Jan., 1961
REACTIVITY OF HYDROGEN ATOMSIN
THE
LIQUIDPHASE
101
To calculate what per cent. of the relaxation is due to each mechanism one estimates the dipole moment change from the energetically most favorable configuration to the least favorable configuration for each mechanism. Since the intensity of absorption is proportional to this difference squared, the ratio of per cent. of total relaxation for two mechanisms is that of the squares of the dipole moment changes.
molecular rotation. Values of E' and 6'' a t 4.35 mm. wave length may be predicted accurately using the dispersion parameters derived from lower frequency data. The behavior of phenyl ether and benzophenone may be explained by the superposition of two Debye processes, one corresponding to over-all molecular rotation and the other to internal rotation accompanied by a mesomeric shift of charge. The same mechanism appears to be responsible for the short relaxation time in both comConclusions pounds. The methoxybenzenes also require two Chlorobenzene and tribromofluoromethane show relaxation times to explain the data, one due to the behavior required by the Debye theory for a over-all molecular rotation and the other to methsingle relaxation process corresponding to over-all oxy group rotation.
THE REACTIVITY OF HYDROGEN ATOMS IN THE LIQUID PHASE BY THOMAS J. HARDWICK Gulf Research & Development Company, Pittsburgh, Pennsylvania Received June $8, 1960
A method is described in which hydrogen atoms, produced in situ by the radiolysis of a saturated hydrocarbon as solvent, are allowed to react competitively with a reactive solute and the solvent. A kinetic expression is derived whereby the relative rate constants for the competing reactions can be calculated from a measurement of the hydrogen gas yield as a function of solute concentration. From previous measurements of the absolute rate constants of H solute, absolute rate constants for the reaction H f paraffin solvent are obtained. These rate constants show an effect of the molecular structure of the paraffin solvent. Specific values can be assigned to primary, secondary and tertiary groups, from which the absolute rate constant of the reaction of hydrogen atoms with any saturated liquid hydrocarbon may be calculated. If rate constants so measured or calculated are compared with the corresponding published values of H paraffin in the gas phase, reasonable agreement is found. It is thus concluded that such radical-molecule reactions occur at the same rate in the gas phase as in hydrocarbon solvent,
+
+
Introduction The reactions of hydrogen atoms with the lower hydrocarbons and other simple organic molecules in the gas phase have been studied extensively during the past twenty-five years. Hydrogen atoms have been produced in a variety of ways; e.g., by electric discharge in hydrogen or other suitable gases, by mercury sensitized photolysis, by photolysis of suitable source materials, such as hydrogen sulfide, etc. These techniques, however, are applicable to gas phase reactions only. This restriction has, in turn, limited the reactants which can conveniently be investigated. In addition, many materials cannot be investigated because of interference with the methods of hydrogen atom generation. The reactions of hydrogen atoms in non-aqueous solutions have received little or no attention. The principal reason appears to be the difficulty of generating hydrogen atoms in situ. Most methods applicable in the gas phase cannot, by their nature, be carried out in solution. One of the few feasible methods of hydrogen atom production is the photolysis of formaldehyde, but as this is restricted to temperatures above loo", the technical problems encountered have limited the value of the method. Several attempts have been made in which an electric discharge was passed through hydrogen gas just prior to bubbling it through solutions.'S2 The results are quite scattered, and the method is not considered promising for general application. (1) F. E. Littrnan, E. M. Car and A. P. Brady, Rad. Reaearch, 7 , 107 (1957). (2) T. W. Davis. 4487 (19aSJ.
