[CONTRIDUTION FROM THE
1)IVISION O F
PHYSICAL SCIENCES, UNIVERSITY O P CALIFORNIA,
RIVERSIDE ]
Free Radical Displacement Processes: Reactions of CH, and CD, Radicals with Crotonaldehyde and with Methyl Propenyl Ketone1 BY J. N. PITTS, JR., D. D. Trionrpsox .WD R. IT. \\TOOLFOLK RECEIVED AUGUST 12, 19.57
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Confirmatory evidence for the free radical displacement CH? f CH3C€I=CHC@CHa = C4H8 C H 3 C 0 was obtained vhen photolysis of a mixture of acetone-& and trans-methyl propenyl ketone gave as a product 2-butene consisting of 78% CDBCH=CHCHBand 22% CH&H=CHCH3. The analogous displacement CHB CH3CH=CHCHO = C4H8 HCO, reaction 1, occurred when mixtures of acetone and crotonaldehyde were photolyzed a t 2654 &, and temperatures from 120 to 360". At 120' the over-all quantum yields were much less than unity, but a t 350' they were CO = 11, C3H6 = 5.5, C4H8 = 0.43, H Z = 0.28 and CH4 < 2. Alternative mechanisms for the chain production of CO and C3H6 are presented. CD3 radicals were generated in the presence of crotonaldehyde, and, in confirmation of reaction 1, tlie butene was exclusively CDICH=CHCHI. "Hot" methyl radicals or photo-excited crotonaldehyde molecules are not essential for (1) since pyrolysis of mixtures of di-t-butyl peroxide and crotonaldehyde a t 170" gave 2-butene as a major product.
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In a recent study of the reactions of methyl radicals with trans-methyl propenyl ketone, 2, i t vrTas necessary to postulate the free radical displacement process (1) to explain adequately the results. Although considerable evidence for ( 1 ) CIIo f CHsCH=CHCOCH3
+CH3CH=CHCHa CHiCO
+ (1)
was accumulated, i t was, in a sense, indirect. X direct test of (1) was proposed4 in which CDJ radicals would be allowed t o react with the ketone and the products would be analyzed for CDeCH= CHCH3. This paper reports the results of this study and of its extension to include reactions of methyl radicals with crotonaldehyde. Experimental
volatile a t -130" (primarilv propylene) and at -128' (butene). .A portion of the Cl cut was analyzed for CO I\ ith a Blacet-Leighton microgas analj sis apparatus. The r e m i n d e r of :he Cl cut and the second and third fractions a niLtss spectrometer. Analjsis of deuwere analyzed with terated compounds \+as complicated b> the lack of pure standards aiid most of these annlj ses must be considered to be onlv semi-quaiititatiTe The parent peak of CD,CH= CHCH3 was considered to have the same sensitivity as (2113CH-=CHCHa.
