The kinetics and mechanism of n-butyraldehyde photolysis in the vapor phase at 313 nm

The Kinetics and Mechanism of n-Butyraldehyde Photolysis in the Vapor

Phase at 313 nm

S. FORGETEG, T. BRRCES, andS. DOBE Reaction Kinetics Research Group, Hungarian Academy of Sciences, 6701 Szeged,

Hungary

Abstract

The photolysis was investigated at 313 nm wavelength,253-529 K temperatures, and 4 X 10-"-2 X mol.photon/cm%ec light intensities by determining the quantum yields of 20 reaction products. Primary quantum yields for the seven primary processes and rate constant ratios, rate constants, and Arrhenius parameters for secondary processes were derived on the basis of the suggested reaction scheme. The dependence of the quantum yields of the four major primary processes on experimental conditions was established.

Introduction

A detailed study of the vapor-phase photolysis of n-butyraldehyde carried out by Blacet and co-workers [l-31 more than twenty years ago eluci- dated many important features of the reaction. The major primary processes were identified and primary quantum yields were obtained, using the iodine trapping technique in case of reactions yielding free radicals. A kinetic study of the secondary processes at higher temperatures supplied rate constants and Arrhenius parameters for some of the important free- radical reactions [4].

The detection of cyclobutanol among the products of n -butyraldehyde photolysis in benzene solution [5] on the one hand, and the possible photophysical effect of iodine [6] on the other hand made desirable the rein- vestigation of the primary quantum yields by methods different from the iodine-trapping technique. Furthermore, very little quantitative infor- mation is available on the kinetics and mechanism of the secondary free- radical processes at low temperatures. Kerr and Trotman-Dickenson carried out their kinetic investigations [4] at higher temperatures where chains are long and radical-radical processes are of minor importance.

A systematic study of the primary and secondary photochemical reactions of the vapor-phase photolysis of n-butyraldehyde between 253 and 529 K has been initiated in our laboratory. The preliminary results on

International Journal of Chemical Kinetics, Vol. XI, 219-237 (1979) 0 1979 John Wiley & Sons, Inc. 0538-8066/79/0011-02 19$01 .OO

220 FORGETEG, BERCES, AND DOBe

product formation at 313 nm and room temperature [7] revealed the formation of altogether 20 reaction products and proved the occurrence of seven primary processes:

(1)

(1’)

(11)

C3H7CHO* - C3H7 + CHO

C3H7CHO* - C3Hg + CO

C3H7CHO* -+ C2H4 + CH3CHO

(11’) C3H7CHO* - cyclobutanol

(111) C3H7CHO* - CH3 + CH2CH2CHO (111’) CsH7CHO* - C2H5 + CH2CHO

(IV) C3H7CHO* + C3H7CHO - C3H7CHOH + C3H7co

By studying the dependence of characteristic product yields on pressure, it was shown [8] that 411 and 411~ increase while 41 decreases with increasing pressure, and a detailed mechanism of the primary photochemical and photophysical processes was suggested.

In this paper we describe the influence of the light intensity and temperature on the product quantum yields, report on the mechanism of secondary processes at low and medium temperatures, and establish the primary quantum yields as well as the rate constants of free-radical reactions.

Experimental

Materials, product identification, and quantitative analysis, as well as experimental technique have been described in detail in a previous publication [7].

The n-butyraldehyde (Fluka), purified by precipitation and repeated distillation in vacuum, contained about 0.5% of isobutyraldehyde and trace amounts of C3Hs and C2H4.

The reaction was carried out in cylindrical quartz cells equipped with planparallel Ultrasil windows on both ends and with Teflon valves. The internal diameter was 36 mm and the length of the optical path was either 25 or 50 mm (depending on the aldehyde pressure). Irradiation was made with a parallel light beam of an OSRAM superpressure mercury arc sta- bilized by a feedback control to give an intensity stability of about f0 .2 percent. A band in the 313 nm region was isolated using a filter combination (3-nm width at the half-height of the band). The intensity was varied by means of wire sieves or ORIEL neutral density filters. Intensity measurements were made by ferrioxalate actinometry or with a calibrated vacuum phototube (Pressler DGL 490a).

After irradiation the reaction cell was sealed to a vacuum line and the

PHOTOLYSIS OF n-BUTYRALDEHYDE 221

products were separated into two parts by leading the mixture through a trap cooled to liquid air temperature. The noncondensable fraction was collected and measured in a gas burette and submitted to analysis [7], while the condensable sample was dissolved in 1 cm3 isooctane containing (iso- pentane and cyclohexanone) internal standards. The composition of the isooctane solution was determined by gas chromatography using various columns [7]. Formaldehyde and glyoxal were analyzed by spectrophoto- metric methods [7].