S. Gordon and E. J. Hart,J . Am. Chsm. SOC.,80,
The absorption of ionizing radiation in liquid hydrocarbons produces hydrogen atoms. These atoms will react with suitable solutes, and their relative reactivity among various solutes can readily be measured. This paper, the first of a series, outlines the principle and methods of generating hydrogen atoms by ionizing radiation, and develops the kinetics required for a quantitative measurement of reactivity. Data are presented on the rate of reaction of hydrogen atoms with simple alkanes and naphthenes in the liquid phase. By combining these results with absolute measurements in the gas phase, individual rates of reaction have been determined. Radiolysis of Saturated Hydrocarbons.-The over-all processes which occur on the absorption of ionizing radiation by saturated liquid hydrocarbone are many and varied. However, since the essence of the proposed method involves a measurement of the hydrogen gas produced, all reactions which are unrelated t o hydrogen gas formation can be ignored. The method requires the use of a saturated hydrocarbon as a solvent, and, as will be shown later, either alkanes or cycloparaffins may be used. In outlining the method, the term paraffin will be considered to include straight chain and branched chain alkanes and cycloparaffins. The pertinent characteristics of a liquid paraffin hydrocarbon absorbing ionizing radiation are the following. (1) Primary products (radicals and some molecules) are formed in proportion to the amount of energy absorbed; i e . , in a zero-order reaction. The amount of chemical change per unit of energy ab-
THOMAS J. HARDWICK
102
sorbed, or yield, is expressed in units of molecules per 100 e.v. absorbed, and is designated by the term G. (2) Hydrogen atoms are produced in the radiolysis of paraffins and disappear by reaction with either solvent or solute. Biradical reactions involving hydrogen atoms appear to be absent. The experimental method of measuring relative reactivities of hydrogen atoms involves measurements of the radiolytic hydrogen yield (a) from the pure paraffin solvent and (b) from this solvent containing a range of low concentrations of a reactive solute. From the kinetics of the reaction, the relative rate constants for hydrogen atom attack on solvent and solute can be obtained directly from the experimental data. Kinetics of Hydrogen Gas Formation in P a r f f i Solute Systems.-In the radiolysis of saturated hydrocarbons, hydrogen gas is produced by two general processes. One is influenced by free radical scavengers, the other is not. The latter process may be represented by the over-all reaction
+
GI
R H -+
HZ+ Products
(1)
In the other process, the first step is hydrogen atom format,ion Gz RH-+R+H
(2)
When the hydrocarbon contains small amounts of a free radical scavenger S, hydrogen atoms disappear in one of the competitive reactions H
ks + RH + Hz + R
(3)
k4
H+S-+HS
(4)
where the nature of S is such that hydrogen gas is not produced in reaction 4. In the absence of a scavenger (Le., pure hydrocarbon) the total amount of hydrogen gas produced, GH~(o),will be GI Gz. With increasing amounts of scavenger present in solution, a decrease in measured hydrogen yield GH%(s) is observed. If a stead:y-state kinetic treatment of the hydrogen atom production is made for the hydrocarbon system with and without scavenger present, the following expression is obtained.
+
Since the rate of product formation is equal to an intensity of radiation term times the yield, the use of yield values in place of rates is permissible and in practice is more convenient. If the kinetics are correct, a plot of the reciprocal of the difference in the hydrogen gas yield, without and with sca,venger, versus the ratio of the concentration of paraffin to scavenger, should give a straight line, the slope of which is 1/G2 X k&a and the intercept 1/G2. From the values so obtained, G2 and k3/k4may be calculated. In this kinetic treatment it has been assumed that the reaction of hydrogen atoms with a scavenger does not produce hydrogen gas. This is true for many scavengers, particularly with vinyl monomers.
Vol. 65
There are many hydrogen atom scavengers, however, where the hydrogen atom can both add to the molecule and abstract hydrogen atoms. Such behavior is particularly common among olefins. In such cases the reaction H
ks + S --+ M + Hz
(5)
must be considered. Applying this further reaction to the kinetic scheme, equation I becomes
(11)
In this case the intercept has a value of 1/G2(k6/k4 1). With a knowledge of G2, obtained using other scavengers, k5/k4 may be calculated. Many previous studies have been made on the effect of various solutes on the hydrogen gas yield in paraffin radi~lysis.~It has not been found possible, however, t o use such results for a quantitative estimate of GI and Gz, for most data have been presented in graphical form. Tabulated data have not been of sufficient accuracy for a satisfactory determination of GI and G2. However, with the exception of systems containing iodine and hydrogen iodide, the results obtained for scavengers in paraffin solutions give in most cases values which are comparable with those to be reported in this and subsequent papers.