Results For equal pressures of the two compounds, acetone absvrbs about three-fourths and methyl propenyl ketone one-fourth of the incident 2G54-2537 radiation. The quantum yields reported in Table I represent the total yields of product per total quatita absorbed by the two ketones. Siniilar considerations'apply for the results of photolyses of crotonaldehyde-acetoiic mixtures in Table 11. Correctioiis were nixie for dark reactions, but these were large only for the photolysis of acetoiiecrotonaldehydc iiiixtures .it 330" where the corrcctioiis were about 155c for CO and 25% for propylene. No dark correction was needed for the 2 butene at 330". A. trans-Methyl Propenyl Ketone.-Over-all quantum yields fbr the photolysis of mixtures of trans-methyl propenyl ketone with acetone or per-
Apparatus.--Xll photolyses and pyrolyses were carried out in the same reaction system.2 The cylindrical reaction cell was made of fused quartz, 20.0 cm. long, 30.0 mm. i.d.; it had a volume of 140 cc. T h e cell was almost filled with a parallel beam of radiation. Most photochemical runs were made with a Type A Hanovin quartz-mercury arc followed by n quartz cell filled with a Rr2-C$ mixture t h a t transmitted, in the ultraviolet, chiefly 2654 A . and 2337 A. radiation. The reaction cell was placed in a metallic block oven with automatic temperature control. The fraction of light transmitted was measured with a n RCA 935 phototube while absolute light intensities were determined by using acetone photolysis at 120" as a n internal actinometer. TABLE I Other apparatus was of conventional design. Materials .-The tmns-methyl propenyl ketone was preQUASTUMY I E L D S O F A-OS-COSDENS.4BLE PRODCCTS FROM pared b y t h e action of dimethylcadmiurn on crotonoyl THE PHOTOLYSIS OF MIXTURES OF METHYLPROPENYL chloride. After three distillations the product gave Z * ~ D K E T O N E IVITII XCETOSE OR K I T H PERDEUTEROACETOSE AT 1.4320 (I.C.T'., 1.4350, n ' j ~1.4370). The crotonaldehyde was Eastman, white label, dried and distilled a t n 365-1-237 .A. ASD 170' high reflux rate through a spinning band column. For the Run 1 distillate, 11% 1.4367 (Beilstein 1-728, ?Z*',~D 1.4362'1. Ace46.5 Pressure, ?IIPI< ~ i i i i i . tone was Mallinckrodt Reagent grade, while the di-f:Kl. ,5 Pres,,tire of iicctoiic, I I I I ~ I . butyl peroxide, furnished through the courtesy of tlie 0.00 Pressure of ncctoiic-&, i i i i i i . Shell Development Coinpaiiy, was purified by bulb to bulb di~tillatioii. Acetone-de with isotopic purity in excess o f fL5I)O 'Tiinc, SCC. 99[;: was kindly shpplicd bj- Dr. J. K . McNesby. 9,84 I,, (quanta al)s.jcc. qcc.) X Analytical Methods.--It the end of each run the products Over-all quailtutti yiclcls and unreacted starting materials were cooled to NZ (liquid) temperature and the CO, CHI and Ht removed with a Toep0.9s c0 ler pump. The remaining material was then separated, 0.22 2-13u t cn e with the aid of a modified Ward-LeRoy still, into fractions ... CDaCH=CHCH3 (1) Presented before t h e Division of Physical a n d Inorganic Chem0.22 Total butene istry, a t t h e Spring meeting of t h e American Chemical Society, Miami. CH4 Fla., 1957. T a k e n i n p a r t from t h e senior thesis submitted for t h e CD3H ... A.B. degree b y D. D. Thompson. a CHiCHa (2) J. Pi. Pitts, Jr., R. S. Tolberg a n d T. W. M a r t i n , THISJOURKAL, 1 6 , 2834 (1954). ... C2D6 (3) J. N. Pitts, Jr., R. S. Tolberg a n d T. W. M a r t i n , ibid., 79, 6370 CDaCHj ... (1907). 1 race Propylene (4) J. N. Pitts, Sr., Conference on Photochemistry and Free R a d c a l s , 1-niversity of Rechestpr. September, 1854. Present but not dctcrinincd.
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FREERADICAL DISPLACEMENT PROCESSES
Jan. 5, 195s
67
TABLE I1 OVER-ALLQUASTUM YIELDSOF NON-CONDENSABLE PRODUCTS FROM THE PHOTOLYSIS OF MIXTURES OF CROTONALDEHYDE AND ACETONEAT 2654-2537 A. AND A SERIES OF TEXPERATURES Run
a
Temp., OC.
3 120 4 170 5 210 6 210 7 250 8 300 9 3 50 10 3 50 11 3 50 Not determined,
Pressure crotonaldehyde,
la,
mm.
Pressure acetone, mm.
