July 20, 1957
?dECHAR'ISM
OF THE I N T E R A C T I O N OF P E R O X Y
methyl-a-D-galactose; X-ray powder diffraction datal4Js (identical \I ith those of authentic 3-O-methyl-or-~-galactoqe): 8.85rn1, 6.93s, 5.747, 4 . 9 8 ~ 4.56vs, , 3.90vw, 3.76s, 3.62vw, ~~ 3.49m, 3.39s, 3.31m, 2 . 8 8 ~ 2, . 8 3 ~ 7 ,2.66m, 2 . 4 1 2.32w, 2.26~. Tetra-0-acetyl-0-p-tohenesulf onyl-P-D-galactopyranose from Isomer A.-The general experimental procedure followed was that of Helferich and K1ein.l6 An amount of 500 mg. of isomer A mas treated with 200 mg. of p-toluenesulfonyl chloride in 3 ml. of dry pyridine. After 24 hr., the mixture was poured into ice and water. The precipitate that formed vxs filtered and dissolved in ethanol. Crystals appeared upon concentration of the solution; yield 500 mg., m.p. 153-155'. The product was recrystallized from ari acetone-ether-petroieum ether (b.p. 30-60') mixture; ~ (c 3, chloroform); X-ray m.p. 151-155', [ a I z 5 +43" powder difiraction 10.52s, 8.44vw, 5.7.2m, 5.31w, 4.7Gw, - I . i X m , 4.19vw, 4.00vw, 3.53m, 3,45m, 3 . 1 4 ~ . A n d . Calcd. for C ? I H ~ & :C, 50.19; H , 5.21; S, 6.35. Found: C, 50.10; H,5.30; S, 6.33. Similar treatment of isomer B with p-toluenesulfonyl chloride and pyridine failed to yield a crystalline product. The p-toluenesulfonate (150 mg.) from isomer A yielded a negligible precipitate (5 mg.) of sodium p-toluenesulfonate on heating a t 100" with 70 mg. (1.5 molar ratio) of sodium iodide in 3 ml. of acetonylacetone for 48 hr. h like experiment utilizing 400 mg. of tetra-0-acetyl-6-0-p-toluenesul-
[COSTRIBLITIONF R O M
THE
387 1
RADICALS
Fonyl-P-D-glucopyranose, 180 mg. of sodium iodidi: arid 8 ml. of acetonylacetone yielded 141.2 mg. (8870) of sodium p-toluenesulfonate. Methyl Tri-0-acetyl-0-P-toluenesulfonyl-B-D-galactopyranoside from Isomer A.-The general procedure of Haworth, Jackson and Smith' was followed. An arrount of 500 mg. of tetra-O-acetyl-0-p-toluenesulfon~-l-~-~-galactopyranose (from isomer A ) m-as treated for 1 hr. with 3 ml. of a solution containing 30YGhydrogen bromide in anhydrous acetic acid:acetic anhydride (1: 1 by vol.). Chloroform was then added to the solution and the extract was mi.lied four times with water and dried over sodium sulfate. The dried solution was concentrated under reduced pressure, and t h e resulting sirup was dissolved in 25 ml. of dry methanol, and 10 g. of freshly prepared silver carbonate i v a q addel. The mixture was shaken for 5.5 hr. whereupon it was filtered, and the filtrate was concentrated under reduced pressure. Crystals of the methyl tri-0-acetyl-O-p-tolueiiesulfon?.l-p~-galactopyranoside appeared during the eoncer trnt ion; yield240 mg., m.p. 129-130", [ a j 2 j+~2 i o (c 3, chlowform); ~~ X-ray powder diffraction data14j1j: !9.77vs, 6 . 2 0 ~5.84m, 5.24s, 4.88m, 4 . 4 6 4.23w, ~ ~ 4.015, .3.79v~v,3.05,: 3.5lm, 3.37m.
Anal. Calcd. for C.oH*,O,,S: C, 50.62; H , 5.52; S, 6.77. Found: C,50.42; H,5.71; S,6.67. COLUMBUS 10, OHIO
GENERAL ELECTRIC RESEARCH LABORATOXI.]
Deuterium-isotope Effects in the Autoxidation of Aralkyl Hydrocarbons. of the Interaction of Peroxy Radicals' BY GLEN
Mechanism
RUSSELL
RECEIVEDFEBRUARY 1, 1957
-
a-Phenethplperoxy radicals interact to form non-radical products 1.9 times as readily as their a-deutero derivatives. C6HjCOCH3 c&,cH(D)OH(D)This is taken as evidence that the termination reaction is 2C&CH(D)CH&OO. CH! O?. Yields of hydroperoxide and acetophenone produced in the autoxidation of ethylbenzene at variou:j kinetic chain lengths are consistent with the indicated termination reaction. Evidence is presented that this reaction involves a cyclic transition state and excluding an alternate process proceeding by the formation and disproportionation of alkoxy radicals. The deuterium-isotope effect in the reaction of a peroxy radical with cuinene and a-d-cumene is k a / k l , = 5.5. The magnitudes of deuterium-isotope effects in the reactions of atoms and radicals with carbon-hydrogen and carbondeuterium bonds are considered. Reactivities of the attacking atoms or radicals and polar effects in the reaction are discussed.
+
Despite the fact that the autoxidation of hydrocarbons has been studied intensively,2 evidence is lacking as to the mechanism and products of the reaction wherein two peroxy radicals are converted to non-radical products. We have, therefore, investigated the products of this termination reaction in the oxidation of ethylbenzene and have demonstrated its detailed mechanism by a study of deuterium-isotope effects. The deuterium-isotope effect in the reaction of a peroxy radical with a-dcumene has been measured, and its magnitude confirms our earlier conclusions in regard to the extent of bond-breaking in the transition state for this reaction. Results Autoxidations of cumene, ethylbenzene and various deuterated derivatives were performed a t 60' in the liquid phase in the presence of (Y,(Y'azodiisobutyronitrile (.4IBN), The rates of oxidation were independent of the oxygen pressure ( 1 ) Directive Effects in Aliphatic Substitutions. IX. Presented before the Division of Organic Chemistry a t the Atlantic City meeting of the American Chemical Society, September, 1956. ( 2 ) J . I,, Rolland, Q M I I Y I . Rrvs., 3 , 1 ( l < I l ! ~ \ : I,. Bateman. ; h ; d , 8 , 1 4 i (1954).
