C a n a d i a n J o u r n a l o f C h e m i s t r y

THE RTPECTIONS OF 1-1 .AND D ATOi\/lS WITH CYCLIC AND PARAFFIN HYDROCARBONS'

Abstract -1'lle reactions of H and D atonis, produccd 1))- thc tlischnrgc m c ~ h o d , \\.ith

benzene. iour cyclo:llkancs, and the correspontlillg nornlal paraffins have I~ccn in\.esiigatecl a t room telnperaturc. h,lct!lanc is t l ~ c major product- oi cach reacr.ior1. Collision yiclcls have I~ecn calcul:~retl on r;hc basis of hydrocarboll clis:~lqwa~ulce and exchange. O i ~ h c c!~cloall;a~re~, c),clopropane is the mui t inert \vi~ll cyclopcntanc the most renccive. C>.clopropanc and cyclobr~t;ine clo not crctiari~c \\fit11 D atornr: but benzenc, c!.clopentarlc, ancl cyclohexane undcrgo consicleral~lc ercllange. In thc case of ~ l l c l lor~nal paraffins the dcgrce of ex- charlge increases \\:it11 increasing nlolecr~lar \\.cigl~t.

I t is concluded h a t thc initial reac~iorl EI + RH-, I< + 111

is responsible lor the p roduc~s iorn~ccl in the reactions I I U I other primary pro- cesses In&:. bc responsible for the exchange o i these Ir)~drocarbot~s.

'The tlischarge methocl of producing 1-1 atoms has been widely ~lsecl to stutly the prirnar~. processes in reactions of E-I atoms wit11 h\;clrocarbons. The con- sensus of opinion a t present is that 1-1 atoms react .rvith paraffins hy hydrogen abstraction to [~rocluce hyclrocarbon I-aclicals ivhicln then undergo I-apicl "at- omic cracking" reactions. This belie[ is largel!. based on the al3pearance of methane as the major procluct in all such I-eactions. I t seemed of some interest therefore to investigate the reaction oi M atoms with benzene and with some of the cycloalkancs.

Materials

CS.lincler 111 drogen n7as freccl from ox>.gcn 114. passage over copper no01 a t 300°C., and through ttvo traps coolecl in l i q ~ ~ i d ~litrogen. About 0.67; ;,nitrogen still remained as an impurity.

Deuterium was obtained under pressure 1))- reacting 99.6% D 2 0 n,ith spe- cially purified calcium t ~ ~ r n i n g s a t 2G0°C. i n an autoclave; traces of NHR formed from the CasK2 present in the c a l c i ~ ~ m were readill- frozen out. Anallsis shon-ccl the cleuterium contained no H? and not more than lOYo H D for n~hicll appro- priate corrections were macle in the calculations.

1 . l I a ~ ~ z ~ s ~ r i p t received Septenzber 6 , 19ii0. Conlribz~tion froin the Division of Clternistry, iliational Rescarclt Labora,torics, Ottawa,

Canada. I.tstLed as IV. R. C. No. 2277. iValiona1 Research. Council of Canada Postdoclorate Fellow. Present address: Dcparti~zent

of Chemistry, AfcCilL University, dfontreal, Quebec.

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2 C A N A D I A N JOURNAL OF CHEMISTRY. VOL. 2 9

C\,clopropane \\as obtained fro111 the Ohio Chemical Conipan~- ancl \\.as rated as 99.5% pure. I t 11-as freed from nonconclensable gases anel s ~ ~ l ~ j e c t e d to several trap-to-trap distillations, the ~niclclle I , 3 fraction Ijeing retained.

Cyclobutane \\;as prc11arccl from c~~clobutanone b>- the 111etllotl oi ICobcrts and Sauer (9). The c~.clobutanonc was obtained from the reaction oi diazo- methane with ketenc. The cyclobutane was tested for in1p~11-itics b!. cal-elully fractionating a sample in a ~~lodihecl Ward still (10) and a~lal!.zing the fractions in the luass spectrometer. C>.clopentane, the largest impi~rit!-. \\.as 111-esent to the extent oi 0.6 mole (id; lighter hydrocarbons a ~ n o ~ ~ n t c c l to less than 0.07 mole yo. An infrared spectrum of the gas was founcl to agrec well ~v i t h pub- lished spectra lescept for the presence of an absorption nlasi~nunl a t ISLO c111.-1 for which no explanation has as yet been found.

National Bureau oi Sta~lclarcls standard samples ol ~>~cloper l ta~ lc and CJ-clo- hexane \\;ere clried bj- passage over pllosphorus pentosicle a11cl ~lsecl witho~lt further purification.

Carefully purified ancl iresllly clistilled benzene was PI-oviclecl ior 11s b\- Dr. L. Marion of this laboratory anel was thoroughly clcgassecl ancl clriecl over phosphorus pentoside beiore use.