52 51 52 53 52 52 54 54 49
46.5 39.5 47 48 47 49 52 47 47
Time, sec.
x
10-3
quanta abs. cc. sec. x 10-'1
10.0 9.01 11.06 7.62 7.06 4.69 2.47 2.17 3.22
deuteroacetone are given in Table I. No CH3D was found. B. Crotonaldehyde.-Over-all quantum yields in the photolysis ofocrotonaldehyde-acetone mixtures a t 2654-2537 A. and a series of temperatures are given in Table 11. The major products were CO, propylene, 2-butene and methane. Small, but significant yields of hydrogen were found. Methane and the traces of ethane were products but were not determined quantitatively. Carbon monoxide and propylene were produced by a chain process, since the over-all quantum yields were of the order of 11 and 5, respectively, a t 350". The value of $Butene of 0.72 in run 11 is completely out of line with the other results. No explanation is apparent other than that it is a gross error. One photolysis a t 250" was made under conditions similar to run 7 except that perdeuteroacetone was used. The following quantum yields were obtained: CO = 2.23; C3H8 = 0.60 (no deuterated propylene was detected) ; CD3CH=CHCH3 = 0.14 (no non-deuterated 2-butene was observed). The hydrogen-C1 cut was lost. A mixture of 15 mm. of di-t-butyl peroxide and 54 mm. of crotonaldehyde was heated for 13 minutes a t 170". The volumes of the major non-condensable products were CO = 647; C3H6 = 81; 2butene = 101; CH4 = 975; and CzH6 = 132 microliters S.T.P. Only a trace of hydrogen (less than 0.5 microliter) was observed.
Discussion trans-Methyl Propenyl Ketone.-Results from the photolysis of mixtures of CD3COCDJ and transmethyl propenyl ketone confirm previous postul a t e ~of~ ,the ~ existence of the free radical displacement process (1). This type of reaction seems necessary to explain the yields of 2-butene, and it is a key feature of the following mechanism for the formation of the non-condensable products. Because of the small quantum yields of decomposition, methyl propenyl ketone primary processes are neglected in this treatment. CD~COCDB + hv +2CD3 + CO + CH3CH=CHCOCHa + CH,CH=CHCDa + CHiCO CHiCO +CH3 + CO CHo + CH,CH=CHCOCHg + CHaCH=CHCHs + CHaCO
(I)
CD3
(2)
(3)
(1)
8.38 7.99 7.83 7.50 8.36 7.63 7.35 3.48 3.48
Quantum yields
co 0.57 1.41 1.94 1.91 2.95 4.33 10.5 11.1 11.4
C3H6
CiHs
0.13 .25 .27 .69 1.45 5.5 5.8 5.2
0.008 .14 .22 .21 .28 .33 .45 .40 .72
Hz
0.0045 ,0092 .011 0
.02s ,074 .25 .31
+ CH3CH=CHCOCH3 + CDBH + CH%CH=CHCOCHz CH3 + C€I3CH=CHCOCH3 + CH, + CH2CH=CHCOCH3 CDI
2CD3 +CzD6 CD3 CH3 +CDsCH3
+
2CH3 +CZHB
(4)
(5) (6) (7) ( 8)
Primary process (I) is well established for acetone and acetone-ds and a t 170"the quantum yield of carbon monoxide may be taken as Perdeuteromethyl radicals formed in (I) may abstract H or D atoms from the substrate, recombine or react a t the alpha or P-carbon atom of the olefinic double bond in the methyl propenyl ketone. This multiplicity of reactions leads to a complex mixture of condensable products which was not subjected to detailed analyses; however, several conclusions can be drawn from the observed yields of the noncondensable products. (a) The large yield of CD3CH=CHCH3 compared to CD3H, C2D6 and CD3CH3indicates that under the experimental conditions employed, the displacement process (2) is favored over competing abstraction and disproportionation reactions of the methyl radicals. Most of the 2-butene is CDJCH=CHCH,, and the presence of the CD3 grqup in this olefin is evidence against butene formation from a direct decomposition of tram-methyl propenyl ketone into 2-butene and CO. Furthermore, the large yield of 2-butene relative to the ethanes is evidence that most of the olefin was not formed by a combination reaction between methyl and propenyl radicals. (b) From the yields of CH3CH=CHCH3, CH4, CzH6 and CD3CH3it is clear that substantial quantities of CH3radicals must have been generated during run 2. Reactions 2 and 3 followed by the displacement process (l), provide a source of CHB radicals and serve to explain the yield of non-deuterated 2-butene. Furthermore, since acetyl radicals decompose rapidly a t temperatures above 120°, reactions 2 and 3 provide a means for CO production by secondary reactions. This additional source of CO is necessary t o account for the fact that the over-all quantum yield of CO is virtually ( 5 ) W. A. Noyes, Jr., and L. XI. Dorfman, J . Chem. Phys., 16, 788 (1948). (6) J. R. hfcNesby a n d A. S. Gordon, THISJOURXAL, 76, 1416 (1954).