+
over the range POO-'T(iO min. indicating thtit only peroxy radicals were involved in the termination reaction.* The kinetics of the reaction under these conditions indicate that the following sequence of events is involved. AIBK
kd + (CH3)zCCS
+ RH R. +
ROO.
0 2
2RO0.
ko
--+
0 2
ka fast
-+
ROO. (Rate = R,)
ROOH
+ R.
ROO.
nrni-radical pruducts f (I?
Under steady-state conditions the rate of oxygen consumption is calculated to be -d[Og]/dt
+
= & [ R H ]( R L ) ' / 2 / ( 2 k ~ ) ' / z K , / L
(I)
The rate of initiation, Ri, is equal to 2kde [AIBN]'ly where e represents the fraction of catalyst molecules that produce two free radicals. In Table I the rates of oxidation of cumene and ethylbenzene are presented as a function of hydrocarbon concentration, AIBN concentration and oxygen pressure. The data of column 6 indicate that (1) iq obeyed :tnd the rate of oxidation is independent of
GLENA. RUSSELL
3872
OF
TABLE I CUMENEA N D ETHY 'LBENZENE AT 60.0" R a t e of oxygen absorption,a pres- mole 1. -1 [RH], [AIBIU], 0% inole mole sure, sec. - 1 1-1 1-1 mm. x 10s
6 90
750 2.55 1.16 500 2.58 1.16 200 2.60 1.17 750 3.66 1.14 .OS0 750 1.75 1.13 Cuineiiz ,100 750 1.53 1.15 Ethyibenzeiie 7 . 8 2 . l o 0 750 0.55 0.207 Ethylbenzene 7.82 ,100 500 .50 ,199 Ethyibeiizene 7.8% ,100 200 .GO ,214 Ethylbenzene 7.83 .I50 750 .74 ,211 Ethylbenzene 7.88 .200 750 .85 .203 Corrected for nitrogen evolved in decomposition of A I B S . * Chlorobenzene used as ail inert diluent. G 90 G 90 G 90 b 90 4.00b
0.100 ,100 .lo0 ,200
VOl. 79
This sample of a-d-cumene oxidized considerably less rapidly than cumene, even though 41.GOj, of the molecules had an a-hydrogen atom. Since, in the presence of 0.1 M AIBN, cuniene was oxidized a t a rate of 2.58 X mole 1.-' set.-' and a-d-cumene (58.470pure) a t a rate of 1.38 X lou6 mole 1.-l set.-', it can be calculated that in the slow propagation reaction the deuterium-isotope effect is kH/kn = 5.5. Based on possible errors in analysis, the isotope effect might be as low as 4.5 and as high as 7.2. ROO. -t R H ROO.
% ROOH + Re
kn + RD + ROOD + R -
kH/kr, = 5.5
There can be no deuterium-isotope effect in the termination reaction in the oxidation of a-d-cumene since both cumene and a-d-cumene yield the same a,a-dimethylbenzylperoxy radical. the oxygen pressure and proportional to k(Ri)"' The data of Table 11 demonstrate that the yields 3. Ri!2. The value of Ri used in the above cal- of hydroperoxide in the autoxidations of cumene culations for 0.1 M AIBN has been taken as 1.38 and a-d-cumene were essentially quantitative irx mole I.-' SeC.-' based on a value of 1.15 respective of kinetic chain length (molecules oxix SCC.-' for h d a t 60" and 0.60 for e. The dized per initiating fragment). This supports the value of k d in an aromatic Solvent Was selected from conclusion of Boozer6that the interaction of cumyla coinpilation of various literature referencess4 peroxy radicals should be formulated while the value of e appears established from the 2CsHsC (CHj)200 ----f work of Hammond, Sen and Boozer.b CsHsC(CHs)rOOH C~HSCCH~=CH 4- ~0 2 Having thus established the course of the reaction, the deuterium-isotope effect in the slow propaThe yield of hydroperoxide in the oxidation of gation step ( ka) \vas investigated. Q-Deuterocu- ethylbenzene was neither quantitative nor inineiie was prepared by the reaction of deuterium dependent of kinetic chain lengths (see Table 11). chloride with a,cY-diriiethylbeiizylpotassiuln. Anal- 'l'h Yield of hydroperoxide decreased with deysis by high resolution nuclear magnetic reso- creasing kinetic chain length suggesting that nance spectroscopy indicated that 58.470of the ter- OxYgen-Containing non-peroxidic Products were tiary hydrogen atoms of cumefle had been replaced formed in the termination reaction. In the oxidaby deuterium atoms. Aromatic deuterium atolns tion of ethylbenzene, under the conditions specified were present (0.24 atom/molecule) in the form of in Table 11, an average of 4.2 molecules of oxygen dl-, dz- and d3-cumenes,but analysis by high resolu- were absorbed per initiating radical. If the tertion infrared absorption indicated that less than mination reaction between two ff-PhenethYlPeroxy 3% of the molecules contained a @-deuteriumatom. radical yields one molecule of oxygen and one molecule of oxygen combined in non-peroxidic products, (3) T h e following d a t a mere used: F. M. Lewis a n d M . S. Mathethe theoreticdl yield of hydroperoxide^ based on s o n , T m S J O U R N A L , 71, 747 (1949); SO', xylene, 1.53 x 10-4 sec,-i; 700, xylene, 4.10 x 1 0 - b (unpublished); 900, xylene, 5.0 x 1 0 - 4 oxygen absorbed, is 88%. The observed yield (cnpubiished): L.hl. Arnctt, i b i d . , 74,202; !1952), 7 7 O , xylene, 9.5 x was 85%. 111 the oxidation of &-ethylbenzene 10-6; 8 2 O , xylene, 1.45 X le-*; X. Talat-Erben and S. Bywater, (prepared by the addition of deuterium to phenyli b i d . , 77, 3712 (1933), 7 0 3 ,toluene, 4.0 X 10-6; S0.4', toluene, 1.55 X 10-8; G. s. acetylene) the kinetic chain length was only 2.0 10-4; 1000, toluene, 1.6 lo-4; 900. toluene, 4.86 and the yield of hydroperoxide was 65%. The Iiammond, J. N.Sen and c. E. Boozer, ref. 5 , 02.50, benzene, 1.42 x 10-6; 62.50, chlorobenzene, 1.54 x 10-6: e.G. Overberger, hi. 'r. theoretical vield calculated on the basis of a terO'Shaughnessy and H. Shalit, ibid., 71, ?GG1 (1919), 69.8a. toluene, mination reaction yielding a molecule of oxygen 3.8 X 10-5; 80.2', toluene, 1.66 X 10-4; G. A. Russell, i b i d . , 78, and a molecule . of oxygen combinedinnon-Peroxidic 104.1 (ISX), 80°,cumene, 1.07 x 1 0 - 4 : J . C. R ~ J. ~ R. N, a b , R. R. products 1s 75y0,. \\'illiams, Jr., and ',V, H. Hamill, i b i d . , 78, 519 (1956), SOo, chlorobenz m e , 1.50 x io-? J. W. Breitenbach a n d A. Schindler. M o n a f s h . A possible termination reaction, consistent with Chem., 83, 724 (l952), 50°, styrene, 2.97 X 10-0; 7 0 ° , styrene, 4.72 X the above observations, is 10-6. These d a t a yield t h e expression kd = 1.75 X 10'6 exp( -31,3001
