J . Phys. Chem. 1993,97, 10989-10995
10989
Kinetics of the Reactions of CH2(g3B1) with Selected Polycyclic Aromatic Hydrocarbons at Temperatures between 296 and 690 K H. Kraus,t C. Oehlers, F. Temps,' and H. Gg. Wagner' Max-Planck-Institut f i r Striimungsforschung, Bunsenstrasse 10, 37073 Giittingen, Germany Received: April 13, 1993; In Final Form: July 12, 1993'
The kinetics of the reactions of CH2 radicals in their W3B1ground electronic state with the aromatic hydrocarbon molecules benzene ( l ) , naphthalene (2), phenanthrene (3), anthracene (4), and biphenyl (5) have been studied. Experimental measurements were carried out for reactions 2-5 at temperatures between 445 and 690 K using an isothermal discharge flow system with far-infrared laser magnetic resonance (FIR-LMR) detection of the CH2 radicals. The results of these investigations as well as those of an earlier study of the rate constant of reaction 1 in the temperature range 296 K I T I 683 K were analyzed in light of recent temperature-dependent data on the competing reactions of CHz(iilA1). The direct experimental data for reactions 2-5 were found to be described by the following rate expressions (f2u): k2,cxp= (2.52:) X 1013exp[-(30.0 f 0.9) kJ-mol-l/RT] cm3/(mol.s) (445 K I T I 660 K), k3,cxp= (36.:;) X 1013exp[-(29.9 f 3.6) kJ.mol-l/RT] cm3/(mol.s) (477 K I T I 548 K), k4,cxp= (8.5f;f) X lo1, exp[-(17.8 f 1.0) kJ.mol-l/RT] cm3/(mol-s) (523 K I T I 6 9 0 K), and k5,cxp= (2.9:;) X 1013exp[-(30.8 f 2.4) kJ.mol-l/RT] cm3/(mol.s) (476 K I T I 638 K). After correction for the depletion of CH@) via excitation to the H state, the rate constants for the reactive channels of CH&) (f2u) with the five aromatics (1-5) were found to be k l T = (1.72,;) X 1013 exp[-(36.4 f 2.5) kJ.mol-l/RT] cm3/(mol-s) (296 K I T I683 K), k2T = (1.22;) X 1013 exp[-(27.7 f 1.0) kJ.mol-l/RT] cm3/(mol.s) (445 K I T I 660 K), k 3 T = (2.2:;) X 1013exp[-(28.5 f 3.9) kJ+mol-l/RT] cm3/(mol-s) (477 K I T I 548 K), k 4 = ~ (62.);: X 10l2 exp[-(16.6 f 1.9) kJ-mol-l/RT] cm3/(mol-s) (523 K I T I 690 K), and kST = (1.32,;) X 1013 exp[-(28.2 f 2.9) kJ*mol-l/RT] cm3/(mol.s) (476 K I T I 638 K).
1. Introduction Reactions of carbenes with aromatic hydrocarbonsare of great interest as a straightforward route for the synthesis of cycloheptatriene derivatives.'-3 However, little has been known quantitatively about the different elementary reactions that play a role in the overall reaction systems. This is true even for the prototypical carbene, CH2, in both its X3B1 ground electronic state and the HIAl first excited state (hereafter referred to as 3CH2 and TH2, respectively). In a recent publication,4 we reported on the kinetics of the reaction of 3CH2 with benzene, the most important aromatic hydrocarbon molecule,
3CH, + benzene
-
products
(1) Both the overall rate constant for the effective depletion of 3CH2 as a function of temperature in the range 296 K I T I683 K and the high-pressure product distribution at room temperature were determined. The apparent experimental activation energy for the total depletion of 'CHI by benzene was measured4 to be EA,^^^ = 37.5 kJ/mol. This value of the activation energy is practically equal to the electronic excitation energy of the lowlying gIA1 excited state of CH2, AH" = 37.7 kJ/mo1.5?6 Thus, since 'CHI reacts with benzene with virtually every collision,7 an unambiguous distinction between reactive depletion of the 3CH2 with benzene versus collisional excitation of the 3CHz to the singlet state followed by the rapid reaction of ICH2 with benzene has been diffi~ult.~ A similar picture was also encountered in a study of the reaction of 3CH2with the simplest alkyl-substituted aromatic molecule, toluene.* In order to throw more light on the reactions of 3CH2 with aromatic molecules, we have now studied the kinetics of the
* To whom correspondence should be addressed.
