n oxygen-freedilute (0.001M )solution of hromine and cinnamic acid (0.04 M )will decolorize rapidly i n light of wavelength about 436 nm. In t h e experiment descrihed t h e reaction is conveniently followed a t about 467 n m using t h e photolysis l a m p a s t h e spectronhotometric lieht source. Few photochemical experiments, suitahle for undergraduates. a r e available. Onlv one current lahoratorv manual (.4.) describes such a n experiment which is concerned primarily with a quantum yield determination. T h e experiment described here illustrates well the kinetic determination of the full form of the rate equation including t h e light intensity dependence, together with t h e relative importances of t h e different chain termination steps i n t h e determination of the reaction kinetics. T h e quantum yield for t h e reaction is readily determined, and also t h e student is introduced t o simple vacuum degassing technique. Further, the reagent cinnamic acid may he first synthesized a n d identified by t h e student. This gives the whole experiment a n appeal a s a n integrated laboratory experience.
Ex~erimental Synthesis of Cinnarnic Acid Several methods are described in the literature hut a convenient one giving experience of steam distillation, with subsequent purification by crystallization, is that described by Buckles (5) as an exemolarvuse of the Perkin reaction. Use of a tertiarv amine. triethvlamine, is recommended to avoid misunderstanding i f the mwhani;m of the aldol condensation. The triethylamine catalyzes the eondensationof henzaldehyde with acetic anhydride (details are given in ref. (5)). After refluxing for 24 hr, with calcium chloride drying tube protection, the mixture is steam distilled. Then, after boiling with decolorizing charcoal, the solution is filtered, the filtrate acidifiedwith 6 M hydrochloric acid while hot, and the cinnamic acid deposits on cooling in about 50%yield. The product may be recrystallized from water or ethanol in good yield (mp 133°C) and may be further examined and identified if this is desired. For example, the infrared spectrum ( 6 )may he examined and the various major vibrational bands identified. The nmr spectrum (7) isalsoof value in identification and, if facilitiesareavailahle, the mass spectrum (8)is well known and may he examined. Reagents Sdution\c,f hnmine i n rrdtrtilled letrarhloromerhane were %tored hy titratinn, wnh in sttjpperrd I -I llaik, and were checked p~ri~~dic311y thit,sulplwr, of the ibdinr hhrrated fmm n rdutiun of ptrtassium iodide by the bromine in an aliquot of the bromine solution. For the experiments described below, cinnamic acid (common "trans" fom) was recrystallized several times from ethanol and gave a sharp melting point of 133'C. A 0.04Mstoeksolution wasprepared and stored in another stoppered flask. Apparatus I.ight t'rum n 12-V 100-WQunrtz.Hdogen pn,jectorlnmp uascollimated by a single lens and passed thr~ughn hlur gelatin iiltrr tu
\
Figure 1. Apparatus for degassing and fw filling photolysis cell by rotation ol ground glass joint. produce a parallel beam at 467 nm with a half-height band width of 40 Dm. The beam then passed through a blackened cylindrical ~ y r e x absorption cell of diameter 3 cm and length 7.3 cm fitted with plane optical end windows, and it was monitored by a phototube (RCA 935A3 connected to a simple and galvanometer, The were also displayed on a 25-mV readings on the recorder. The amplifier output was linear and gave the light transmitted direct.lv. -.....~ The same c& filled with only tetrachloromethane was used for the reference readings for incident light intensity, lo. calibration Known solutions of the purified bromine in tetrachloromethane, raneine in concentration from lo-' M to 2.0 X lo-3 M ,e w e a eood straTg6 line when their optical densities (ahsorhances), G a s u r l d on the apparatus described, were plotted against the concentration. The constant, h, in the Beer-Lambert law It = loexp(-hlc) was found to he 339 1mole-' em-'. Because of the large band width used, this is lower than the extinction coefficient of 448 1mole-' em-' found by Bauer and Daniels (3) for monochromatic liaht a t 436 nm.