+
KT) with serious deviation for the 8 2 O point of Arnett
only. Interestingly, t h e decomposition rates of C. E. H . Bawn and S. H. Mellish [ T r a m . F a r a d a y Soc., 47, 1210 (1951)], obtained by use of diphenylpicrylhydrazyl, which supposedly is a measure of k d s , h s are in excellent Deagreement with this r a t e expression over t h e range 40-70'. composition rate constants in nitrobenzene solution are consistently higher. T h e d a t a of Hammond, ref. 5, 1.79 X 10-8 sec.-l a t 62.5', a n d of K. Ziegler, W. Deparade and W.Meye [Anm., 667, 141 (1950)1, 2.0 X 10-4 at SO", 2.25 X 10-8 a t looo. give t h e expression k d = 4 , 9 X 10'6 exp(-31,200/RT). (4) C. WallinK. J . Polymer Sci., 14, 214 (1954); J. C. Bevington. A ' n i w e , 176, 4 7 7 ( I 9 5 5 ) , J . C/je'!i. S . C . ,1117 (1956). ( 5 ) G . S . Hainmond, J . K , Sen and C. E, Boozer, TUIS JuUnxAI.. 77, 3244 (19.55).
2CrHsCHCI-1300. +C ~ H ~ C O C H ~ $ - C ~ H ~ C H OfH0C2 I ~ I
Hydroxylic and ketonic products are generally formed in small amounts in low-temperature autoxidations, but these products may represent only the decomposition of some of the initially formed hydroperoxide. However, the formation of exactly one mole of acetophenone per two kinetic chains would give considerable support to the above termination reaction. The oxidates of a-d,-ethyl(t?) C.E . Boozer, I3 K'. Fonder, J . C . TrisIer and C. U, \\'rightinan, i b i d . , 78, 1500 (19.50;.
July 20, 1957
TABLE I1 OXIDATION OF ARALKYL HYDROCARBONS AT 60°, 0.1 M AIBN Hydrocarbon (mole)
3873
MECHANISM OF THE INTERACTION OF PEROXY RADICALS
[RH], mole 1.-1
Rate” mole 1.” sec.-l X 106
Kinetic chain lenghb
Oxygen absorbed0 mole X 108
Hydroperoxide found Stannous Iodimetrice chlorided
1.80 1.87 19 1.86 6.90 2.58 Cumene (0.1380) 1.10 1.10 25 1.12 1.75’ Cumene (0.1380) 6.90 0.86 0.88 10.0 0.90 6.90 1.38 “a-dt-Cumene” (0.0690) 1.70 2.00 1.80 4.2 0.58 Ethylbenzene (0.1562) 7.82 0.72 0.75 0.915 0.47 3.4 7.83 a-&-Ethylbenzene (0.0783) 0.42 0.64 0.53 2.0 7.85 0.27 &-Ethylbenzene (0.0785) Reference 41. Based on oxygen consumed. a Corrected for nitrogen evalued in decomposition of AIBN. 0.05 M AIBN. 42. Calculated from hydroperoxide found by stannous chloride.
*
%
0xy:en found as hydroperoxldea
9;‘ 98 96 85 79 64 Reference
benzene and d4-ethylbenzene were therefore an- mean of the rate constants for the other two reac~ D = (kHkD)’/:, it can be calculatled that alyzed for carbonyl content by infrared absorption. t i o n ~ k, H I n the oxidation of d4-ethylbenzene the kinetic the deuterium-isotope effect in the ternination chain length was only 2 . Therefore, three mole- reaction is k H / k D = 1.9. The magnitude of this cules of hydroperoxide and one molecule each of deuterium-isotope effect, which occurs when a acetophenone and a-phenethanol were expected deuterium atom is substituted for an a-hydrogen per pair of initiating radicals formed. Of the ka mole, ‘,/a total oxygen absorption of 0.64 X 2CsHjCIICHsOO. + should be present as acetophenone if the suggested termination reaction occurs. By analysis 0.13 CsHjCHCHsOO. CaHsCDCHtOO. + X 10-3 mole of acetophenone was found which is ko in fair agreement with the theoretical value of 0.16 2C6H6CDCHaOO. --t ( 2 X 0.64/8) X lov3 mole. Based on a kinetic atom in the a-phenethylperoxy radical, suggests mole of aceto- that an a-carbon-hydrogen bond of the (r-phenchain length of 3.4, 0.135 X phenone was expected in the oxidation of a-dl- ethylperoxy radical is broken in the rate-determinethylbenzene wherein 0.915 X mole of oxygen ing step of the termination reaction. This, of was consumed. The observation that 0.13 X loF3 course, is in agreement with the termination reaction mole of acetophenone was formed lends strong sup- suggested wherein two a-phenethylperoxy radicals port t o the suggested termination reaction. interact to form oxygen, acetophenone and a-phenThe mechanism of this termination reaction was ethanol. investigated further by a study of deuterium-isoDiscussion tope effects in the oxidation of ethylbenzene. If Mechanism of Chain Termination.