Chemistry

E. E. Milliarsi, V. A. Izmailskii

Spectra of Derivatives of 2,4-Dinitroaniline

The Presence of Quasi-Autonomous Cochromophoric Systems

(Presented by Academician B. A. Kazanskii, 20 VI 1962)

The spectra of derivatives of 2,4-dinitroaniline (DNA) (I) and 2,4-dinitrodiphenylamine (2,4-(NO₂)₂-DPA) (II) were investigated. For analysis of structural effects, the principle of decomposing the structure of a molecule into polar chromophoric systems* was applied. It proved possible to assign individual \(\lambda_{\max}\) values to definite systems: \(B^1KA^1\), \(B^2KA^1\), \(B^2K\), \(A^1K\), \(A^1\Phi\), \(A^1\Phi A^2\) (Tables 1 and 2). The quasi-autonomy of individual chromophoric systems upon excitation by light in molecules with complex cochromophoric systems was demonstrated experimentally.

\[ \begin{array}{c} \text{(schemes I and II)} \end{array} \]

\(B^1 = n\text{-}NO_2;\quad B^2 = o\text{-}NO_2;\quad K\) — benzene nucleus \(a\); \(\Phi = n\text{-}C_6H_4\) — nucleus \(b\);
\(A^1\) in I: \(NH_2\), \(NHEt\), \(NMe_2\), \(NEt_2\); \(A^1\) in II: \(NH\); \(A^2 = H\), \(OCH_3\), \(NH_2\), \(NMe_2\).

Table 1

Genesis of the bands of derivatives of 2,4-dinitroaniline (I) and (II, \(A^2 = H\)) (in 95% ethanol. \(C = 10^{-4}\))

Note. The compound numbers correspond to the numbers of the curves in Figs. 1 and 2. \(\lambda_{\max}\) and \(\varepsilon_{\max}\) determined from the bend in the curve are denoted by the sign \(\sim\).

* Literature source (\(^8\)). ** Literature source (\(^9\)).

In the spectrum of DNA (I, \(A^1 = NH_2\)) we find bands of two systems: \(n\text{-}O_2N\Phi NH_2\) (\(B^1KA^1\)) and \(o\text{-}O_2NC_6H_4NH_2\) (\(B^2KA^1\)) (Nos. 4, 5, 6, Table 1, Fig. 1).

* This principle, advanced in (\(^1\)), proved useful in the study of spectra of benzene derivatives (\(^{2-6}\)).

The main \(2_a\)-band at 375 mµ in the spectrum of \(n\)-O\(_2\)NΦNH\(_2\) is associated with the \(\pi \to \pi^\ast\) transition in a mesosystem of type BKA (in I B\(^1\)KA\(^1\)). The \(2_a^\prime\)-band, \(\lambda_{\max} 227\) mµ, may be assigned chiefly to a \(\pi \to \pi^\ast\) transition, but to the next higher excited state of the \(\pi\)-electron system (\(^{7}\)). When an \(o\)-NO\(_2\) group is introduced into \(n\)-O\(_2\)NΦNH\(_2\) (B\(^2\)), the main 2-band is retained, but is strongly shifted toward the ultraviolet (\(\Delta\lambda = -40\) mµ) with a strong decrease in \(\varepsilon\) (\(\Delta\varepsilon = -11\,800\)). The \(o\)-NO\(_2\) group removes the NH\(_2\) group from coplanarity; the degree of conjugation of the \(p\)-electrons of the N atom with the benzene ring decreases. At the same time, three bands of the \(o\)-O\(_2\)NC\(_6\)H\(_4\)NH\(_2\) system appear, also shifted toward the u.-v. part of the spectrum: the \(1_a\)-band of the system B\(^2\)KA\(^1\), \(\Delta\lambda = -28\); the \(2_a\)-band of the system KA\(^1\) (C\(_6\)H\(_3\)NH\(_2\)), \(\Delta\lambda = -6\); and the \(2_a\)-band of the system B\(^2\)K (C\(_6\)H\(_3\)NO\(_2\)), \(\Delta\lambda = -19\) mµ.