Present address: Laboratorium fiir Technische Chemie, Eidgenbsische Tcchnische Hochschule, 8092 ZGrich, Switzerland. * Abstract published in Aduance ACS Abstracts, September 15, 1993. f
reactions between 3CH2and a seriesof larger polycyclic aromatics, namely naphthalene, phenanthrene, anthracene, and biphenyl,
3CH, + naphthalene
-
3CH, + phenanthrene
-
+ anthracene 'CH2 + biphenyl
'CH,
products
(2)
products
(3)
products
products
(4)
(5) Clarg and Zander10 have proposed qualitative rules for the reactivities of the polycyclic aromatics compared to benzene. According to their rules, the larger aromatics should exhibit significantly higher reactivities toward attack by free radicals than benzene. In addition, it is of interest that differences are also predicted for the reactivitiesof the isomers anthracene versus phenanthrene because of their different electronic structures.9JO Thus, reactions 2-5 should have smaller activation energies than reaction 1. In the well conceivable case that these activation energies were smaller than the CHI singlet-triplet electronic energy splitting, the aforementioned two competing processes (reactions of 3CH2 with the aromatic hydrocarbons versus collisional excitation to the singlet state followed by reactions of ' C H 3 could be separated experimentally. This means that it would be possible to establish the reactions between carbenes and aromatics quantitatively on firmer grounds. In the present publication,we would like to report on the results of the experimental investigationsof reactions 2-5 in comparison to those for reaction 1. We have carried out measurements of the rate constants for reactions 2-5 using the isothermaldischarge flow technique. The depletion of 3CH2by the respective aromatics was monitored with a far-infrared laser magnetic resonance (FIRLMR) spectrometer.ll Temperatures were varied between 445 and 690 K. The experimentaldata for the reactions were analyzed taking into account recent temperature-dependent kinetic datal2 for the respective reactions and the collisional deactivation of
0022-3654/93/2097-10989%04.00/0 0 1993 American Chemical Society
Kraus et al.
10990 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
1CH2. In addition, theexperimental data for reaction 1 described previously4 were reanalyzed correspondingly for a quantitative comparison. The combined results provide a systematic picture which can be used for predictions of the kinetics of the reactions of 3CH2 with other reactant molecules.
2. Experimental Section A detailed description of the experimental apparatus has been given earlier.4.13 Briefly, the flow system used in this work consisted of a 4-cm i.d. quartz tube with a moveable injector. Prior to installation, all glass parts were cleaned with 5% H F solution and rinsed thoroughly with distilled water. A constant temperature in the reactor was maintainedwith a resistively heated Ni oven. Variationsof the temperature along thereaction distance did not exceed f3 K. Helium served as inert carrier gas. Flow rates were controlled with calibrated mass flow meters (Tylan). The pressure was measured directly in the LMR sample cell with a capacitance vacuum meter (MKS Baratron). Correctionsfor viscous pressure drop and diffusion along the length of the reaction zone were made as u s ~ a 1 . l ~ 3CH2radicals were generated inside the moveable injector via the reaction of O(3P) atoms with a large excess of CH2C0.15J6 The 0 atoms were produced in a microwave discharge of 02 diluted in He. The concentrations of 3CH2were monitored with the FIR-LMR spectrometer13 operating at X = 158 pm (I3CH3OH), BO= 0.323 T with A polarization of the electric field vector of the laser radiation relative to the magnetic field.” The CH2CO was prepared by pyrolysis of acetone and purified by repeated trap-to-trapdistillations. Its flow rate was controlled with a needle valve and determined from the pressure rise in a calibrated volume. The other gases were of the highest commercially available purities (He, 99.9999%; Ar, 99.9999%; 0 2 , 99.998%;2% 02premixedin He; all Messer-GriesheimorUCAR). The aromatic hydrocarbons were used as supplied (Aldrich) without further purification. Specified purities were 99% for naphthalene, >98% for phenanthrene, 99% for anthrancene, and 99% for biphenyl. For the measurements, the aromatic compounds were vaporized into a He flow in a two-stage saturator. Complete saturation of the gas stream was achieved by keeping the temperature of the second stage 10 OC below that of the first. All connecting tubes to the flow system were heated to a temperature -50 OC above the temperature of the saturator to avoid condensation. Seals were made with Kalrez O-rings (Du Pont). The concentrations of the aromatics were evaluated from their vapor pressures as given in ref 18. The temperature range of the experiments was limited to avoid decomposition of the aromatics at higher temperatures. The lower temperature limits were set by the high melting points and low vapor pressures of the compounds.
