Steric Effects on Aldehyde Group Relaxation
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
1091
Steric Effects on Aldehyde Group Relaxation in Some Aromatic Aldehydes A. Lakshmi, S. Walker,“ N. A. Weir, Department of Chemistry, Lakehead University, Thunder Bay, Ontario, P7B 5E 1 Canada
and J. H. Calderwood Department of Electrical Engineering, University of Salford, Salford, England (Received December 19, 1977) Publication costs assisted by Lakehead University
Dielectric absorption studies have been made of aldehyde group relaxation of o-tolualdehyde, 2,4,6-trimethylbenzaldehyde, 1-naphthaldehyde,4-quinolinecarboxaldehyde, 9-anthraldehyde, lO-chloro-9-anthraldehyde, and phenanthrene-9-carboxaldehyde in a polystyrene matrix. All these molecules are characterized by having either a methyl or a peri hydrogen adjacent to the carboxaldehyde group which, in principle, could offer some steric hindrance to group relaxation. Aldehyde group relaxation has been established for o-tolualdehyde, 1-naphthaldehyde,and 4-quinolinecarboxaldehyde, and activation parameters have been estimated. The dielectric data for 9-anthraldehyde, lO-chloro-9-anthraldehyde, phenanthrene-9-carboxaldehyde, and 2,4,6-trimethylbenzaldehyde are less straightforward since there appears to be significant overlap of the group and molecular relaxation processes.
Introduction It is the purpose of this work to examine how steric interaction between the aldehyde group and neighboring substituents in the aromatic ring would affect aldehyde group relaxation. A number of dielectric and spectroscopic studies has been made previously on the influence of steric factors on energy barriers and relaxation times of group relaxation. Das Gupta and Smythl reported a rotation of the chloromethyl groups in bis(chloromethyl)durene, although they encountered steric hindrance from the adjacent methyl groups. Forest and Smyth2 reported on 1-chloromethylnaphthalene that the chloromethyl group was prevented from rotating on account of steric hindrance offered by the peri-hydrogen atoms. However, a t a later date Gupta and Smythl observed rotation of the group, accompanied by large distribution of relaxation parameters, pointing to steric interference from the peri hydrogen. The aldehyde group, being smaller than the chloromethyl group, may well experience less steric hindrance to the peri-hydrogen atom. Miller et al.3 examined the infrared spectrum in vapor and liquid phases of o-methylbenzaldehyde and ofluoroacetophenone. An NMR study by Dhami and &others4 of mono-ortho-substituted acetophenones suggests that these molecules are not planar in the ground state, and they have found evidence for the existence of steric hindrance to coplanarity of the acetyl group with the aromatic ring. Silver and Wood5 from their far-infrared study conclude that in o-tolualdehyde the oxygen cis form is favored by polarity and by any hydrogen bonding that may be present. Infrared evidence favors an out-of-plane acetyl group in 2,3,5,6-tetramethyla~etophenone.~ It seemed of interest to examine the peri-hydrogen effect in naphthalene and anthracene systems on an adjacent aldehyde group. In addition, the steric influence by an adjacent methyl group in benzaldehyde seemed pertinent to such studies. As a consequence o-tolualdehyde and 2,4,6-trimethylbenzaldehydewere chosen for examination. The studies were carried out at low concentrations in a polystyrene matrix where the possibility existed of separating completely the group relaxation contribution from the molecular relaxation. Such a procedure appeared more promising than studying the dielectric absorption in so0022-3854/78/2082-1091501 .OO/O