Fig. 1

A complete analogy is observed in the spectrum of 2,4-dinitrophenol (Table 1, No. 3). The introduction into \(n\)-O\(_2\)NΦOH of a second NO\(_2\) group, while the \(n\)-O\(_2\)NΦOH (B\(^1\)KA\(^1\)) bands are retained, leads to the appearance of a system of \(o\)-O\(_2\)NΦOH bands (B\(^2\)KA\(^1\)) (Table 1, Nos. 1–3). The smaller hypsochromic shift of the \(\lambda_{\max}\) of the 2-band of the B\(^1\)KA\(^1\) \(n\)-O\(_2\)NΦOH system upon introduction of \(o\)-NO\(_2\) (\(\Delta\lambda = -19\) mµ) is apparently connected with stabilization of the spatial structure by the hydrogen bond of OH with \(o\)-NO\(_2\) and with the smaller volume of the OH group. Hypsochromic shifts are also observed on passing from No. 2 to No. 3 for the system B\(^2\)KA\(^1\): for the \(2_a\)-band of the B\(^2\)K system \(\Delta\lambda = -19\) mµ; for the 1-band of the system B\(^2\)KA\(^1\), \(\Delta\lambda = -16\) mµ.

Fig. 2

* In accordance with (\(^{2,4,7}\)) we consider the \(1_a\) band as the shifted 1-band of benzene (\(\lambda_{\max} = 255\) mµ, \(\varepsilon_{\max} = 205\)), and the \(2_a\) bands as shifted 2-bands of benzene (\(\lambda_{\max} = 200\) mµ, \(\varepsilon_{\max} \sim 7000\)). The signs \(a\) and \(b\), added to the designation of the band type, indicate affiliation with nucleus \(a\) or \(b\).

As we assumed, replacement of NH₂ by the groups NHEt, NMe₂, NEt₂ causes a gradual increase in \(\lambda_{\max}\) of the \(2_a\)-band of the system B²KA¹: \(\Delta\lambda = +12, +32, +40\) mµ (Table 1, Nos. 6–9; Fig. 1). All three bands of the system B²KA¹ disappear, and the previously overlapped \(2'_a\)-band of the system \(n\)-O₂NΦNH₂ B¹KA¹ appears. However, such a considerable shift of the 2-band of the system B¹KA¹ upon replacement of NH₂ by the groups NMe₂, NEt₂ cannot be explained only by an increase in the donor character of the amino group (\(\Delta\lambda\) in the transition O₂NΦNH₂ → O₂NΦNEt₂ is only \(+23\) mµ). Dialkylation of the NH₂ group leads not only to strengthening of conjugation of the N atom with the benzene nucleus \(a\), but also to restoration of its coplanarity with the benzene nucleus owing to an increase in the volume of the amino group and destruction of the hydrogen bond and complete disruption of the coplanarity of the \(o\)-NO₂ group. As a result of the destruction of the interaction of \(o\)-NO₂ with the benzene ring in Nos. 8, 9, the spectrum returns to the spectrum of No. 4 \(n\)-O₂NΦNH₂ (system B¹KA¹).

The study of the genetics of the bands of 2,4-(NO₂)₂-DPA (II, A² = H) on the basis of comparison of spectrum No. 10 with the spectra of Nos. 6, 11, and 12 (Table 1, Fig. 2) confirmed the conclusions from the study of the spectra of 4-NO₂-DPA (⁶). The spectrum contains bands not only of the systems of nucleus \(a\), B¹KA¹ and B²KA¹, but also of the system of nucleus \(b\) (A¹Φ). A single conjugated system is absent. Owing to the noncoplanarity of the benzene nuclei \(a\) and \(b\), normal conjugation of the \(p\)-electrons of the N atom simultaneously with both nuclei is impossible. The existence of geometrical isomers of conjugation (³) IIIa and IIIb, as in (⁶)*, is possible.