-
3. Results The rate constants of the title reactions were determined by monitoring the depletion of the 3CH2 concentration along the flow tube in the presence and in the absence of a large excess of the aromatic hydrocarbon reactants, R = benzene, naphthalene, phenanthrene, anthracene, and biphenyl, respectively. In the absence of the reactants, the decay of [3CH2] could be described by effective first-order “wall” rate coefficients kwsff which account for the heterogeneous loss reactions of the 3CH2 on the surface of the reactor. The values of kw,cnwere found to be on the order of -20 s-I at room temperature, increasing to -90 s-I at T = 690 K. For low initial radical concentrations ([3CH2]052 X 1&13mol/cm3), sidereactionssuchas thereactions of 3CH2with undissociated 0 2 , the reaction with CHzCO, or the mutual combination of ’CH2 are practically negligible. In the presence of a large excess of the reactants, the reactions of 3CH2with the respective aromatics constitute the main 3CH2
TABLE I: Rate Constants for lCH2 species benzene naphthalene phenanthrene anthracene biphenyl CHzCO
He
k~ + ks [cm’/(mol.s)] 5.6 X 5.6 X 5.6 X 5.6 X 5.6 X 1.4 X 6.5 X
10’5(T/296 K)4”5 10’5(T/296 K ) 4 3 10i5(T/296 K)-O’”’ 10Is(T/296 K)-0.S5 1Ol5(T/296 K)-0.55 1Ol6(T/296 K)-0.*O 1Pg(T/296 K)1.Z6
a
kD/
(kD + ksP
0.30 0.30 0.30 0.30 0.30 0.25 1 .oo
ref 22,23
estimated, 23 estimated, 23 estimated, 23 estimated, 23 20,23 20,23
a = k ~ / ( +k ks) ~ is the fractionof the collisionaldeactivation channel for T = 296K. Thevalueofaisassumed tobetemperatureindependent.M @
consumption pathways. The reactionswere studiedunder pseudofirst-order conditions ([R] >> [3CH2]0),which prevailed throughout all experiments. The 3CH2decay profiles were observed over a range of more than 1 order of magnitude change of the concentrations. Over this range, there wereno apparent deviations from first-order behavior. First-order 3CH2decay rate constants k$,exp( i = 1-5) were evaluated for the conditions of each run from plots of In( [ ~ C H ~ ] +[R ~C / H ~ I - Rversus ) reaction time ([3CH2]+~ and [3CH2]-~denote the concentrations of 3CH2in the absence and presence of the reactants, respectively). In all cases, these plots yielded good straight lines. The desired experimentalsecond-order rate constants ki,cxp were then obtained for the different temperatures from plots of the first-order rate constants versus the concentrations of the reactants. These plots yieldedstraight lines through theorigin. Within the experimental uncertainty limits, the values of the intercepts were virtually zero, which can be taken as an indication for the absence of significant systematic errors. The further analysis and interpretation of the direct experimental data has been outlined in detail in several of our earlier papers.4J9-20 Briefly, the experimentallymeasured rate constants k1,exp-k5,cxp describe the overallconsumptionof the 3CH2radicals by the reactant molecules via all possible pathways. In the first place, one has the direct reactive pathways of the 3CH2with the different substrates. These are the channels which are of most interest. They have been referred to as the direct “triplet reactionsn4J9and are denoted in the following as reactions 1T5T. Here, addition reactions of ’CHI to the ring systems of the aromatic molecules are considered as the main channels (see, e.g., ref 4 and the discussion below). However, a second depletion pathway for 3CHg arises by collisional excitation of the 3CH2 radicals from the X ground state to the B excited electronic state (lCH2) followed by fast reactions of lCH2 with the aromatics. This channel, which is well appreciated by now,20isa consequence of the low electronic excitation energy of the BIAl state5v6of CH2 and the known high rate constants2I-2’ for the collisional intersystem crossing transition, lCH2 + M + 3CH2 M, which leads to a partial thermal equilibration between the two spin states. Considering this mechanism, it has been the first task in the present study