+
Experimental Materials.-All hydrocarbons were Phillips Pure Grade. As nearly all contained a degree of unsaturation sufficient to engender erroneous results, removal of these unsaturated materials was necessary. Each hydrocarbon was shaken overnight with 1/10 its volume of concentrated sulfuric acid. The two layers were separated and the organic layer stored over sodium carbonate. In a number of cases the hydrocarbon was further purified by washing, drying and distilling. Experience showed however, that results obtained with and without this last phase of purification were identical. In all cases the unsaturation in the purified material, as measured by bromination, was less than 0.5 mMD. (-O.O05%). Rohm and Haas methyl methacrylate monomer was used without further purification. Bromination measurements showed it to be within 1%of the theoretical unsaturation. The stabilizer, 0.006% hydroquinone, was not removed. Hexyl methacrylate, from Rohm and Haas, was used directly. Tetrachloroethylene was supplied by Eastman Organic Chemicals. Hydrogen Gas Production and Measurement.-The experimental method involved irradiating an evacuated sample of liquid paraffin solution and subsequently measuring the hydrogen gas produced. The technique outlined below is only one of many suitable for this type of experiment. The techniques used for sample preparation, for irradiation and for hydrogen gas analysis have been described in detail.4 Briefly, 100 ml. of liquid was placed in an irradiation cell and degassed. Irradiations were made using Xrays from a 3 MeV. Van de Graaff accelerator, with suitable monitoring of the absolute energy absorption. The hydrogen gas produced by the radiolysis was transferred to a gas analysis apparatus, isolated from impurities, and measured quantitatively in a McLeod gauge. The total energy absorbed in each sample was about 0.3 j ./g., resulting in the production of 5-15 micromoles of ohydrogen gas. The temperature of irradiation was 23 f 1 At least three determinations of the hydrogen yield were
.
.
(3) J. P. Manion and M. Burton, THISJOURNAL, 66, 560 (1952);
R. H.Schuler, ibid., 61, 1472 (1957); H.A. Dewhurst, ibid., 68, 15 (1958); P. J. Dyne and W. M. Jenkinson, Can. J . Chem., 38, 539 (1960). (4) T. J. Hardwick, THISJOURNAL, 64, 1623 (1960).
Jan., 1961
REACTIVITY OF HYDROGEX ATOMSIIS THE LIQUIDPHASE
103
TABLE I COMPARISON OF CALCULATED AND EXPERIMENTAL VALUESOF k3/kra Scavenger, methyl methacrylate; T = 23’ Solvent
n-Pentane Cyclopentane n-Hexane 2-Methylpent ane 3-Methylpentane 2,2-Dimethylbutane 2,3-Dimethylbutane Methylcy clopentane n-Heptane 2,4Dimethylpentane Methylcyclohexane n-Octane 2,2,4-Trimethylpent:tne n-Nonane
Molecules/100 e.v. GHZ(O) Gz
6.35 5.78 5.28 4.47 4.56 3.12 4.02 4.62 6.06 4.19 4.76 6.18 2.91 6.05
4.25 3.06 3.16 2.86 2.90 1.96 2.33 2.63 3.70 2.60 2.71 3.33 1.67 3.53
,---No. Primary
2 2 3 3 4 4 1
2
of group typesSecondary Tertiary
3 5 4 2 2
1 1
1
4 5
2 1
4
1
2
1 2 5 2
5 6 1 7
1
1
--ka/kn Exptl.
X 108Calcd.
