Abstract
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PHYSICAL CHEMISTRY
P. P. SHORYGIN and Z. S. EGOROVA
THE INFLUENCE OF SUBSTITUENTS ON THE PROPERTIES OF MOLECULES OF MONOSUBSTITUTED BENZENES
(Presented by Academician V. N. Kondrat’ev, July 6, 1957)
Monosubstituted benzenes constitute one of the most important groups of organic compounds. The mutual influence of the substituents and the benzene ring in these compounds determines to a considerable extent the specific character of their chemical and physical properties. It is manifested in the heat of formation, interatomic distances, vibrational frequencies, ultraviolet absorption spectra, magnetic susceptibility, transition probabilities in infrared and combination spectra, dipole moments, and other properties.
In this work various monosubstituted benzenes PhX were investigated, containing as substituents X alkyl groups, halogen atoms, and other groups of various types.
We shall focus our main attention on whether there is any commonality in the various indicators of the influence of substituents on the benzene ring. One such indicator is the increased intensity of the characteristic lines of the benzene ring ((1000 \pm 10\ \text{cm}^{-1}) and (1600 \pm 20\ \text{cm}^{-1})) in combination-scattering spectra (Raman spectra). The line (\sim 1600\ \text{cm}^{-1}) has a small intensity in benzene and alkylbenzenes and a much greater one in derivatives containing substituents capable of more or less strong interaction with the benzene ring. The intensity of the (1600\ \text{cm}^{-1}) line can serve for certain orienting judgments about the polarizability in that part which depends on the nuclear coordinates of the benzene ring.
In Table 1 are compared: 1) the results of measurements of the coefficients of integral intensity of the benzene-ring line (\sim 1600\ \text{cm}^{-1}) in combination-scattering spectra ((I_{1600})); 2) the values of exaltation of molecular refraction for (\lambda = 5893\ \text{Å}) ((EMR_D)) and for (\lambda = 4361\ \text{Å}) ((EMR_\gamma)^); 3) the position of intense absorption bands (wavelength of the maximum (\lambda) in Å); in parentheses are given the values (\varepsilon/1000), where (\varepsilon) is the molar (decimal) absorption coefficient at the band maximum*; 4) anomalies in the dipole moments ((\Delta\mu,) the difference of the (\mu) vectors of the compounds PhX and AlkX ((\text{Alk} = \text{Me}, \text{Et})) in debyes, according to data for (\mu) in benzene); 5) Hammett constants (\sigma_n), which determine the influence of substituents on the reactivity of groups located in the para position. The table uses averaged literature data for (\mu, MR, \sigma, \lambda); absorption-spectra data marked with an asterisk are from our measurements.
The intensity coefficients of the combination-scattering lines were measured by the photographic method in solutions in (\mathrm{CCl}_4); dimethylaniline and nitrostyrene were investigated in cyclohexane. Most of the coefficients were also redetermined by the photoelectric method by V. P. Bazov. In the event of discrepancies, mean values were taken. As the unit was adopted—
* (EMR) values were obtained by comparing the refraction of PhX and AlkX; the exaltation already present in AlkX, for example in (\mathrm{Alk \cdot CH{:}CH \cdot NO_2}), is not included here.
