Dec., 1960
INFRARED STUDYOF SUBSTITUTION IN THE BENZENE RING
194 1
AN INFRARED STUDY OF SUBSTITUTION TN THE BENZENE RING BY A. CABANA, J. L. PATENAUDE, C. SANDORFY AND P. M. G. BAVIN* Ddpartement de Chimie, TJniuersitB de Montrial, Canada, and Department of Chemistry, The University of Hull, England Received July 8,1060
Limited but representative series of phenol and benzonitrile derivatives (ortho meta and para) were assembled and the OH and CFX band frequencies and intensities were determined. The method of “segments” was used to evaluate the latter. Substituents are shown to divide themselves into three classes on the basis of their effect on the electronic distribution of the unperturbed ground state of the molecules, and these classes are the same as those obtained on the basis of chemical reactivity. There is, however, no complete analogy between the phenol and the benzonitrile derivatives. The observed frequencies and intensities are related to electronic distribution, and it is shown that the existence of the three groups is entirely compatible OH, with a smooth variation of the electronic charges sent by the substituent into (ortho),meta and pura in the series ”2, F, C1, Br, I, CHO, C=N, XOZ.
Introduction The problem of benzene substitution constitutes one of the oldest problems of quantum chemistry and molecular spectroscopy. It would be very difficult to summarize all the previous literature and being fair to everybody would probably require a review paper. Most of the credit should be given to early works by Wolf and Herold,‘ Wolf and Strasserz and Conrad-Billr~th,~ who observed the well known fundamental regularities in the ultraviolet spectra, and to Sklar,4 who made the first full-scale attempt to explain them. From the chemical side, Hammett’sj work and the introduct’ion of his u-factor provided a solid basis for comparative studies of spectra and chemical reactivity (see also ref. 6). Infrared and Raman spectra of benzene derivatives have been studied by many authors (ref. 7 to 36). From t’he point of view of the present work, * I.C.I. Fellow. (1) K. L. Wolf and W. Herald, Z . physik. Chem., B13, 201 (1931). (2) K. L. Wolf and 0. Strasser, ibid., B21, 389 (1933). (3) H. Conrad-Billroth, ibid., B29, 170 (1936). (4) A. L. Sklar, J . Chem. Phys., 10, 135 (1942). (5) L. P. Hammett, “Physical Organio Chernistw,” McGraw-Hill Book Co., New York, N. Y., 1940, p. 184. (6) H. H. Jaffe, Chem. Reus., 63, 191 (1953). (7) J. Lecomte, J. phys. radium, 9, 13 (1938). (8) A. Depaigne-Delay end J. Lecomts, ibid., 7, 38 (1946). (9) K. W. F. Kohlrausch, Monatsh. Chem., 76, 231 (1946). (IO) G. M. Barrow, J . Chem. Phys., 21, 2008 (1953). (11) L. L. Ingraham, J. Gorse, G . F. Bailey and F. Stitt, J . A m . Chem. Soc., 74, 2297 (1932). (12) N. Fuson, M L. Josien and E. M. Shelton, ibid.. 76, 2526 (1954). (13) J. F. Brown, ibtd., 77, 6341 (1955). (14) M. W. Skinner and H. W. Thompson, J . Chem. Soc., 487 (1!355). (15) H. ‘Ar. Thompson and G. Steel. Trans. Faraday Soc., 62, 1451 (1956). (16) P. J. Stone and H. W. Thompson, Speclrochzm. Acta, 10, 17 (l!357). :17) H. W. Thompson, R. W. Needham and D. Jameson, ibid., 9, 208 (1957). (18) hf. R. Mander and H. W. Thompson, Trans. Faraday Soc., 63, 1402 (1957). :19) P. J. Krueger and H. W. Thompson, Proc. R o y . Soc. (London), 8 3 4 3 , 143 (1958); 8 2 6 0 , 22 (1959). ( 2 0 ) P. Sensi and G. G. Gallo, Cazr. chim. ital., 8 6 , 235 (1955). (21) S. Califano and R. Moccia, kbid., 87, 805 (1957). R. Moccie. and S. Califano, ibid., 88, 342 (1958). ! 2 2 ) R. N. Jones, W. F. Forbes and W. A. Mueller, Can. J . Chem., 36, 504 (1957). 1:23) C . Garrigou-Lagrange. J. M. Lebas and M. L. Josien, Spectrochrm. Acta, 12, 305 (1958). 124) J. M. Lebas, C. Garrigou-Lagrange and M. S. Josien, *bad., 225 (1959). and previous papera. (25) L. J. Bellamy, J . Chsm. Soc., 2818 (1956). (26) M. S t . C. Flett, Speclrochim. Acta, 10, 21 (1957). 127) E. Lippert, Z . Elektrochsm., 69, 534 (1955).