1.83 2.36 1.92 3.67 3.45 0.94 7.50 5.89 2.59 8.18 5.85 3.43 4.33 3.87
1.66 2.40 2.14 4.49 4.49 0.92 6.84 5.23 2.62 7.72 5.91 3.10 4.43 3.58
% Dev. from oalcd. value
+ 9 - 1
- 10 - 18 - 23 $ 4 + 9 +11 - 1 + 6 - 1
+10 Mean
0
- 2 + 7 110
k,/k4 primary = 0.11 X 10-3; secondary = 0.48 X 10-3; tertiary = 3.20 X
made for pure paraffins. Agreement was always within *2%. In the case of systems containing a scavenger, 100-ml. solutions of appropriate concentration were irradiated. In the usual case, six different concentrations of scavenger ranging between 20 and 80 mmoles/l. were used. Where necessary, minor corrections in the energy absorption were made to allow for the direct absorption of energy by the solute.
Results Hydrogen Gas Yield from Pure Parffis.-The radiolytic hydrogen gas yield for a number of pure paraffins is shown in column 1,Table I. I n general, the values are somewhat higher than have been reported by others.6 The possible reasons for this discrepancy are the following. (1) Presence of unsaturation in the pure liquid. In a previous paper* it was shown that 1.8 mM/l. hexene in n-hexane would lower the hydrogen yield by 1%. Since preliminary results indicated an effect of similar magnitude in other hydrocarbons, care was taken to reduce the unsaturation below 0.5 mM/l. In previous work purification procedures were given, but the degree of purity achieved was not reported. ( 2 ) Unsaturation is produced as a part of the general radiolytic reactions. At higher doses the measured value of GH>decreases as a result of the scavenging action of these unsaturates for hydrogen atoms.4 For instance, with n-hexane an energy absorption of 1 Mrad. (10 j./g.) results in a 2% decrease in the measured value of GH~. Not all previous workers have been cognizant of the magnitude of this decrease in hydrogen yield. It is therefore not advisable to compare the present data with hydrogen yields obtained on irradiation t o more than 1 Mrad. energy absorption. From the results in Table I it is apparent that the radiolytic hydrogen yield from pure paraffins is affected by chemical structure, the more highly branched alkanes having lower hydrogen yields. Such a finding is in qualitative agreement with other work.5 Detailed comment on this will be reserved for a subsequent paper. (5) H. A. Dewhurpt, .I. Am. Chrrm. Soc., 86, 5607 (1958).
Kinetic Results-One Para& with Several Scavengers.-The addition of a hydrogen atom scavenger decreases the measured hydrogen gas yield (GHJ. A typical curve is shown in Fig. 1, where the hydrogen yield for the system n-hexanemethyl methacrylate is shown as a function of scavenger concentration. These same results are shown in a kinetic plot (Fig. 2) where the reciprocal of the difference in hydrogen yield, without and with scavenger, is plotted against the concentration ratio [Hexane]/ [Methyl methacrylate]. Similar straight lines were obtained for all systems studied. From the kinetic scheme, it follows that for a given paraffin solvent, the same value of Gz should be obtained, regardless of the scavenger used. Confirmation of this is shown in Table 11.where for each of three paraffins, the values of Gz obtained using the various scavengers are in quite good agreement. As a result of this agreement, more confidence can be placed on the kinetics. Kinetic Results-One Scavenger with Several Paraffins.-Values of G2 obtained from kinetic plots of the data obtained on the radiolysis of the systems: paraffin plus methyl methacrylate, are shown in Table I. Comment on the individual values of G2, together with their relation t o GH~(o) will be deferred to a subsequent paper. The values of the ratio of the rate constants, k3/k4,obtained from the product of the slope of the kinetic plot and G2, are listed in column 6, Table I. The results indicate that methyl methacrylate is 100-1000 times more reactive toward hydrogen atoms than are paraffins. The data also bear out Steacie’s statement6 that the “maximum variation in reactivity a t room temperature (of H atoms) for all paraffins other than methane is only a factor of about ten.” Relation of k3/k4to Molecular Structure.-An attempt was made to relate the values of k3/kq to the molecular structure of the solvent. Since the same scavenger (methyl methacrylate) was used through(6) E. W. R. Steacie, “Atomic and Free Radicai Reactions,” Reinhold Publ. Corp., New York, N. Y.,1954, p. 497.
THOMAS J. HARDWICK
104
5.5r--
2.0
1 1-
20
1
40 [METHYL
1
60
I
80
METHACRYLATE]
I
loo
I 120
mM/L.
