3177
CATALYTIC DEUTERIUM EXCHANGE REACTIONS WITH ORGANICS
Catalytic Deuterium Exchange Reactions with Organics. XIV.' Distinction between Associative and Dissociative ,-Complex Substitutjon Mechanisms by J. L. Garnett and W. A. Sollich-Baumgartner Department of Physical Chemistry, T h e University of New South Wales; Sydney, Australia (Received M a y 26,196.4)
The relative importance of the associative and dissociative n-complex substitution mechanisms in group VI11 transition metal catalyzed exchange reactions between heavy water and aromatic molecules has been investigated by comparing the rate of D20-benzene exchange reactions to the randomization rate of a mixture of normal and 98.3% deuterated benzene. Kinetic equations have been derived for the randomization process on the assumption that random deuterium incorporation occurs a t all stages of the reaction. A test of these equations showed that this assumption was not valid. A semiempirical method has therefore been developed for the calculation of randomization rate constants. The results show that randomization occurs at a rate which is 70% as fast as deuterium oxide exchange. This difference in the two reaction rates is readily explained by water activation of the catalyst. It is concluded that the dissociative ,-complex substitution mechanism is the predominant process by which deuterium oxide exchanges catalytically with aromatic molecules. Isotope effect studies show that the reaction between chemisorbed benzene and chemisorbed hydrogen is the rate-determining process of the exchange reaction.
Introduction
Dissociative n-Complex Substitution Mechanism
Two new reaction mechanisms, the associative and dissociative a-complex substitution mechanisms, have been proposed for group VI11 transition metal catalyzed hydrogen exchange reactions between aromatic mole-~ tules and heavy water (eq. I-IV). General 1-Complex Adsorption
(119
M
M
M
~
J.
'M
r
M
a-Bonded to catalyst (M) with lane of ring parallel to catalyst surFaco
Associative r-Complex Substitution Mechanism
9 M
+
p
M
[.! M
++
M
*-Bonded
7 @I)
Inclined to catalyst surface (45")
Plane of ring a t right angles to catalyst surface (edge-on u-bonded)
I x-.- -J
M
Both mechanisms involve n-complex adsorption of the aromatic reagent (e.g. benzene) so that the plane of the ring is parallel to the plane of the catalyst surface (eq. I). I n the associative mechanism the aromatic mole-
M (1) Part XIII: Australian J. Chem., in press.
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J. L. GARNETT AND W. A. SOLLICH-BAUMGARTNER
3178
cule is attacked by a chemisorbed deuterium atom originating from the dissociative chemisorption of water or deuterium gas (eq. 11). I n the dissociative mechanism (eq. I11 and IV) the 7r-bonded aromatic reacts first with a metal radical (active site) by rotating through 90’ to form a carbon-metal c-bond so that the plane of the ring is now at right angles to the plane of the catalyst. While u-bonded, the compound undergoes another rate-controlling2 substitution reaction a t the carbon-metal bond with a chemisorbed deuterium atom, so that it returns to the original n-bonded state. From previous data3+ it was not possible to distinguish clearly between the relative importance of these two mechanisms; however, special experiments, which are described in the present communication, were designed for this purpose. These experiments depend on the fact that a-complex adsorbed aromatics cannot furnish deuterium atoms to the catalyst. The associative mechanism requires therefore a second substance, water or deuterium gas, which can supply the attacking reagent by dissociative chemisorption. Exchange between one hydrocarbon and another (e.g.) benzene-ddiphenyl) is thus precluded by the associative mechanism, but feasible by the dissociative mechanism. Both mechanisms can operate simultaneously in benzenedeuterium oxide exchange where it has been shown that noncompetitive reagent adsorption occurs.’ To ascertain whether one of the two possible mechanisms is exclusively responsible for exchange, it is necessary to perform exchange reactions between a normal and a deuterated species where mutual reagent displacement effects are unimportant. This eliminates systems such as benzene-d-naphthalene, where no significant exchange occurs due to benzene displacement, or even benzene-d-diphenyl, where exchange is significantly retarded at low temperatures.2 The ideal system with respect to reagent displacement is to perform exchange reactions, i.e., randomization, between normal and 100% deuterated benzene. This system presents however a number of difficulties in the evaluation of the reaction rate constant. Nathematical procedures by which this randomization rate constant can be calculated and consequently compared to that of the deuterium oxide reaction are described in the following section.
with deuterium oxide at 120’. Exchange and randomization products were analyzed mass spectrometrically (-10 e.v.) and the seven benzene isomers (do,dl . . .do) corrected for the contribution of the C13isotope peak.