Photolysis Working in a darkened room to prevent daylight photolysis and under the illumination of a red photographic safelight, the various mixtures of cinnamic acid and hromine in tetrachloromethane were each carefully measured out into the 250-ml degassing flask. The latter was then attached, together with the absorption cell, to the vacuum line (see Fig. 1). The mixtures were frozen in the flask carefully using a liquid nitroeen method . . bath ~ ~ and then deeassed bv~. the freeze-oum~-thaw . . using only a nltary oil pump. Thew npp-ars ro he no danger of the 1la.k breaking a,long na the I'yrrx round-bottomed flack is alwsvs leis than hnlf-full. Neverthcles~,the normal rnfety precautmns ihuuld he taken as for standard vacuum line working. The degassing process was repeated five times which ensured complete degassing and then the evacuated system was flushed with B.O.C. "white spot" oxygen-free nitrogen gas (99.9%purity) and the reaction mixture transferred to the cell by rotation of the flask about its ground glass joint. The liquid runs into the cell which is then removed and quickly stoppered under the nitrogen flow. With the shutter closed the cell is placed in its p i t i o n in the optical train just in front of the phototube. The reaction was started by opening the shutter and allowing the light from the photolysis lamp to fall on the cell. The change in the transmitted light intensity was monitored and displayed on theehart recorder which was set to give a continuous trace tbrouzhout .. the reaction. The shutter was then rlmed and the I . , valur found hy lill~ngthr cell with pnre tetrarhlo. nmwthane and mearurinr the transm~rtPdintrns~t). ~
~
~
~
~~
~~~~
~
~
Volume 54, Number 12, December 1977 / 771
TheVdark"reaction rate was found to be insignificant when compared with the photolytic rate. Results Analysis The recorder trace of transmitted light intensity against time was converted, using Beer's law, to optical density values. Some typical results are given in Figure 2 which corresponds to the starting conditions given in the table. Initial rates were determined by measuring tangential slopes of the initial part of the reaction curves covering about the first 30sof the reaction time. This procedure was used for the different concentrations of bromine and cinnamic acid to determine the order in each of these reactants. (See the table). Neutral density filters allowed a determination of the order in laburm.Typical results from Figure 2 give values of the order in bromine as 0.97, in cinnamic aeid(A) as 0.033, and in labSnrbed as 0.51. These values are estimated to he accurate to within 39'0, are in good agreement with published work ( I ) , and are very close to the rate law
-do = h(Br2)1.0(A)0.0~,b,0.S
dt With this rate law, students should be able to produce a reaction scheme (essentially reactions (2)-(4) below) whichgives the rate law after a simple steady-state analysis. In so doing, the effect of the various termination steps on the kinetics becomes apparent.
Graph 2 [Br,l=117mM
1.2
Graph 3 [Br,l=O.75mM
I
100
200 Time, seconds
Figure 2. Typical reaction curves. Starting conditions given in 1 and 4 are omined for clarity.
300 table. Graphs
Acthometry The experiment may be used todetermine the light source intensity Rates of Reaction and hence the quantum yield of the experiment by standard solution aetinometry. The technique has been fully described (5)and the acRate of cepted potassium ferrioxalate solution actinometer worked very well Concenhation Concenhatian Reaction/ in the apparatus described. Bowman and Demas (101, however, warn GTaph Bromindmole I-' Cinnamic Acid/ Reactions mole I-' s-' of possible problems in this method and recommend using fresh 1:10 Number (X lo-') mole I-' Conditions (X phenanthroline (stored in the dark), and both the addition of buffer before the phenanthroline, and the omission of NHIF. Calculation .... ..... ~.~~ . 2 1.17 0.0057 Normal 1.27 showed that our filtered light source gave about lOI4quanta s-' ~ m - ~ 0.887 Normal 0.0057 at a distance of about 7.5 cm from the lamp and quantum yields for 3 0.75 Neufral Density 0.991 lass of bromine in excess of a hundred are typical. These were calcu4 1.17 0.0114 lated from the initial bromine loss rate and with the labcalculated Filter from lj,,id.., and our experimental absorption coefficient for the % Trans. 62.8 waveband used. The li..v.., ........... was found from the actinometer exoerm m l . The ..rnnlliorre.twn rrrt (91p -9 11 required i t w rdlcwrrl k h t nl,wrptmn w n i ignored. I t s h c ~ l dIre n u t 4 that the quantum yield ABr radicals; for intermediate values of this ratio, we cannot IS drpcndent, :appnjximatrly inwrwly. u p w the rrridual oxyzcn neglect reaction (6) of ABr and Br, and for large values of this concentration ( I ). ratio, t h e recombination of bromine atoms becomes t h e DreDiscussion dominant chain breaking step. From inspection of t h e results in t h e table, i t can be seen In t h e absence of oxygen t h e reaction may he represented t h a t for t h e reaction conditions investigated, t h e ratio of ( I ) by t h e following simple chain kinetic scheme i n which (Brz)I(A) used was of t h e order of 0.1. cinnamic acid is represented by A. From the kinetics described here, this dictates t h a t t h e rate expression should he of t h e form ~~~
A+Br.-ABr.