lution where the molecular and group absorptions ~ v e r l a p . ~ Experimental Results The dielectric measurements have been made on a General Radio 1615-A capacitance bridge in the frequency range 50-105 Hz. The measuring cell was a three-terminal circular parallel plate capacitor mounted in a temperature-controlled chamber flushed with dry nitrogen gas. The cell may be operated from liquid nitrogen temperature to about 400 K. For a few of the polystyrene matrix samples additional measurements were made with a Hewlett-Packard 4342A Q-meter with a temperaturecontrolled, two terminal cell in the range 2.5 X 104-1.5 X lo7 Hz. The apparatus and measurement techniques have been described p r e v i o ~ s l y . ~ ~ ~ The polystyrene matrix was prepared in the way described by Davies et al.,lOJ1and we have adopted their procedures both experimentally and also in the evaluation of the relaxation time and distribution parameter by means of the Fuoss-Kirkwood equation and the enthalpy of activation from the Eyring equation. The polystyrene used here had a weight average molecular weight Mw = 230 000. The solutes have each been examined in a polystyrene matrix and the procedures and accuracy have been described p r e v i o ~ s l y . ~The results of Eyring equation analyses of these data are given in Table I. In the fitting of the data of relaxation time as a functional “temperature to the Eyring equation”, standard statistical techniques12 have been employed to obtain the slope and intercept of the line and the variances of these two parameters. The data yielded values of the enthalpy (AH,) and entropy (AS,) of activation for the relaxation process along with confidence intervals for each of these two. The 95% confidence intervals on AH, were typically *lo% of the nominal values or less, in agreement with the work of Davies and Swain.lo For the ASE term these confidence intervals were of the order of &50% of the nominal values in some cases, since this term is often a small part of the intercept (In ( h / k B ) - AS,/R) of the In (7’7) vs. (l/T) plot. For this reason, an extensive study of the matrix was undertaken to obtain a larger number of points on the Eyring plot extending to higher temperatures. The error limits on ASE were thus reduced to @ 1978 American Chemical Society
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Walker et at.
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
TABLE I : Relaxation Data for Results for Several Molecules in Polystyrene Matrices and for Phenanthrene-9-carboxaldehyde as a Pure Solid ASE,
Molecule
mol -'
JK-' mol-'
4-Quinolinecarboxaldehyde o-Tolualdeh yde o-Bromotoluene 2,4,6-Trimethylbenzaldehyde 2,4,6-Trichloronitr~benzene~ 1-Naphthaldehyde 1-Bromonaphthaleneb 9-Anthraldehyde 10-Chloro-9-anthraldehyde Phenant hrene-9-carboxalde h yde (a) Pure solid ( b ) In polystyrene 9-Bromophenanthrenea
22 28 15
14 19 0.03
21 24 15
29 34 18
1 13
29 23 18
kJ
AHE,
Reference 19.
AGE(200If)j k J mol-
-4
0
31
31
Temp range, K
200K >
8.5x 5.8x 2.3x 1.6x 9.2X 2.3x 1.4x 5.0x 4.5x
10-7 10-7 10-9
10-5
128-159 144-184 93-117 136-186 166-200 147-173 108-128 207-258 187-225
8.0x 10-5 7.4x 10-5 3.3x 10-5
191-230 185-225 170-210
10-7 10-7
10-4
Reference 8.