Results and Discussion Equation for Randomization Reactions. During randomization the mass spectrum of the normal benzenebenzene-d (98.3%) mixture changes from that shown approximately in Fig. l a at t = 0, through various intermediates (e.g., Fig. 5 ) until it reaches the classical equilibrium distribution depicted in Fig. 2. The close agreement between the calculated and the experimental spectrum (Fig. 2) shows that isotope effects cause negligible departures from the classical equilibrium distribution. Approximate “phase’ boundaries
I I I
84
82 80 Mass number. (a)
78
84
82 80 Mass number. (b)
78
Figure 1. Approximate “phase boundaries” in the mass spectrum of an equimolar mixture of 100% deuterated benzene and normal benzene: ( a ) prior to commencement of reaction; ( b ) at early stages of randomization.
I n order to calculate a reaction rate constant it is necessary to express the changes in the mass spectrum of the normal benzene-benzene-d (98.3%) mixture by a first-order equation. The model adopted for this purpose involves the division of the mass spectrum into two hypothetical “phases,” analogous to the water and benzene phases, viz., the deuterium and hydrogen “phases.” I n the initial stages of the randomization reactions these “phases” are visible in the mass spectrum (Fig.
Experimental
(2) J. L. Garnett and W. A. Sollich, to be published. (3) J. L. Garnett and W. A. Sollich, Australian J . Chem., 14, 441
Exchange and randomization reactions were performed a t 32’ in evacuated ampoules with platinum catalysts by previously described procedures.8 The randomization mixture consisted of equimolar quantities of normal and 98.3y0 deuterated benzene (thiophene free), the latter being prepared from repeated exchanges
(4) J. L. Garnett and W. A . Sollich, ibid., 1 5 , 5 6 (1962). (5) J . L. Garnett and W. A. Sollich, J . Catalysis, 2 , 350 (1963). (6) J. L. Garnett and W. A. Sollich, A‘ature, 201, 902 (1964). (7) T . I. Taylor, “Catalysis,” P. €1. Emmett, Ed., Reinhold Publishing Carp., New York, N. Y., 1957, p. 257.
The Journal of Physical Chemistry
(1961).
(8) J . L. Garnett and W. A. Sollich, J . Catalysis, 2 , 339 (1963).
CATALYTIC DEUTERIUM EXCHANGE REACTIONS WITH ORGANICS
3179
I n order to compare the randomization rate with that of the deuterium oxide exchange reaction, the normalized rate constant ~ X Rmust be calculated, ie., the rate constant expressed in terms of fractional completion of reaction per unit time. For randomization reactions, this is given by the equation 78
79
80
81
82
83
84
Maas number.
Figure 2. Mass spectrum of randomized benzene ( t -+ m ) containing 49.067, deuterium: 0,observed; ,. calculated.
Ib), and the calculation of rate constants approximates a situation where such phases really exist in the reaction vessel. Serious overlapping of “phases” occurs, however, a t later stages of “exchange,” and consequently the physical manifestation of these mass spectral phases disappears. The composition, however, may still be calculated by the mathematical procedures,shown below, so that a theoretical “separation” of phases is still possible. If random deuterium distribution is assumed a t all stages of the randomization reaction, the spectra of the two hypothetical pha,ses can be calculated from eq. 1
where p 1 3 ~ ~is - the ~ ~ fractional abundance of benzene molecules containing 0, 1. , .6 hydrogen atoms (m) in the hypothetical hydrogen phase of the mass spectrum, giving rise to mass numbers of 84,83. . .78, respectively; PHand QH are the fractional abundances of hydrogen and deuterium atoms in the hydrogen “phase” a t time t. An analogous equation, involving pD78-84, QD, and PD,describes the deuterium “phase.” Further, PD=. (1 - PH)in the case where normal and 100% deuterated species are involved. During randomizat,ion, in which equimolar quantities of normal and 98-37’, deuterated benzene are used, PHchanges exponentially from 1.0 a t t = 0 to 0.508 a t t = and can consequently be described by the first-order equation
while that for exchange is
where D is the deuterium content of the benzene phase. The method by which P H values were determined involved measuring the ratio of peaks occurring in the spectra a t mass 78 and 79, ie., those corresponding to the normal and nondeuterated species. Normal benzene peaks (PH+D7’) of the reaction mixture are however composed of two parts; vix., one fraction is due to normal benzene in the hydrogen “phase” of the spectrum which has not yet exchanged (pH7*), while the other fraction (pD7’) originates from the deuterium “phase,” ie., from an initially completely deuterated molecule which has exchanged all six deuterium atoms with hydrogen. Using a similar argument for the 79 peak, eq. 5 was derived which relates the peak ratio to the P H value a t time t.