All the quantum yields given in this paper are averages of three or more determinations based on runs carried out up to 2%-7% conversion.

Results and Discussion

Apart from the polymer compounds (see below), altogether 20 reaction products can be identified in the vapor-phase photolysis of n-butyraldehyde at 313 nm wavelength [7]. These are: C2H4 (0.17) CH3CHO (0.16) CYC~O-C~H;IOH (0.026)

C3Hs (0.34) C3Hs (0.019)

(C~H;I)~CO (0.007) (C3H.iCO)z (0.00005) C~H~CH(C~HS)CHO (0.003) Hz (0.012) CH20 (0.021) (CH0)2 (0.001) CH4 (0.0004) C2H6 (0.0002) n-C4Hlo (0.0007) n-C5H12 (0.0003) n-C4HgOH (0.02) (CsH7)2CHOH (0.02)

CO (0.55)

n-CGH14 (0.10)

C ~ H ~ C H ( O H ) C O C ~ H ~ (traces) The figures given in parentheses indicate the product quantum yields obtained at room temperature with 100 torr initial aldehyde pressure. The dependence of the quantum yields of these products on the aldehyde concentration (given in a previous publication [S]), together with their light intensity and temperature dependences (presented below) will be used to establish the mechanism of the secondary processes and to derive the primary quantum yields.

222 FORGETEG, BfiRCES, AND DOBc

Dependence of the Product Quantum Yields on Light Intensity

The incident light intensity was varied between 4 X and 2.3 X mol = photon/cm2-sec at room temperature and 100 torr initial aldehyde pressure.

The quantum yields for the products of Norrish type (11) decomposition (CzH4 and CH3CHO) and those for cyclization reaction (11’) (cyclobutanol) are plotted against the rate of light absorbtion in Figure l(a). As expected, these yields are found to be independent of the light intensity.

Reaction (IV) is a photoreduction process in which an excited aldehyde reacts with a ground-state molecule yielding C3H7CHOH and C3H7C0 radicals. The characteristic products of this reaction are 4-heptanol and n -butanol formed in combination and disproportion reactions of C3H7CHOH radicals with C3H7 and CHO, respectively. The independence of the n-C4HgOH and (C3H7)zCHOH yields from the intensity [shown in Fig. l(b)] supports the assumptions that these products (a) are formed in radical-radical reactions in the whole light intensity range studied, and (b) account for the fate of the C3H7CHOH radicals produced in reaction (IV). B u ~ ~ ~ o ~ ~ [ C ~ H ~ C H ( O H ) C O C ~ H ~ ] , a cross combination product of

0

0 0 0

0.1 5 0

OjO t a

t c

01

0 50 100 150 01

l,/169mol photon dm’j’

Figure 1. ucts (a) of reactions (11) and (11’); (b) of reaction (IV).

Light intensity dependence of quantum yields for characteristic prod-


C3H7CHOH and C3H7C0 radicals, which is a well measurable compound in the photolysis in isooctane solution [7], is formed only in trace amounts close to the detection limit in the vapor phase.

The quantum yields of the three products presented in Figure 2 show similar and very strong dependence on light intensity. The decrease of @ c ~ H ~ and @co with increasing intensity from values close to 1 to limiting yields of about 0.2 and 0.4 respectively, indicates that these products are formed for the most part in chain propagation steps (reactions first order with respect to radical concentration) a t low I,, while disproportionation reactions (reactions second order with respect to radical concentration) prevail a t high I,. The intensity dependence of the C3H7CH(C2H5)CHO quantum yields is in accordance with the assumption that C3H7CH(C2H5)CHO is formed by hydrogen abstraction from the cy position of n-butyraldehyde and subsequent combination of the resulting C2HSCHCHO radical with C3H7.

The quantum yields of products of radical-radical reactions involving C3H7 are plotted against I , in Figure 3(a). The steadiness of the self- combination and self-disproportionation products of C3H7, that is, n -C6H14 and C3H6, may be explained by the fact that propyl radicals are neither consumed nor produced in reaction chains. The C3H6/CGH14 = 0.19 ratio derivable from the data is somewhat higher than kd/k , = 0.154, the value of the disproportionation/combination ratio given for n- propyl radicals by Terry and Futrell [9], indicating a minor contribution to propylene formation (quantum yield 60.003) which originates from disproportionation reactions of C3H7 with other radicals.

The yield of combination of C3H7 and C3H7C0 to (C3H7)ZCO is seen to decrease with decreasing intensity, which is probably caused by the in- creased probability of n-butyryl radical decomposition at low I , (where

a

Q a5

0 0 50 100 150

1,tlO” mol photon dm%’

Figure 2. CsH&H(CzH&HO formation.