to obtain direct experimental measurements of the overall depletion rate constants for 3CH2 with the reactant molecules of interest. The second task was then to separate the reactive consumption19 of the ’CH2 radicals by the aromatics (Le. the “triplet reactions”) from the contribution to the overall 3CH2depletion that is due to the collisional excitation pathway to the singlet state and reactions of ICH2 under the conditions of the experimental measurements. Toward these ends, the rate constants k l r k s for ~ the direct 3CH2 reactions were evaluated from the experimental data, as we have described several times b e f ~ r e . ~ The J ~ . reader ~ ~ is referred here to ref 20 for the exact details, to avoid repetition. The rate constants for the 1CH2 reactions which are needed for the analysis are collected in Table I. 3.1. Reaction of j c H z with Benzene. The experimental conditions and raw data for the measurements of the reaction of
+
Reactions of CH,(%Bl) with Aromatic Hydrocarbons
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 10991
TABLE Ik Experimental Results for the Rate Constant of the Reaction 3CHz T (K)
p (mbar)
445 445 445 480 480 480 541 541 541 603 603 603 660 660 660
4.83 4.83 4.83 3.76 4.36 4.84 3.12 3.45 3.80 2.72 3.12 3.83 2.72 2.73 3.13
[Ozl mol/cm3)
v (m/s) 15.7
[CHzCOl (10-9 mol/cm3)
[naphthalene] (it9 mol/cm3)
k 'zoxp
1.9 1.7 1.5 1.4 1.3 1.3 1 .o 1 .o 1 .o 0.7 0.7 0.7 0.8 0.7 0.9
6.31 8.28 10.90 4.00 7.01 9.58 3.06 3.92 5.08 1.53 2.76 4.49 0.79 1.41 2.16
52 66 85 56 89 122 92 117 160 94 167 288 94 149 239
1.4 1.3 1.1 1.4 1.3 1.2 1 .o 1 .o 1 .o 0.9 0.9 0.8 0.9 0.8 0.9
17.5 20.2 15.8 17.2 18.2 23.0 22.8 23.6 24.5 25.6 27.1 23.8 26.7 25.9
+ Naphthalene (2) (d)
k2,oxP
(lolocm3/mol.s) 0.82 0.79 0.78 1.40 1.27 1.27 3.01 2.98 3.15 6.14 6.05 6.41 1.90 0.60 11.10
TABLE III: Experimental Results for the Rate Constant of the Reaction CH2 + Phenanthrene (3) [021 [CHzCOl [phenanthrene] k'3,axp k3, T (K) p (mbar) v (m/s) mol/cm3) (lt9 mol/cm-') ( lt9mol/cm3) (s-l) (lolo cm3/mol.s) 477 488 477 548 548 548
3.49 4.03 4.03 3.89 3.89 3.64
16.2 16.7 21.0 15.8 18.8 24.8
1.9 1.8 1.4 1.4 1.2 0.9
1.5 1.5 1.2 1.6 1.3 1 .o
2.03 3.88 5.92 1.61 2.98 4.31
40 79 104 82 162 205
1.97 2.04 1.76 5.09 5.44 4.76
k2T
(lolocm3/mol.s) 0.72 0.70 0.69 1.18 1.06 1.07 2.38 2.37 2.55 4.47 4.51 4.95 8.47 7.40 8.15
k3~
(lolocm3/mol-s) 1.76 1.83 1.56 4.29 4.70 4.07
reactant concentrations were varied in the range 7.9 X lO-lOmol/ cm3 I[naphthalene] I1.1 X 10-8 mol/cm3. On the basis of estimates of the initial 3CH2 concentrations, these conditions correspond to ratios of [naphthalene]/[3CH2]o 1 8 X 103. The first-order rate constants are plotted versus reactant concentration in Figure 1. The experimental second-orderrate constants which were obtained from these plots can be approximated in an Arrhenius diagram by a straight line which gives a rate expression of the form
x io1, x
k2,exp= (2.52;)
exp[-(30.0 f 0.9) kJ.mol-'/RT] cm3/(mol-s) 0
4
2
6
a
IO
12
[naphthalenell (lO-'mol. ~ m - ~ )
Figure 1. Plot of measured pseudo-first-order rate constants for the reaction 3CH2+ naphthalene (2)versus reactant concentration.
3CH2with benzene were given in detail earlier.4.28 Measurements of the depletion of 3CH2by benzene had been carried out in the temperature range 448 K IT I683 K with pressures in the regime 1 mbar Ip I8 mbar. In addition, the rate constant for reaction 1 at room temperature could be inferred from the results of end product measurements. A least squares regression to a linearized Arrhenius function (In k = In A - E*/RT) using equal weights yielded the experimental expression
kl,cxp= (4.7:;)
x ioL3x exp[-(37.5 f 2.4) kJ-mol-'/RT] cm3/(mol-s)
After a reevaluation in the present work of the correction for the depletion of CH2(X) via excitationJo the8 state, the rateconstant for the reactive channel of CH2(X) excluding the pathway via 'CH2 was found to be
k,, = (1.7;)
X lo', X
exp[-(36.4 f 2.4) kJ-mol-'/RT] cm3/(mol.s)