** We here neglect the presence of fine structure in the first band.
Table 1
| Group X in PhX | $I_{1600}$ | $EMR_D$ | $EMR_\gamma$ | Ultraviolet absorption spectra (in heptane), $\lambda_1$ | Ultraviolet absorption spectra (in heptane), $\lambda_2$ | Ultraviolet absorption spectra (in heptane), $\lambda_3$ | $\Delta\mu$ | $\sigma_\pi$ |
|---|---|---|---|---|---|---|---|---|
| $\cdot\mathrm{C:C.NO_2}$ | 3500 | 4,0 | — | — | 3000 (17) | 2290 (10) | — | — |
| $\cdot\mathrm{C:C.Ph}$ | 3000+ | 5,0 | — | — | 2950 (25) | 2250 (15) | $\sim0$ | 0,3 |
| $\cdot\mathrm{C:C.CHO}$ | 2000 | 3,3 | 5,1 | — | *2790 (25) | 2200 (13) | −0,2 | — |
| $\cdot\mathrm{C:C.COOEt}$ | 1400 | 3,1 | 4,6 | — | *2720 (21) | 2160 (17) | — | — |
| $\cdot\mathrm{C:C.C:C}$ | 1100 | 3,0 | — | — | 2800 (25) | 2230 (12) | $\sim0$ | — |
| $\cdot\mathrm{Ph}$ | 360+ | 2,0 | — | — | 2470 (18) | 2070 (27) | $\sim0$ | 0 |
| $\cdot\mathrm{C:C}$ | 240+ | 1,4 | 2,0 | 2820 (0,8) | 2450 (13) | 2030 (22) | −0,1 | 0 |
| $\cdot\mathrm{NO_2}$ | 210 | 0,9 | 1,4 | 2800 (1,5) | 2520 (9,6) | — | −0,9 | 1,0 |
| $\cdot\mathrm{CHO}$ | 200+ | 1,1 | 1,45 | *2800 (1,5) | 2420 (14) | — | −0,55 | 0,8 |
| $\cdot\mathrm{NHCHO}$ | 190 | — | — | *2750 (1) | 2400 (13) | — | — | 0 |
| $\cdot\mathrm{COR}$ | 180+ | 0,85 | 1,15 | 2800 (1) | 2380 (13) | — | −0,5 | 0,7 |
| $\cdot\mathrm{NR_2}$ | 160 | 1,6 | 2,3 | 2970 (2) | 2500 (14) | 2000 (22) 1760 (36) |
1,5 | −0,6 |
| $\cdot\mathrm{CN}$ | 140+ | 1,0 | 1,2 | 2740 (0,6) | 2250 (12) | — | −0,4 | 0,8 |
| $\cdot\mathrm{COOR}$ | 120+ | 0,75 | 0,95 | 2770 (0,9) | 2290 (12) | — | −0,2 | 0,6 |
| $\cdot\mathrm{NHR}$ | 110 | 1,25 | 1,75 | 2880 (1,8) | 2380 (14) | 2000 (21) | — | −0,6 |
| $\cdot\mathrm{SR}$ | 110 | 0,80 | 1,10 | 2790 (1,5) | 2550 (10) | 2050 (12) | 0,5 | 0 |
| $\cdot\mathrm{SH}$ | 100 | 0,50 | 0,70 | 2690 (0,7) | 2360 (9) | — | 0,5 | — |
| $\cdot\mathrm{NH_2}$ | 85 | 0,95 | 1,30 | 2850 (1,7) | 2330 (8) | 2000 (20) | 0,9 | −0,5 |
| $\cdot\mathrm{NEt\cdot COR}$ | 80 | — | — | — | *2380 (5,5) | — | — | — |
| $\cdot\mathrm{CCl_3}$ | 80 | 0,35 | — | *2680 (0,6) | 2240 (7) | — | −0,55 | 0,45 |
| $\cdot\mathrm{OR}$ | 50+ | 0,35 | 0,50 | 2720 (1,8) | 2200 (8) | — | 0,9 | −0,2 |
| $\cdot\mathrm{OH}$ | 40 | 0,28 | 0,37 | *2700 (1,8) | 2130 (6) | — | 0,7 | −0,25 |
| $\cdot\mathrm{I}$ | 42 | 0,15 | 0,20 | 2570 (0,7) | 2310 (12) | — | 0,2 | 0,3 |
| $\cdot\mathrm{C.C.Ph}$ | 45 | — | — | 2600 (0,5) | $\sim$2050 (19) | 1870 (100) | — | — |
| $\cdot\mathrm{R}$ | 39 | 0,27 | 0,31 | 2620 (0,3) | 2050 (8,1) | 1870 (55) | 0,35 | −0,15 |
| $\cdot\mathrm{CR_3}$ | 32 | 0,20 | 0,24 | 2600 (0,3) | 2060 (9) | 1880 (80) | 0,5 | −0,2 |
| $\cdot\mathrm{SO_2R}$ | ($\sim$30) | — | — | *2700 (1) | 2160 (8) | — | −0,4 | 0,8 |
| $\cdot\mathrm{Br}$ | 33 | 0,10 | 0,15 | *2650 (0,3) | 2150 (9) | — | 0,2 | 0,2 |
| $\cdot\mathrm{Cl}$ | 33 | 0,15 | 0,17 | *2650 (0,3) | 2150 (8) | — | 0,2 | 0,2 |
| $\cdot\mathrm{F}$ | $\sim$28 | −0,05 | −0,15 | 2670 (1,2) | 2010 (7,3) | 1810 (50) | 0,2 | 0 |
| $\cdot\mathrm{D}$ | 35 | 0 | 0 | — | — | — | 0 | 0 |
| $\cdot\mathrm{H}$ | 35 | 0 | 0 | 2550 (0,2) | 2020 (7,3) | 1820 (50) | 0 | 0 |
Note. The H atoms attached to carbon in the formulas have been omitted; R is a methyl group. One one-hundredth of the integrated intensity of the 313 cm$^{-1}$ line of CCl$4$ was taken in the calculation per mole. Accuracy ±10%. The spectrum was excited by the mercury line 4358 Å. The data for phenylbutadiene were obtained by V. M. Medvedeva, for anilines by Z. Alaune, and for ethylacetanilide by T. N. Shkurina; the figures marked with the sign (+) are according to data from ($^1$). For dibenzyl, stilbene, and diphenyl, the value given is $0{,}5\cdot I$*.