In the early stages of the reaction, i.e., with negligibly overlapping spectra, this approximates to pH+D7’
PH)
P H
The solution of eq. 5 for a given peak ratio is complicated ; consequently, the reverse procedure was adopted, Le., peak ratios were calculated for chosen PHvalues. Since randomization experiments were performed with 98.3% deuterated benzene, ie., P D = { (1 - PH) 0.017 eq. 5 had to be slightly modified for residual hydrogen. Results of the calculations are shown in Fig. 3, where peak ratios are plotted against PHvalues so that solutions of eq. 5 could be directly determined. The graph contains also approximate solutions as calculated from eq. 6 (dotted line). These agree satisfactorily with the exact solutions over a considerable range of the reaction.
+
Integrating (2)
= 6(1 -
pH+D”
1,
Volume 68, Number 11
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J. L. GARNETT AND W. A. SOLLICII-BAUMGARTNER
1 .o
0.3 d,
2
0.9
.
3
$ 0.2
%
rr
I
0.8
8
4
3
.e
f 0.1
.
&
0.7
n
0’
78
0.6
0.5 0
1
2
3
4
5
6
p H + D78/PH+D78.
Figure 3. Theoretical peak ratios us. fractional hydrogen abundance in “hydrogen phase”: calculated by accurate equation (full line); calculated by approximate equation (dotted line).
79
80
83
82
81 Mass number.
84
Figure 4. Benzene mass spectrum from a P h r . randomization reaction at 32”: 0,observed; H, calculated from PH = 0.938, Q a = 0.062, PD = 0.079, QD = 0.921. 0.2 d
1
a
B
Verification of Kinetic Equations. The basic assumptions involved in the derivation of the kinetic equations were subsequently investigated. The analytical procedure was confirmed by comparing experimental results with those calculated by eq. 1. The test was performed on a (49.06%) deuterated and completely randomized (t = a ) benzene sample. Figure 2 shows an excellent agreement between the calculated and the experinientally determined spectrum. The assumption that deuterium enters benzene molecules in a random manner a t all stages of the reaction (t # 00) was examined by analyzing the products of both randomization (Fig. 4 and 5 ) and deuterium oxide exchange reactions (Fig. 6). In the former case the peak ratio 79/78 was measured, and the corresponding PH value was determined from graph 3 ; separate spectra for hydrogen and deuterium “phases” were then calculated from PH and PD. These were combined and compared with the experimentally observed spectrum. The benzene spectrum from deuterium oxide exchange was calculated from the known deuterium content by eq. 1. Figures 4 and 5 , which correspond to early and late stages of the randomization reaction, respectively, show poor agreement between calculated and observed spectra. These discrepancies cannot, however, be attributed to faults in the model of randomization reactions (Le., the division into hypothetical phases) since these discrepancies occur also in deuterium oxide exchange (Fig. 6) where this model does not apply. One must conclude, therefore, that deuterium does not enter benzene in a random fashion, but reaches this state only when lLexchange” between the hypothetical The .Journal of Physical Chemistry
% 0.1 3
B
.e c
z
c 0
78
79 80 81 Mass number.
83
82
84
Figure 5. Benzene mass spectrum from a 17-hr. randomization reaction a t 32’: 0, observed; M, calculated from PH = 0.765, QH = 0.235, PD= 0.252, QD = 0.748.
78
79
80 81 Mass number.
82
83
84
Figure 6. Benzene mass spectrum from a deuterium oxide exchange reaction at 32”: 0 , observed; H, calculated; deuterium content = 10.5y0.
phases, or with D20, has been L1completed.” Consequently, PH values determined from graph 3 give only an approximate measure of the reaction progress, there being a tendency to underestimate the reaction rate. Randomization Reactions Involving Multiple Exchange Processes. The above nonrandom deuterium distribu-
CATALYTIC DEUTERIIJM EXCHANGE REACTIONS WITH ORGANICS
tion in benzene can be explained by the participation of multiple exchange 1processes.9 I n multiple exchange, a parameter, M , may be calculated corresponding to the mean number of deuterium atoms entering each benzene molecule under initial conditions. This parameter is defined by two rate constants.
3181
0
h
2 -0.2 I 0
2 7 -0.4
Y ro
I .,
n
v
-0.6
M
One of these, IC,, was shown on theoretical groundslo to be defined by the first-order equation
s
-0.8‘
’
0
’
’
’
2
”
4
’ 8
’
6
.
’ 10
Time, hr.
+
+ + +
where 4 = u 2v -t 3w 4s 5y 62, “u”to “2” being the percentage of total benzene present as benzene-dl to -d6. However, the first-order equation defining ICb has no theoretical foundation, but is an empirical relationship which applies over a considerable range of a number of exchange reactions -log ( b - b,)
=
-:--k b t
2.