TABLE 11: Fuoss-Kirkwood Analysis Parameters for Some Molecules in Polystyrene Matricesa
T,K
106r,s
p
e,
103~",
1-Naphthaldehyde (0.46M )
147.6 152.5 157.5 161.8 167.7 172.6 176.9
523 176 70.6 29.9 12.9 5.9 5.1
0.15 0.16 0.17 0.18 0.20 0.22 0.23
18.40 19.48 20.47 21.44 22.48 23.69 23.80
207.1 212.3 217.4 222.2 228 232.3 237.2 242.2 247.6 252.4 257.7
245 112 56.2 25.3 15.1 4.5
0.15 0.15 0.17 0.15 0.17 0.19
2.11 2.16 2.24 2.26 2.33 2.43 9-Anthraldehyde (0.24M ) 248 0.21 8.82 155 0.21 9.00 99.1 0.21 9.21 58.3 0.20 9.40 37.2 0.21 9.68 26.2 0.21 9.80 20.1 0.20 9.39 13.5 0.21 9.65 10.1 0.22 9.91 6.7 0.22 10.07 4.4 0.23 10.41
2.669 2.666 2.666 2.663 2.663 2.658 2.578 2.574 2.570 2.566 2.563 2.561 2.561 2.559 2.559 2.555 2.553
Pure Solid Phenanthrene-9-carboxaldehyde
191.5 197 204.9 212.8 225.2 230
118 92.6 61.4 53.0 26.7 24.6
1067,s
P
103~",
4-Quinolinecarboxaldehyde (0.44M)
2.578 2.574 2.572 2.567 2.568 2.567 2.556
10-Chloro-9-anthraldehyde (0.11 M )
187 192.4 199.7 204.4 209 220.5
T,K
0.42 0.38 0.62 0.54 0.44 0.40
3.20 2.428 3.18 2.422 4.09 2.419 3.88 2.414 3.53 2.401 3.47 2.399 9-Phenanthrenecarboxaldehyde (0.30M) 186.7 336 0.17 10.91 2.705 191.1 219 0.17 11.46 2.703 196.0 112 0.18 11.73 2.702 200.9 66.2 0.17 12.33 2.695 205.8 41.0 0.19 12.69 2.696 210.6 22.2 0.19 13.39 2.694 215.5 17.4 0.19 13.61 2.689 220.4 9.7 0.19 14.27 2.684 225.3 7.1 0.19 14.72 2.682
128 133.6 137.2 141.2 145.5 149.6 153.4 158.7 144.1 150.6 153.5 157.1 161 167.6 173.2 176.4 133.7 88.5 93.7 95.7 101.7 108.7 114.9 117.3 122
226 104 52.8 28.6 18.7 7.8 6.2 3.9
0.371 4.22 0.356 4.35 0.318 4.43 0.302 4.53 0.307 4.74 0.290 5.00 0.291 5.06 0.293 5.32 o-Tolualdehyde (0.5104M ) 721 0.266 12.78 179 0.282 13.27 119 0.278 13.46 69.03 0.314 13.89 41.12 0.309 14.10 18.11 0.333 14.69 9.80 0.335 14.97 6.52 0.329 15.25 2.96 0.324 15.75 o-Bromotoluene (0.319M ) 580 0.175 4.19 192 0.157 4.51 107 0.164 4.61 25.1 0.173 5.09 5.1 0.180 5.72 3.1 0.213 6.25 2.5 0.2LO 6.26 1.9 0.234 6.54
2.656 2.652 2.648 2.645 2.643 2.640 2.637 2.634 2.603 2.614 2.613 2.620 2.619 2.626 2.628 2.627 2.628 2.631 2.629 2.627 2.628 2.625 2.630 2.631 2.632
2,4,6-Trimethylbenzaldehyde(0.63M ) 136.8 122.7 0.142 14.81 2.529 141.6 64.22 0.146 15.45 2.532 145.5 37.04 0.136 15.94 2.521 149.9 16.95 0.143 16.60 2.522 153.1 10.53 0.146 17.11 2.521 157.4 6.20 0.430 17.78 2.514 179.1 1.17 0.200 19.05 182.6 0.55 0.189 20.43 186.2 0.49 0.203 20.15
Solute concentrations are in parentheses.
f 1 0 J K-l mol-l or less by making additional measurements on the Q meter. This procedure yielded values of AH, = 28.2 f 1.56 kJ mol-l and AS, = 19 f 9.6 J K-l mol-1 (95% confidence intervals) for o-tolualdehyde in polystyrene. For other samples, the values have been quoted to two
significant digits in line with the error estimates as outlined above. The Eyring activation parameters are listed in Table I for the aldehyde molecules, where it seemed that the group process predominated. For the other aldehyde molecules only the relaxation time has been listed.