the most significant results were the relationships between infrared band frequencies and intensities of the substituent groups, and chemical reactivity as represented by Hammett’s a-factor. (See especially ref. 14 to 20.) I n the spite of the great amount of work previously done by others we decided to take up the problem again for the following reasons: (1) It was hoped that by choosing a limited but otherwise complete series of compounds (monosubstituted, ortho, meta and para-disubstituted) a more solid basis for future discussions could be given. Therefore, a great effort was made to secure all the necessary compounds. This was successful with the phenol derivatives but less so with the benzonitrile derivatives. (2) Two of us recently published an improved method for computing infrared intensities,a7which is a modification of Ramsay’s direct integration method.88 It consists of dividing the band area into segments and integrating for every segment with a width parameter taken from the segment itself, applying corrections in RamsayJs manner. This method has now been applied to the case of the benzene derivatives. Two biatomic groups, OH and C=N, were chosen as first substituents and were combined with, as a second substituent, OH, ”2, F, C1, Br, I, C z N , NOz and CHO. Only the stretching vibrations of the OH and C=N groups were used. Triatomic groups were avoided because these possess two stretching vibrations and their angular dependence would make comparisons uncertain. Experimental The measurements were made with a. Perkin-Elmer model 112 single beam, double pass spectrometer using a lithium fluoride prism. The whole instrument was put into an atmosphere of dry nitrogen. Solutions of less than 0.001 M in CCl, were used for the phenol derivatives and less than 0.004 M in tetrachloroethylene for the benzonitriie derivatives. A 3 cm. cell was used. The computed spectral slit widths were less than 3 cm.-’ in the OH region and less than 2 em.-’ in the CzN’ region. (28) E. Lippert and W. Vogel. Z . physile. Chem., 9, 133 (1958). (29) N. S.Bayliss, A. R. H. Cole and L. H. Little, Spectrochim. Acta, 12, (1959). (30) A. W. Baker, THISJOURNAL, 82, 744 (1958). (31) T. L. Brown, ibkd., 61,820 (1957). (32) T. L. Brown, J . A m . Chem. Soc., 80, 794 (1958). (33) D. H. Whiffen, J . Chrm. Soc., 1350 (1956). (34) D. H. Whiffen. Speckwhim. Acta, 7 , 253 (1955). (35) A. Stojiljkovic and D. H.Whiffen. %bid.,111, 47, 57 (1958). (38) A. R. Kstritaky and P. Simmons, J , Chsm. Soo., 2051 (1859). (37) A. Cabana and C. Sandorfy, Spectrochim. Acta, 335 (1960). 74, 7 2 (1952). (38) D. A. Ramsay, I . Am. Chem. SOC.,
A. C A B A N A , J. L. PATENAUDIT,
1942
c. SANDORFY AND P. M. G.BAVIN
Vol. 64
The following compounds were prepared by one of us (P.M.G.13 .) : m- and p-cyanophenol, p-fluorobenzonitrile, m- and pchlorohenzonitrile, 0-, m- and p-bromobenaonitrile, o-, m- and p dicyenobenzene and p-aminobenzonitrile. o-Cyanophenol and p-nitrobenzonitrile were sent us by Dr. P. Sensi and G. G. Gallo from the research laboratories of Lepetit S.P.A., Milano, Italy. p-Cyanophenol came from Dr. W. V. Thorpe from the University of Birmingham, England. o-Iodophenol, p-cyanophenol, p-chlorobenzonitrile and o-dicyanobeneene were sent us from the collection of E. I. du Pont of Nemours and Company from Wilmington, Delaware, by the courtesy of du Pont of Canada limited. The other compounds were commercial products. Before taking the spectra, the compounds were recrystallized or redistilled and their purity was checked by taking the melting or boiling points.
