-(-RADIATION-INDUCED HYDROGEN EXCHANGE REACTION IN LIQUID CYCLOHEXANE - HYDROGEN CHLORIDE SOLUTIONS

1. W. FLETCI-IEII AND G. 13. FREELIAK Dcpa~tnzent of Che?nzstry, University of Alberta, Edilzonto?t, Alberta

Rcceived June 15, 1966

.-IBSTRACT

The overall exchange reactions

MCI f C-CGDI? --t DCI f c-C,~DIII-I

and DCl f C-C6HI? --t HCl f c-C6HlID

have been initiated by radiolysis and measured by a nuclear magnetic resonance spectroscopic method. 'The reactions occur by a chain mechm~ism, with a chain length of N 104 in a 1 Ad solution of hydrogen chloride in cyclohexane. The mechanism is complex.

'I'he exchange reaction can also be initiated by the photolysis of chlorine or of hydrogen chloride.

-4 chain exchange reaction does not occur between c-C6H12 and DI, DzS, or ND3. These results are consistent with the suggestion that the exchange reaction in the cyclohexane - hy- drogen chloride system occurs by a free radical chain mechanism.

This type of exchange reaction can cause difficulties in isotopic tracer studies.

INTRODUCTION

111 recent studies of the effects of mineral acids on the radioIysis of organic cornpounds (J. W. Fletcher, unpublished results) i t was noted that a radiation-induced hydrogen exchange reaction occurred between hydrogen chloride and cyclohexane. The reaction has a long chain-length in the liquid phase. Since this type of exchange reaction may give erroneous results in isotopic tracer studies, it was of interest to conduct a more detailed kinetic investigation.

The platinurn-catalyzed exchange of 1-1 and D between G-CGEI~~ and DC1 is a relatively slo~v reaction (I).

EXPERIMENTrlL ~lduteriols

Eastman Spectrograde cyclohexane and Phillips Research grade cyclohexane treated with sulfuric acid to remove olefit~s were used in these experiments. They both gave the same results. The cyclohexane was saturated with DzO then dried in vacuum with P?O; which had been transferred in a dry atmosphere.

The perdeutero-cyclohexa~~e, deuterium chloride, perdeutero-ammonia and deuterium sulfide were used as received from Merck, Sharpe and Dohme. Matheson hydrogen chloride and chlorine were fractionated in vacuum before use.

Deuterium iodide was prepared by heating deuterium and iodine with a platintun oxide poxvder catalyst.

Tecl~niqzre The eschange reaction was followed by nuclear magnetic resoilance (11.m.r.) spectroscopic observation of

the buildup or loss of IICI. The n.m.r. tubes were cleaned with a 3 : l solution of sulfuric and nitric acids and rinsed several times with

doubly distilled water. They were then evacuated and heated to about 450 "C, then cooled. Approximately 0.1 ml of D 2 0 was distilled into each tube and left for 24 h t o exchange with the tIsO on the walls. The D2O was then pumped out and the tubes were heated to about 450 "C.

In the filled cells, the vapor space was lcept to less than 10% of the total cell volur~le, to reduce the amount of hydrogen chloride in the vapor. The high acid concentration sa~nples were contained in heavy wall n.m.r. tubes, to prevent breaking by the HCI pressure.

The acid concentrations are reported in units of rnolarity (114).

Calladial1 Journal of Cllemibt~y. Volume 44 (lCI(i(j)

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Geo

rgia

Ins

titut

e of

Tec

hnol

ogy

on 1

1/15

/14

For

pers

onal

use

onl

y.

2646 CAS.ADIAS JOURS.ZL O F CIIEMISTRY. VOL. 4.1, 1DGG

RESULTS

The results of each experiinent were plotted in such a way that the pseudo-first order exchange rate constants were obtained.

In the c-CGI-I~~-DCI systeln, the formation of I-ICl \\-as observed. Representative sain- ples, \\;it11 different DC1 concentrations, are shown in Fig. 1.

