CHEMISTRY

V. A. IZMAILSKII, P. F. POLESHCHIKOV

ON THE PROBLEM OF THE MICROSTRUCTURE OF THIOPHENE

AND THE GENETICS OF SPECTRA

(Presented by Academician M. I. Kabachnik on 29 VI 1964)

The genetics of the spectra of 3-nitro-5-acetyl-2-aminothiophene \((\mathrm{HATNH_2})\) \((\mathrm{I}, \mathrm{B}^1=\mathrm{CH_3CO})\), 3,5-dinitro-2-aminothiophene \((\mathrm{DHTNH_2})\) \((\mathrm{I}, \mathrm{B}^1=\mathrm{NO_2})\), and their N-substituted derivatives \((\mathrm{II}, \mathrm{III})\) has been established by comparison with the spectra of the corresponding benzene derivatives: 2-nitro-4-acetyl-aniline \((\mathrm{HAFNH_2})\) \((\mathrm{IV}, \mathrm{B}^1=\mathrm{CH_3CO})\), 2,4-dinitro-aniline \((\mathrm{IV}, \mathrm{B}^1=\mathrm{NO_2})\), and their N-substituted derivatives \((\mathrm{V}, \mathrm{VI})\). Applying, for the analysis of structural effects, the principle of decomposition of the molecular structure into polar chromophoric systems \((^{1,2})\), we attempted to assign individual \(\lambda_{\max}\) values to definite auxochromophoric systems \((^2)\):
\(\mathrm{B^1KA^1}\), \(\mathrm{B^2KA^1}\), \(\mathrm{B^2K}\), \(\mathrm{KA^1}\) \((\mathrm{B^1}=\mathrm{NO_2},\ \mathrm{CH_3CO};\ \mathrm{B^2}=\mathrm{NO_2};\ \mathrm{K}\) is the benzene or thiophene nucleus \(a\); \(\Phi = n\text{-}\mathrm{C_6H_4}\) nucleus \(b\); \(\mathrm{A^1}=\mathrm{NH_2},\ \mathrm{NMe_2}\); in III and VI \(\mathrm{A^1}=\mathrm{NH}\), \(\mathrm{A^2}=\mathrm{H},\ \mathrm{OCH_3},\ \mathrm{NH_2},\ \mathrm{NHCOCH_3},\ \mathrm{NMe_2}\)).

In the spectrum of 2,4-dinitroaniline (No. 4), Fig. 1, bands of two chromophoric systems \((^2)\) were found: the para-system \(\mathrm{B^1KA^1}\) \((n\text{-}\mathrm{NO_2\Phi NH_2})\), the \(2_a\)-band* \((\lambda_{\max} 335\ \mathrm{m}\mu)\), and the ortho-system \(\mathrm{B^2KA^1}\) \((o\text{-}\mathrm{O_2N\Phi NH_2})\) (three absorption bands \((^3)\), No. 2, Table 1). In the spectrum of 2-nitro-4-acetyl-aniline (No. 3) we find four absorption bands of the same systems. The intense band \(\lambda_{\max} 300\ \mathrm{m}\mu\) belongs to the \(2_a\)-band of the para-system \(\mathrm{B^1KA^1}\) \((n\text{-}\mathrm{CH_3CO\Phi NH_2})\) (band \(2_a^1\), corresponding to \(x^1\) \((^4)\), is overlapped). All three absorption bands of the ortho-system \(\mathrm{B^2KA^1}\) \((o\text{-}\mathrm{O_2N\Phi NH_2})\) appear: the \(1_a\)-band of the whole conjugated system \(\mathrm{B^2KA^1}\), \(\lambda_{\max} 395\ \mathrm{m}\mu\); the \(2_a\)-band of the \(\mathrm{B^2K}\) system \((\Phi\mathrm{NO_2})\), \(\lambda_{\max} 265\ \mathrm{m}\mu\); and the \(2_a\)-band of the \(\mathrm{KA^1}\) system \((\Phi\mathrm{NH_2})\), \(\lambda_{\max} 227\ \mathrm{m}\mu\), in agreement with \((^3)\). Replacement of the strong electron-acceptor \(\mathrm{NO_2}\) by the weaker \(\mathrm{CH_3CO}\) group does not affect the character of the spectral curve: in No. 3 the types of characteristic chromophoric components contained in molecule No. 4 are retained (see Table 1).

The spectra of 3-nitro-5-acetyl-4-aminothiophene \((\mathrm{HATNH_2},\ \text{No. }5)\) and 3,5-dinitro-2-aminothiophene \((\mathrm{DHTNH_2},\ \text{No. }6)\) \((^5)\) proved to be very close to the spectra of their benzene analogues, Nos. 3, 4, Fig. 1, both in the character of the spectral curves and in the number of bands. This allows us, in the spectra (I), the intense

* The numbers of the compounds in Figs. 1–3 correspond to the numbers in Table 1.
** In benzene derivatives we regard the \(1_a\)-band as a shifted benzene 1-band, \(\lambda_{\max} 254\); the \(2_a\)-band as a shifted benzene 2-band, \(\lambda_{\max} 203.5\). The prefixes \(a\) and \(b\), added to the designation of the band type, indicate belonging to nucleus \(a\) or \(b\).