Cabana and Sandorfy3' for more details about this method. It yields values about 10 to 15% lower and closer to reality than those obtained by integrating directly a Lorentz curve with the half width as the only width-parameter. Stone and Thompson'6 give the frequencies and intensities of 9 compounds out of those in Table I, and Mander and Thompsonls those of 5 compounds out of those in Table 11, in the same solvent. Our frequency values agree within 1 cm.-' with theirs, except for phenol, p-cyanophenol and pformylphenol, where our values are by 2.5, 3.5 and 6.2 cm.-l higher, respectively. The intensities were computed with different methods, but even so, in Results only four cases do our values differ by more than The results are contained in Table I for phenol lo%, and in most cases they differ by less than 5%. derivatives and Tahle I1 for benzonitrile derivaDiscussion tives. In the case of the ortho compounds, the freThe substituents of the benzene ring can be quencies of the intra-molecularly hydrogen bonded divided into three classes on the basis of their orientgroups are included. In these cases the intensities ing effects. Those of the first class, such as OH were not computed as we do not know the exact and NH2, orient a second electrophilic substitution proportion of free and bonded isomers. In four into ortho or pura and a t the same time accelerate other cases the intensities are not given because of the reaction with respect to benzene itself. In the the very low solubility of the compounds. third class are those which, l i e NO*, C=N, CHO, orient a second electrophilic substitution into metu, TABLE I slowing down the reaction. The second class has FREQUENCIES A K D INTENSITIES OF THE OH STRETCHING not always been so clearly distinguished. It comVIBRATION I N PHENOL DERIVATIVES prises F, C1, Br and I, which orient into ortho and THEINTEYSITIES ARE COMPUTED BY THE METHODOF SEOpura but slow down the reaction (see for example MENTS. T H E Y A R E DIVIDED BY T W O FOR DIPHENOLS. ref .59S4O) ortho-pura ratios were examined by Dewar.41 Sub----mela------parastitur-ortho------. As X As X The Phenol Derivatives.-It appears clearly from ent I, em.-' crn.-' v, c m - 1 10' v. em.-' IO7 Table I11 that the infrared frequencies and intenNone 3610.5 No band 3610.5 5.6 3610.5 5 . 6 sities give again three well-defined classes, and that NH2 3617.7 No band 3612.4 . . 3618.0 .. they are the same as the ones based on chemical OH 3616.5 3567.2 3610.5 5 . 8 3616.7 5.7 reactivity. This means that substituents like OH F Noband 3590.8 3607.5 5.8 3613.5 5.4 or NH2 have one type of influence on electronic C1 3807.7 3544.6 3606.2 6.4 3608.8 7.0 distribution in the benzene ring, NOz, +N, CHO Br 3604.0 3522.3 3604.3 6.6 3607.2 7 . 1 have another type of influence on it and the haloI 3600.3 3498.5 3604.0 . . 3605.8 7 . 5 gens a,nother type again. This seems to indicate CHO N o band 3180 3604.1 6 . 7 3598.2 8 . 4 that there are certain relations in this case between C E N 3597.6 3555.5 3602.9 6 . 3 3597.5 8.1 the electronic distribution of the unperturbed NOz No band 3240 3600.5 7.7 3593.3 8 . 6 ground state and the activation energies. It is TABLE I1 perhaps significant in this respect that Y ~ a n ' ~ A N D INTENSITIEF OF the C s N STRETCHINQ found an analytical relationship between free vaFREQUENCIES lences and activation energies when both are comVIBRATION IN BENZONITRILE DERIVATIVES THEINTENSITIES ARE COMPUTED BY THE METHODOF SEG- puted by the Huckel method. Y,
MENTS.