Steric Effects on Aldehyde Group Relaxation
Discussion 4-Quinolinecarboxaldehyde was chosen for the perihydrogen study as the reference compound which offered one of the best possible chances for the detection of aldehyde group relaxation. In benzaldehyde the magnitude of the weight factors governing the molecular (C,) and group (CJ processes may be estimated from its dipole moment of 2.92 D and the angle of inclination of the aldehyde group moment of 38' to the principal axis through the 1 and 4 carbon atoms.13 In fact C, IC, = (cos 38/sin 38)' and C1 + C2 = 1 and it follows C1 = 0.56 and Cz = 0.44. In this case the aldehyde group is planar with the ring. If this assumption is made for 4-quinolinecarboxaldehyde and the moment of the hybridized nitrogen atom is taken to be -2.2 D (similar to that of pyridine and quinoline14),then a rough estimate of C1 and Cz may be gained as follows: C1 a (2.92
COS
38" - 2.2)2
and Cz a (2.92 sin 38")2 and thus C1 a (0.1)2and Cz a (1.80)2and C1 = 0.003 while Cz = 0.9g7. Hence, the group relaxation process predominates, and the molecular process should make only a very small contribution indeed to the dielectric absorption. Even if the aldehyde group is inclined to the plane of the ring, due to the peri-hydrogen effect, the position would not alter substantially in that the fixed component of the dipole moment along the principal axes would still be small relative to that of the flexible moment. In fact, for a similar molecule, l-naphthaldehyde, the angle15between the planes through the group and the rings is small (-27'). Further, although there may be a fixed component moment from the second ring into the heterocyclic ring, its magnitude would be small and would not appreciably alter the weight factor value for group relaxation. That such a moment is small is borne out by the virtually identical dipole moments of pyridine and quinoline. We were able to find only one set of absorption peaks within the frequency range of the bridge a t various temperatures and none in the range of the rigid molecule, l-bromonaphthalene, which is similar in shape and size but absorbs in the temperature range 108-128 K. The only absorption for 4-quinolinecarboxaldehyde was in the higher temperature region, 128-159 K, which is part of the range previously observed for aldehyde group relaxation.16 This indicates that the free energy of activation for 4-quinolinecarboxaldehyde is greater than that for the corresponding rigid molecule and suggests a different process for 4-quinolinecarboxaldehyde, that is group relaxation. Comparison of the relaxation times of l-bromonaphthalene and 4-quinolinecarboxaldehyde revealed that the relaxation time of the latter, which is slightly smaller in size than the l-bromonaphthalene,has a t 200 K a relaxation time about 60 times longer, which again supports the group process in the aldehyde case since in a polystyrene matrix the molecular relaxation time increases very rapidly with size for a particular shape of molecule (see later). This may be accounted for as follows. In the polystyrene matrix there is probably more than sufficient free volume in which the aldehyde group can rotate; hence the group rotation is relatively less affected by the presence of the macromolecules.
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
1093
It seemed justifiable to carry out an Eyring analysis on 4-quinolinecarboxaldehyde data for a group process. This yielded A H E = 22 kJ mol-l, ASE = 15 J K-l mol-l, AGE = 21 kJ mol-l, and these values are to be compared with those for 1,4-benzenedicarboxaldehyde(i.e., terephthaldehyde) of AHE= 29 k J mol-l, ASE = 15 J K-l mol-l, and AGzm= 26 kJ mor1, where only aldehyde group relaxation occurs since the component moment along the long axis is and consequently molecular relaxation does not occur. Thus, the energy barriers have decreased somewhat for this heterocyclic aldehyde compared with terephthaldehyde. Miller