The normal paraffins were Pl~illip's "Research Grade" nlaterials ratecl as 99.88Y0 pure. These \\-ere degassed and subjectecl to se\,eral tra11-to-tral, clis- tillations prior to use.

Experimental Procedure

The clischarge tube was of the familiar Wood-Bonhoefier t! pe clescribed in previous communications from this laboratory (11, 15). The h-clrogcn pressure was recluced from at~nospheric to about 0.35 mm., ancl its Llon- rate (58 cc. a t N.T.P. per min.) was controlled by a capillary leal; a t tile inlet enel and a tllrottle a t the pump end of the system; tilrottling a t the pump encl n.ns Lound to be necessary i n orclcr to ~nini~nize pressure fluctuatiot~s. .-\tom concen- trations were measured betore and after each experiment a t the top and bottoln of the reaction tube by lneans of M'recle-I-Iarteck gauges (5, 17) ~~secl in con- junction with a thermocouple type Pirani gauge which was frecli~entl~. Cali- brated for hydrogen and cleuteriuin. The average of these readings Ivas considered to represent the average atom concentration obtaining during the experiment.

The flow rate of the hydrocarbon was controlled by nlai~~taining a fisec] vapor pressure in a side tube of known volun~e. The pressure of the gas a t room temperature in this volurne was measured before and after each experiment; the difference then gave a measure of the total anlount of hydrocarbon useel in the from which the flow rate could be calculatecl. The reactant gases were mixed a t a distance of 20 cm. from the discharge ancl flowed together down a cjilindrical reaction tube; hydrogen atoms leaving the tube recorn-

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SCHIFF Ah'D S T E A C I E : RE:LCTIOSS OF H A N D D ATOMS 3

binetl on platinum gauze. From the ~lo\v rates of the gases. their [,artial Ires- surrs, ancl the volume of the reaction tulle it \\.as possil~le 10 c-idcul,ltc> the ax-er;lgc time spcnt 11)- a moleculc in the t~111c; this I-eartion time \\,IS ot tlie ortler of 0.6 sec.

The condensable products \\-ere trap1)ecl out a t liquid n i trogcn tempera ture, their \.olurne measured a t room temperature, ancl their colnposition t le tcr~~~inct l 1)). separation in a Ward still follo\vetl t)). mass spectrometcl- anal!.sis of the various tractions.

The nonconclensable proclucts I,assecl throi~gli a"Lc~~bolcl".tliltt~sion pump ancl ivel-c aclsorbed on silica gel a t liqi~icl llitrogen temperature. After an csl,eriment the silica gel \\;as maintained a t liquitl nitrogen t-crnj~crat~~l-e ancl ol,cnetl to a "megavac" oil pump for 15 min. i l l orcler to tlesorb most of the li\.cl~-oge~~. "Davco" technical grade silica gel \\-as touncl to be the most si~ital>le n~aterial for this purpose since the use ot a more "actix-c" gel ~naclc the clesor[)tion esceeclillgly slow. The amounts oi methane ancl ethane lost 1,). this procctlul-c lvere sho\vn to be negligible bj- blank esl~cl-iments 111acle wit11 kno\vn gas mis- till-es ancl by the excellent material halances ol,tainecl in all the ezpcrimcllts. The trap was then warmecl to room tempcrati~rc and the gas toeplerccl into a gas burette where its volulne \vas ~neasurecl. An alicluot [vas isolat:.cl in a saml)le bulb for subsequent mass sl,cctrometc~- analysis. From the amount of 11)-clrocarbon which hacl unclergone reaction (suI>seq~~entl~- rcicl-recl to as the a m o ~ ~ n t clecomposed) ancl the r cac t i o~~ tinw it u.as possible to calculate the rate of tlie initial process.

There are several objections i r c q u e ~ ~ t l ~ . raisecl against tliis methctl oi cal- culating the rate of the initial reaction. First the concentration of I 1 atoms decreases clo\vn the reaction t i ~ l ~ e as the r cac t i o~~ takes place; calci~latetl cor- rcctiol~s tor this eifect are complicatccl ancl gellerall!; i~~accura tc . Ho\vc\,er, the use of the mass spectrometer ant1 other im1)ro\~ecl anal>.tical I)roccclures In s nlacle it possit~le to work \\.it11 ver). s~iiall cli1a11 ti ties of 11)-tlrocarl~on gases. Consecluentlj-, uie \\,ere able to rctluce the tloiv rate of 'he 11yclroca1-bon to less than 1 '50th that of the 1-1 atonis, thc~-cb). greatly minimizing this soLlrce oi error-.