Fig. 1.-Hydrogen yield in n-hexane radiolysis as a function of scavenger concentration (methyl methacrylate).
Vol. 65
for a particular molecule depends on the total number of primary, secondary and tertiary hydrogen atoms present in the molecule, suitable values of k3/k4 were assigned to each of the three structural units, viz., primary 0.11 X secondary 0.48 X tertiary 3.40 X These values were so chosen as to give over-all the smallest deviation between calculated and observed data. These values refer to the probability of attack at a given type of carbon atom. The reactivity of an individual primary hydrogen is one-third the primary value; that of an individual secondary hydrogen is one-half the secondary value. The number of structural groups in each individual molecule is listed in columns 3, 4 and 5, of Table I. In column 7 the values of ks/kr, calculated from assigned values, are given. In column 8 the percentage deviation of the experimental from the calculated value is given. The agreement between observed and calculated values is in general quite good, for the mean deviation is only =klO%. It is concluded that using these data for primary, secondary and tertiary groups, the relative reactivity of hydrogen atoms with liquid paraffins may be calculated with reasonable accuracy from a knowledge of the molecular structure. Absolute Values of the Rate Constants.-It is of course desirable to establish the value of the rate constants on an absolute basis and to determine the effect of temperature on these values. There are several scavengers available for which the rate constants of reaction 4 (and 5 where pertinent), have been measured. (a) Methyl Methacrylate.-Allen, Melville and Robb have measured the rate of the hydrogen atom reaction with a variety of unsaturated compounds in the gas phase at Bo.' Among the reactants studied was methyl methacrylate, for which the rate constant for reactivity with hydrogen atoms was determined as 4.4 X 10" cc. mole-' sec.-'. This value, corrected to 23" using an assumed activation energy of 5 kcal., is 5.1 X 10" cc. mole-' set.-'. Applying this result to the calculated values of ka/k4 for paraffins (Table I), individual rate constants for the reaction H paraffin were found. Values for typical paraffins, viz., cyclopentane and n-hexane, are given jn Table 111.
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TABLE I1 THERMAL HYDROQEN ATOMYIELDS(G2)FOR DIFFERENT SCAVENOERS IN PURE ALKANES G*
Scavenger
I
100 [HEXANE]
I
200
/ [METHYL
I
300
I
400 METHACRYLATE].
Fig. 2.-Kinetic plot of the data in Fig. 1 to determine hydrogen atom yield (G2)and the relative reactivity of solvent (n-hexane) and solute (methyl methacrylate) toward hydrogen atoms.
out, differences in the values k0/k4 reflect differences in the rate constant of reaction 3. The rate constant k4 is not expected to vary from one paraffin solvent to another, and evidence for this will be presented later in the paper. Assuming that the reactivity of hydrogen atoms
Methyl methacrylate Hexyl methacrylate Tetrachloroethylene Carbon tetrachloride
-Molecule/100 e.v.+Hexane n-Heptane Neohexane
3.18 3.15 3.16 3.13
3.68 3.75 3.70 3.70
1.96 1.96
(b) Cyclohexene, cis-Pentene-2 and Benzene. -In the same paper Allen, et al., investigated the reaction of hydrogen atoms with cyclohexene, cispentene-2 and benzene, all of which may be conveniently used as scavengers in the present irradiation techniques. To check the data obtained using methyl methacrylate, these materials were used as (7) P. E. M. Allen, H. W. Melvllle and J. C. Robb, Proc. Roy. SOC. (London). AB18, 311 (1963).
Jan., 1961
REACTIVITY OF HYDROGEN ATOMSIN TABLE I11 REACTIVITY OF H ATOMSWITH SCAVENGERS IN
T
Me methacrylate Benzene Cyclohexene cis-Pentene-2
3.06
Me methacrylate Benzene Cyclohexene cis-Pentene-2
3.16
=
+
LIQUIDPHASE
105
CYCLOPENTANE AND n-HEXANE
23'
2.40 40.7 7.65 8.94
Cyclopentane