Spb-
THEY ARE DIVIDEDBY Two A r t h -
st1tu-
ent
None NHs OH
F C1 Br C=X NO2
Y.
om.-'
AsX
101
2231.0
6.7
2233.8 2221.2
..
2234.6 Z32.2 2235.4 2234.7
FOR THE
EENES -metaY,
cm-8
2231.0 2235.1
DICYANOBEN-
-para-
As X 108
Y,
cm.-L
As X 108
6.7 7.8
2231.0 2223.4 2229.0
6.7 32.2 18.2
5.3 6.1 3.0 2.9
2233.6 2233.0 2231.5 2234.3 2235.4
7.8 8.2 8.5
.. 4.3
.. I .1
..
2235.1 2234.5 2238.3 2238.5
..
1.9
Intensities ( A ) are given in absolute units (cm.-2 molecule-' sec.-l), using natural logarithms and were obtained by the method of segments where the band area is computed as a sum of four segments (or 8, if the hand is asymmetric), each one with its own Au. The reader is referred to an article by
TABLE I11 THE GENERALTREND OF FREQUENCIES AND INTENSITIES FOR meta AND para DERIVATIVES OF PHENOL (Q STANDS FOR PHENOL) 1 s t Class "I,
Frequency Intensity
OH
>m = p m>p=cp p > m > v
p
3rd Clam
2nd Class CHO CmN, (F),C1, Br, I I40, Q > p >m p>m > p
p>m>v
As early as in 1935, Wheland and Pauling4' computed the r-electronic charge distribution in benzene derivatives using the LCAO molecular orbital method. As is well known, this type of (39) C. E.Ingold, Chsm. Reus., 16, 225 (1934). (40) M. J. S. Dewar, "The Electronic Theory of Organio Chemistry," Oxford University Pmn, 1949. (41) M.J. 8. Dewar, J . Cham. Soe., 463 (1949). (42) P. Yvan, J . cbm. pAua., 49, 457 (1952). (43) G. W. Wheland and L. Pauling, J . Am. Chem. Soc., 67, 2086 (1935).
Dec., 1960
INFRARED STUDY OF SUBSTITUTIONIN THE BENZENE RING
calculation yields small negative charges in ortho and para and zero effective charge in meta for first class substituents and positive charges everywhere but less in meta for third class substituents. So far, no calculation has been able to give the charge distribution we have to infer from the infrared data for the second class, the halogens, namely creation of positive charges everywhere but less in ortho and para than in meta. (R. D. stressed this fact.) Looking a t the data of Table I we see that for both para and meta derivatives (and the unassociated ortho derivatives) there is a smooth variation of the OH frequencies and intensities (in the opposite sense) if we put the halogen derivatives in what appears to be their natural place: between the other two classes. This is, of course the fact underlying the relationships found between frequencies and Hammett's o-factor. For the para-derivatives the frequency varies between 3618 and 3593 and for the meta-derivatives between 3612 and 3600. They decrease in the order KH2, OH, F, C1, Br, I, CHO, C=N, NO*. This would correspond to a gradual decrease of electron density in the same order both in para and meta, the electron density in meta becoming higher than in para between I and CHO. It means that the three classes are not really fundamentally different and intermediate cases (such as the one of F) are possible. The charge in meta varies between narrower limits than the charge in para, but it does vary, and in the case of the halogens it is even more affected by substitution than the charge in ortho and para. It is a challenge to quantum chemists to match the regularities discussed here by more refined calculations. Figure 1 shows schematically how this smooth variation is compatible with the existence of the three groups and the diagrams of Fig. 2 give the charge distributions which would correspond to the observed infrared spectra on the basis of conventional ideas inspired by approximate wave mechanical calculations. (+1 or -1 represent a given amount of electronic charge thought to be about one or two hundredths of the charge of an electron.) A pattern very similar to the one in Fig. 1was given by Pople, Bernstein and Schneider who obtained it from nuclear magnetic resonance data.45 The behavior of the halogens is explained by a charge distribution as on Fig. 2d. In particular, one can explain the apparent exception of fluorine which, in para, raises the OH frequency but, in meta, lowers it with respect to phenol. All we have to suppose for this is a charge distribution in fluorobenzene shown in Fig. 2c, that is to say, negative charges still in ortho and para but positive already in meta. The heightening of the OH frequency in maminophenol is explained by a charge distribution shown in Fig. 2a. It seems that as it is usually admitted in the case of the OH group the force constant decreases regularly with increased bond polarity and that the variation of the bond dipole moment during the (44) R. D. Brown, Quar%.Rev.. 6, 63 (1952). (45) J. A. Pople, W. 0. Sohneider snd H. J. Bernstein, "High-resolution Nuclear Magnetic Resonance," McGraw-Hill Book Co., New York, N. Y . . 1959. p. 260.