In the c-C~D~~-I-ICI system, the destruction of I-ICl was observed. Representative samples, with different I-ICl concentrations, are shown in Fig. 2.

To verify that the reaction was indeed an exchange reaction, both the forination of c - C ~ D ~ ~ I - I and the disappearance of IlCl were measured in one C-CGDl2-HC1 sample.

[I] 6-CsDla + HC1 -+ C-CGDIIH $ DCI

The amount of c - C ~ D ~ ~ H forined n-as fouild to be equal to the amount of HC1 destroyed. In Fig. 3, one set of points represents log ([HCJlo/[I-ICI]), the other represeilts log ([HCl]o/[HCl]o - [c-CGD~~I-I]), and the two sets of points are equivalent to each other.

Irradiation Time (m~n)

FIG. 1. Exchange of DCI with C-CGHI? [DCIIO represe~lts initial DCI concentration. [IICl] = [DCllo - [I-ICI]. Dose rate = 6.08 X 1017 eV/ml min. (0) [DCIIo = 0.177 BI; (A) [DClIo = 0.300 116; (0) [DCl], = 0.310 M.

FIG. 2. Exchange of HCI with C-CGD~?. [HCIIo represents initial I-ICI concenirdtion. Dose rate = 6.08 X 1017 eV/ml min. (0 ) [HCIjo = 0.073 -11; (@) [IIClIo = 0.139 11; (A) [I-ICl]o = 0.351 116; (0) [I-ICl]o = 1.780 M.

The initial rates of the exchange reactions \\-ere determined by nlultiplying the initial acid concentrations by the pseudo-first order rate constants obtained from the slopes of plots such as those in Figs. 1-3 (k = 2.3 X slope). The initial rates of exchange are plotted against initial acid coi~centration, a t different dose rates, in Figs. 4 and 5 . At all dose rates, and in both the c-c~I-I l~-Dcl and the c-CsD12-I-ICI systems, the rate-concentration plots have maxima.

'I'he dose rate dependence of the c-C&I12-DC1 exchange reaction is sho~vn in Fig. 6. The rate of the reaction is proportional to approximately the first power of the dose rate.

The length of the chain is indicated by G(exchange) = 1 X lo5 a t [DCIJo = 0.5 Fd, and a dose rate = 6.08 X lo1" eV/m1 min. The addition of carbon tetrachloride to the systein greatly decreased the rate of the exchange reaction (Table I). Carbon tetrachloridc was more than 50-fold better as an inhibitor than was CDC13 (Table I).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Geo

rgia

Ins

titut

e of

Tec

hnol

ogy

on 1

1/15

/14

For

pers

onal

use

onl

y.

FLETCHER A N D FRBEhI.\X: -y-R.\DI.\TIOS-ISDUCED IIYDROGES ESC1I.-WGE RE.ICTIOS 2G4i

Irrad~ation Time (min)

I:IG. 3. Formation of C-CGDIII-I and destruction of HC1 i r ~ c-CtjD1?-HCI. Dose rate = 15.08 X lo1' eV/rnl mili. Co = initial concentration of HC1 = 4.25 M. ( A ) C = [IICl]; ( 0 ) C = CO - [c-CGD~~H]..

FIG. 4. The rate of exchange of DCl with C-CGHI? as a function of DC1 concentration a t d~fferent dose rates. ( A ) 6.0s X 1017 eV/inI min; (0) 5.35 X 10IG eV/ml m i n ; ( 0 ) 6.25 X 1014 eV/~nl min.

TABLE I

Rate of eschange of C - C G H ~ P with DC1 ill the presence of CCll and of CDCl3 (IDCI] = 0.5 111; dose rate

= (3.05 X 10'7 eV/ml min)

Concentra~ior~ of additive (]I[) (dc/dt) X lo3

The rate of forination of c-C,jD1lH ill n-CGH14 containing 20 inole Oi/, c-CGD12 and 0.30 n J-Icl \\-as 1000-fold smaller than it \\;as in a solution of 0.30 h! I-ICl in c - C G D ~ ~ (Table 11). The corresponding reduction in neo-CGI-114 solvent was 200-fold (Table 11).