The practice of equating the ratc ot clisappeara~~cc 6f the 111 clrocarl~on to the ratc of the primary step call also be criticizecl. A scconcla~-) reaction \\ hich could regenerate the hyclrocarbon, s i~ch as thc combination of an atom u i t l ~ the h~drocarbon radical ~voulcl, of course, vitiate this I ~ ~ - o c e c l ~ ~ ~ e . T'his objec- ti011 ma) be overcome by the use of cleuterium in place of h>clrogen; thc amount of h~clrocarbon which has been e\changecl can t l i e ~ ~ be clcterminecl b> means of a Inass spectrometer and acldecl to the amount clecomposed in order to cal- culate the true rate of the initial step. I n order to interpret the mass spectra it \vas necessary to assume that the ion sensitivity (ion current per centimeter partial pressure) for the parent peak of a liyclrocarl~on does not change appre- ciably when the hydrocarbon is deuteratcd: evidence from ~vork or1 ~nethane

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4 CANADIAN JOURNAL OF CHEJIISTRY. I.OL. 2 9 .

ancl ethane inclicates tha t this assumption is reasonal.)l!- \-alicl. Corrections iirere nlacle for the c o ~ ~ t r i b u t i o n s to each parent peal; i l - o ~ ~ ~ the cracking i l l t h e mass spectrometer of more cleuteratecl h~.c l rocar l~or~ molecules, ancl for the CL" isotope. This in\-oli-cs the aclclitional assumption t h a t all the cleuteratecl hydrocarbons ha\-e sin~ilal- cracking patterns, b ~ ~ t sinc-c thcsc corrections were always small for the 1~j~clroca1-bolls investigatecl, this assumption does no t introduce a serious error. I t sl~oulcl be e~nphasizecl tha t the use of the mass spectrometer PI-ovicles a n l ~ ~ c h more positive ancl ser~sitii.c test for cleuteration than any other metl~ocl. 'The anlount of each isotopic s I~ccies of the c o m p o ~ ~ n c l can be measurecl inclepc~lclently ancl aclclecl together to gi\-e a true value of t h e percentage of the h ~ ~ d r o c a r l ~ o n which has ~i r lc ler~one eschange, ivlzereas the procedure of comb~rsting the l~yclrocarbon ancl arlal5-zing the resultant water gives onll- the ox-er-all d e ~ ~ t e r i u ~ n content of the sarnple. Fur ther , there is no possibility of error arising froin the presence of cleuterium or c l c~~ te ra t ed reaction products in the sample.

Products of the Reactions

Methane \\-as I )>- tar the most inlportant procluct in the reactions of H and D a toms wit11 all the l~~~c l roca rbons . -A.\nal\-sis of the methane procl~icecl in experiments with D atoms sho\ved i t to be over 80% CD, , ivith the remaincler being mainlj- CD31-I.

Methane ivas f o ~ ~ n c l to be the only product of the reaction of EI or D a toms with cyclopropane. This is neither in ag-reenlent ~ v i t h G ~ ~ n n i n g ancl Steacie (4), i i~ho found no e~riclence for reaction ol cyclopropanc \\-it11 H atoms produced by mercury photosensitization, nor with Dingle ancl LeRoy (3) who found no reaction with 1-1 a toms procluced by the thermal dissociation of I-Ir. I t n-as necessary, t l~crcfore, to investigate the reaction in cletail t o ensure t h a t t he methane in O L I ~ esperiments clicl not arise froln any estraneous sources. Blank esperiments, conclucted in the absence of cyclopropane, procl~~cecl some me- thane by the reaction of 1-1 a toms with the "apiezon" grease on the stanclard taper joints connecting the Wrecle gauges and the l~yclrocarbon inlet t ~ ~ b e with the reaction vessel. These joints were, tl~ereforc,'elin~inatecl from the appara tus and subsequent blank esperinzents proclucecl no appreciable amounts of

1 As previousl!- ~rnentionecl, , the h).clrogen contninecl abou t 0.6% nitrogen \vhicl~ formed a m m o ~ ~ i a ill the discharge t ~ ~ b e . T o eliminate an]- possible effect o f this inzpirrity one e rpe r i~nen t \\-as conducted b ~ . first passing the hj-drogen through a long, thin glass helis immersed in licl~~icl hyclrogen. This \vas found to remove the nitrogen :but had \no effect on the amount of methane p r o d ~ ~ c e d .

T o ensure t h a t no cyclopropane was diffusing back in to the discharge the hydrocarbon inlet tube was bent away from the discharge ancl experiments were conductecl over a considerable range of cyclopropane flow rates; in each case the amount of methane produced was the same percentage of the cyclo- propane used.

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I t nras later disco\-eretl t h a t s n ~ a l l methane peaks were present ill the mass spectra of tank h > clrogcr: e l el1 a t tcr the 115 drogcn hat1 been purit~ccl 111 passage th roug l~ a pallatlit1111 t t ~ b c I ' r c ~ ~ l ~ ~ l a b l y , the methane a s tor~nctl in some \I a y in the mass spectromctcx~ ~ t ~ e l l 'To correct this , a stantlard h \c l~ogcn s a ~ n p l e n a s mcasurecl in the mass s p e ~ t ~ o m e t e r belore each anal! sis a t a ~)rcssure equal to the partial h>~clroge~l pressure in the sample to be anall zed T h e height of tlle mass 16 peak obtainecl troll1 the hyclrogen spectrum 11 as then taliel1 a s the residual peak l l e~gh t to be subtractecl from the sample spectri1111.