-The obthere were no deuterium-isotope effect in the termination reaction of a-deutero-a-phenethylperoxy servation that a-phenethylperoxy radicals terradicals, the ratio of ka/ks’/* for the oxidations of minate more readily than their a-deutero derivaethylbenzene and a-dl-ethylben~ene~ should be tives allows the assignment of a detailed mecha1:0.60 provided that the deuterium-isotope effect nism to this reaction. A reaction involving a in the propagation reaction is k H / k D = 5.5. The cyclic transition state seems most likely. observed ratio was 1:0.78. This ratio would be consistent with a termination reaction involving no isotope effect only if the deuterium-isotope effect in the propagation reaction (k,) was k H / k D = 1.8. This isotope effect is so inconsistent with the isotope effect of 5.5 determined in the oxidation of cumene that there must be a deuterium-isotope effect in the termination reaction of a-deutero-a-phenethylperoxy radicals.8 Using an isotope effect in the I propagation reaction of k H / k D = 5.5, i t follows that a-phenethylperoxy radicals terminate 1.7 I may actually be an intermediate that is formed times as readily as the‘peroxy radicals formed in the rapidly and reversibly and which decomposes oxidation of a-dl-ethylbenzene. The peroxy radi- slowly in an irreversible manner. The seaction cals formed in the oxidation of a-dl-ethylben~ene~path illustrated is consistent with the yield of hyshould be 78.8% a-deutero-a-phenethylperoxy and droperoxide and acetophenone observed in the 20.2% a-phenethylperoxy based on the magnitude oxidation of ethylbenzene and in view of the exotheris consistent with a low of the isotope effect in the propagation reaction. micity of the (9) See for example, G. M. Burnett, “Mechanism 01’ Polymer If the crossed termination reaction (kHD) is assumed Interscience Pub., Inc., New York, N. Y., 1951, p. 290. t o occur with a rate constant that is the geometric Reactions,” (10) Using the bond dissociation energies of T. L. Cottrell [“The
+
(7) Containing 1.1% ethylbenzene. (8) Since a hydrogen atom of ethylbenzene is less reactive than a hydrogen atom of cumene toward a peroxy radical [G. A. Russell, Tars JOURNAL, 18, 1047 (1956)], It is expected that if there is a difference in the deuterium-isotope effect in the propagation reaction, then the largest deuterium-isotope effect will occur in the attack of a peroxy radical on ethylbenzene.
Strengths of Chemical Bonds,” Academic Press, New York, N. Y., 19641 D(-o-o-), 36 kcal. mole-’: D(Ec-H. benvyl), 78; I>(%), 117; E(-C-O. ketone) E(-C-O-), 87; D(Ro-H), 100 [P. G r w , Trans. Faraday SOC., 62, 344 (1056)1, i t is calculated that 157 kcal. is liberated in the interaction of two moles of peroxy radicals. This estimate is high if the oxygen-oxygen bond dissociation energy of a peroxy radical is greater than 35 kcal. mole-1. Since estimates of D ( ~ 0 - 0 . )
-
GLEKA. RUSSELL
3874
1’01. i 9
deuterium-isotope effect ( k H / k u = 1.9j when a deuterium atom is substituted for an a-hydrogen atom.‘? Vaughan and co-workers have suggested t h a t prim. and sec.-alkylperoxy radicals terminate by the formation and disproportionation of alkoxy radicals.I3
radicals terniiriate more readily than cumylperuxy radicals.8t15 Because of this, under conditions of constant rate of initiation, the concentration of peroxy radicals is greater in an oxidation of cumene than in an oxidation of ethylbenzene. l t follows, from Table I, that the concentration of peroxy radicals in the oxidation of cuniene (0.1 AIBN, 6.90 M cumene) is 6.8 times the concenkr kt 2RzCHOO. + 0 2 $- 2Il:CHO. --+ K2CO T R?CHOH tration of a-phenethylperoxy radicals in the oxidaThis suggestion lacks experimental verificatiori, tion of ethylbenzene (0.1 f1/1 AIBIi, 7.82 M ethylparticularly in regard to the actual participation benzene). Therefore, for a 0.1-If AJBN-catalyzed of alkoxy radicals in the reaction. However, in oxidation of cumene, k , l k r < 6.6/6.8= 03. This the liquid phase oxidation of ethylbenzene a ter- indicates that if kf > K t and if ki is the same for mination reaction involving the formation and dis- cumylperoxy and a-phenethylperoxy radicals, that proportionation of alkoxy radicals can be excluded. cumylperoxy radicals formed in a 0.1ill AIBNA deuterium-isotope effect of the magnitude ob- catalyzed oxidation would interact to yield alkoxy served would not be expected if alkoxy radicals were radicals inore rapidly than they would react with formed slowly and disproportionated rapidly (kf cumene. Such a situation is unlikely in view of < K t ) . On the other hand, although a deuteriuni- the essentially quantitative yield of hydroperoxide isotope effect would be expected if hi > K t , this obtained in the oxidation of cumene (Table 11). Another possible termination reactionIG can I:e process can be eliminated on the basis of stoichiom“ROO. + K’CFHj ---+ROOH C R’C&IaOOR etry of the oxidation reaction, since, if alkoxy radicals were formed rapidly but disproportioriated excluded on the basis of the observed deutcriumslowly, a majority of the alkoxy radicals would isotope effect in the termination reaction and also have to enter into chain propagation reactions, on the basis of the argument that this reaction e. g. should occur as readily for cumylperoxy radicsls C6HsCHCHaO. t K H --f CoH3CWCHaOH + l i . as for a-phenethylperoxy radicals. In this case The observation that only one molecule of oxygen the kinetic chain length observed in the oxidation ~ was present in non-peroxidic products per two kine- of cumeiie (0.1 Jf AIBN) should be 1 ’ ~(6.90; tic chains in the oxidation of ethylbenzene ex- 7.W X 1:’1.20 X 1!.’6.8) that observed in the oxidation of ethylbenzene (0.1 r l l AIBN) whereas the cludes this possibility. The improbability of h i > kt can be deiiioii- kinetic chain length is actually much greater i n the strated bj- a soniewliat less direct approach. ITre oxidation of cumene (Table 11). Finally, we would like to poitit out that the tercan write the followinq expressions for the oxidation of ethylbenzene performed in the presence of niiriation mechanism suggested for a-phenethylperoxy radicals is possibly general for prini. ant1 sec.0.1 AI ~ 1 ~ s . 