Light intensity dependence of quantum yields for CO, C3H8, and

224

a03

ao'

FORGETEG, BERCES, AND DOBE

0 -

CH20 0

0 0 2 - 4 : 0 b m

m e m H2

- (CHO);! -/-

3

l,/lOqmol photondm3s'

Figure 3. tion reactions; (b) for CHO + CHO reactions.

Light intensity dependence of quantum yields (a) for C3H7 combina-

radical concentrations are small). The same reason explains that (C3H&0)2 could not be measured and was hardly detectable at low I,.

The quantum yields of products formed by CHO + CHO reactions are given in Figure 3(b). A reaction between CHO and n-butyraldehyde (see below) is assumed to be responsible for the small decrease of the quantum yields observable for all three products at low intensities. An additional minor formaldehyde-producing reaction (either heterogeneous CH2O formation at the walls or some contribution from H abstraction by CHO) may explain the apparently smaller decrease in case of CH20.

The light-intensity dependence of the quantum yields of simple hydrocarbon products (CH4, CzHs, n-C4Hlo, and n-C5H12) is not shown on graphs, since the values are small and the dependences are not great enough to be established with fair accuracy.

Dependence of the Product Quantum Yields on Temperature

Measurements were made at temperatures of 253,273,298,363,426, and 529 K using 2.4 X 10-10 mol-photon/cm2.sec incident light intensity. A dark reaction (presumably a heterogeneous one) prevented us from ex- tending the investigations to higher temperatures. The initial n-butyr-


aldehyde pressure was lOOtorr at 298 K and above; however, 40 and 10 torr aldehyde pressures had to be used at 273 and 253 K, respectively, because of the low vapor pressure of the compound at these temperatures. Since primary quantum yields depend on the overall pressure [8], and some product quantum yields are influenced by the aldehyde concentration, the lower initial pressures used at 273 and 253 K have to be taken into account in the interpretation of the results shown below.

The quantum yields for the products of reactions (11) and (11’) are plotted against temperature in Figure 4(a). The decrease of the yields at low temperatures should not be interpreted as a temperature effect but rather should be regarded as the result of pressure dependence of the primary quantum yields observable below 100 torr [8]. In contrast with the results of Blacet and Calvert [l,2], we definitely observed the decrease of type (11) as well as type (11’) quantum yields with increasing temperature above 298 K. This decrease is very well accounted for by the decrease of the triplet

01 5

@ OjO

005

C

I I

b a

(C3H7)zCHOH

@ w2h a01

, b , ,, ’ 250 350 450 550 TIK

Figure 4. ucts (a) of reactions (11) and (IF); (b) of reaction (IV).

Temperature dependence of quantum yields for characteristic prod-

226 FORGETEG, BERCES, AND DOBI?

yield with increasing temperature [lo]. In accordance with the greater role played by the triplet state in cyclobutanol formation [compared to that in type (11) decomposition] [ll], one finds a more significant decrease in the cyclo-C4H70H yields.

Characteristic products of the photoreduction process (IV) are shown in Figure 4(b). The explanation for the dependence of the quantum yields on the temperature is similar to that given in the case of C2H4, CHsCHO, and cyclobutanol. Photoreduction has been found to occur from the triplet state [ll], and the decrease of the quantum yields above room temperature may be explained by the decrease of the triplet yields. Since the rate of photoreduction (IV) depends on the aldehyde concentration, the initial aldehyde pressures smaller than 100 torr, which had to be used below room temperatures, cause an apparent “temperature dependence” at low temperatures.

The considerable increase of the C3Hs and CO quantum yields with temperature shown in Figure 5 clearly indicates the rapidly growing importance of formyl-hydrogen abstraction and subsequent butyryl radical decomposition reactions with increasing temperature. The increase in the rate of a-hydrogen abstraction is also evident from the increase of the C3H-,CH(C2H&HO yields with T; however, the change in the shape of the curve above 430 K indicates that the rate of decomposition of the

TI K Figure 5. C3H&H(C&I5)CHO formation.

Temperature dependence of quantum yields for CO, C&, and


0.10

CZH5CHCHO radical into CHs and CH2:CHZCHO becomes comparable with the rate of combination with C:3H7.

The quantum yields for combination and disproportionation products of C3H7 are presented in Figure 6(a). The common feature of these curves is the maximum occurring around room temperature. The increase of the quantum yields between 253 and 298 K may be accounted for partly by the increase of the primary quantum yield 41 with the temperature and partly by the increase of the radical concentration with the aldehyde concentration. With further increase of the temperature up to about 430 K, reactions first order with respect to the radical concentration are favored causing the decrease of the quantum yields of combination and disproportionation products. Finally, the increase of C ~ H G yields at high temperatures is an indication of an additional propylene source. This is probably the p- hydrogen abstraction from the aldehyde and subsequent decomposition of the CHSCHCH~CHO radical into C3Hs and CHO.

i I

jw,, ,, " ~ ~~

250 350 450 550 T1 K

Figure 6. nation and disproportionation reactions; (b) for CHO + CHO reactions.