Finally, to test n-hetl~er tl-tc ~ u e t h a ~ l c was t'ormetl from an). i1up11rit)- present in the cyc lopropa~~e , ~~nreactec l c>-clopropa11e \\:as recoverecl iron1 one esperi- ment ancl ~ ~ s e c l in a subsecl~~c.nt- csperiment; the percentage of nnethaue fo r~ned was fount1 to be the same in each case. We can safely concl~~clc, thereiol-e, t h a t methane is a t rue procluct ot t11c reaction of H atoms a.ith cyclopropane ~rntler our experiment c o ~ ~ c l i t i o ~ ~ s . S o clcuteratecl c>,clopropane was reco\-erecl from any of the deuterium cspcrimcr~ts.

In the case of c! .c lo l>~~ta~~c, ~inethanc a-as again the sole product of the reaction lvith H and D atonla, a l ~ i l e 110 m e a s ~ ~ r a b l c amount of cleuterated cyclobutane coulcl be cletcctccl.

\\'it11 c y c l o p c ~ l t a ~ ~ e a11tl c\ clolne\ane, ~ n e t h a n e accotuntecl for over 00% of the reaction protlucts, \\ 1111 s ~ n a l l amo~unts of other light hydrocar l~ons also being formed. T o illustrate tlle order of ~ n a g n i t ~ ~ c l e of these products, the r e s ~ ~ l t s ot the mass spectrornctcr anal>ses of the p roc l~~c t s of the H atom- c>,clopenta~le r e a c t i o ~ ~ a rc rcl~ortecl in Table I. In the deuterium euper i~nents co~nsiclerable exclna~lgc \ \ a s found to occur. I t was found, ho~vel-er, t ha t the ratio of the amount of 11: clrot ,~r l )on decomposed to the a r n o ~ ~ n t e\changed was not constant b ~ ~ t incrcasecl n it11 increasing D a tom concentration.

Tl-te green fluoresce~~ce rcportetl \I> Bonhoeffer nncl Har teck ( I ) for the reaction of EI a toms n it11 benzene n a s obscrvecl in several of our expcrinients. .About 8 5 5 of the clecomposetl l)e11zene appeared a s ~netl-tane, t he otller 15% being co~nprisecl of all the strClight chain 1iyclroca1-bons containing siu carbon

Cyclopentane Row, cc. per min. N.T.P.

I-I atom Roiv, cc. per min.

N.T.P. Mass

balance

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6 CANADIAN JOUFLVAL OF CIIEIVIISTRY. L.OL. 29.

atoms or less. This is a good inclication of the complexity of the scconcla~-:- reactioils involl-ecl; apparentl>. oncc the h>.clrocarbon radical is formed, ,it undcrgocs rapicl atomic craclcing and h\-drogcnation reactions until the rela- tivcly inert niethanc niolecule is formccl. ?'her-c \\.as no evidence for the for- mation oi an\. c>-clohexane or c)-clohcxene, \\-liicl~ might have been espectecl in the event of hyclrogenation of the benzene \\.ithout ring split. The benzene reco\.erccl after reaction \\-it11 D atoms \\-as iountl to ha\.e uncl~rgonc exchange to a consiclcrable extent.

Single csperi~~lents \yere macle with each oi the normal paraffins for co111- parison purposcs. The reaction products 11 erc almost exclusively methane in each case. -111 the no]-ma1 paraffins \yere founcl to be partially exchanged in the deuterium esperirnents.

Calculation and Discussion of Collision Yields

The collision >.ield can be defined as: Thc rate of the primar\ process

The number of collisions per cc. per sec. The nu~nerat-or is cletern~incd experimentally; the clcnolninator, the collision number, can l ~ e calc~~latecl from kinetic theor:.. The values for the collisio~l dianlete~-s i~secl in these calculations were obtained from viscosity data (13); 2.1 a for H , 4.2 for CaHe, 4.8 for C41HLo, 5.2 lor CjHl?, 5.8 for C6H14, $0 for cyclo-C:,H,;, 5.2 for c ~ c I o - C ~ H ~ ~ , and 5.0 for Cr,HG. iralues of 4.4 and 4.8 A were obtainecl b ~ . interpolatio~l for cyc10-C4H3 and c!-clo-CjHlo respectively. Thc de- tails of tile calc~~lntions of collision yields for the c~~cloalkanes are shown in Table 11. 'The experiments markecl D in column 1 ol the table refer to cleuteriun~ experi-