1 4 alkylperosy radicals. Previously we have demk , ,k - rate of chain propagation - kr[KII] [ROO.] = onstrated that most prim.- and sec.-proxy radical t 2ks[R00:] z-rate of chain terminatiorl terminate much more readily than sec.-alkylperoxy ’Therefore, if ki > kt, R,, ’hi < 7.4. It would seem radicals. This suggests that many prim.- and likely that cuniylperoxy and a-phenethylperoxy sec.-alkylperoxy radicals terminate in the same manradicals would interact to form alkouy radicals a t ner and that a differenttermination reaction is itiabout the same rate. I n this case, and at the volved for tu.-alkylperoxy radicals. Such a circumsai?ic radical concentration, WE could write for an stance is readily appreciated if prim- and sec.-alkyloxidation of cutnene peroxy radicals terminate by the cyclic interaction pictured wherein an a-carbon-hydrogen bond is ruptured. Ter.-alkylperoxy radicals, lacking an acarbon-hydrogen bond, cannot undergo this trrmiwhere 6.90 and i . S 2 are the concentrations of cu- nation xeaction.G mene and ethylbenzene, :md the ratio 1:120 repBolland and C,ooper have suggested that in the resents the relative reactivities of cumene and photo-sensitized oxidation of ethanol acetic acid ethylbenzene towards a peroxy radical.8 u’e is formed in a termination reaction involving have previously shown that a-phenethylperoxy peroxy radicals.” In the absence of any data to in the rauge 4%7-55 are not unreasonable (see footnote 22), A H , may the contrary, and in view of the present findings, he as low as 59 kcal. per mole of p e r o x y radical. we would prefer to formulate the reactions as shown. f11) C. H. Bamford a n d M. J 9 . Dewar [PYOC. Roy. Sur. (Lo;idu;z)2 Deuterium-isotope Effects in Aliphatic Sub198A, 232 (1940)l have assigned an energy of activation of about 1, kcal. mole-’ to the termination reaction between two a-tetralylperoxy stitutions.-The observation that the deuteriuniradicals. isotope effect observed in the attack of a peroxy (12) E;. \Viberg, Chetn. Recs., 56, 713 (195.5) radical upon an a-hydrogen atom of an aralkyl (13) E.R. Bell, J. H. Raley, 1:. F. Rust, F. H. Seubold, J r . , and hydrocarbon has the magnitude k a / h n = 5.5 is in \Y, E. L’auchan, D i s c . F a r a d a y S o r . , 10, 242 (1051); F. H. Seubold, ~,~
I
Jr., F. F. Rust a n d b‘. E. Vaughan, THISJ O U R X . A L , 73, 18 (1951). (14! In the oxidation oi ethylbenzene 4.2 molecules of oxygen was cnn5unied per initiating radical. One molecule of oxygen was consumed in the iiiitiating step and one molecule of oxygen was liberated i n the terminatinn nf twn kinctic chains. Therefore, on the average. rdillcn! ab5trdCtCd a hrdrogi.11 olnln 2 . 7 tini?. an ( x ;ihei?ct~lq.l~~eros?. before it entered into a termiration rcactjon
( i i ) G . .A Rnssell. ’ ~ I : I F J C I U R N A L , 77, 4583 (10.i33. ( l i i ) T h i i reaction is similar to the termination reaction t h a t D C C U ~ J in perosiilations inhibited by phenols or siibstitiited-anilines [ C . E. H a m m o n d , i,’>i,?,, 76, 3 E C l ( l < 2 2 4 ) , 77, 3 2 3 3 , 3238 i1 9.5 5 I
!
(17) J 1.. Uullaiid and H. R.Cooper, P r J r . I< ,j..%:.( L o u d o i i ) ,2 2 5 8 , 105 (1954j; .Tomre, 172, 413 (19S3,
MECHANISM OF
July 20, 1957
THE
INTERACTION OF PEROXY RADICALS
TABLE I11 DEUTERIUX-ISOTOPE EFFECT, ENERGY O F ACTIVATION,EXOTHERMICITY AND SELECTIVITY O F hTOMS AND REACTIONX. R H 4X H R.
+
X
RH
Exothermicityo
+
Energy of activation
3875
RADICALS 1 9 THE
Relative reactivity of toluene (prim.-hydrogen) t o cumene (ter.-hj drogen)
kdku
1:4 ( S O " ) d 24.7 2-3 2 . 0 (80')" C6HjCHz c1. 1 : 5 (135)O 2 5 . Oe .. 3 . 7 (107-116°)f C6H5CHzCH3 (CHa)3CO. Br . C6HsCHs 9.0 7.2h 4 . 8 (77")i ...... .. 1 : 13 (90')': C&IjC(CH&H 16-26 6.7' 5 . 5 (60") ROO. CH,. CaHsCH3 23.5 8.3' 5 . 5 (1250)" 1: 1 2 . 5 (91.5")" * Based on bond dissociation energies listed by Cottrell (ref. 10). * H . 0. Prichard, J. B. Pyke and A . F. TrotmanD(Ro-H) Ref. 24. H. C. Brown and G. A. Russell, ibid.,74,3995 (1952). Dickenson, THISJOURSAL, 77, 2629 (1955). taken as 100 kcal. mole-', P. Gray, ref. 10. f E. L. Eliel, F. T. Fang and S.H . Wilen, Preprints qf General Papers, Div. Pef. Chem., 1, 195 (1956). 0 A. L. Williams, G. A. Oberright and J. W. Brooks, THISJOURSAL, 78, 1190 (1956). * H. R . Anderson, Jr., H . X. Scheraga and E. R. J'anArtsdalen, J . Chem. Phys., 21, 1258 (1953). i Ref. 12. j H. \V. blelville and S. Ref. 8. 2 A. F. Trotman-Dickenson and E. R. Steacie, -7, Chem. Phys.. 19, 329 Richards, J . Chem. Soc., 944 (1956). (l95l), this energy of activation may be largely a measure of the reactivity of aromatic hydrogen atoms, see reference cited in Toward the trichloromethyl radical, E. C. Kooyman, Disc. Faraday Soc., 10, 163 (1951). footnote f .
excellent agreement with an isotope effect of about 5 reported by Max and Deatherage for the relative rates of autoxidation of cis-9-octadecene and its 8,8,ll,ll-tetradeutero derivative.'8 The isotope 2RCHOHOO.
R-C
----f
0 +RCO?H I !