Photol~.sis of a Dcl-c-c~I-I~? solution in a quartz tube with a 600 W mercury resonance lamp gave complete hydrogen exchange in (3 min. Under these coilditions in an initially 1 114 DC1 solution, the final concentration of the major chain-termination product CGI-I,lCl was 1 X 114. This indicates that the chain length was >, lo3.

Rate of exchange of r-CGD12 with HCI in solution in 11-hesane and in neo-hesane ([c-CGIll?] = 25 inole e5; [HCl] = 0.3 14; dose rate = 7.8 X 1017 eV/g inin)

Solvent (cl[c-c~DllH]/dt) X 103 G(exchange)

?L-C~I-II.I 0 .44 5 0 Xeo-C6111., 2.35 260 Pure c-CGDlr 520 5.2X10"

The exchailge between DCI and 6-c~I-112 was also induced by the photolysis of Cl?, using a high-pressure mercury lamp and a Pyrex reaction cell. Coillplete exchange \\-as observed in a solution 1 114 in DCl and lo-? M in C12. This result indicated a chain length of greater than lo2. hIore dilute solutions of C12 were not used because of the difficulty of measuring sillall quantities of C12.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Geo

rgia

Ins

titut

e of

Tec

hnol

ogy

on 1

1/15

/14

For

pers

onal

use

onl

y.

2648 CANADIAN JOURSAL O F CHEMISTRY. VOL. 44, l D G G

A 1 171 solution of DI in c-CGI-I~?. was irradiated for 40 min a t a dose rate of 6.08 X 1017 eV/m1 min. Within experimental error no exchange was observed, so G(exc11nnge) < 30 in this system.

Similarly, no exchange was observed in a 0.3 M solution of D2S in c-C~H12, after 11 11 irradiation at 6.08 X 1017 eV/ml mill. Thus, G(exc11ange) < 1.5 in this system.

In a 0.3 M solution of ND3 in c-C8WI2, G(ND2I-I) = 0.8, within a factor of t\\-o.

DISCUSSION

The chain length of the hydrogen exchange reaction between hydrogen chloride and cyclohexane is of the order of lo4 when the hydrogen chloride concentratioil is about 1 M. No reasonable ionic chain mechanism could be devised to explain the results. The inost reasonable free radical chain-propagation steps are

C-CGHII + DCI + c-CcH1LD + CI

C1 + C-COIII? + HCI + c-Callll

I-Iigh concentrations of IlCl (or DC1) inhibit the exchange (Figs. 4 and 5). This might be due to the formation of I-ICl2,

[4] C1 + HCI HClz,

which would be less efficient in propagating the chain than is C1, but which could enter into a chain-termination reaction.'

If chain ternlination occurred by the reactions [5]-[7],

26-CsHI1 -, products

Cl + c-CcH1l + products

2C1 + A,I -, Cl? + h,I,

the rate of exchange should have varied as (dose rate)0.5. However, the rate of reaction varied approximately as (dose (Fig. 6). This is probably due to inhibition oi the reaction by a trace of impurity, which is often observed in long chain processes (2, p. 356).

-1.0 -0.8 -06 -0.4 -02 0 0.2 0.4 15 1 h 17

LOG [HCllo LOG Dose Rate (eV/ml min)

I . 5 . T h e rate o f exchange o f HCI with c-CaD11 as fur~ction o f IlCl concentration at a close rate o f 6.08 X 1017 eV/rnl mill.

FIG. 6. T h e rate o f exchange o f DCI with c-CaHl? as a functio11 o f dose rate. [DClIo = 0.: -11. ?'he slope o f t he broken line is 1.0; t ha t o f t he dotted line is 0.5.