[[structural schemes IIIa and IIIb]]

The band of nucleus \(b\), NHΦ, appears at the same \(\lambda_{\max}\), 257 mµ (Fig. 2), in all three DPA derivatives Nos. 10, 11, and 12. This indicates the identical

Table 2

Genetics of the bands of derivatives of 2,4-dinitro-DPA (II). Comparison of compounds R¹A² and R²A²*
\((\mathrm{R}^1 = 4\text{-}\mathrm{O_2NC_6H_4NH\Phi}-;\quad \mathrm{R}^2 = 2,4\text{-}(\mathrm{NO_2})_2\mathrm{C_6H_3NH\Phi}-)\)

| No. of compound | A² | \multicolumn{3}{c}{\(2_a\)-bands of nucleus \(a\): R¹—A² (B¹KA¹)} | \multicolumn{5}{c}{\(2_a\)-bands of nucleus \(a\): R²—A²} | \multicolumn{3}{c}{\(2_b\)-bands of nucleus \(b\): R¹—A² (A¹ΦA²)} | \multicolumn{5}{c}{\(2_b\)-bands of nucleus \(b\): R²—A² (A¹ΦA²)} | \multicolumn{2}{c}{Me₂NΦA², for comparison} |
|---:|---|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|---:|
| | | \(\lambda\) | \(\varepsilon\) | \(\Delta\lambda\) to No. 1 | \(\lambda\) | \(\varepsilon\) | \(\Delta\lambda\) to No. 1 | \(\Delta\lambda\) to R¹A² | \(\lambda\) | \(\varepsilon\) | \(\lambda\) | \(\varepsilon\) | \(\Delta\lambda\) to No. 1 | \(\lambda\) | \(\varepsilon\) | \(\Delta\lambda\) to No. 1 | \(\Delta\lambda\) to R¹A² | \(\lambda\) | \(\varepsilon\) |
| 1 | H | 395 | 29220 | 0 | 352 | 1800 | 0 | −43 | 232 | 1420 | 257 | 13500 | 0 | 257 | 11200 | 0 | 0 | 250 | 14150 |
| 2 | CH₃ | — | — | — | 352 | 1710 | 0 | — | 232 | 13000 | — | — | — | 260 | 10500 | +3 | — | 253 | 14150 |
| 3 | OCH₃ | 400 | 25800 | +5 | 355 | 1480 | +3 | −45 | 225 | 14600 | 257 | 12300 | 0 | 255, 260 | 11600, 11000 | 0 | 0 | 250 | 13490 |
| 4 | NH₂ | 405 | 21100 | +10 | 360* | 12750 | +8 | −45 | overlapped | overlapped | 260 | 13780 | +3 | 247 | 16150 | −10 | −13 | 250 | 12020 |
| 5 | NMe₂ | 410 | 22650 | +15 | 365** | 12340 | +13 | −45 | overlapped | overlapped | 265 | 19220 | +8 | 260 | 21000 | +? | [[unclear]] | 26[[unclear]] | 16220 |

Notes. The numbers of the compounds R²A² correspond to the numbers of the curves in Fig. 3. Color of the crystals: R²A²: No. 1 — orange, No. 3 — red, No. 4 — reddish-brown, No. 5 — brown-black; R¹A²: No. 1 — yellow, No. 3 — brown-yellow, No. 4 — reddish-brown, No. 5 — red-orange. For compound R³A³ the long-wave \(1_a\)-band of the system B¹KA¹ of nucleus \(a\), which appears as a bend in compounds Nos. 1, 2 at 400 mµ, has not been entered; in compounds Nos. 3–5 this band is apparently overlapped by the \(2'_a\)-band of the system B¹KA¹; the bands of the B²K system of nucleus \(a\) are overlapped. In compounds R¹A² the \(2_a\)-bands of the system B¹KA¹ of nucleus \(a\) are overlapped.

* \(\lambda_{\max}\) by the half-width of the band, since there are 2 peaks: \(\lambda_{\max}\) 257, 362 mµ (\(\varepsilon_{\max}\) 12700, 12800).