Table 1

Absorption spectra in 95% ethanol, \(C \cdot 10^{-4}\)*

Abbreviations shown in the table:
\(\mathrm{NA\Phi}=\mathrm{CH_3CO}\)-substituted nitrophenyl derivative; \(\mathrm{NAT}=\mathrm{CH_3CO}\)-substituted nitrothienyl derivative; \(\mathrm{DN\Phi}=\mathrm{O_2N}\)-substituted nitrophenyl derivative; \(\mathrm{DNT}=\mathrm{O_2N}\)-substituted nitrothienyl derivative; \(\Phi=\mathrm{C_6H_4}\) or \(\mathrm{C_6H_5}\).

* The melting point is given only for undescribed compounds. Values marked with \(\infty\) were determined approximately from the curve (inflection).

bands \(\lambda_{\max}\) 322–323 mµ as the \(2_a\)-bands* of the “para-systems” \(\mathrm{B^1KA^1}\) (5-\(\mathrm{CH_3CO}\)-2-\(\mathrm{NH_2}\)-thiophene (No. 5) and 5-\(\mathrm{NO_2}\)-2-\(\mathrm{NH_2}\)-thiophene (No. 6)), analogous to the \(2_a\)-bands of the benzene para-systems \(\mathrm{B^1KA^1_B}\) (IV). As in the case of benzene derivatives (IV), the \(2_a^1\)-bands of these systems in (I) are overlapped. The ortho-systems in (I) \(\mathrm{B^2KA^1}\) (3-\(\mathrm{NO_2}\)-2\(\mathrm{NH_2}\)-thiophene) appear in Nos. 5 and 6 as three bands of three quasi-autonomous systems: the \(1\)-band of the entire conjugated system \(\mathrm{B^2KA^1}\), \(\lambda_{\max}\) 375 mµ; the \(2_a\)-band of the partial system \(\mathrm{B^2K}\) (3-\(\mathrm{NO_2}\)-thiophene), \(\lambda_{\max}\) 262–270 mµ, and the \(2_a\)-band of the partial system \(\mathrm{KA^1}\) (2-\(\mathrm{NH_2}\)-thiophene), \(\lambda_{\max}\) 245–242 mµ (Fig. 1, Table 1). In the amines of the thiophene series, as in the amines of the benzene series, replacement of the \(\mathrm{NO_2}\) group by \(\mathrm{CH_3CO}\) does not change the general character of the spectrum.

Fig. 1

The similarity in the general character of the spectra of derivatives with benzene and thiophene nuclei in the presence of identical chromophoric components is a consequence of a certain analogy in their electronic structures. However, this analogy applies only to the structure of para-systems of the type \(\mathrm{B^1KA^1}\), having the structure \(\mathrm{B^1{-}C{=}C{-}C{=}C{-}NR_2}\) (IV, V and I, II). In the microstructure of thiophene there is no such equalization of the lengths and orders of the bonds, no such symmetry and equivalence of angles, as in benzene. Geometrical data**, obtained by the electron-diffraction method \((^8)\) and by the molecular-orbital method \((^6)\), clearly indicate the possibility that a butadiene quasi-autonomous chromophoric system is manifested in the microstructure and in the spectrum of thiophene derivatives (I, II).

We see confirmation of this conclusion in the differences found by us in the spectra of HATNH\(_2\) No. 5 and DHTNH\(_2\) No. 6 (I, II) for the \(1_a\)-bands of the ortho-system \(\mathrm{B^2KA^1}\) (\(\mathrm{O_2N{-}C{=}C{-}NH_2}\)). In contrast to (IV, V) HAFNH\(_2\) No. 3 and DHFNH\(_2\) No. 4, \(\varepsilon_{\max}\) of the \(1_a\)-band in the thiophene derivatives is approximately twice as large as in the benzene derivatives (Table 1, Fig. 1). Whether we regard the \(1_a\)-band of the \(\mathrm{B^2KA^1}\) system as the band of an electronic transition polarized along the \(\mathrm{C{=}C}\) bond toward the \(\mathrm{NO_2}\) group \((^9)\), or as a charge-transfer band from the \(\mathrm{NH_2}\) orbital to the \(\mathrm{NO_2}\)-group orbital \((^{10})\), the increase in \(\varepsilon_{\max}\) is a consequence of the shift of the bond order of \(\mathrm{C_2\ldots C_3}\) in (I, II) toward that of a double bond, with enhancement of its polarization under the influence of the S atom. The internuclear distances in the thiophene molecule are \(\mathrm{C_2\ldots C_3}\) 1.35 Å; \(\mathrm{C_3\ldots C_4}\) 1.44 Å; \(\mathrm{S\ldots C_2}\) 1.74 Å.