Benzene, toluene, fluorobenzene, and phenol have two lines in the region of 1600 cm$^{-1}$; the table gives the values of the summed intensity of both lines. Apparently, PhX has two forms of vibration of the benzene ring that are very close in frequency, one of them symmetric (with a significant influence of substituents the $\sim$1600 line is rather strongly polarized ($^2$)).
* The intensity coefficients in Table 1 for some compounds studied previously differ from the old data and are more accurate (see data for dibenzyl and aniline). According to measurements in cyclohexane, the value $0{,}5 I_{1600}$ for dibenzyl is about 70, i.e., greater than in CCl$4$. For toluene, identical values of $I$ were obtained in both solvents.
As can be seen, alkyl groups have little effect on the chemical, electrical, and optical properties of PhX. Substituents containing double bonds C=C and benzene rings strongly affect the optical properties and only slightly the chemical properties and dipole moments. Strongly electropositive and strongly electronegative substituents significantly affect all these properties.
Among the indicators of the influence of substituents on various optical properties there is a certain commonality: the closer and more intense the absorption bands, the higher, in most cases, the intensity of the 1600 lines and the EMR. At the same time, there is an undoubted connection between anomalies in dipole moments and the constants $\sigma$; electronegative substituents in most cases have positive $\sigma_n$, while electropositive ones have the opposite (roughly speaking, the larger $\Delta \mu$, the smaller $\sigma_n^*$). However, the optical properties mentioned are not in obvious correspondence with the electrical and chemical properties.
From the position and intensity of the absorption bands one can determine the contributions $R_i$ of individual electronic levels to the value of the molecular refraction $MR$ according to the formula $R_i = 18 \cdot 10^9 \cdot f_i(\nu_i^2 - \nu^2)^{-1}$, where $\nu_i$ is the frequency of the $i$-th absorption band, $\nu$ is the frequency of the incident light in cm$^{-1}$, and $f_i$ is the oscillator strength.