** \(\lambda_{\max}\) by the half-width of the band, since there are 4 peaks: \(\lambda_{\max}\) 355, 362, 367, 375 mµ (\(\varepsilon_{\max}\) 12350, 12400, 12350, 12250).

origin of the band and the independence of NHΦ in IIIb from the system of nucleus \(a\). Since in DNA the NHΦ system is absent, the band at 257 mµ in No. 6, by analogy—

* In connection with this, the phenomena of chromoisomerism may perhaps be found.

apparently may be assigned, by analogy with No. 5, to the band of the system \(o\)-\(\mathrm{O_2NC_6H_3NH}\) (\(\mathrm{B^2KA^1}\)), namely to the 2-band \(\mathrm{B^2K\,O_2N\Phi}\) (\(\lambda_{\max}\) of nitrobenzene \(\sim 260\) m\(\mu\)).

A study of the influence of \(\mathrm{A^2}\) (II) on the spectrum of 2,4-\((\mathrm{NO_2})_2\)-DPA showed the following. Introduction of the \(o\)-\(\mathrm{NO_2}\) group causes shifts of \(\lambda_{\max}\) toward the ultraviolet part of the spectrum by the same amount \(\Delta\lambda = -45\) m\(\mu\) (Table 2, Nos. 1–5); in the case of groups \(\mathrm{A^2}=\mathrm{H}, \mathrm{CH_3}, \mathrm{OCH_3}\), the spectrum contains the bands of all three systems \(\mathrm{B^1KA^1}\), \(\mathrm{B^2KA^1}\) (nucleus \(a\)) and \(\mathrm{A^1\Phi A^2}\) (nucleus \(b\)) (Table 2, Nos. 1, 2, 3; Fig. 3); with an increase in the donor character of \(\mathrm{A^2}\) (\(\mathrm{NH_2}, \mathrm{NMe_2}\)), the spectrum shows bands of only two systems, \(\mathrm{B^1KA^1}\) (nucleus \(a\)) and \(\mathrm{A^1\Phi A^2}\) (nucleus \(b\)), as in the case of the derivatives of 4-\(\mathrm{NO_2}\)-DPA (Table 2, Nos. 4, 5; Fig. 3). The short-wavelength bands of the \(\mathrm{B^2KA^1}\) system of nucleus \(a\) are overlapped; strengthening of the donor character of \(\mathrm{A^2}\) (\(\mathrm{H}<\mathrm{OCH_3}<\mathrm{NH_2}<\mathrm{NMe_2}\)) causes an insignificant shift of \(\lambda_{\max}\) of the 2\(a\)-band toward the infrared part of the spectrum of the \(\mathrm{B^1KA^1}\) system (of the same order as in 4-\(\mathrm{NO_2}\)-DPA). This is explained only by the inductive effect of \(\mathrm{A^2}\) (cf. \({}^{6}\) and \(\Delta\lambda\) for No. 1 for the bands of nucleus \(a\), Table 2, Nos. 1–5); the shift of bands of type 2 of the \(\mathrm{A^1\Phi A^2}\) system of nucleus \(b\) in (II) Nos. 4, 5 (Table 2) is not amenable to explanation, since in the ultraviolet part of the spectrum we have a complex system of overlapping bands.

Fig. 3

The deeper color of the crystals of the derivatives of 2,4-\((\mathrm{NO_2})\)-DPA (Table 2, Nos. 1–5), in comparison with 4-\(\mathrm{NO_2}\)-DPA, is due to enhancement of intermolecular interactions in the solid state. In the case of \(\mathrm{A^2}=\mathrm{NH_2}, \mathrm{NMe_2}\), exo-interaction also occurs in solutions (see the strong shifts of the long-wavelength branches of the curves toward the infrared part of the spectrum, Nos. 4, 5, Fig. 3, as also in \({}^{10}\)).

Laboratory of the Chemistry of Dyes and Problems of Color
at the Lenin Moscow Pedagogical Institute

Institute of Organic Intermediates and Dyes

Received
17 IV 1962

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