The temperature dependence of quantum yields (a) for C3H7 combi-

228 FORGETEG, BERCES, AND D6BR

The quantum yields for the products of the CHO + CHO reactions are given in Figure 6(b). The shape of the curves may be explained in a similar way as in the case of the combination and disproportionation products of the propyl radicals. The apparent decrease of the quantum yields between 273 and 253 K indicates that the CHO radicals diffuse to the walls and disappear in heterogeneous reactions at low pressure (10 torr) used at 253 K. The decrease of the quantum yields with increasing temperature above 273 K may be explained-in an analogous way to that given in the case of the products of propyl radical reactions-by the appearance of a reaction first order with respect to the CHO radical concentration. This reaction, which can be identified with the reaction between CHO and n-butyraldehyde mentioned above, seems to set in at rather low temperatures and shifts the maximum toward lower temperatures compared to that found in case of C3H7 products (see, for example, C6HI4 or C3H6). Hydrogen abstraction by CHO and decomposition of CHO are suggested to be the additional reactions which contribute to CH2O and H2 formation, respectively, at even higher temperatures.

The dependence of the quantum yields of (C3H7C0)2 and C3H&H(OH)COC3H7 on temperature were not shown on graphs. The former ones are small a t 298K and rapidly decrease with T due to the thermal instability of C3H7C0, while the latter is hardly detectable at all temperatures studied.

The variation of the quantum yields of the simple hydrocarbon products has also been studied. The CH4 and C2H6 yields are small a t low temperatures and increase with temperature. The increase of CH4 occurs at the expense of the n-C4H1O formation as expected if CH4 and C4H10 are formed by the reactions of the CH3 radicals produced in primary process (111). However, at the highest temperature CH4 yields as large as 0.1 are obtained, indicating the occurrence of a new CH3 source and perhaps formation of CH4 in reaction chains. The reaction responsible for this is most probably the decomposition of the C2HsCHCHO radicals into CH3 and CH2:CHCHO.

Mechanism of Secondary Photochemical Processes

Free radicals formed in primary procasses (I), (111), (111’), and (IV) disappear in several elementary reactions as indicated by the large number of reaction products identified. The detailed study of the effect of the aldehyde concentration, light intensity, and temperature allows us to suggest the following reaction scheme for low and medium temperatures: (1) 2C3H7 - n-C~H14 (2) (3) 2CHO + (CH0)z

2C3H7 -+ C3Hs -k C3H6


2CH0 -+ CH20 + CO

2CH0 + H2 + 2CO

CHO - H + CO

C3H7 + CHO - C3Hs + CO

C3H7 + C3H7CHO + C3Hs + C3H7Co

C3H7 + C3H7CHO + C3H8 + CH3CH2CHCHO

C3H7 + C3H7CHO -+ C3Hs + CH3CHCH2CHO , C3H7CHO

C3H7 + C3H7CHO - PI------+ polymer

CHO + C3H7CHO - CH2O + C3H7Co C3H7CHO

CHO + C3H7CHO - CO + Pp -polymer

H + C3H7CHO + H2 + C3H7C0

2C3H7C0 -+ (C~HTCO)~

C3H7C0 + C3H7 + (C3H7)2CO

C3H7C0 - C3H7 + CO

C2H5CHCHO + C3H7 + C~H~CH(CZH~)CHO

CsH5CHCHO - CH3 + CH2:CHCHO

CH3CHCH2CHO -+ C3H6 + CHO

C3H7CHOH + C3H7 - (C3H7)zCHOH

CsH7CHOH + CHO - n-C4HgOH + CO

Reactions of CH3 and C2H5 radicals formed in primary processes (111) and (111'), respectively, are not included in the reaction scheme. These reactions are of minor importance and the mechanism of product formation is more or less obvious. Disproportionation (with CHO and in a small degree with C3H7) and hydrogen abstraction reactions of methyl radicals yield CH4; and CH3 + C3H7 combination formes n-butane. Analogous reactions of C2H5 radicals produce C2H6 and n-pentane. The dependence of the n-CSH12 quantum yield on conversion [7] indicates that some addition of C3H7 to CzH4 does occur. Ethyl radicals may be formed by addition of H to C2H4 at higher temperatures where CHO decomposes into H and c o .

The experimental results show that the rates of production and the concentrations of propyl and formyl radicals are considerably higher than

230 FORGETEG, BERCES, AND DOBE

those for the rest of the radicals. Thus among radical + radical reactions of the minor free radicals only reactions with C3H7 and CHO are included in the scheme. This assumpt,ion is supported by the observation that self- or crosscombination products of the minor radicals are either formed with very low quantum yields (see, for example, dibutyryl) or can hardly be detected (see, for example, butyroin). It is further assumed that minor radicals combine with C3H7 and disproportionate with CHO.