J. N. PITTS,J R . , D. D. THOMPSOY AXD R.ll-.SVC)OLFOLK
G8
1-01. 80
unity for the mixture, yet the fmns-methyl propenyl ketone, which is quite stable compared to acetone, absorbs about one third of the incident 265-4-2537 A. radiation. Reactions 0 and 10 furnish an alternate method for producing CH3 radicals in run 2. Although there is no direct evidence for this sequence, it may be significant, particularly a t high teniperatures.
the displacement of acetyl by methyl, reaction 1 . The results of this research confirm the evisteiicc of (11) as a secondary reaction of significance in systems containing methyl radicdls and crotoiialdehyde. The following evidence is oflcrecl iii sui)port of this statement. (a) It is evident from the results iii Table 11 that at 170" %butene is the major olefin product of photolyses (if crotonaldehyde-acetone mivtures Hac\ a t 26.34-2537 A. Its quantum yield increases 31C1)3 + CH3CH=-CHCOCH:j + jCHCHCOCH3 most linearly from 0.14 a t 170" to .ihout 0.1, dt 1)3C ( 9) 350". It seems most unlikely that such substantial yields of ?-butene could arise chiefly from the r'3C>CHCHCOCH3 --+ CHI + CDaCH=CHCOCH3 IISC combination reaction between CH3 arid C H C H = (10) CH radicals, in systems containing a free r'idical (c) The fact that all the methane formed was "trap" such as crotonaldehyde. The fact that either CD3Hor CH, demonstrates that only methyl only traces of ethane, presumibly formed by a coinpropenyl ketone was the substrate for H atom ab- bination reaction between methyl radicals, were straction by CH3 or CD3 radicals. This seems found in these photolyses, suhstmtiates this viewreasonable since the activation energy for abstrac- point. tion of an H atom from methyl propenyl ketone (b) The C4 olefin formed in the photolysis of a should be appreciably less than that for abstrac- mixture of CD3COCD3 and crotonaldehyde was tion of a D atom from perdeuteroacetone. In ad- solely CD3CH=CHCHa. This is further indicadition to a lower activation energy for CD3 attack tion that 2-butene is formed bj- direct methyl on a C--H bond LIS. a comparable C-D bond, ab- radical attack on crotonaldehyde. straction of a r-hydrogen of the unsaturated ketone (c) Pyrolysis of a mixture of di-i-butyl peroxis favored by the formation of a strongly reso- ide and crotonaldehyde a t 170' yielded a substantial amount of ?-butem, about 207% in excess nance-stabilized radical as a product. In agreement with previous work,3 an actual in- of the propylene formed. Thus "hot" methyl creaseoin the fraction of light absorbed a t 2654- radicals from a primary process or photo-excited 2537 A. was observed during the course of each crotonaldehyde or acetone molecules are not necesphotolysis. This suggests that saturated ketones sary for the displacement reaction 11. are products. Evidence available to date3s7 in(d) The yields of hydrogen per quantum abdicates that one of these is CH&H(CD?)CH?- sorbed by the crotonaldehyde are at least three times COCH3. This could be formed by @-additionof a larger in the photolysis of mixtures than in the CDa radical to the unsaturated ketone to give photolysis of crotonaldehyde alone.1° This can CH3CH(CD3)CHCOCH3 which in turn could ab- be explained by assuming that ( I I) acts as another stract a y-hydrogen from the substrate to give the source of formyl radic,ils. Decomposition of thesc final product. Further discussion of this phase of radicals into H atoms a i d carbon monoxide, folthe reaction mechanism must be deferred until lowed by an abstraction rextion, could furnish the more detailed analyses of the condensable fractions additional hydrogen fouiid in cvcess o f that 01)are available. served in direct photolysis. Crotona1dehyde.