C.ILCCLATION 01' COLI.ISI0K YIlYLDS FOR TIlE C\.CI.O.ILI<AIVES .- -- -- -

Par~ial Per .Z Per cent cent; Per Collision Collision

t.irlle, ,a,:iorl nle~llane otllcr y;';Li;; ex;;;;gc sec. t,inle(a) formrd pro- cl'al~ge X 10' X 10'

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SCHIFF A N D STEACIE: RE.4CTIOArS OF H A N D D ATOMS 7

l'.\nr.rs:il l-Co~r!inried -

( a ) T h e nrrrrzber of collzsions o j 1 kydrocnrbo7z i7rolecr~lc w d l ~ H or D atonrs i ? z !he reactiota tz t~lc . ( b ) I n the dezr ter~um exper inrents olhcr prodr~rls when drler7nrned are groziped logellrcr roilh llre

7rlethanc.

1

,692 1 . 8 ,373 j 16.6 ..5OS i 13.3 .5lS / 10.1 ,201 20.0 IliY 1 14.7 . -d '793(D)1 8.30 .143(~))I 8.30 .O96(D) 3.87 . lilO(D) 5-46 .2fiG(D>, 3.75 .27O(I~)!j 3.36 ..?02(D)' Dittusion 2 ( 1.91 .2002(1))1 2.49 otio(n)i 1.91 . l l9 (D) 3.20 .319(D) 1.47 I ( ! 1 0 . 2 ~ 0 ( l ? / 1.75

I

ments. The collision yields for all the hydrocal-bons along with their mean deviations are shown in Table 111. Coluxun 2 of Table I11 gives the collision yields for the hydrogen experililents calculated from the rate of hydrocarbon disappearance. Column 4 gives the same data for the deuteriuiii esperiments,

1 PC, CoIIirioli >,ield for rczction x 107

111

pro- nlrn. Hg. jeC.

ducts(b,

-

Cyclol~rsniri:

.049 ,059 .041 .031 .OGG 0 5 2 ,077 ,077 ,037 ,050 .032 .032 into ,017 ,020 ,016 ,028 ,013 .013 .017

CoIIision ).icld

cscllrtnge x 10;

ccnt ex-

C I ~ ~ " ~ C

C~c loD~r!a i~ . i~

I

8 2 / 1 - I 0

.85 t i . 1 / 24.0 Discl~arge 1

.79 3 . 5 - -

.82 2 . 6

+. - --

. i t 2 . 1 12.4 ?. -

--

.207 i 16.1 .097 13.5 .082

9 4 .059 :4:i 1 0 . ,065 .13-1 13.8 ,082

I 6 7 I 17.9 13. I 1 >O.l .67 15.1 1 12.8 7 0 12.6 17. 1 7 1 1 10.9 i 1 i . s .7O 11.9 1 12.9

. I i l (D) : 10.0

.253(1111 9 . 2 0 3 ( 12.0

I

26.4 31.0 35.7 41.7 51.0

.. - 1 3

.85 1 . -1. 1 .0 1.1

0 6 4 0 5 3 ,073

I I

3.4 .1.0 5 . 9 3 .6 3 .9

37.5 40.2

0 2 . 6 0 3 . 2 0 1 . 6 0

.69 1 1-1.6 14.9 1

1 . 9 2:i 5 . 1 -1.4 8 . 3

5 .9 1 17.5 ti.0 I 14.4

1.00 I 2 7 3 3 6 1.02 1 1 0 7 3-LO

9 9 1 14.3 23.0 I

>O. 1

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8 CANADIAN JOCRNAL OF CHEMISTRE'. I'OL. 29.

while column 6 gives the collision ).ields for thc eschange process; column i is the ~11111 of coli~mns 4 and 6. The average \-slue lor t11c percentage of h\-clro- carbon I\-hich has unclergone exchange is gi\.en in column G ; this will, of course, clcpe~icl i1po1-1 the D atom concentration. .Icti~-ation energies arc gi\-en in colunin 3 ~ O I - the 1-1 atom reactions ancl in colunln S lor the D atom I-eactions: the latter \\.el-e calculatecl from the combinccl collision yielrls for exchange ancl decon~position. In ~naliing these calci~latiolls a value of 0.1 has been chosen for the steric factor, in the expression

collision yielcl = ~ . e - ~ * ' " ~ since this has been tlle custom in the past in clealing with H atom react io~~s. I-ionrever, a great deal of evidence is accun~i~lat ing which indicates that many free radical reactions have very lo~v steric iactors (ca. lo-'). There appears to be no wa!. a t present of deciding if steric factors are low for H atom reactions. If tire). also should prove to be very Ion. then the activation energies reportecl may be greatl), in error. However, this has no effect on any arguments con- cerning relative reactivity which are basecl on collision jrields.