' I t was pointed out by a referee tlrat inhibition col~ld also be caz~sed by association of the HC1 ~t~olecltles in tlze Inore concentrated HCI solz~tions. Tlze rate of reaction of c - C G H ~ ~ radicals witlr free IlCl would probably be faster than with l~ydrogen-bonded HC1. Tire n.tt2.r. absorption freqz~ency of the HC1 proton was observed to shift to lower 7-ualues as tlze HC1 concentration was ~ncreased. This conjirins that tlze anzor~nt of association z?lcieased with increasing HC1 concentration.

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Geo

rgia

Ins

titut

e of

Tec

hnol

ogy

on 1

1/15

/14

For

pers

onal

use

onl

y.

FLETCHER AND FREEMAN: r-RADIATIOX-INI~UCED HYDROGEN EXCHANGE REACTION 2649

The inhibition caused by carbon tetrachloride (Table I) call be explained by reactions [SI-i91.

CCIB is evidently a less efficient chain carrier than is c-CGH~~ and the chain length is shortened.

CDCI3 is a less efficient inhibitor than is CCI'I because the former is a less efficient chain-transfer agent than is the latter (2, p. 152).

The exchange of hydrogen between C-CGD12 and HCl occurs five times more efficiently in neo-hexane solvent than it does in n-hexane solvent (Table 11). This is consistent with the proposed free radical mechanism if the C1 atoms react predominantly with secondary hydrogeils on the hydrocarbons.

[lo] C1 + CH3CH?CHzCHnCHrCH3 --t HCI + SCC-CGHll

[ I l l Cl + CH3C(CH3)nCH2CI-13 --t HC1 + C H ~ C ( C I ~ ~ ) ~ C H C H ~

The number of secondary hydrogens on a n-hexane molecule is four times as great as the nuinber on a neo-hexane molecule.

Photolysis of DCl by the 1 847 A mercury line or of Cl2 by visible light produces atoms \vIlich initiate the chain exchange reaction. The C1 atoms undergo reaction [3] and the D atoms also react to produce cyclohexyl radicals.

One would not expect a free radical chain exchange reaction to occur between C-CGFI12 and DI , D?S, or ND3 because of the inefficiencies of the endothermic propagation steps [12]-[14].

[I2] 1 + c-CGI~~.' -f HI + C-CGI~L~

[I31 DS + c-CeI-Iln --t EIDS + C-CGH11

No chain exchange reaction was observed in these systems. All the experimental results are thus consistent with the suggestion that the hydrogen

exchange bet~veen cyclohexane and hydrogen chloride occurs by a free radical chain mechanism.

Deuterated polar compounds have been (3, 4) and are currently being used as ion scavengers in radiolysis systems. The amount of ion scavenging that occurs is estilnated from the yield of I-ID or of D z produced during radiolysis. This study illustrates the necessity to be aware of the possibility of exchange reactions in these systems. \Vc con- clude, however, that exchange reactions did not vitiate the results obtained by using ND3 (3) or D2S (4).

IZCICNOWLEDGMEN'T

We are grateful to the Natiollal Research Council for giving finailcia1 assistance to this worlr.

REFERENCES

1. C. I~ORRBX, R. I<. GREBNHALGH, and i\I. I'OLANYI. 'Trans. 1;araday Soc. 35, 311 (1939). 2. C. WALLING. Free radicals in solutions. John Wiley & Sons, Inc., New Yorlc. 1957. 3. F. ~VILLIAMS. J. Am. Chem. Soc. 86, 3054 (1964). 4. A. MEISSNER and A. IIEXGLBIS. Ber. Bunsenges. Physil;. Chem. 69, 264 (196.5).

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

Geo

rgia

Ins

titut

e of

Tec

hnol

ogy

on 1

1/15

/14

For

pers

onal

use

onl

y.