There are definite experimental evidences available for the hydrogen- atom abstraction from the alkyl chain of n-butyraldehyde. Accordingly, both formyl and alkyl H abstraction by propyl radicals are given in the scheme. However, among the less important reactions between H + C~HTCHO, only the major reaction, namely, the formyl-hydrogen atom abstraction, is included.

In the interpretation of the experimental results certain reactions have to be assumed to occur between CHO and butyraldehyde and between C3H7 and butyraldehyde, which, however, are not the hydrogen atom abstraction reactions. These are symbolized by reactions (11) and (13) in the scheme. The CHO + aldehyde reaction is assumed to explain the intensity dependence of the CH20, H2, and (CH0)2 yields at very low light intensities and the decrease of these yields with increasing temperature above 273 K. This reaction may explain also the decrease of the products of CHO + CHO reactions [that is, CH20, H2, and (CH0)2] with increasing aldehyde concentration observed at room temperature @]. On the other hand, the C3H7 + aldehyde reaction is assumed to be responsible (beside alkyl hydrogen abstraction) for the decrease of the C6H14 and C3H6 yields with increasing temperature above 298 K. Reactions (11) and (13) give products not detectable by the careful analysis carried out on six different gas chromato- graphic columns [7]. In addition to the kinetic evidences above, the occurrence of reactions (1 1) and (13) is supported also by some deficiency in the balance equations given below, and by the consumption of n- butyraldehyde in excess of that accounted for by identified products. Both reactions (11) and (13) symbolize complex processes initiated by the addition of C3H7 and CHO, respectively, to the CO double bond and followed by several successive additions of the complex radicals to the aldehyde. Group balance considerations require the first step in reaction (13) to be rather a special hydrogen atom transfer reaction (CO elimination) than radical addition.

The soundness of analysis and the reaction scheme may be checked by balance equations. A CO and C3H7 group balance equation can be given as

(E,l) CO - C ~ H B = 2C6H14 + C3H6 + (C3H7)2CO + (C3H7)2CHOH + C~H~CH(C~HS)CHO - CH20 - Z(CH0)2 + @(ll)

while a CHO and C3H7 radical balance, derived from the reaction scheme, can be formulated as


(E,2) CHzO + H2 + (CH0)z + C~HSOH = C6H14 + C3H6 + (C3H7)2CO + (C3H7CO)z + C ~ H ~ C H ( C Z H ~ ) C H O + '/z{a(ll) - @(13)/

In these equations and thereafter formulas designate the quantum yields for the formation of the appropriate products, and a(11) and a(13) are the quantum yields of the first steps in reactions (11) and (13), respectively.

One can easily distinguish between the propylene originating from the two sources since it was shown that reaction (20) is negligible up till about 426 K, while at 529 K propylene formed in reaction (2) may be neglected compared with that formed in reaction (20). Only the quantum yields of reactions (11) and (13) are unknown in eqs. (EJ) and (E,2). By substituting the required data in eq. (E,l) it may be shown that a(11) is negligible at room temperature and below, 0.08 at 363 K, and 0.3 at 426 K. On the other hand, the yield of reaction (13), obtained from eq. (E,2), seems to be close to 0.1 in the whole temperature range, with a maximum around 298 K. From the comparison of these figures with the yields of aldehyde consumption not accounted for by measured products one estimates an average polymeric chain length for the products formed in processes (11) and (13) of about 6.

Primary Quantum Yields Among the primary processes, Norrish type (11) decomposition and cy-

clization proceed through a 1,4-biradical intermediate [ 121 and give products characteristic for the primary reactions. Thus we obtain 4x1 = 0.17 f 0.01 and 41p = 0.025 f 0.003 at room temperature and 100 torr n-butyraldehyde pressure. These quantum yields were found to be independent of light intensity [Fig. l(a)] and pressure above 100 torr. However, a decrease in the quantum yields of about 35-40% occurs due to the change in the population of electronic and vibrational energy levels, if pressure changes from 100 to 5 torr [8]. The decrease of the triplet yields with increasing temperature causes a decrease in both 411 and 411j above 298 K [Fig. 4(a)]. Thus the quantum yields of reactions (11) and (11') are found to depend on the experimental conditions.

The rest of the primary reactions produce free radicals which disappear in various secondary processes. The primary quantum yields may be obtained in such cases by using the iodine trapping technique [2]. The pos- sibility of a photophysical effect that may be caused by the added iodine [6], and a possible chemical reaction between iodine and the aldehyde (131, makes it desirable to use a different method. We shall attempt to derive the primary quantum yields from the product yields measured. This technique depends on the full analysis of the reaction products and on the detailed understanding of the mechanism.