Differences in the microstructure and valence angles in thiophene and benzene are also manifested in the different influence of replacement of \(\mathrm{NH_2}\) by the \(\mathrm{NMe_2}\) group. In the spectrum of DHFNMe\(_2\), the bands of the \(o\)-system \(\mathrm{B^2KA^1}\) disappear completely, and only the \(2_a\)-band of the \(n\)-system \(\mathrm{B^1KA^1}\) appears (No. 7, Table 1) \((^2)\). This was explained by the establishment of coplanarity of \(\mathrm{NMe_2}\) and the complete withdrawal of \(o\)-\(\mathrm{NO_2}\) from conjugation. In the spectrum of HATNMe\(_2\) (No. 8), however, in addition to the \(2_a\)-band at 342 mµ of the \(n\)-system \(\mathrm{B^1KA^1}\) 5-\(\mathrm{CH_3CO{-}C{=}C{-}C{=}C{-}NMe_2}\)-2, there is present the \(1_a\)-band

* Taking into account that the electronic structure of thiophene is intermediate between benzene and butadiene, but closer to that of benzene \((^6)\), it may be assumed that the thiophene bands at 220–260 mµ and 200–220 mµ correspond to the benzene bands at 254 and 203.5 mµ \((^7)\).

** Bond lengths in thiophene, butadiene, and cyclopentadiene: for \(\mathrm{C{=}C}\), 1.35, 1.37, 1.35 Å; for \(\mathrm{C{-}C}\), 1.44, 1.47, 1.46 Å.

of the o-system \(B^2K A^1\) \((3\text{-}NO_2\!-\!C{=}C\!-\!NMe_2 - 2)\), \(\lambda_{\max}\sim 375\) mμ. This is a consequence of the incomplete removal of \(3\text{-}NO_2\) from coplanarity and conjugation with the thiophene nucleus, owing to the fact that the valence angle in thiophene between the bonds to the \(NO_2\) and \(NMe_2\) groups is \(67^\circ 30'\) greater than in benzene \((60^\circ)\).

Fig. 2

The study of the effect of replacing \(NH_2\) groups by \(NH\Phi\) and \(NH\Phi A^2\) showed that, upon phenylation of the \(NH_2\) group in amines of the thiophene series (III) and the benzene series (VI), a complete analogy is observed in the spectral changes. We are dealing with compounds of the \(BKQKA\) type, having two chromophoric systems, nucleus \(a\) and nucleus \(b\), separated by the group \(Q = NH\) (BKNHKA). A single conjugated system involving the \(p\)-electrons of NH with a single excitation vector along the \(\pi\)-system is absent. There are bands of the para- and ortho-chromophoric systems of nucleus \(a\) and a band of the system of nucleus \(b\), \(A^1\Phi A^2\) \((NH\Phi,\ NH\Phi A^2)\) (Table 1, Figs. 2 and 3). The interaction of the chromophoric systems of nuclei \(a\) and \(b\) occurs as in inductively interacting systems \((^{2,11})\), with NH entering into conjugation with nucleus \(a\) and with nucleus \(b\). Increased polarization of nucleus \(b\) under the influence of \(A^2\) creates a charge \(\delta^-\) on the \(n\)-C atom of nucleus \(b\), which inductively increases the charge \(\Delta+\) on NH and enhances the electronic displacements in the system of nucleus \(a\), as in \((^{2,11})\).

In the spectra, in addition to the \(2_a\)-band of nucleus \(a\), the \(2_b\)-band \(A^1\Phi A^2\) appears clearly, with the largest \(\varepsilon_{\max}\) in the case \(A^2 = NHCOCH_3, NMe_2\) (Nos. 19, 21, Fig. 3). In contrast to No. 7, the spectra of compounds Nos. 9, 15, 17 contain a \(1_a\)-band (Nos. 9, 17, Figs. 2, 3). This is a consequence of the lower energy of conjugation of \(4\text{-}CH_3CO\) with NH through the benzene nucleus than in the case of the \(4\text{-}NO_2NH\)-system.

In the spectra of the N-phenyl derivatives DNTH\(_2\), Nos. 12, 14, 21 (Figs. 2, 3), a low-intensity band was found in the region 535 mμ, \(\lambda_{\max}\sim 1920\). Its origin is unclear. It may be associated with intermolecular interaction or with tautomerism. The latter may be the cause of the fine structure of the long-wavelength bands \(\sim 350\)—450 mμ of thiophene compounds Nos. 13, 14, 19, 21.

Laboratory of the Chemistry of Dyes and the Problem of Color
at the V. I. Lenin Moscow Pedagogical Institute

Received
27 VI 1964

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