+ RCHO + H?O + 0,
RCHOH RCHOHOO
+ HOO. + 0 2 0 \
H O d
H effect reported here has more significance, however, because in the case of the octadecenes isotope effects in both propagation and termination reactions may have been present as well as uncertainties in regard to the rates of initiation. Wiberg has discussed the correlation between the magnitude of the deuterium-isotope effect in free radical substitution reactions and the reactivity of the attacking radical or atom.I2 In Table I11 are listed a compilation of data concerning the isotope effect and the reactivity of chlorine atoms, alkoxy radicals, bromine atoms, peroxy radicals and methyl radicals. The exothermicities listed in Table I11 were calculated assuming a bond dissociation energy for an a-carbon-hydrogen bond of toluene of 77.5 kcal. m ~ l e - ~ . 'This ~ may be as much as 10 kcal. too low.2o The exothermicity of (18) R. A. M a x and F. E. Deatherage, J . A m . Oil Chcm. Soc., 28, 110 (1951). (19) From t h e pyroiysis of toluene, M. Szwarc [ J . Chcm. Phys., 16, 128 ( 1 9 i S ) l . From electron-impact measurements a value of 77 =t3 has been obtained [D. 0. Schisder a n d D. P. Stevenson, ibid., 21, 1.51 (1954)l. (20) T h e following bond dissociation energies for a benzyl hydrogen atom have been recently reported: electron-impact. 95 kcal. mole-'. LJ. B. Farmc:, i. H. S. Henderson. C. A . McDoweil and L. P. Lossing, i b i d . , 22, 19.18 (19541, J. B. Farmer, F. P. Lossing, 0. G. H. h f a r s a e n
the reaction between a peroxy radical and a n i e n e listed in Table I11 has been calculated assuming D(ROO-H) to be 90-100 kcal. mole-'. This estimate is based on the comments of Vaughan2' and as 100 the recently determined value of D(R,,-H) kcal. mole-'. 2 2 From Table I11 i t is apparent that the deuteriumisotope effect increases with the energy of activation of the reaction and with the selectivity of the attacking atom or radical. We believe tha.t in reactions involving a low energy of activation and a low deuterium-isotope effect, the carhon-hydrogen (or deuterium) bond is only slightly broken in the transition state. As the atom or radical becomes more selective, the energy of ac1;ivation increases, the deuterium-isotope effect increases and the transition state for the reaction involves a greater amount of bond breaking. The argument that the transition state in these hydrogen-abstraction reactions varies between one resembling the reactants and one involving a carbon-hydrogen (or deuterium) bond that has been extensively ruptured can be placed on a more quantitative basis. Evans and Polanyi suggested that for processes such as -AB DB
+ C +d t BC + C +D + BC
(1) (2)
involving similar A and D that AEl* -- AEz* = a(AH1 - AB2) where a can vary between 0 and 1 and decreases with an increase in the reactivity of the attacking radical or a t o m z 3 It now appears and C. A. h~cDoNel1,ihid.. 24, 5 2 (1956)l: bromination ,>f toluene, 88 kcal. mole-] [H. R . .4nderson, Jr., H. A. Scheraga and E. R . Van Artsdalen, i b i d . , 21, 1258 (1953)l: pyrolysis of toluene, 90 kcal. mole-' [H. Blades, A. T. Blades a n d E. W. R . Steacie, C a n . J . Chem., Sa, 298 (1954)l. (21) W. E. Vaughan, D i s c . F a r a d a y Suc., 10, 314 (1961). (22) Recently published heats of hydroperoxidation of a number of hydrocarbons [W. Pritzkow a n d K. A. Miller, B c r . , 8 9 , 2318 :195G)] are not in disagreement with a bond dissociation energy of 90- 100 kcal. if t h e reaction R. 4- Os ROO. is exothermic by 20-30 k c d . mole-'. For example, t h e reaction C6H:% 0 2 -+ C6H1100H is cxo:hermic b y 28 kcal. mole-1 while t h e bond dissociation energy of a cyclohexane carbon-hydrogen bond is about 92 kcal. mole-' [J. L. F a n k l i n , J . Chcm. P h y s . , 21, 2029 (1953)l. I f D(ROO-H) is 90-100 kcal , it follows t h a t D(Ro-o).) is in t h e range 45-55 kcal. since D(R..o.) i s 92 kcal. [P. Gray, ref. 101. (23) M. G. Evans and M. Polanyi, T r a n s . F a i a d a y S 8 x . , 34, 11 (1938): E. C. Baughan and 11. G. Polanyi, X a t u w , 146, C85 (1940); E. T. Butler and M. Poianri, 71ci:s. F w c d a ? ' Soc., 39. 19 (1943); 11 G. Evans, Disc. F a r a d a y Soc., 2 , 2;' j191ij.
-
+
3876
GLENA. RUSSELL
VOl. 79
that the value of a can be used not only as a meas- nance forms such as ure of the reactivity of the atom but also as a R.+H:X R.+H:OR R.+H-OOR measure of the extent of bond breaking in the transishould be important in the attack of atoms or radtion state. We have shown previously that in photochlorination a is 0.1 and suggested that this icals having a high electron affinity on a carbonshould be interpreted as meaning that the carbon- hydrogen bond. Of course, the exothermicity of hydrogen bond is only about 10% broken in the the reaction cannot be completely ignored. For transition state for the attack of a chlorine atom example, when the polar effect is held constant, such on a branched-chain hydrocarb0n.~4 On the other as in alkoxy and peroxy radicals, the more exotherhand, a is 0.5 for the attack of a methyl radical on mic reaction (attack by alkoxy radical) proceeds an aliphatic hydrocarbon indicating that in this more readily and with a lower selectivity. case the original carbon-hydrogen bond is half Experimental broken in the transition state.25 For peroxidation Materials.