Determination of the primary quantum yield for the type (I) decomposition requires the knowledge of propane formed in reaction chains con- sisting of steps (8) and (17). The chain contribution may be given by

232 FORGETEG, BERCES, AND DOBJ?

The increase of the chain length with temperature may be estimated from Table I where overall C3Hs yields and chain contributions are given. The light intensity was lo = 2.3 x 10-10 mol-photon / cm2.sec, and the initial aldehyde pressure was 100 torr a t 298 K and above, 40 and 10 torr a t 273 and 253 K, respectively.

Quantitative treatment of the suggested reaction scheme leads to the correlation

(E,4) C3H8 + 2C6H14 4- C3H6 2(C3H7)2CO + ~ ( C ~ H T C O ) ~ + C3H7CH(C2HS)CHO - C4HgOH

or by taking into account eq. (E,l) to

(E,5) CO + CHzO + 2(CH0)2 + (C3H7)zCO + ~ ( C S H ~ C O ) ~ - C4HgOH

For the sake of simplicity we have neglected reactions (6), (ll), (12), (14), (19), and (20) in deriving eqs. (E,4) and (E,5). The maximum error caused on the left-hand side of the equations does not exceed 1% at 298 K and below, is around a few percent at 363 and 426 K, and exceeds 10% only at 529K.

Relationships (E,4) and (E,5) were applied at room temperature using the data of experiments made at different light intensities and 100 torr initial aldehyde pressure. The results presented in Figure 7 determine a straight line with an intercept of 0.36 f 0.02. Taking $I/ = 0.02 [2], one obtains $1 = 0.34 f 0.02 at 298 K, which agrees very well with 41 = 0.35 obtained by Blacet and Calvert with the iodine-trapping technique [2]. This agreement indicates that (a) no serious quenching by iodine occurred, and (b) recombination Of C3H7 and CHO into n-butyraldehyde was negligible.

Since light intensity was not varied at other temperatures, only rough estimations for 41 could be made. Using the values of k81k11/2 calculated from eq. (E,6), one obtains $1 = 0.3,0.37, and 0.3 at 253, 273, and 363 K, respectively. Taking into account the greater inaccuracy of these data,

TABLE I. Overall and chain propane quantum yields.

T/X 253 273 298 363 426 ,529

C3H8 (overall) 0.13 0.25 0.34 1.04 2.30 7 .56

C3HR ( cha in ) 0.01 0.08 0.23 0.88 2.16 7.51


Figure 7. tum yields as given on the left-hand side of equations. (E,5); 0-eq. (E,7).

Verification of eqs. (E,4), (E,5) and (E,7). 20 = sum of product quan- 0-eq. (E,4); 0-eq.

no definite temperature dependence of the type (I) primary quantum yield is apparent from our investigations. Finally it is mentioned that 41 decreases with increasing pressure due to the change in the population of electronic and vibration levels [8].

At low temperatures (253-363 K) the primary quantum yield for reaction (111) may be obtained as the sum of CH4 and n-C4H10 yields, and in a similar way $111. can be calculated as C2H6 + n-CsH12 (extrapolated to zero conversion). Thus $111 = 0.0014 and & I I ~ = 0.0004.

The quantum yields for reaction (IV) may be given as the sum of the yields of n- butanol and 4-heptanol. The data obtained at 253 K (10 torr), 273 K (40 torr), and 298 K (100 torr) are 0.002,0.014,0.044, respectively; the increase is mainly caused by the large variation of the aldehyde concentration. Above 298 K (constant aldehyde pressure of 100 torr) there is a strong decrease in &v observable due to the decrease of the triplet yields with temperature.

Primary quantum yields determined at room temperature and 100 torr initial n- butyraldehyde pressure are summarized in Table 11.

234 FORGETEG, BERCES, AND D6BE

TABLE 11. Primary quantum yields a t 298 K and [aldehydelo = 100 torr.

9 1 91’ 911 911, ?I11 4111’ 9IV

0.34 0.02[21 0.17 0.025 0.0014 0.0004 0.044

Rate Constants

Three hydrogen atom abstraction reactions from n -butyraldehyde by C3H7 radicals were detected and measured in this work. The major one is the formyl hydrogen abstraction reaction. The value of k8/k11/2 at room temperature was determined from the slope of the straight line of Figure 7. The ks/k11/2 data for 273 and 363 K were obtained from eqs. (E,4) and (E,5) assuming $1 + $I( to be similar to the room temperature value. A t the highest temperatures, at 426 and 529 K, where reaction chains are long, the rate of reaction (8) may be given simply by the rate of C3H8 formation, or by Rco - R C H ~ O - ~ R H ~ - R13. In the latter case allowance has been made for C3H7CO formation in reactions other than (8) and for CO formation in some CHO reactions. From the plot of log(ks/k11/2) versus 1/T a good straight line is obtained, from which log(ktJk11/2) = (3.1 f 0.3) - (6600 f 500)/2.303 RT. Estimating log kl(dm3/mol.sec) = log A1 = 9.8 on the basis of the rate constant obtained for ethyl recombination [14] and reported for ethyl [15] and isopropyl [15] self-combination, one obtains

