The Photochemical Synthesis, Kinetics, and Reactions of Nitrosomethane Dimer A Physical-Organic Experiment H. Kozubek, B. Marciniak, and S. Paszyc' Institute of Chemistry, A. Mickiewicz University, 60-780 Poznah, Poland Theory When a molecule ahsorhs energy, the process is referred to as excitation. The molecule is raised from its ground state of minimum energy to an excited state of higher energy. Excited singlet and triplet states which are formed will either undergo chemical reaction or lose their energy by radiative or nonradiative processes A+hv-A* A*-A+hu A*-A A*-B
excitation emission of light nonradiative processes photochemical reaction (formation of product B)
(1) (2)
(3) (4)
Photochemical reaction is, therefore, one of the routes of deactivation of excited molecules. The efficiency of a photochemical process is defined by a quantum yield. The quantum yield may he expressed as the ratio of the reactant (A) disappearance or product (B) formation to the intensity of light ahsorhed by the reactant (A) ( I ) dnaldt 'PA=-IeA
where nA, n~-number of molecules of reactant (A) and product (B) IaA-intensity of light absorbed by reactant (A). From eqn. 5 it is clear that the quantum yield also may he defined by the ratio of number of molecules decomposed (or product formed) to the numher of photons ahsorhed by the reactant (A). Both quantities refer to time and volume units. In practice, very often the so-called integral quantum yield is applied which is given by the following expression:
where I. = IaAIuand u is a volume of irradiated sample. Assuming that the reactant is the sole absorbing species and introducing the well-known Lamhert-Beer law, the above equation can he expressed as follows
where lois the intensity of incident light; r, the absorption coefficient; 1, the light path (the cell length); and c, the concentration of the absorbing substance. Equation (8) can he simplified for two different cases: (1) When the reactant ahsorhs almost completely the incident light (I. =lo), i.e., when rlc >> 1, one obtains
which can be integrated to e(0) - e(t) = q ~ l d
(10)
where c ( 0 ) is the initial concentration of the reactant. The photochemical reaction then follows a zero-order rate law. (2) When rcl is lower than 0.1, 10-"~ can he expanded in a series and neglecting higher order terms of the series one obtains
which can he integrated to
In such a case, photoreaction follows a fiist-order rate law. The course of such a photoreaction is presented in Figure 1.
' Author to whom correspondence should be addressed
of reactant (A) by intensity of light absorbed Substituting IaA by irradiated system (reactant and products) the overall quantum yield may be obtained (2). The primary quantum yield may be ohtained by the extrapolation of the overall quantum yield to zero percentage conversion of reactant (A). According to the definition of quantum yield (eqn. 5), the rate of reactant disappearance in the course of the simple photochemical reaction hu
A +B is given by
h"
Figure 1. The course of photochemical reaction A +B on irradiation wilh A = 313 nrn (t < tl reaction follows zero order rate law, t > t reaction follows first-order rate law).
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889
Photochemical reactions sometimes play a significant role in the synthesis of organic compounds particularly when thermal synthesis is difficult to perform or its efficiency is low. They have some advantages over thermal, catalytical, and other methods due to selective activation of individual reactants. specific reactivitv of electronicallv excited molecules. low theimal load on the reaction system; etc. Photochemical synthesis can he exemplified by nitrosomethane dimer formation from alkyl nitrites (3).The bond e n e r w of RO-NO in alkvl nitrites is 170 kJImol. Therefore. theycan he photolyzed easily with wavelengths A = 254 n& (470 kJ1mol) or 366 nm (327 kJImol). RON0
+ h~
-
RO* + NO
(13)
T h e quantum yield of the above reaction in the case of tBuONO in the gas phase is 1. The hot radicals RO* formed in this case are known to decom~oseto ketone and methvi radical (CHhCO*
+
CHaCOCH3 + CH3
(14)
Further reactions lead to nitrosomethane dimer formation:
+
CH3 NO 2 CH3NO
-
-
CH3N0
(15)
(CH3N0)~
(16)
T h e total reaction which takes place during the photolysis of t-BuONO is
The Experiment PhotochemicalSynthesis of Nitrosomethane Dimer
Nitrosomethane dimer is obtained by the photodissociation of t-BuONO as described in (7). Low pressure immersion mercurv lam^ emittine A = 254 nm (mercurv resonance radiationj, e . g . ; ~15732 ~ ~ Original ~ k a ~ug l a m enclosed p in the reaction vessel (Fie. 2) is used for the irradiation of the . reactant. Procedure
Place approximately 1ml of t-BuONO in the reaction vessel and irradiate the sample for 3 hr at 40% (The compound vaporizes and nitrosomethane dimer crystals deposit on the inner walls of the reaction vessel.) Collect the crystals and recrystallize them from pure ethanol. Measure the UV spectrum in ethanol in the spectral range 220-400 nm trans(CHsN0)~A, = 269 nm, cis (CHZNO)? X,, = 283 nm (8).Store the compound in a refrigerator. Photochemical Reactions of (CH3N0)2
In this experiment the quantum yields of isomerization and decompositkm of trans niirosomethane dimer irradiated with X = 313 nm is described. The optical bench svstem with source of light and suitable filter (9jpermitting the passage of 313 nm radiation was used (Fig. 3). [In this experiment the authors used an interference filter UV KSIF 313 (Zeiss Jena).] Uranyl oxalate actinometry was used for the measurement of the incident light intensity lo (10). Beckett and Porter's (11) equation was applied for the estimation of light intensity ahsorbed by the investigated system
Nitrosomethane dimers are photochemically active (4-6)
AA is where l o is intensitv of incident light (auanta/l.min.) .. the absorption change caused by exciting light and T(o), T ( t ) are initial and time-dependent transmittances. Procedure
Fill a l-em quartz cell with an ethanolic solution of trans nitrosodimer (c = -l.10-4M) and fix it on the optical bench. Irradiate a sample with UV light and measure the absorption values at X = 269 nm and X = 283 nm for various times of exposure (10-60 min). The temperature of the irradiated solution should he maintained in the range of 20 i 1°C. Calculate and plot cis and trans nitrosomethane dimer concentrationfor different times of irradiation. Determine the ordrr of the rewtiun km~tiraf u r ashart time of irrndintion andcal. culatc the rate constant of nitrusumcthane dimer photodccumpuait i o n Estimate the quantum yield uf trans n:rrosomrthane dimer decomposition and cis one formation for various irradiation times. Appropriate runs should be made for the actinometer. Extrapolate quantum yields obtained to zero percentage conversion of trans dimer. For the quantum yield determination a darkroom is required. ~~
~
Results and Discussion The trans dimer (CH3N0)2was obtained after irradiation of t-BuONO and recrystallization of the precipitate formed.