3.2. Reaction of J c H 2 with Naphthalene. The experimental conditions for the measurements of the reaction of 3CH2 with naphthalene29 are compiled in Table 11. Investigations were carried out in the temperature regime 445 K I T I 660 K with pressures comprising the range 2.7 mbar I p I 4.9 mbar. The
The corrected rate constant expression for k 2 was ~ calculated to be
k,, = (1.22;)
X
lo', X
exp[-(27.7 f 1.0) kJ.mol-'/RT] cm3/(mol.s)
3.3. Reaction of JCHZwith Phenanthrene. The reaction of 3CH2 with phenanthreneBhas been investigated with excess ratios of [phenanthrene]/[3CH~]o1 1 X lo4. The pressure was varied in the range 3.5 mbar Ip I4.0 mbar. The temperature range was 477 K IT I 548 K. The experimental results are presented in Table 111,and the pseudo-first-orderrate constantsas a function of reactant concentration are displayed in Figure 2. The experimental data yield an Arrhenius expression of
k3,exp= (36.:;)
x io1, x exp [-( 29.9 f 3.6) kJ.mol-'/RTj cm'/(mol-s)
After correction of the experimental rate constants one obtains
k,, = (2.2:;:)
x 1013 x exp[-(28.5 f 3.9) kJ.mol-'/RT] cm3/(mol.s)
3.4. Reaction of 3CH2 with Anthracene. Measurements for the reaction of 3CH2 with anthracene3O were carried out in the temperature regime 523 K IT I 690 K with pressures 1.5 mbar I p 5 2.0 mbar and reactant excess ratios [anthracene]/ [3CH2]~ 2 6 X 102. The experimental conditions and results are listed in Table IV. Figure 3 shows the pseudo-first-orderrateconstants
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10992 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
TABLE IV: Experimental Results for the Rate Constant of the Reaction -2 + Anthracene (4) LO21 [C H zC 01 [anthracene] kG,ap k4,a k4~ T(K) p (mbar) u (m/s) mol/cm3) mol/cm3) mol/cm3) (s-l) (1011~m~~m01.s)(10" cm3/mol-s) 1.53 1.83 1.63 1.83 2.03 1.97 1.74 1.98
523 523 523 605 605 655 690 690
TABLE V T(K)
1.o 1.2 0.9 1.o 1.1 1.2 0.9 1.o
22.1 18.9 24.7 21.6 20.5 21.7 24.9 22.2
2.83 5.49 6.79 2.73 5.36 3.29 1.17 2.65
1.3 1.9 1.4 1.5 1.6 1.5 1.3 1.6
41 78 99 62 121 103 48 104
1.45 1.42 1.46 2.27 2.26 3.13 4.10 3.92
+
Experimental Results for the Rate Constant of the Reaction e H 2 Biphenyl (5) P 2 1 [CHzCOl [biphenyl] k's.up kSAX p (mbar) u (m/s) mol/cm3) (10-9 mol/cm3) ( l t 9 mol/cm3) (d) (lolo cm3/mol-s)
416 476 479 479 547 547 546 616 616 616 673 673 673
1.40 1.38 1.42 2.13 2.12 2.87 3.72 3.56
1.o 1.1 1.1 1.1 1.o 0.9 1.1 0.9 1.o 0.8 1.o 0.9 0.7
24.9 22.6 23.0 23.0 26.2 26.4 23.2 28.3 26.2 29.7 24.1 29.0 35.6
3.25 3.77 4.04 4.04 3.25 3.65 4.45 2.87 3.67 3.67 2.72 2.73 2.76
1.2 1.4 1.3 1.3 0.9 0.9 1.o 0.6 0.7 0.6 0.8 0.6 0.5
3.85 6.00 8.22 8.22 2.25 4.41 6.13 1.58 2.96 4.12 0.60 1.33 2.20
50 83 106 107 65 122 184 110 181 260 83 172 287
kST
(lolo cm3/mol.s)
1.30 1.38 1.29 1.30 2.89 2.77 3.00 6.96 6.1 1 6.31 13.80 12.90 13.00
1.10 1.20 1.10 1.11 2.13 2.09 2.35 4.94 4.25 4.57 9.78 9.20 9.64
548 K
0
2
4
0
6
10
12
!
Iphenanthrene]/(lO-gmol.cmm3)
[biphenyll/(lO-g m o l . c ~ n - ~ )
Figure 2. Plot of measured pseudo-first-order rate constants for the reaction 3CH2 + phenanthrene (3) versus reactant concentration.
2oo
t
k,T = (62).;: 690 K
605 K
2
4
6
X 10'' X
exp[-( 16.6 f 1.9) kJ*mol-'/RTj cm3/(mol.s)
3.5. Reaction of CH2 with Biphenyl. The reaction of 3CH2 with biphenyP9 was investigated at temperatures in the range 476 K IT 5 638 K with pressurescomprisingthe range 2.7 mbar Ip I 4.5 mbar and reactant excess ratios [biphenyl]/[TH2]0 2 6 X lo3. The experimental conditions and results are given in Table V. The first-order rate constantsare plotted versus reactant concentration in Figure 4. A least squares fit to the experimental data yielded the Arrhenius expression
.523K
0
Figure 4. Plot of measured peudo-first-order rate constants for the reaction 3CH2 + biphenyl ( 5 ) versus reactant concentration. The corrected term for the rate constant was found to be
S
10
12
[anthracene] / (10"0mol.cm'3)
k5,cxp = (2.9:,';)
Figure 3. Plot of measured peudo-first-order rate constants for the reaction 3CH2 + anthracene (4) versus reactant concentration.