et al.3 from far-infrared studies noted a decrease in the energy barrier ( V zvalue from the cosine formula) of 3.49 kJ mol-l between benzaldehyde and pyridine-4-carboxaldehyde. Probably this may be attributed to a mesomeric effect related to the nitrogen atom. Another factor, which could influence the value of the energy barrier, would be the peri-hydrogen atom. Consideration of the l-naphthaldehyde data should be pertinent to this. For the moment we shall stress that the main feature which emerges for the 4-quinolinecarboxaldehyde data is that aldehyde group relaxation occurs despite the presence of the peri-hydrogen atom. The AHE = 34 kJ mol-l for l-naphthaldehyde may be compared with that of the slightly larger rigid molecule, l-bromonaphthalene, the AHEof which has been reported by Tay and Walker8 to be only 17.6 kJ mol-l. This again suggests that the dominant process in l-naphthaldehyde is aldehyde group relaxation. The AHE appears slightly higher than the typical value -28 kJ mol-' for aldehyde group relaxation in an aromatic aldehyde15 which does not have any mutual conjugative effects. It then becomes necessary to query whether the neighboring peri hydrogen has caused the increase in the activation enthalpy from the usual aldehyde group value. One further point to be noted is the low value of 23 kJ mol-l for AGE a t 200 K compared with that of 31 kJ mol-' for 4-biphenylcarboxaldehyde. It is striking how similar the AGE a t 200 K for l-naphthaldehyde is to the value of 21 kJ mol-l for quinolinecarboxaldehyde. Such lowering of AGE may well be related to the steric effect of the peri hydrogen. It would seem likely, though, that the lowering of the AHE for aldehyde group relaxation in 4-quinolinecarboxaldehyde over that in terephthaldehyde or 4-biphenylcarboxaldehyde is to be attributed to the conjugative influence of the nitrogen atom rather than the peri-hydrogen atom. Comparison of the dielectric relaxation times a t 200 K of l-bromonaphthalene,8 4-quinolinecarboxaldehyde, and l-naphthaldehyde, which are 1.4 X 8.5 X W, and 2.3 X lo-' s where the size of the Br atom is slightly greater than that of the aldehyde group, would suggest that, since the relaxation time of aldehyde group relaxation is 2 X 10% s in the fully conjugated aldehyde, then a t least some contribution from aldehyde group relaxation occurs in 4-quinolinecarboxaldehyde and l-naphthaldehyde. Another molecule, which has a peri hydrogen adjacent to a carboxaldehyde group, is phenanthrene-g-carboxaldehyde. If it is assumed that there is only one relaxation process, the activation parameters are AHE= 33 kJ mol-', ASE = -14 J K-l mol-l, and AGE a t 200 K is 36 k J mol-l, and these are to be compared with the corresponding values for the aldehyde group in l-naphthaldehyde of 34 kJ m o P , A S = 13 J K-I mol-l, and 23 kJ mol-l. Although the AH, are in good agreement, the AGE value differs considerably from both l-naphthaldehyde and 4-quinolinecarboxaldehyde (AGE a t 200 K is -22 k J mol-I). Examination of the rigid molecule, g-bromophenanthrene,17which is just a slightly larger but similarly shaped
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The Journal of Physical Chemistry, Vol. 82,
No. 9, 1978
molecule to phenanthrene-9-carboxaldehyde,gave AHE = 31 kJ mol-', A S E = 0 J K-' mol-', and AGEzm = 31 kJ mol-l. The A H E matches that of phenanthrene-9-carboxaldehyde within experimental error, and the AGE'S are much closer than that for 1-naphthaldehyde. This would suggest at least some contribution from a molecular relaxation process for phenanthrene-9-carboxaldehyde. Indications that group relaxation occurs may be gained from work in the which pure solid phase for 9-phenanthrenecarboxaldehyde, yielded a relaxation time a t 200 K of 8.0 X 10-5 s. This is virtually identical with that of 7.4 X 10-6s obtained in the polystyrene matrix. The fact that the relaxation times are so similar in the matrix and the