---------------- - - - - -. Figure 2. An immersion reactor 1. immersion quartz lamp 2 . cylindrical qumz envelope 3. water jacket 4. reaction vessel 5. sample (t-BuONO) 6. water thermostat bath
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Journal of Chemical Education
~igure3. An apparatus suitable for quantitative work 1. Source of light (100 W w 200 W spherical high pressure lamp) 2. diaphragm 3. thermal filter (quanr cell tilled with distilled water) 4. qumz lens 5. filter 6. quark reaction cell 7. thermostat iacket
mercury
Ahsorotion maxima of trans and cis dimen in ethanol are 283 nm and 269 nm, respectively. The trans dimer was irradiated with X = 313 nm as desrrihed earlier. The intensity of incident light, estimated by means of uranyl-oxalate ac&ometry, is 1.3.1019 auantam-'-min-'. The molar coefficients values for trans and cis nitrosomethane dimers are A283 = 10600 M-lcm-l, A269 = 7900 M-'cm-I and Xxs3 = 6250 M-lcm-l, A269 = 8640 M-'cm-I, respectively. Hence, the concentrations of trans and cis dimers for various time of irradiation may he estimated. Concentration changes for trans and cis nitrosomethane dimers versus the time of irradiation are presented in Figure 4. The quantum yields of trans dimer disappearance and cis dimer formation were calculated from the concentration chanees - unon . irradiation, intensity of light absorbed by dimers, and eqn. (19). The values obtained were extranolated to zero nercentaee of reactant conversion. The Quantum vields of trans nitrosomethane dimer disappearance and cis dimer formation are 0.35 and 0.26, respectively. These results suggest that the main photoreaction in the system investigated is isomerization of trans nitrosomethane dimer. The decomoosition of the dimer is neeligihle. The ex~eriment.as described here. has been in use for more than two years in our laboratory andhas shown a high degree of reliability in student hands. There are very few photochemical experiments suitable for use in teaching laboratories and this exoeriment brings students into contact with some problems or physical andhrganic photochemistry. Acknowledgment
Literature Cited p.
134.
2.
~on~enlralion of cis dimer .,B a h i V., Momi.L., '"AdvaoeeinPhotaehamisw."V.9., L.
1374, P. 147.
Interasienn,
Onmic
A,,.."Preparative Phatachhhietry." Springer-Vetlag, Berlin, .~ ~eidelberg,New Y0.k. 1368, p. 252. (4) Gowien1ock. and Ll3Ltke. W.. Ouon. Re".. 12.3'21 ~
The authors thank Professor M. Vala (Denartment of Chemistry, University of Florida, ~ainesville;~ l o r i d afor ) the critical reading of the paper and for valuable comments. 11) M a w . H.. '"Formale Kindik:' BerteIsmnn Uniwrsit6t verb& DIiaaeldorT,
Fbwe 4. Concentration chame of trans and cis dimers ICHINOIO durino irra-
B. G.. (19581. (51 Farnsledt,L.,Hekman,M.,~indquit,S..~ho~.~er,12,93(1977l. (61 Kozubek. H..Pasrye.S..Bull. Ac. PoLSci. Chsm.,XXV111.481 11980). (71 Drake, N. L.. Or* Synfheser. Wiley Inc., New York, 24.26 (1944). (8) Gowonlock, B. G..Trotman,I.. J Chom. Soc.. 1670 (1956). (9) ca1vert.d. G. and Pitu. J. N.. "Photaehemi.try," John Wiley, New York, ",m
1974,
7""
Muruu, S. L.,"Haodbaok of Photochehistry." M. Dckker Inc, New York, 1973, p. ,"A
(111 Becketf A,, Porter, G. B.. ROW. Faradqy Soc., 59,2038119631
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Number 10 October 1982
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