-
--,-
L
hii
1
"
1943
~
br
YLT
---i--L
~~
L
N 11
I
Fig. 1.-Schematic illustration showing how the smooth variation of charges is compatible with the existence of three groups of substituents. The charges are inferred from the frequencies and/or band intensities of the OH stretching vibrations. H
N
H \
-1
I
/"
rd
, -3
0
-I
+I
0 0 -2
E
Br
'\, +I + z .. \J ;+* -I
*I
+3
.lt3 f5
Figure 2
(a)
(b)
(C)
(d)
(e 1
Fig. 2.-Electronic charge distributions in some monosubstituted benzene derivatives (in meta and para). The charges are inferred from the frequencies orland intensities of the OH stretching vibrations.
vibration increases regularly a t the same time. Electronic charge distribution in the benzene ring is very often discussed in terms of inductive and mesomeric effects. We had no need to use these notions explicitly as the diagrams of Fig. 2 represent the resultant of both effects. We have to ask the question, however, where the high electron attracting power of the halogens corresponding to their inductive effect comes from. We suggest that it is related to digonal hybridization of the halogens, which makes them more electronegative, giving them an asymmetric lone pair which is largely responsible for the dipole moments of the halogenobenzenes. The Benzonitrile Derivatives.-The nitrile group has a very high dipole moment, about 3.5 to 4.0 Debye. This is about twice as high as, for example, the dipole moment of a carbonyl group (see for example ref. 46). In spite of this, the intensity of the nitrile band is about 10 to 50 times lower than the intensity of an average carbonyl band. Therefore, this is a case where we have a high bond dipole which changes little during the vibration. As normally the stretching of the C=N link would tend to increase the separation between the centroids of the positive and negative charges with electronic charge flowing toward the nitrogen atom, there must be a competing flow in the opposite direction, leaving little resultant change in dipole moment. We suggest that this is related to digonal (4F) C. P. Smyth, "Dielectric Behavior and Structure," McGraw -Hill Book Co., New York, N. Y.. 1955.
1944
FRANKLIN
J. WRIGHT
hybridization and the existence of an asymmetric lone pair on the nitrogen. Digonal hybridization is energetically favorable as long as it assures a stronger overlap than the unhybridized orbitals would. Thus, when the C=N bond is stretched, the overlap diminishes and less stabilization will come from digonal hybridization. There will be a partial return to the unhybridized state with the lone pair being pulled back toward the nitrogen nucleus and the bonding electrons toward the carbon. This change would not require a change in bond angles as both p and sp bonds would give a linear arrangement. The remaining change in dipole moment would therefore be the result of a delicate equilibrium between two opposing effects which could easily be modified by a slight change in electronic distribution caused by a substituent. This would give a qualitative explanation of the great sensitivity of the C=N band intensity to substitution. It would also be in line with the fact that a t the same time the values of the frequencies change only slightly, as this change of hybridization would only occur during the vibration and would not influence the force constant. From Table IV it is seen that we have again three classes of substituents, the same as in the case of the phenol derivatives. There is no complete analogy, however, between the two cases as is immediately clear if we compare Tables I11 and IV. The one is not identical with or just the inverse of the other.