crystalline solid favors the involvement of group relaxation in both media since such variation of the medium ought to have appreciable influence on the molecular relaxation time. In fact, both values are of a similar order to that obtained for aldehyde group relaxation (2 X lo4 at 200 K) in terephthaldehyde.16 It would seem likely that in the crystalline solid only group relaxation occurs. However, in the polystyrene matrix case there may well be an overlap of the group and molecular process since the relaxation time of a similar sized rigid molecule, 9-bromophenanthrene, is 3.3 X s. Thus it seems that the data for phenanthrene-9-carboxaldehyde can best be accounted for by overlap of the group and molecular process. Hence, the AH,, I S E , and A G E deduced for this molecule may not be meaningful in terms of one relaxation process. In 9-anthraldehyde restriction of reorientation of the aldehyde group may be anticipated on account of the even greater steric interference from the peri-hydrogen atoms on either side of the aldehyde group. The geometry of the system, bond lengths, and bond angle for 1-anthraldehyde and 1-naphthaldehyde are given by Sutton.16 The molecule is slightly buckled while the aldehyde group is twisted 27" out of the plane. From the geometry, it would seem likely that the group does not twist past the plane of the ring t o encounter the steric hindrance from the two peri-hydrogen atoms. The molecules, which have been examined in the study of this problem, in order of increasing size are 9-anthraldehyde,9-bromoanthracene, and 10-chloro-9-anthraldehyde. At 200 K these molecules have relaxation times of 5.0 X 3.7 X and 4.5 X s, respectively. For a given type of molecule the increase of relaxation time with size for a molecular process is quite appreciable. For example, a t 200 K in the sequence bromobenzene, p-bromotoluene, and 4-bromobiphenyl the molecular relaxation time values increase substantially, 2.2 X 1.5 X W5,2.4 X lo4s.18 Thus, for solely a molecular process the increase in from 9-bromoanthracene to 10-chloro-9-anthraldehydeshould be appreciable, whereas, in fact, the increment is negligible; this suggests a t least some contribution from aldehyde group relaxation. Certainly if A G E values a t 200 K are estimated for 9anthraldehyde, 9-bromoanthracene, and lO-chloro-9anthraldehyde in sequence of increasing size, the values of 36, 31, and 32 kJ mol-' do not make sense in terms of solely a molecular relaxation process. It would seem that a t least some overlap of a group and molecular process occurs. The most likely group process would be the switching of the aldehyde group between two nonplanar positions without rotating past the peri-hydrogen atoms. Thus, if the molecular geometry is borne in mind, the aldehyde group could reorient through an angle something of the order of B O o - 2 X 27" 125". We shall now consider the influence of one and two o-methyl groups on aldehyde group relaxation, as in otolualdehyde and in 2,4,6-trimethylbenzaldehyde.Besides
-
Walker et al.
the twofold barrier arising mainly from overlap between R orbitals of the carbonyl bond with those of the aromatic ring, the reorientation of the aldehyde group may be further restricted by steric interference in ortho-substituted benzaldehydes, the extent of the interference depending upon the size of the neighboring substituent and its van der Waal radius. We obtained a AH, of 28 kJ mol-' for o-tolualdehyde in polystyrene. Comparison with the slightly larger rigid molecule, o-bromotoluene, the AHE of which is 15 kJ mo1-l rules out any substantial contribution from a molecular process in o-tolualdehyde. Further, the temperature range of absorption for o-bromotoluene of 93-117 K on the bridge when compared with that of otolualdehyde of 114-184 K bears this out. The Eyring parameters for o-tolualdehyde of AHE= 28 kJ mol-', ASE = 19 J K-l mol-', and AGEzm = 24 kJ mol-l compare favorably with those