(E,6) log ks(dm3/mol-sec) = (8.0 f 0.3) - (6600 f 500)/2.303 RT in very good agreement with the results of Kerr and Trotman-Dickenson

Hydrogen atom abstraction from the a-carbon atom of the alkyl chain of the aldehyde [reaction (9)] is shown to occur by the formation of the combination product C3H7CH(C2H&HO at low temperatures and by the strong increase of the methane yield due to decomposition (19) at high temperatures. The kg/k11/2 ratio may be obtained from the equation

(E,7)

[41.

C 3H7CH(CzHdCHO + (CH4 + C4Hio - $111)

Determination of kg/k I1J2 is shown in Figure 7 for room temperature where C4H10 + CH4 - 6111 = 0. From the Arrhenius plot of the data determined between 273 and 529 K, log(kg/k11/2) = (2.4 f 0.5) - (8300 f 700)/2.303 RT is obtained. Taking into account the higher A factor determined for reaction (81, one may prefer the upper limits and derive, with A1 and El used above,

(E,8) log kg(dm3/mol.sec) = (7.6 f 0.5) - (8600 f 700)/2.303 RT Hydrogen atom abstraction from the P-carbon atom of the alkyl chain


of the aldehyde could be detected only at the two highest temperatures by the increase of the C3H6 yields caused by the decomposition of the CH3CHCH2CHO radicals in reaction (20). From the propene yields measured a t 529 K, after correction for the minor contribution to C3H6 formation by disproportionation reactions, one obtains log(k lo/k l1I2) = -1.55 dm3/2/mo11/2.sec1/2 in excellent agreement with the results of Kerr and Trotman-Dickenson [4]. Assuming A10 = Ag,

(E,9) log klo(dm3/mol-sec) = 7.6 - 10,300/2.303 RT

The rate of abstraction of a hydrogen atom from the formyl group, from the a and (3 position of the alkyl group, decreases in the given order, and y-hydrogen abstraction would be expected to be slow even a t 529 K. We have found no indication for the occurrence of the latter reaction; however, a minor amount of CzH4, formed in the decomposition of CH2CH2CH2CH0, might have survived detection.

Among the free radicals formed as a result of hydrogen abstraction reactions, two disappear in competitive reactions. Thus, for instance, n - butyryl radical may decompose or react in combination reactions (15) and (16). Estimating the quantum yield of CO formation in reaction (17) from CO(17) = CO - {$I - 2(CH0)2 - CH20) - $I/, one may calculate log(k17h1~/~/k16) = (6.3 f 0.4) - (12,500 f 600)/2.303 RT for the temperature range of 273-426 K. Taking log A 16 = 10.6 and E 16 = 0 as suggested for ethyl + propionyl combination by Watkins and Thompson [16],

(E,lO) log k17(sec-l) = (12.0 f 0.4) - (12,500 f 600)/2.303 RT

is obtained. This activation energy is low compared to 14.4-14.8 kcal/mol determined for the high-pressure limiting activation energy of propionyl radical decomposition [16,17]; possibly reaction (17) is in the pressure- dependent range a t the pressures (40-100 torr) used in our experiments.

The rate of n- butyryl decomposition may be compared to that of butyryl seG-combination a t 273 and 298 K where quantum yields for dibutyryl formation can be measured. From the k 17/k 151/2 ratios calculated and the k17 values derived from (E,10) a recombination rate constant log k15(dm3/mol.sec) N 10.6 is obtained. This value agrees with the rate constant suggested by Watkins and Thompson [16] for ethyl + propionyl combination, on which k15 is actually based.

The CH3CH2CHCHO radical formed by hydrogen abstraction may decompose or combine with C3H7. The difference CH4 - 4111 is a measure for the former, C3H7CH(C2HS)CHO for the latter. Values of about 1 X

are estimated for the h19k11/2h18 ratio at 426 and 529 K, respectively, which correspond to log(kIg/k18) N -10.9 (426 K) and -9.6 (529 K).