Vol. 64
TABLE IV THE GENERALTREND OF FREQUENCIES AND INTENSITIES FOR meta A N D para DERIVATIVES OF BENZONITRILE (N STANDS FOR BENZOWITRILE) 2nd Class C1, Br
1st Class
OH
Frequency Intensity
m
p
>N >p >m >N
m
p
>p >N >N >m
3rd Class
C-N, NOz
m
>p >K
N >>m 7 p
There is again a smooth variation of frequencies and intensities and the halogens are again between the other two classes. Whatever the second substituent, the nitrile frequency of the meta isomer is always the highest. This and other anomalies show that, in the case of the nitrile group, the change in dipole moment during the vibration does not always follow the changes of the dipole moment itself from one substituent to the other, and the relation between the polarity of the bond and the force constant does not seem to be a simple one. We conclude that qualitative explanations should not be pushed any further. Acknowledgment.-We owe sincere thanks to all those who helped our work with samples. Their names are given in the Experimental section. We are grateful to Dr. E. K. Plyler from the U.S. National Bureau of Standards who kindly agreed to check the frequencies of our calibration bands. We are indebted to Mrs. I. R. Jegyud for helping in the preparation of the manuscript. Finally, we acknowledge financial help from the National Research Council of Canada.
GAS PHASE OXIDATION OF THE XYLENES. GENERAL KINETICS BY FRANKLIN J. WRIGHT Esso Research and Engineering Company, Linden, New Jersey Received July 18, 1960
The kinetics of the slow combustion of the three xylene isomers have been studied manometricall in a quartz vessel, under static conditions a t subatmospheric pressures over the temperature range from 410 to 550°, empyoying 1:l to 1:20 hydro= k.Pn where P is the total initial pressure, W,, the maximum rate carbon:oxygen mixtures. It was shown that W,, developed and n is 2.8, 1.9 and 1.5 for m-, 0- and p-xylene, respectively. Although Wmsxjsaffected by changes in mixture composition and temperature, the value of n is independent of these parameters. Arrhenius ploh were linear between 410 and 550" and the activation energies for the over-all oxidation process were colculated to be 38, 39 and 40 kcal./mole for 0-, m and p-xylene, respectively. The greater ease of oxidation of o-xylene was ascribed to the greater reactivity of the chain branching intermediate. The lifetime of this intermediate was calculated to be 2 min. for o-xylene as contrasted to 20 and 17 min. for m- and p-xylene, respectively. Addition of inert gases (He, A, N) and SFs) increased W,, thus suggesting that this rate ie governed by a diffusion controlled process a t the walls of the reaction vessel. Studies of the competitive oxidation of binary mixtures of the xylenes indicated that the chain propagation reactions proceed a t essentially equal rates in all three oxidations and have nearly equal activation energies.
The vapor phase oxidation of hydrocarbons has been much studied in the past 30 years. Yet, in spite of their increasing use in fuels for internal combustion engines, relatively little attention has been paid to t,he slow combustion of the aromatic hydrocarbons. -4number of investigations of the oxidation of benzene have been reported. Fort and Hinshelwood' concluded that the oxidation took place by a mechanism involving chains of short length. Newitt and Burgoyne2 studied the slow oxidation (1) R. Fort and C. N. Hinshelwooa, Proc. Roy. SOC.U c n d o n ) , 127A, 218 (1930). (2) D. M. Newitt and J H. Burgoyne, rbid., 16SA, 448 (1936).
of benzene, toluene and ethylbenzene a t high pressures and showed that in the case of the substituted benzenes both nuclear and side-chain oxidation occurred simultaneously whereas in the case of benzene only a single series of hydroxy intermediates preceded the breakdown of the ring. Norrish and Taylor3 showed that the oxidation of benzene proceeded via a successive hydroxylation of the ring to the dihydroxy stage whereupon ring splitting and rapid degradation of the fission products ensued. Burgoyne4 examined the oxidative behavior of benzene and a series of substituted (3) R. G. W. Norrish and 0.W. Taylor, tbid., W4A, 160 (1956). (4) J. H. Burgoyne, ibid.. 176A. 539 (1940).