The reason for the i~ncertaintl in the case nt benzene becomes apparent when the stoicheiometry of the reaction is consiclered. At least 18 H atoms arc required to transform one ~nolecule oi benzene into methane. Since this reaction is rapid there is a serious depletion of H atonls even a t the flow rates used in these experiments, nrhicI-1 results in too lo\\- a value for the collision yielcl. (Robh and 3Ielville ( 7 ) have recentl~, reportecl a collisio1-1 yield of 7 X lo--' for this reaction cnlculatecl from the rate of 1-15 clrogen atom clisappearance.)

It will be seen from Table I I I that c).clop~-opane is more inert than the othel- cycloallianes, with c~~clopentaue being the most reactive. I t is interesting to note that this is the same orcler of reactivity founcl by Trotman-Dickenson ancl Steacie (16) for the reactions of meth\:l radicals with the c)~cloalIianes. The inertness of cyclopropane may seem son-1eivhat surprising although the rate

COI.LISION YIELDS A N D AC.TI\'\.TION ESLRGIES - - -. - - . -- I I

Cyclopropane Cyclobutane Cyclopentane Cyclohexane

Benzene

Propane n-Bu tane n-Pentane n-Hexane

EI atoms D atonrs - - - . -- - - --

Ac~iva- Collision Collision Total j Collision iion yichl for yield for Collision

Yield X lo7 energy, reaction ! c x c h a ~ ~ g e 5-ielcl Iccal. X loi

-- -- -p

a- lc~i \~z~- tion

energy, kcnl.

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SCHIFF A N D STE;ICIE: RE.'ICTIONS OI.' H AND D A 7 0 5 1 5 9

cletermining step probably clocs not in\.ol\.e the strainecl ring co~l f ig~~ra t io r~ 1,ut I-ather the I - L I ~ ~ L I I - e of a C-FI t)o~lcl \\.hich coulcl be strongel- than the C-fl I~ond i l l 11orlnal paraffins. Il'ith the csccption oi c~.clopropane the c!.cloall;a~les react sorne\vliat more ~-apicll~- than tileis straig-llt chain cot~nterpa~-ts. 'I'he tact that the c!.clic h~~clrocarbons possess orll\- scconcla~-y boncls scarcel!. seems sutticicnt cause for these clifferences.

Since only single esperimcu is \\.ere concluctecl \\.it11 each of the ~lormal par,~f- fins no significance can be attril~utecl to the cliflere~lces in their collisio~l ! icids, \\ tlich are \I ithin the cstimatecl e\l)crimental error. The collision ! iclcls conl- pare quite favorabl!. \\it11 prel io~~sl!. p~~bl i shcd values.

The percentage decompositions in the c l e u t e ~ - i ~ ~ ~ u espel-iments al-c all sonle- hat larger than for the corrcsponcling H atom experiments. Since the atoms

(lo not possess zero point energies t-he clifferences, i f real, n z ~ ~ s t be attl-ibutccl to the energy clifferences in the acti\.atecl complexes. Ho\vel.el-, these tlill'ercl~ces ma)- be largel~. superficial since it: \vas noted during blank experiments that tht= D atom concentrations clecreasccl Inore slowly with time than die1 t-l~e I3 aton1 cor~ce~ltrations. Herice the procecl~~re oi sing the average of the at0111 concell-- trations measi~red befol-e ancl after each esperinlent ma). involve el-rors sufli- cientl!. large to account for the cliliel-ent sates." The niai11 point oi interest in the cleuterium esperiments lies i l l the degree to ~ ih ic l l the I1!-cIrocal-bons al-c escllangecl. In the case of cl-clopsopane ancl c.~~clobutane no eschange occurs, ivhile c\,clopentane, cj,cIohesane, and benzene ~~nclergo consiclerablc eschange. I n the case oi the normal paraflins it will be seen that tllc clegl-ec oi escllange increases with illcreasing molecular weight. This appears to be in clisagl-eenle~~t with the xork of Steacie ancl I'al-lee (12) \vl~o founcl tha t C;;I-Is docs not es- change \\,it11 D atolns a t room tcmperat i~~-e, ancl with Trenner, h'Ioril;a\\,a, ancl 'l'a~-lor (14) \vho iound no escllange \i.itll eitller CaHs or'li-CJIl,,. I-Iowcver, \ye ha\.e alreacl). mentioned that the mass spectrometer metllocl is nlr~ch nlol-e positive allel sensitive a test than those e;nplo);ccl by these \\rorl;crs. Thus the

escllallge which u.e obser\.ecl in the case of propane (\\~hicIl \vas mainly C:rH;D) \\;auld correspond to less than lyo c l eu t e r i~~~u content b!. the nlctliocl oi Steacie ancl Parlee.