Apart from C3H7, formyl radicals play a major role in the secondary processes of n-butylraldehyde photolysis. A t low temperatures (CH0)2,

and 2 X

236 FORGETEG, BERCES, AND DOBE

CH20, and H2 are formed only by CHO + CHO reactions, and the rate constant ratios k4/k3 = 17 f 5 and k5/k3 = 8 f 2 can be calculated from the quantum yields in the temperature range of 253-298 K. Quee and Thynne reported [18] k d k 3 = 132, which was derived, however, in an indirect way and not from measured glyoxal yields. None of the three rate constants is known with fair accuracy. Accepting as a tentative value log k5(dm3/ mol-sec) = 10.3 suggested by Quee and Thynne [18], we obtain log k3 = 9.4 f 0.1 and log k4 = 10.6 f 0.3.

The most important reaction a t low temperatures is the disproportionation step (7). Estimating the quantum yield for reaction (7) by a(7) =

~C~H~CH(C~HS)CHO, which is expected to be valid for 298 K and below, the following rate constant ratios are obtained: k7/(k1k3)ll2 = 9.1 f 2.1, h7 / (k1kq)~ /~ = 2.3 f 0.3, and k7/(k1kg)1/2 = 3.8 f 0.5. Taking the rate constants obtained above,

(EJ1) log k.i(dm3/mol-sec) = 10.5 f 0.2

in the temperature range of 253-298 K. It should be taken into account that this value and all other rate constants derived for the formyl radical reactions depend on the assumed value of kg.

Approximate values for rate constants of CHO decomposition and formyl hydrogen abstraction by CHO can be obtained by estimating CO(6) = Hz - (k5/kd(CH0)2 and CH20(12) = CHZO - (kq/k3)(CH0)2 at 363 K where glyoxal quantum yields are still measurable. With these estimations kG/k31/2 N 3 X

Finally the rate constant ratio for reactions of the C3H7CHOH radicals formed in reaction (IV) can be determined in three different ways. From the equation

( E , W

(k22/k21)(kl/k3)1/2 = 6.0 f 2.4 is obtained for 253-298 K. In a similar way (k22/k21)(k1/k4)l/~ = 1.5 f 0.5 and (k22/k21)(k1/k5)1/2 = 1.5 f 0.5, and (k22/k21)(k1/k5)1/2 = 2.3 f 0.6 were determined. Taking into account the known rate constants, one obtains k22/k21 = 4.1 f 1.4, independent of the temperature between 253 and 298 K.

$1 + C4HgOH - 2C~H14 - 2C3H6 - 2(C3H7)2CO - 2(C3H7C0)2 -

and k12/k13~/~ = 0.07.

C4HgOH CSH14 112 kz2 k l 112

(C3H7)ZCHOH (m) = h21 (hi)

Bibliography

[l] F. E. Blacet and J. G. Calvert, J. Am. Chem. SOC., 73,661 (1951). [2] F. E. Blacet and J. G. Calvert, J. Am. Chem. SOC., 73,667 (1951). [3] F. E. Blacet and R. A. Crane, J . Am. Chem. SOC., 76,5337 (1954). [4] J. A. Kerr and A. F. Trotman-Dickenson, Trans. Furuday Soc., 55,572 (1959). [5] J. D. Coyle, J. Chem. SOC. B, 2254 (1971). [6] R. B. Cundall and A. S. Davies, in “Progress in Reaction Kinetics,” Vol. 4, G. Porter, Ed.,

Pergamon, New York, 1967, p. 149.


[7] S. Forgeteg, T. Bbrces, and S. D6b6, Acta Chim. Acad. Sci. Hung., 96,321 (1978). [S] S. Forgeteg, S. D6bB, and T. BBrces, React. Kinet. Catal. Lett., 10, (1979), in press. [9] J. 0. Terry and J. H. Futrell, Can. J . Chem., 45,2327 (1967).

[lo] M. Tolgyesi, A. Nacsa, and T. BBrces, React. Kinet. Catal. Lett., 7,45 (1977). [ l l ] M. Tolgyesi, T. BBrces, and A. Nacsa, Acta Phys. et Chem. Uniu. Szeged, 22, 57

[12] P. J. Wagner, Acc. Chem. Res., 4,168 (1971). [13] R. K. Solly and S. W. Benson, J . Am. Chem. SOC., 93,1592 (1971). [14] D. M. Golden, K. Y. Choo, M. J. Perona, and L. W. Piskiewicz, Int. J . Chem. Kinet., 8 ,

[15] D. A. Parkes and C. P. Quinn, J.Chem. Soc., Faraday Trans, I , 72,1952 (1976). [IS] K. W. Watkins and W. W. Thompson, Int . J . Chem. Kinet., 5,791 (1973). 1171 J. A. Kerr and A. C . Lloyd, Trans. Faraday SOC., 63,2480 (1967). [IS] M. J. Yee Quee and J. C. J . Thynne, Trans. Faraday Soc., 63,1656 (1967).

(1976).

381 (1976).

Received February 6,1978

Documents

The kinetics and mechanism of n-butyraldehyde photolysis in the vapor phase at 313 nm