-Cumene and ethylbenzene (Phillips 99 mole a value of a of 0.39 has been f o ~ n d . ~'I'alues ~ , ~ ~ % minimum) were rectified in a large Podbielniak column of the deuterium-isotope effect (Table 111) are in (>50 plates) and middle fractions having constant refractive and boiling point selected. These fractions were agreement with these conclusions. The deute- index combined, chromatographically filtered through activated rium-isotope effect is small for the reactive chlorine silica gel (Davidson, 80-100 mesh) and stored under nitroatom and large for the less reactive peroxy or gen at 0'. Cumene, b.p. 69-69.5 at 42 mm., itZo1)1.4912; inethyl radicals. The deuterium-isotope effect, ethylbenzene, b.p. 138.5-139', nPoD 1.4959.31 a-dl-Ethylbenzene was prepared by the reaction of lithium which occurs because of the freezing out of certain aluminum deuteride (Metal Hydrides Corp.) with a-phenvibrational frequencies in the transition state, ethyl chloride.32 The a-dl-ethylbenzene was purified by should be 1.4 a t a = 0 and, although temperature rectification through a spinning-band Podbielniak column dependent, can be calculated a t a = 0.5 (e.g., (ce. 50 plates) and chromatographically filtered through gel, b.p. 133-133.5', TZ% 1.4954. Analysis was perk ~ / k= ~ 4.7 a t 100°).12*28Attempts have been silica formed by mass spectrometry at an ionizing voltage summade to calculate a from the values of k H / k D ciently low that the ratio of peak heights for mass numbers by assuming a linear relationship between these 106 and 105 was After correcting for the presence of quantities over the range a = 0 to a = 0.5 (reac- isotopic carbon, the following analysis was obtained, etliyl1.1%, dl-ethylbenzene, 98.8%, dp-ethylbenzene, tions of the type under discussion with a > 0.5 benzene, 0.17c. proceed with great difficulty). From the data of d4-Ethylbenzene mas prepared by the addition ol deuTable I11 the following values of a were calcu- terium (Stuart Oxygen Co.) t o purified phenylacetylene ~ at 25' in the presence of a lated, C1. (SO'), a = 0.08: (CR3),CO., (DOo), (b.p. 64.5 a t 48 mm., n Z O1.5489) platinum oxide catalyst. The product was rectified a = 0.43; Br. ( 7 7 O ) , a = 0.44; ROO. ( G O O ) , through a spinning-band Podbielniak column and chromatoa = 0.50; CHa. (l25'), a = 0.50. These values graphically filtered through silica gel, b.p. 136-136.5", I Z ~ D of CY are in surprisingly good agreement with the 1.4943. Analysis by mass spectrometry at reduced ionizing potential indicated the following composition, ethylbenzene, experimental values. 0.05%, &-ethylbenzene, 0.8%, dz-ethylbenzene, 5.2%, daPolar Effects in Aliphatic Substitutions.-Table ethylbenzene, 20.570, dd-ethylbenzene, 41.5%, &-ethylI11 demonstrates that there is not a direct rela- benzene, 31.9%. Extensive shufffing of hydrogen and tionship between the reactivity of an atom or deuterium atoms in the presence of a hydrogenation cataradical and the exothermicity of its reaction with a lyst was expected.34 a-d-Cumene was prepared by the addition of deutcriurn carbon-hydrogen bond. The value of a is a far chloride to the organometallic compound formed by the better measure of the reactivity of an atom or reaction of sodium-potassium alloy with a,a-dimethylbenzyl radical than is the heat of the reaction. Chlorine methyl ether.35336 The a,a-dimethylbenzyl methyl ether atoms and methyl radicals react with a hydrocar- was prepared by the addition of hydrogen chloride t o puria-methylstyrene (b.p. 81.542' a t 40 mm., n% 1.5382) bon with nearly equal exothermicity, but chlorine fied followed by methanolysis in the presence of potassium hyatoms ( a = 0.1) are far more reactive than methyl droxide. The ether was rectified through a small packed radicals ( a = 0.5), as judged from the energy of column (ca. 10 plates), b.p. 84.5-85', TZ% 1.4958, dz01, activation or deuterium-isotope effect, and less 0.946. a,a-Dimethylbenzyl methyl ether (100 9.) was t o 1.77 equivalents of potassium in the form of the selective.29 This is due undoubtedly to stabiliza- added sodium-potassium alloy (Xa:IC = 1:5, 54.5 g.) in 5 1. of tion of traiisition states by polar resollance ether. The deep red solution of the organometallic conlfornis.8.24-ao For example, transition state reso- pound was stirred for 2 1 hr. under a dry nitrogen atmos(24) G. A. Russell and H. C. Brown, ' I a r s J O U R N A 77, L , 4031, 4578 (1955). (25) A. F. Trotman-Dickenson, Q u e r f . Reus., 7, 193 (1953). (26) J. L. Bolland, Trens. F a r a d a y SOC.,46, 358 (1950). (27) a, of course, is not necessarily a constant for a given atom or radical b u t may increase with a decrease in t h e reactivity of the carbon-hydrogen bond involved. For this discussion we will consider a t o be nearly independent of the reactivity of the hydrocarbon. (28) This calculation assumes t h a t a t a = 0.5 all zero point energy differences in the transition states have been lost. This is thus a maximum deuterium-isotope effect. (29) The energy of activation for attack of a chlorine atom and a methyl radical on methane are 3.8 and 14.3 kcal. mole-', respectively [H. 0. Prichard, J. B. Pyke and A. F. Trotman-Dickenson, THIS J O U R N A L77, , 2629 (1955); J. R. McNesby and A. S. Gordon, ibid., 76, 4196 (1954). (30) Other independent evidence for polar effects in radicalhydrocarbon reactions has been presented [E. C. Kooyman, R. Van Helden and A . F. Bickel, Koninkl. X e d . A k a d . Welenschnp., Proc., B66, 75 (1953); R . Van Helden and E. C. Kooyman, Rcc. trav. < h i m . , 73,
phere before neutralization with 1.9 equivalents of deuterium chloride (prepared from benzoyl chloride and 11.3 g . of 99.54-75 deuterium oxide).37 The deep red color of the
, 1927 269 (1954); C. Walling and E. McElhill, THIS J O U K N A L73, (1951); C. Walling and B. Millcr, ibdd., in press. (31) All boiling points were determined with uncalibratcd ironconstantan thermocouples. 71, 3970 (1949); J. E. Johnson, (32) E. L. Eliel, THISJOURNAL, R . H. Blizzard and H. C. Carhart, ibid., 70, 3664 (1948). (33) D . P. Stevenson and C. D. Wagner, ibid., 72, 5612 (1950); R. E. Honig, Anal. C h e m . , 22, 1474 (1950); F.13. Field and S. H. Heatings, ibid., 28, in press (1950). All mass spectra were obtained a t the General Engineering Laboratory of the General Electric Company by hlr. G . Schachter. (34) R. L. Burwell, Jr., and A. B. Littlewood, ibid., 78, 4170 (195G). (35) W. G. Brown, C. J. Mighton and M. Senkus, J . 0 7 8 . Chetn.. 3, 02 (1938). ( 3 6 ) R.Ziegler and B. Schnell, A n n . , 437, 222 (1934). (37) H. C. Brown and C. Groot, THISJ O U R N A L64, , 2223 (1942).