aldehyde group relaxations of terephthaldehyde, which are respectively 29 kJ mol-l, 15 J K-l mol-', and 26 kJ mol-l. The relaxation time a t 200 K of 5.8 X s is similar to the value of 8.5 X lo-' s for 4quinolinecarboxaldehyde for group relaxation. It would seem likely that relaxation for the aldehyde group occurs without any increase in AHE, and that the dipole flips between two nonplanar positions without rotating past the 2-methyl group. When additional methyl groups are placed in o-tolualdehyde in the 6 position, as in 2,4,6trimethylbenzaldehyde, it may be thought that the steric hindrance to aldehyde group would increase. However, the relaxation time at 200 K is 1.6 X s, that is shorter than that for group relaxation in o-tolualdehyde (5.8 X s). The possibility exists, though, that the former value is a molecular relaxation one, However, the relaxation time value of 9.2 X lo4 s for the similar sized and shaped but does not bear rigid molecule, 2,4,6-tri~hloronitrobenzene,'~ this out. Altogether, the most likely interpretation would seem to be that there is an overlap of group plus molecular relaxation. Although an Eyring analysis of this system would thus not appear to be justified, the parameters of A H E = 22 kJ mol-l, ASE = 0 J K-l mol-l, and A G E a t 200 K of 22 kJ mor1 which emerge differ appreciably from that of the corresponding rigid molecule, 2,4,6-trichloronitrobenzene, A H E = 29 kJ mol-', A S E = 1J K-' mol-l, and A G E a t 200 K of 29 kJ mol-l to demonstrate convincingly that the group process must contribute in part, at least, to the observed dielectric absorption. In fact, it is striking how similar this A G E value is to that for the cases where restricted aldehyde group relaxation definitely occurs as in o-tolualdehyde, 4-quinolinecarboxaldehyde, and 1naphthaldehyde where the values are 24, 21, and 23 kJ mol-', respectively. Conclusions The predominant process in the observed dielectric absorption of o-tolualdehyde, 1-naphthaldehyde, and 4-quinolinecarboxaldehyde is group relaxation, and for these the Eyring activation parameters have been estimated and given in Table I. The dielectric data for 9anthraldehyde, lO-chloro-9-anthraldehyde,and 2,4,6-trimethylbenzaldehyde may be interpreted as giving some indication of aldehyde group relaxation, and for these molecules the Eyring parameters which have been estimated may well not be meaningful; hence, they have not been tabulated in Table I. References and Notes (1) S. Das Gupta and C. P. Smyth, J. Am. Chem. Soc., 90, 6318 (1968). (2) E. Forest and C. P. Smyth, J . Am. Chem. SOC.,86, 3474 (1964). (3) F. A. Miller, W. G. Fateley, and R. E. Witkowski, Spectrochim. Acta, Part A , 23, 891 (1967). (4) K. S. Dhami and J. 8.Stothers, Can. J . Chem., 43, 479 (1965). (5) H. G. Silver and J. L. Wood, Trans. Faraday SOC.,80, 5 (1964).
The Journal of Physical Chemistry, Vol. 82, No. 9, 1978
Communications to the Editor
(6) R. N. Jones, W. F. Forbes, and W. A. Mueller, Can. J , Chem., 35, 504 (1957). (7) P. F. Mountain and S.Walker, Adv. Mol. Relax. Processes, 7, 105 (1975). (8) S. P. Tay and S . Walker, J . Chem. Phys., 64, 1634 (1975). (9) C. K. McLellan and S. Walker, Can. J. Chem., 55, 583 (1977). (10) M. Davies and J. Swain, Trans. Faraday SOC.,67, 1637 (1971). (1 1) M. Davies and A. Edwards, Trans. Faraday Soc., 63, 2163 (1967). (12) B. Ostie, "Statistics in Research", 2nd ed,Iowa State University Press, Ames, Iowa, 1963.
1095
(13) P. F. Mountain and S. Walker, Can. J . Chem., 52, 3229 (1974). (14) A. L. McClelian, "Tables of Experimental Dipole Moments", W. H. Freeman, San Franclsco, Calif., 1963. (15) L. E. Sutton, Chem. Soc., Spec. Pub/., No. 11 (1965); No. 18 (1958). (16) A. Lakshmi, S.Walker, N. A. Weir, J. H. Caiderwood, J. Chem. SOC., Faraday Trans. 2 , 74, 727 (1978). (17) C. K. McLelian, Ph.D. Thesis, University of Man., Canada, 1977. (18) A. Kwaja, private communication. (19) M. A. Mazid, private communication.