x 1013 x exp [-( 30.8 f 2.4) kJ-mol-' /RT] cm3/(mol-s)
The corrected rate constant as calculated to be given by as function of the reactant concentration. The experimentaldata excluding the measurementsat the highest temperature, for which some indications for a slow decomposition of the biphenyl were observed, can be described by the Arrhenius expression
k4,cxp= (8.5:;)
k5T
(1.3z.i) x 10" x exp[-(28.2 f 2.9) kJ.mol-'/RTJ cm3/(mol.s)
4. Discussion X 10l2 X
exp[-(17.8 f 1.0) kJ*mol-'/RTl cm3/(mol*s)
The experimental results of the present work establish the kinetics for the important class of reactions of 3CH2with a series
Reactions of CH2(A3Bl) with Aromatic Hydrocarbons
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 10993
TABLE VI: Summary of Arrhenius Parameters for the Chemical Reactions 1T-ST of 3CHz with the Polycyclic Aromatic Hydrocarbons Studied' aromatic hydrocarbon A (cm3/mol-s) E A (kJ/mol) T (K) 37.5 1.9 296 S T I 683 benzene (2.3+'.3) x 1013 j ) x 1013 27.7 1.0 445 S T I660 naphthaleneb ( 1 &$9 phenanthrene (2.2; ,') x 1013 28.5 3.9 477 I T 5 548 anthracene (6,2!$) x 1012 16.6 1.9 523 IT I690 biphenyl (1.3fo.f) X 1013 28.2 2.9 476 S T I 6 7 3
* *
*
Uncertainty limits refer to the 95% ( 2 4 confidence intervals. b The smaller error limits for the reaction with naphthalene compared to those with the other aromatics are due to the larger temperature range of the measurements. TIK 1000
700 600
3CH2+@l/
OI
= -
-100-
-
-200-
E"
7
Y \
co m
LOO
500
0-
I
-5001
3
2
T-'/ i io-? K-' 1 Figure 5. Arrhenius plots of the rate constants k l r k s for ~ the reactions of 3CH2 with the five selected unsubstituted aromatics studied.
of polycyclic aromatic hydrocarbon molecules on a quantitative basis. These reactions are of substantial interest in organic synthesis in view of the unique route to products containing a cycloheptatrienering.14 The rate constants for the five reactions which were investigated in the present study,
3CH, + benzene
-
products
(1)
3CH, + naphthalene
products
(2)
3CH, + phenanthrene
products
(3)
--
+ anthracene 3CH, + biphenyl
'CH,
products
products
(4) (5)
are summarized in Table VI and illustrated in an Arrhenius diagram in Figure 5 . The uncertainty limits quoted in Table VI refer to the 95% statistical confidence intervals (2u precision). The quoted 2u uncertainty for the reaction with naphthalene is lower than that for the other polycyclic aromatics mainly because of thelarger temperature range which could be covered. Including estimated possible systematic errors, the absolute values of the experimental rate constants for the different reactions at the individual temperatures are estimated to be accurate to within f20%. Considering the combined data, one can see the systematic trends in the reactivities of the reactants and obtain insight into details of the reaction mechanisms as well as information on the possible products which can be formed under different conditions. It is useful to discuss at first the reaction of 3CH2with benzene (1) in some more detail. The measurements4for reaction 1 have = 37.5 kJ/ established a considerable activation energy of mol. This value is virtually equal to the CH2 singlet-triplet energy splitting, AHas~ = 37.7 kJ/mol. The value of the rate constant for the reaction of 3CH2with benzene at room temperature, k l r (296 K) = 6 X 106 cm3/(mol.s), which has been inferred from measured product yields: has to be compared to the overall rate
Figure 6. Energy diagram for reaction intermediatesand products of the reaction 3CHz + benzene (1). constant of the corresponding ICH, rea~tion,~' (kls + k l ~=)2.4 X 10'4 cm3/(mol*s). Thus, the two CH2 spin states show a differencein reactivity of more than 7 orders of magnitude. These vastly different reactivities contrast with the apparently quite similar product distributions. The results of the product analysis4 indicate that, in the high-pressure regime, both the TCH2and the lCH2 reactions with benzene lead to the formation of cycloheptatriene or toluene. However, for a complete analysis of these results, one has to allow for the collision-induced intersystem crossing (ISC) transition between the two CHI spin states, which affects the rate measurements as well as the product yields. As indicated above, under the present experimental conditions, the partial equilibration between the CHZ spin states followed by reaction of T H 2with benzene competes with the reactivechannels of 3CH2 benzene. BBhland et al. showed that, on average, the excitation pathway of 3CH2and reaction of ICH2 account for approximately 65% of the observed 3CH2decay and therefore dominate the experimentally measured kinetic^.^ The present evaluation of the experimental kinetic data seems to indicate a slightly higher activation energy for the reactive depletion of 3CH2 than that obtained before! However, the inferred values are so close to the value of the CH2 singlet-triplet energy splitting that the proposed parameters for the T H 2 benzene reaction ought to be considered with some caution. The overall rate constants as directly measured are of course fully valid. Pertaining to the detailed mechanisms of the reactive channels of 3CH2with benzene, the first step is considered to be an addition to the aromatic ?r-bondsystem. This would lead to a 'chemically activated" highly vibrationally excited triplet diradical as a first intermediate. The subsequent formation of stable products can be rationalized on the basis of unimolecular rate theory.4 It is assumed that ISC of the initial triplet to a singlet diradical can proceed with a rate of 107-108 s-l, which would be fast compared to possible chemical isomerization processes.32 An unstable norcaradiene species is considered as a plausible further intermediate. 1,2 H atom migration in the triplet diradical to toluene in a triplet state should be too slow to be of significance;it may
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+
10994 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
Kraus et al.