Table 2
| Compound | 1st $\lambda_1$ | 1st $R_1$ | 1st $\Delta R_1$ | 2nd $\lambda_2$ | 2nd $R_2$ | 2nd $\Delta R_2$ | 3rd (and 4th) $\lambda_3$ | 3rd (and 4th) $R_3$ | 3rd (and 4th) $\Delta R_3$ | $\Sigma R$ | $\dfrac{MR_\gamma - MR_D}{\Sigma \Delta R}$ calc. | $\dfrac{MR_\gamma - MR_D}{\Sigma \Delta R}$ found |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Benzene | 255 | 0.06 | 0.01 | 204 | 1.3 | 0.16 | 184 | 7.3 | 0.70 | 8.7 | 0.87 | 0.96 |
| Fluorobenzene | 267 | 0.4 | 0.11 | 200 | 1.2 | 0.15 | 181 | 7.0 | 0.67 | 8.6 | 0.93 | 0.87 |
| Toluene | 262 | 0.1 | 0.02 | 205 | 1.6 | 0.20 | 189 | 7.9 | 0.78 | 9.6 | 1.00 | 1.10 |
| tert-Butylbenzene | 260 | 0.1 | 0.02 | 206 | 1.6 | 0.22 | 188 | 11.9 | 1.26 | 13.6 | 1.50 | 1.36 |
| Styrene | 282 | 0.3 | 0.11 | 245 | 3.6 | 0.73 | 203 | 4.0 | 0.50 | 7.9 | 1.33 | 1.80 |
| Dimethylaniline | 297 | 0.9 | 0.37 | 250 | 3.8 | 0.82 | 200 | 3.9 | 0.46 | — | — | — |
| Dimethylaniline | 297 | 0.9 | 0.37 | 250 | 3.8 | 0.82 | 176 | 4.7 | 0.43 | 13.3 | 2.08 | 2.06 |
| Cinnamaldehyde | — | — | — | 279 | 10.1 | 3.20 | 220 | 3.0 | 0.48 | 13.1 | 3.68 | 3.55 |
In Table 2 are given the values of $R_i$ in cubic centimeters, calculated for the sodium $D$ line, and the increments $(\Delta R_i)$ of these values on going from the $D$ line to the hydrogen line $H_\gamma$ (4361 Å).
As can be seen, $\Sigma R_i$ for some compounds greatly exceeds $\Sigma R_i$ of benzene; in this case the difference is much greater than the EMR found experimentally. If the calculation formula and the experimental data for $\varepsilon$ are correct, this means that consideration of the first three absorption bands is entirely insufficient for explaining exaltation and that further bands may influence EMR in the direction of decrease (i.e., be weaker and farther away than in benzene). Undoubtedly, the exaltation cannot be ascribed to a single absorption band.
For the dispersion and intensity of the lines of combination scattering, the proximity of absorption bands should have greater significance than for refraction (because of the peculiarities of the resonance denominators). The same bands in the region $>1700$ Å can already, in the main, explain the experimentally observed values of $MR_\gamma - MR_D$; indeed, the latter do not differ greatly from the calculated values $\Sigma \Delta R$ (see Table 2).
* The correspondence, however, is very approximate. Here one should bear in mind not only the specificity of chemical processes (the significance of which can partly be judged from the fact that the constants $\sigma$ for different reactions vary over wide limits), but also differences in solvents.
Judging from the known data on the dependence of (I_{1600}) for styrene and benzonitrile on the frequency of the incident light (\nu), the electronic levels having the greatest significance for (I_{1600}) must lie in the region 1700–2500 Å.
The observed similarity in the changes of (EMR) and (I_{1600}) may be interpreted as an indication of commonality in at least some of the parameters on which these quantities depend; such parameters may be (\nu_i) and (f_i) of the intense absorption bands.
The attribution of the exaltation of refraction of PhX to the atoms of the substituents X (Ingold(^{(3)})) or to the bonds (C_{\mathrm{ar}}—X) (^{(4)}) is not justified. Its attribution to the benzene ring is more justified, although nevertheless it is rather conventional. The intensity of the 1600 cm(^{-1}) line is probably associated to a greater extent with the benzene ring. It should be borne in mind that the intense absorption bands in the region 1800–3000 Å belong to a considerable degree to the benzene ring or to the “benzene ring—substituent” system.
Table 1 shows that no general parallelism is observed between the various manifestations of the mutual influence of atomic groups in PhX molecules. In this connection it should be noted that the concepts of a “stronger” or “weaker” interaction of atomic groups can be applied only in a narrow and conventional sense, with reference to definite manifestations of it in series of similar compounds in which it has approximately the same character.
Between the manifestations of the mutual influence of groups in the various optical properties of PhX, a greater correspondence is observed. For more detailed comparisons, more complete information on all levels of electronic excitation is necessary.
Scientific-Research Physicochemical
Institute named after L. Ya. Karpov
Received
1 II 1957
CITED LITERATURE
(^{1}) L. Osityanskaya, Dissertation, Moscow, 1955; A. Khalilov, P. Shorygin, ZhFKh, 27, 330 (1953). (^{2}) P. P. Shorygin, DAN, 78, 469 (1951). (^{3}) C. K. Ingold, Structure and Mechanism in Organic Chemistry, London, 1953. (^{4}) A. Vogel et al., J. Chem. Soc., 1952, 514.