+
-
(2)
+
ABr. Br2 ABrr Br. (3) T h e chain is carried by reactions (2) and (3). It is stopped by one or more of t h e following
T h e concentration of t h e bromine atoms and t h e free radicals, ABr., rapidly reach their steady state values and under these conditions i t can be shown ( 2 ) t h a t
and t h e full kinetic expression for rate of loss of bromine is -
where
When the ratio of (Br2)/(A) is small t h e most efficient chain stopping mechanism is t h e recombination, reaction (41, of the 772 / Journal of Chemical Education
~
which approximates t o where k = kslh4'/2. However, due t o t h e (kfiks(Br2)lk2(A)) term, one would expect the order in hromine to be slightly less than one. and the order in cinnamic acid sliehtlv ereater than zero. his was indeed found t o he true a n d fol ;he reaction conditions of the tahle i t was observed t h a t
For t h e reaction conditions of the tahle t h e variation in Iabc during t h e first 30 s of t h e reaction was only a few per cent. Verification of the full kinetic expression above can he made by varying t h e (Brt)/(A) ratio above a n d below t h e 0.1 value used here, since a decrease in t h e ratio should give closer agreement with t h e kinetic expression
while a n increase in t h e ratio should decrease t h e order in bromine while increasing t h e order in cinnamic acid. T h e only constraint operating on such a n investigation
would he to maintain the bromine concentration high enough to prevent a large variation of labsorbed with time. The reaction rate for oxygen-free solutions appears to be essentially temperature independent (I). This implies that, for the activation energy parameters, EQ= %E4 and means that good temperature control is unnecessary for the determination of the orders of reaction. Brown and Daniels ( I ) suggest that the inhibiting effect of oxygen must he explained on the basis of some chain stopping mechanism much more efficient than reactions (4), (5)or ( 6 ) and involving the formation of bromo-peroxy cinnamic acid free radicals
Further investigations may he carried out if it is desired to make the experiment open-ended. For example, the effect of oxidation inhibitors on the quantum yield may be studied, errors due to stray light may be investigated, and the kinetic analysis a t significant conversion may he examined. After reacting stoichiometric quantities of bromine and cinnamic acid, the subsequent identification of the dibromo product (and its stereochemistry) is another possible line of investigation. We wish to thank the referees for helpful comments and The Carnegie Trust for a Vacation Scholarship for
M.R.F.B. Literature Cited
These peroxy radicals then subsequently react to give a variety of products (1,2). When the stationary state is reached the concentrations of ABr02, ABr, and of Br radicals are essentially constant and it bas been shown ( I ) that the rate of loss of bromine is
(11 Brown,R. F.,andDsni&,F.. J. Amrr. Chem.Soc, 62,2820, (1911. (2) M8gop.J. L..andDanielr,F., J , Amer Cham. Soe., 62,2826,11940). (31 Bauer, W.H.,andDanieln,F., J. Arner. Chem. Soc., 56,378, (19341. ( 4 ) Daniels. F.. Williams, J. W..Bende?. P..Alherty, R. A,, Cornwel1.C. D..and Harriman, J. E.. '"Emerimontal Physiesl Chemistry." McGraw-Hill, Kwakusha, Tokyo. 7th Ed, 1970, b. 376. ( 5 ) Buekles,R. E . , J CHEM. EDUC..21,210. (19501. 16) Lippincott, E. R., Welsh, F. E., and Weir, C. E., A n d Chem., 33,140,(1961). (7) "Solmted N.M.R. Data:'Themdynamia Research Centre Data Pmjecf.Teras,Serial No.358, 1962. 18) "Eight Peak Index of Masa Spectra."M.S.D.C., A.W.R.E.. Aldermaston, U.K.. IatEd., 1970. Vnl. 11, p. 1226. (91 Caluert. J. G.. and PitL1, J. N.. Jr.. "Photochemistry." John Wiley and Sons h e . . New York. 1966.p. 783. (101 Bawmsn, W. D.,andDemss, J. N.. J. Phys Chem., 80.2434, (1976).
Volume 54. Number 12, December 1977 1 773