loo/
-i
3CH2 +phenanthrene
500}
3CH2 +anthracene
Figure 7. Energy diagram for the plausible reaction products of the reactions of 'CHI with phenanthrene (3) and anthracene (4).
in fact even be ruled out by a high barrier. Under this condition, the chemically activated norcaradiene is expected to isomerize to cycloheptatriene, which, at room temperature and pressures above a few mbars, should be stabilized by collisions with the bath gas or, at in the low-pressure regime, could isomerize to t0luene.~3 Note that norcaradiene has been suggested as an intermediatein the unimolecularisomerizationof cycloheptatriene to toluene.34 This reaction exhibits an activation energy of nearly 200 kJ/mol, which is attributed to precisely the 1,2 H atom migration in the diradical intermediate. On the other hand, a C-H insertion channel should not be possible for T H 2 . This seems to rule out direct formationof toluene from 3CH2 benzene. Hence, according to these arguments, under high-pressure conditions, the reaction of 3CH2with benzene should yield only cycloheptatriene. The reaction channel which yields toluene should play a role only in the low-pressure regime. The observed toluene4fromthereaction is attributed to the collisional excitation of 3CH2to 'CHI and the well-known direct C-H insertion reaction of the 'CH2. An energy diagram for the reaction products and intermediates for the CH2 + benzene system is given in Figure 6. Naphthalene, phenanthrene, and anthracene are simple larger polycyclic aromatic systems. Comparing the rate constants for the benzene reaction with those for the other aromatics, benzene is very clearly the least reactive molecule. Although naphthalene and phenanthreneare differentin aromatic character, they behave very similarly. In contrast, anthracene is reacting significantly faster than phenanthrene. Nevertheless, the mechanisms of the reactions of 'CHI with naphthalene, phenanthrene, and anthracene can be assumed to resemble the one for the corresponding reaction with benzene. Plausible exothermic reaction products35 for naphthalene are
+
'CH,
-
+ naphthalene
7H-benzocycloheptene,
A,H0298
= -306 kJ/mol
I-methylnaphthalene,
v 2 9 8
= -423 kJ/mol
2-methylnaphthalene,
Y O 2 9 8
= -424 kJ/mol
For phenanthrene, the following products can be expected 'CH,
-
+ phenanthrene
SH-dibenzo[a,c]cycloheptene,A,.Ho,,,
9-methylphenanthrene, A J P 2 9 8 = -422 kJ/mol 2-methylphenanthrene, A p 2 9 8 = -43 1 kJ/mol
Possible reaction products for the 'CH2 are CH,
-
= -312 kJ/mol
+ anthracene reaction
+ anthracene
SH-dibenzo[a,dJcycloheptene,A J P 2 9 8 = -349 kJ/mol 9-methylanthracene,
v 2 9 8
= -418 kJ/mol
2-methylanthracene,
AJP298
= -43 1 kJ/mol
Assuming a mechanism analogous to that for benzene, %H2 should react via addition to the respective aromatic ring systems to form a triplet biradical which, after ISC, should lead to a norcaradiene-like compound. The next step can be the isomerization to a stablecycloheptatriene-likecompound. Isomerization of these structures to the methyl derivatives should be too slow to be of significance under the conditions used. In view of the known high aromatic C-H bond e n e r g i e ~H-atom , ~ ~ abstraction by 'CHI from the aromatic rings is expected to require activation energies19 of more than 40 kJ/mol, so that this possibility can be neglected. According to these arguments, the 'CH2 reactions with polycyclic aromatics are expected to yield mainly products that are analogous to the cycloheptatriene formed in the reaction with benzene. Nevertheless, the formation of other less obvious polycyclic products with different structures might also play a role for these larger ring systems. An important result is the higher reactivity toward 3CH2 of anthracene compared to the isomeric phenanthrene. Impurities of the anthracene are unlikely to be the reason for the difference, as similar trends have been observed in other studies of these
Reactions of CHz(R3Bl) with Aromatic Hydrocarbons
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 10995