COMMUNICATIONS TO THE EDITOR Hydroxyl Radical Reactions in the Gas Phase. Products and Pathways for the Reaction of OH with Toluene Publication costs assisted by the Environmental Protection Agency
Sir: Aromatic hydrocarbons are important constituents of polluted urban atmospheres1 and the extent to which they contribute to the formation of photochemical smog is of considerable concern.26 Of particular interest are the reactions of hydroxyl radical with aromatics, since they are the major route for involvement of these hydrocarbons in the chemistry of the troposphere. The kinetics of gasphase OH-aromatic reactions have recently been determineda6-11The products and precise mechanisms of the reactions remain undetermined; however, to identify them is essential for determining the fate of aromatics in the environment and developing accurate chemical models of urban airsheds. For alkylbenzenes, we suggest two major reaction pathways. Shown for the case of toluene, these are benzylic hydrogen atom abstraction, reaction 1,and radical addition O H t C,H5CH3
-
H20 t C G H ~ C H ~
C,H,CH,*
I
to the aromatic ring, reaction 2. Davis et aL8 investigated the pressure dependence of the OH-toluene reaction a t 298 K and concluded that k l / ( k l + k,) is less than 0.5. On the basis of the temperature dependence of the same reaction, Perry et al.ll deduced that a t ambient temperature this ratio was 0.14 f 0.06. In neither case were the actual reaction products identified. Hydroxyl radicals were generated from hydrogen atoms formed in an argon discharge flow system by reaction 3. H
+ NO, -+OH t NO
and secondary reactions negligible. Products were collected by condensation in a cold trap or by adsorption onto a Chromosorb G-packed column. Gas chromatographymass spectroscopy and flame ionization gas chromatography were used to identify and quantitatively analyze the products of the reaction. The OH-toluene reaction was run 20 times under conditions of varying initial NOz, Oz, and total pressure. In all cases the only major gas phase products were benzaldehyde, benzyl alcohol, m-nitrotoluene, and 0-,m-, and p-cresol. Nitrotoluene and total cresols formed in rougly equal amounts, and predominated over benzaldehyde and benzyl alcohol. o-Cresol predominated over the para and meta isomers. Although the relative amounts of the individual products depended on the reaction conditions, the ratio (C6H5CH0+ C6HSCHzOH)/(total products) was invariant a t 0.149 f 0.019. The total amount of products was approximately 5 X lo-'% of the toluene remaining, consistent with the low conversion expected for the system. Two minor products were p-methylbenzoquinone and an unidentified compound less volatile than toluene. Each of these amounted to approximately 0.75% of the total products. No phenol was observed (less than 0.5% of products); thus, displacement of the CH, group by OH is not an important reaction. To account for these observations, we propose reactions 1 , 2 , and 4-10. Reactions 4-8 are analogous to the known
(3)
Argon (5 Torr) was used as the carrier gas. The linear flow velocity was 1 X lo3 cm s-l. The method used is described in detail e1swhere.l2-lE Approximately 0.1 Torr of toluene and 1-10 Torr of Op, to trap the radicals formed in reactions 1and 2, were introduced 10 cm downstream from the point of addition of NO2, thus ensuring that reaction 3 would be complete. Since the amount of toluene initially added is large compared with the amount of OH available (1 X Torr),lgconversion of toluene to products is low 0022-3654/78/2082-1095$01.00/0
+ 0,
-+
C,H,CH,O;
C,H,CH,O,* t NO C,H,CH,O* t NO, C,H,CH,O* + 0, C,H,CHO + HO; +
-
-+
C,H,CH,O.
+ NO
C,H,CHO t HNO
C,H,CH,O. t NO, C,H,CHO I + 0, HOC,H,CH, t HO,. -+
+ HNO,
+
I
+ NO,+m-NO,C,H,CH,t
H,O
(4) (5 1
(6) (7 1 (8) (9)
(10)
reactions of methyl,20methylperoxy,21~22 and methoxyZ3J4 radicals. Reaction 9 closely resembles the reaction of cyclohexadienyl radicals with 02.25 Reaction lop6as well as similar addition-elimination reaction^^^^^^ have been observed in solution, although rate constants have not been measured. Given the above mechanism, the distribution of the individual products should depend on the reaction conditions (i.e,, the relative amounts of NOz and 0,) but the amount of abstraction relative to addition product should not." Thus, our ratio (C6H6CH04- C6H5CHzOH)/(total products) is a valid measure of the amount of reaction 1 relative to total reaction, i.e., kl/(kl + k z ) . 0 1978 American Chemical Society