The eschange of a 113-clrocarbon ma)- be conceivecl to occur by one ot' the iollo\ving mechanisms:

A. RH + D -+ I3 + I-ID (1) R + D , - + R D + D (2)

R. RH + D -+ R + IHD (1) R + D - , R D * (2) RD" + M -, RD + ni1 (3 )

C. RH + D -+ (13-R-D) -+ R D + H

* It ,,cay be argued that the reactior~ tinle is greater i n the case of dezrteriz~ti~ owing to ils slo7ucr ,/ow rate. Esperinzental nieaszlretnents showed tlcc relativejlo~v rates of H? and D? to be 1.41, i.e., the inverse ratio of the sqzrare roots of their nzasses. From the kinetic theory of z f i s c o ~ ~ s -flow it colt be shou~n that this corresponds to eqlral collisior~ dianzeters for H? and D?. Since the redzrccd i ~ ~ a s s e s i n tics expression for the collision nattlber are nearly i n the inuerse ratio of the square roots of the ~nasses of Hz lo D? these etfect.~ should nearly cancel.

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10 C A N A D I A N J O U R N A L OF CIIE.1IISTRY. VOL. 29

The reaction of methyl radicals ~ i t h H? has been sho~vn to occur with an activation energy of about 9 kcal. and a Ion- steric factor. I t is to be expected that the activation energy sho~~lcl probabl?. increase and the steric factor cle- crease for reaction A (2) with more co~nplicated 11)-clrocarbo~i raclicals. I-Ience mechanism A would not be espected to pla!- a very important part in exchange processes a t room temperature. Iion.ever, the increase in the ratio of hydro- cal-bon clecon~posed to hydrocarbon eschangecl with increasing atom concen- tratio~l observed in the case of c>.clopentanc suggests that this mechanism may con t r i b~~ te slightly to the exchange process.

The radical recornbination mecha~lism B provides an alternate fate for the hyclrocarbon radical to that of "ato~uic cracking". I t is similar to the mech- anism first proposed by Taylor (14) to explain the high clegree of cle~~tel-ation of thc methane formed in D atom reactions:

CH, + D -+ CH3D* -+ CI-IZD + H etc.

'I'he quasi-deuterornetl~ane can be stnbilizecl only bj. collision \\.it11 the nall or some other third body. The lifetime 01 the quasi-molecule is probat>l!. q ~ ~ i t e s~nall and so there is a good probabilit!. of the splitting off of an H atom. This process recurs until there are only CDa ratlicals reacting with a large excess of D atoms and eventuall>. a three boclj- collision \\.ill occur to procluce a stable CD.I molecule.

Trenner, Moriltawa, and Taylor (14) suggest that the three bod!- rescricrion is also important for the combination of D atoms with prop!.l arltl b~~t!-l raclicals, although their experimental evidence is not verjs clear. If cxcliange cloes occur by mechanism B then our I-esults shon. tha t tllc three I)ocl!- re- striction is much less important for the con~pouncls investigated. Fig. I repre- sents a typical mass spectrum of the cyclohesane rccoverecl after reaction n-it11 D atoms. h~Iass 84 is the parent peak oi c).clohexane ancl so the subsequent pealts will be nearly proportional to the amount of each clcuteratecl c!.clo- herane present. We see that they decrease reglllarl~. from CcHllD to C6D19, the re\.erse orcler to that observed for the cleutcromcthanes formed from this re- action. On the basis of mechanism B this simply means tha t for so complex a n~olecule suficient clegrees of freedom exist for the sharing of the excess energ)- that the collision time exceecls the average lifetime oi the quasi-rnolcc~~le, ancl the amounts of the various de~~ te rocyc lo l~esa~~es are those cspectecl on a proll- abilit:!. basis. The absence of deuteration in pairs also sciggests that the excess energy does not remain localized a t any single carbon atom in the molecule. Completely analogous spectra were obtainec~ from the other h>.tlrocarbons reacted with D atoms.

The reason for the absence of cleuteration in the cases oi c j clopropane and cyclobutane may lie in the strained s t ruc t~~re s of these co~upouncls. T~ILIS i f the cyclopropyl and cyclobutyl radicals possess strain energj- then rhey may be espected to decompose upon collision n-it11 a D atom with little likelihood of being stabilized. Cyclopentyl and cj-clohesyl radicals on the other hand are relatively free from strain and could be readily stabilized.

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SCHIFF A N D STEACIE: REACTIONS OF H A N D D rlTO.L[S

MASS NUMBER

FIG. 1.

In the case oi the normal paraffins, mechanism I3 suggests that as the nunibel- oi degrees of freed0111 increases, the average liictinle ot the quasi- molecule shoulcl also increase. This predicts increased deutcrntioll 111) the series which is in accol-cl with experimen tnl observation. Ho\vever, n c \\ oultl espect the amount of exchange to incl-ease at the expense ol the amount clecornl,osed if the rates ot the initial process \\..ere the same for all the normal paraffins. Experimentally, the amount decomposed remains essentiall!. constant through- out the series. T o be tenable, mechanism B theretore requires all increasing rate for the initial process equal to the increasing rate of eschange. I i this is so, then the collision yields for exchange reaction must be adclccl to the collision yield for the decomposition reaction in order to obtain thc collisio~l yield of the primary process.