molecules.9J0 As illustrated by the energy diagram for the products of the anthracene and phenanthrene reactions in Figure 7,thereareslight differencesin the exothermicities for thedifferent channels, but these alone should not affect the activation energies very strongly. On thermodynamic grounds, attack of the 3CH2 at the middle rings of phenanthrene and anthracene seems to be favored. On the other hand, it is noted that the activation energies for the 3CH2 reactions may be correlated with the vertical ionization potentials of the aromatics. This has also been found for 0 atoms.3' Here, it is of interest that the ionization potential for anthracene is significantly smaller than that for phenanthrene.38.39 The *-sextet model of Clar9 and Zander10 for the reactivities of polycyclic aromatics can explain the rising reactivity from benzenevia naphthalene, phenanthrene, and anthracene to larger aromaticcompounds. Accordingly, the reactivity should increase with the size of the reactants. A clear reactivity gap can be expected between naphthalene and anthracene. This has been confirmed in our experimental studies. Furthermore, although anthracene is isomeric to phenanthrene, it should be more reactive because phenanthrene is angled and, therefore, full aromatic ?r-electron sextets can be assigned to two rings of this molecule but only to one ring of anthracene. Then, because of the olefinic character of the one "double bond", reaction with phenanthrene should take place primarily at carbon atoms C-9 and C-10. A different attempt to explain the increasing reactivity of the larger aromatics can be found in ref 38. Biphenyl is a polyarylic molecule which does not belong to the polycyclic aromatics, because neither of the two rings shares two carbon atoms with another ring" The two rings which are connected via a C-C single bond are twisted by 45 f 5 O . The bond between the rings is 0.09 I% longer than those between C atoms within the rings, which show the same length as those in benzene. Thus, the two rings do not appear to influence each other too significantly. Nonetheless, the reaction of 3CH2with biphenyl is faster than the one with benzene. Instead, somewhat surprisingly, the activation energy of the reaction is closer to that of the reaction with naphthalene. At first glance, one might expect a reaction mechanism similar to that for benzene. Stable isomerization products can be 7-phenyl-l,3,5-~ycloheptatriene (ArHo298 = -278 kJ/mol) and 3- or 4-methylbiphenyl (ArH0298 = -421 kJ/mol). However, an interesting other possibility would be an attack of CH2 to both rings with formation of a new bridge between the two rings. Clearly, direct end product analysis would be of interest to throw more light on the reaction mechanisms. 5. Conclusions
The reported measurements for the reactions of 3CH2with the series of polycyclic aromatic hydrocarbon molecules show distinctly lower activation energies for the larger reactants compared to that of the prototypical reaction of 3CH2 with benzene. The strongestdeviation was found for anthracene, which has an activation energy of nearly 17 kJ/mol versus nearly 38 W/mol for benzene. The observed trends are in accordance with different models that explain the increasing reactivities from benzene to the larger aromatic c o m p o ~ n d s . ~ Biphenyl J ~ . ~ ~ seems to behave like the polyarylic aromatics rather than benzene, if the same reaction mechanism is assumed. Here, a further analysis of the reaction products would be of interest. It should be noted that, at higher temperatures such as in combustion processes, the product distributions may be different from those proposed here for lower temperatures.
Acknowledgment. We thank Professor H. C. Heinrich Nbth for cooperation over many years and would like to dedicate this paper to him on the occasion of his 65th birthday. This work has been supported by the Fond der Chemie. References and Notes (1) Kirmse, W. Carbene Chemistry;Academic Press: New York. 1971. (2) Jones, M.; Moss, R. A. Carbenes; Wiley: New York, 1973. (3) Zeller, K.-P.; Gugel, H. In Houben- Weyl, Methoden der Organischen Chemie; Regitz, M., Ed.; Carbenes, Part 1; Thieme: Stuttgart, Germany, 1989; Vol. E 19b, p 165. (4) Bbhland, T.; Htberger, K.; Temps, F.; Wagner, H. Gg. Ber. BunsenGes. Phys. Chem. 1989,93, 80.
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