--Blacet and Roof have Analytical techniques were not available to deshowns that crotonaldehyde is unusually stable termine directly the st,ible condens ible products toward photo-dissociation a t room temperature and resulting from @-addition of methyl and other wave lengths from 3130 to 2380 fi. Subsequently, radicals. However. the pronounced increase obBlacet and Lulralleg found that a t elevated t e q - serve? in the percentage of light absorbed a t 265 $peratures this al4ehyde is still stable a t 3130 A. 2537 4 . during the course of a run is indirect evi but that a t 2350 A. and 265" the quantum yield of dence for the formation of saturated aldehydes. carbon monoxide is about unity. Among the hy- since the latter absorb strongly in that region corndrocarbon products, chiefly propylene, they found pared to crotonaldehyde. small amounts of methane. Recently in a brief In addition to supplying evidence for the tlisinvestigation Tolberg and Pitts7 carried out two placement process (1 l ) , the data in Table I1 yield photolyses of crotonaldehyde a t 265" and 2350 A. other pertinent facts. These include. 11ass spectrometric analysis of the non-condens(a) Propylene is produced in a chain process ables confirmed the presence of methane and (or processes) that is strongly temperature dependshowed that ?-butene was another "minor" prod- ent. @c*H, is of the order o f 0.13 a t 170", 0.69 uct. They suggested that the 2-butene might be at 250" and over five ,it 350". Thc ratio (quanformed by a methyl radical displacement of formyl turn yield of 2-butene) (quantum yield of propylr:dical, reaction 11, which would be analogous to ene) is 1.1 a t 170" but drops to less than 0.1 a t 350". (b) The quantum yield of carbon monoxide exCIIa + CII:;CTT-CHCHO + ceeds unity a t 110" aiid is about 11 at 350". In CHsCH=CHCH3 + HCO (11) . the case o f crotonaldehyde alone, tinder similar ( 7 ) I.
(14) J. R. McKesby a n d A. S. Gordon, Abstracts Div. Phys. a n d Inorg. Chem. Spring I l e e t i n g , Am. Chem. Soc., Miami, 1957. (15) P. Kebarle a n d I\'. A. Bryce, C a w J . Ckem., 36, 576 (1957).
[COSTRIBUTION FROM
+ CHaCH=CH2 1%
+ CrH5
CII3CHpCH=CHz
+ CO
The recent accumulation of this body of evidence for free radical displacement processes indicates that secondary reactions of this type may be of considerable importance in a number of other systems similar to those described. Current studies in this Laboratory include an investigation of the reactions of methyl radicals with the a,p-unsaturated ester, methyl crotonate. Acknowledgments.-The authors are indebted to the National Science Foundation for a grant in support of this research. JVe are also indebted to Dr. R. S.Tolberg for assistance in assembling much of the apparatus employed in this problem. Mass spectrometric analyses were conducted a t UCLA through the courtesy of Professor F. E. Blacet, Dr. R. Holroyd and N r . R. Vanselow.
n'ithin a few weeks the reverse reactions were reported by Kebarle and Bryce.'5 In both instances CIIS
CiHP
and LVinkler17and reported by Forst and n'iiik1c.r. **
e CD3CH2CH=CH2 + CH3
-L
--+
This process can be classified as a displacement of carbon monoxide by the ethylidene radical. Two examples of what might be termed H atom displacements are proposed by Iiabinovitch, Davis
LIcXesby and Gordon photolyzed acetone-& in the presence of I - b ~ t e n e a' ~t 375 and 500" and explain their results by a mechanism which includes the displacement process CD3
so
Nesby and Gordon visualize the two step process