If mechanism C is operative then the collision j,ields shoulcl not be added since they represent different initial steps. The absence of exchange with cyclo- propane and cyclobutane must be attributed to the inabilitjr of the strained

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12 C.4.VdDIA"i J O C W d L OF CHE.VllSTRY. VOL. 29.

structures to iindergo the IYalclc~l t ~ p e inversion, whereas the incrcasi~lg clegrce of exchange in the norrual parattins recli~il-e the inversion to occur Inore reaclil~. the greater the number ot C!L'~I.CCS of freedom in the molecule. :iccortling to this ~nechanism cleuteration ncc:cl not occur a t the expense of the clccor~l~)osition reaction. I t will also be ilnncccssar!. to in\.ol;e the co~lcept ol the thrcc-bocl!. collision in this case to explain the mass spectra of the tlcuteratcrl 11)-clro- carbons. A comparisorl of our results \\.it11 the two experinzents of liol)l~ ancl hIelville ( G ) n;ith IZ-11cs:tnc alttl c.)-clohexane le~tds support to this mechanism. The\- calculatecl collision !.iclcls from the rate of disappearance of H atorns which are i l l iair agrcemerlt ~vi th our collision yields calciilatecl on the basis of clecon~position alone, i f the same values for collisio~~ diameters are i~seti in both calculations. Tllere is corlsitlcral~le clisagreement, however, when their results arc conlparecl with the sum O F our collisio~~ yields for cle~om~~osition anci exchange. The use oi rtlcclla~~isrn C senloves this discrepancy since there is no net decrease in atom concentra~ion as a result of the exchange pmcess.

Summing up then \vc nla)- sa)- that the reaction

II + IiH -+ R + H, follo\ved by "ato~llic cracki~~g" reactions is i1ncloubtecl1~- respo~lsible for the products formed in the reactions oi H and D atoms \vith benzene ancl the c!-cloalkanes as \\-ell as n.i~ll ~lornlal pal-afins. Other initial processes, ho\vever, nlay be responsible for the exchangc of these l~\:clrocarbons \\,it11 D atoms.

Acknowledgments

The authors are inclebtecl to Dr. Id. I,eitch for the preparation of the c!~lo- butane and the tleuteriunl usecl in this research, to Dr. 4. Cole for the infrared spectrunl analysis of c!-clobutalle, ancl to Dr. F. P. Lossirlg ant1 hIiss Frances Gauthier for the mass spectrometer analyses of the reaction products.

References 1. UOSHOEPFER, I<. F. anrl ~-IAR.SECI<. P. Z. physik. Chern. A, 139: 64. 1928. 2. BONHO~FPER, I<. F. a r ~ d H.x~,recs, p. Pl~otochen~ie. Leipzlg. 1933. 3. DINGLE. I . R. Ph.D. Thesis. Ilniv. oi Toronto. 19-19. . -

-1 GTNN~NT- H. E. ant1 Srwcrr;. 1.3. \\'. I i . [. Chem. Phvs. 17: 351. 1349. - - . -, 5. HARTECIC, P. %. phys~li. C ~ I ~ ~ I I . .\. 139. 98. 1928. 6. ~IELVILLC, H. LV. allti Korio, J . C. Proc. Roy. Soc. (London), .A, 196: 443 1049. 7. BIELVILLE, H. LV. and Konn, 1. C. Proc. Ro)-. Soc. (London), :I, 202. 181. 1950. 8. N.\srxr, -4. G. Proc. Roy. S;c. (Lor\tlon), A , 123: 692. 1929. 9. ROBERTS, J. D. and S.IUEK, C. \\'. J. .\III. Cheln. Soc. 71. 3925. 1949.

10. SAVELLI, J . J., SEYFRIED, \V. D., drld FILBERT, B. M. Ind. Eng. Chen~., Anal. Ed. 13: 868. 19-11.

11. STEACIE, E. W. R. Call. J. Research, B , 15: 264. 1937. 12. STEACIE, E. W. R. and P.XRLEE, N. A. Can. J . Research, 13, 17: 371. 1939. 13. TITANI, T. Bull. Itnst. Phys. Chem. Research (Tokyo), 8: 433. 1929. 11. TRENNER, N. R., ~ I O R I I C A W . ~ , I<., and TAYLOR, H. S. J. Chem. Phys. 5: 203. 1937. 1.5. TROST, W. R. and STEACIE, E. W. R. J . Chem. Phys. 16: 361. 1948. 16. TROTMAN-DICKENSOX, 3. F. and STEACIE, E. W. R. J . Chem. Phys. In press. 17. WREDE, E. Z. Physik, 54: 53. 1929.

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