July 20, 1957
MECHANISM OF THE INTERACTION OF PEROXY RADICALS
3877
organometallic compound was discharged completely by the prepare pure or-d-cumene and to elucidate the mechmism of first equivalent of deuterium chloride. The solution of a- the formation of ring-deuterated cumenes. d-cumene in ether was filtered and rectified in the spinningAIBN was recrystallized twice from acetone, m.p. 104band Podbielniak column, b.p. 70-70.5' at 47 mm., n m ~ 105'. The AIBN was used as a 0.01 M solution in benzene 1.4908. The product was analyzed after chromatographic which was stored a t 0'. The oxygen was 99.3-99.570 filtration by high resolution nuclear magnetic resonance pure, the principal impurities being nitrogen and hydrogen. spectroscopy, high resolution infrared spectroscopy and by Procedure.-Oxidations were performed in the general mass spectrometry. manner described previously.88 The desired amount of The high resolution NMR analysis was performed and 0.1 M AIBN solution was first pipetted into the oxidation interpreted by Dr. J. N. Shoolery of Varian Associates. flask and the benzene removed under reduced pressure a t 0". Three different peaks in the spectra of cumene and a-d- The desired amount of hydrocarbon was then weighed into cumene, each a measure of the concentration of tertiary the flask ( f O . O O 1 g.) and the oxidation flask attached to a hydrogen atoms, were compared. The relative heights of gas buret. The flask, which was a modified 50-1111. erlenall three peaks in the spectra of a-d-cumene and cumene meyer flask, was evacuated a t Dry Ice temperature and (run under comparable conditions) was 1:2.4 f 0.1. Thus, filled with oxygen five times. After warming to room tem41.6% of the cumene molecules contained an a-hydrogen perature in the presence of an atmosphere of oxygen, the atom and 58.47, of the cumene molecules contained an CYflask was placed in a shaking rack in a thermostated oildeuterium atom in the sample of a-d-curnene. A possible bath and vigorously shaken while the absorption of oxygen error of f 3 7 , seems possible in this analysis. was followed by manual control of the mercury level in a Analysis by mass spectrometry a t low ionizing voltages gas buret. (ratio of mass numbers 120 to 119 > lo4)indicated that 0.85 The densities of the deuterated hydrocarbons a t fjO' were deuterium atoms were present per molecule of cumene and calculated from the densities of cumene, d604 0.83C1,~Q and that the distribution of deuterium was as follows, cumene, ethylbenzene, dw, 0.831,39 by the equation of McLean and 32.8%, dl-cumetie, 52.07,, dz-cumene, 14.070, dr-cumene, Adams.Q 1.27,. Analytical.-The analyses of the deuterated h,gdrocarThe high resolution infrared absorption spectra were ob- bons have been discussed already. The oxidates Rere anatained and interpreted by Dr. R. S. McDonald of this lyzed for peroxidic oxygen by the iodimetric procedure of Laboratory using a Perkin-Elmer Model 13U spectrometer Kokatnur and Jelling'l and the stannous chloride p:rocedure with a lithium fluoride prism. The spectrum in the 3-4 w of Hargrave and The iodimetric proce(iure inregion was resolved into absorption bands for carbon-hydrovolved the addition of 1 ml. of oxidate to 25 ml. of 99% gen stretching vibrations which are characteristic for aro- isopropyl alcohol containing 1 mi. of glacial acetic :acid and matic, methyl and tertiary hydrogen atoms. Carbon1 ml. of saturated potassium iodide solution. This solution deuterium stretching frequencies were resolved in the 4-5 was refluxed for 5 minutes under an atmosphere O F carbon p region without resort to high resolution techniques. The dioxide, diluted with 25 ml. of water and titrated with 0.1 N assignment of frequencies is given in Table IV. sodium thiosulfate. There was no blank correction. The stannous chloride procedure used consisted of the reaction of 1 ml. of the oxidate with a mixture of 10 ml. of glacial TABLE IV acetic acid and 10.0 ml. of 0.05 M stannous chloride. After ASSIGNMENTOF C-H AXD C- D STRETCHING FREQUENCIES IN 1 hr. a t room temperature under a nitrogen atmosphere, CUMENE AND "a-d-CUMENE" the solution was titrated with 0.1 N potassium triiodide R-D Deuteriumsolution. In this analysis a blank was always run and the R-H stretching frequency, isotope concentration of hydroperoxide in the oxidate was proporR frequency, cm. - 1 cm.-L shift tional to the difference between the blank titer and the titer CeHs3100 (weak) .. .. of the reaction containing the oxidate. The agreement between the two methods (Table 11) is somewhat better 3080 2280 1.35 than expected from the data of Hargrave and 3040 2250 I .35 Analyses for acetophenone in oxidized ethylbenzene were CeHsCHCHgCHz2960 2220 1.345 performed by infrared absorption. An appropriate series 2920 (weak) .. .. of standards were prepared and the absorption a t 5.9 measured by a Perkin-Elmer model 21 spectrom-ter and CaHsC (CHdz2880 2145 1.34 plotted as a function of concentration. Attempts to anaAssuming that C ~ H ~ C H C H ~ C H Z.and - D CBHIC(CH8)2-D lyze for a-phenethanol by a similar procedure were not suchave similar extinction coefficients, it is calculated that not cessful because of the lack of strong and unique atsorption more than 37, of the cumene molecules contained a 8- in a-phenethanol. deuterium atom. SCHENECTADY, NEW YORK From these observations we conclude t h a t 0.24 (0.85 - 0.58 0.03) deuterium atoms were present as aro(38) G. A. Russell, THIS JOURNAL, 78, 1041 (1956). matic deuterium atoms per cumene molecule in the sam(39) G. Egloff, "Physical Constants of Hydrocarbons," Vol. 111, ple of a-d-cumene. This is in agreement with the ob- Reinhold Pub. Corp., New York, hT. Y., 1946. servation that in the NMR spectra the area of the aro(40) A. McLean and R. Adams, THIS J O U R N A L , 58, 804 ((1936). matic-hydrogen peak was slightly reduced for the sample of (41) V. R.Kokatnur and M. Jelling, ibid., 68, 1432 (1941). a-d-cumene. The neutralization of a,a-dimethylbenzyl(42) K. R. Hargrave and A. L. Morris, T r a n s . F a r a d a j , SOL.,52, potassium is being investigated further in an attempt to 89 (1956).
-
