Infrared Absorption Spectra and Structure of Urea Derivatives
Abstract Generated abstract
Infrared absorption spectra were recorded and systematized for 24 monosubstituted urea derivatives of the general type RNHCONH2 in Vaseline oil and in benzene and chloroform solutions. The study compares alkyl, arylalkyl, and diarylalkyl substituent classes with urea, assigning characteristic NH, NH2, CO, CN, alkyl, and aromatic-ring bands across 700 to 1800 and 2700 to 3600 cm-1. The spectra show that substituents shift urea bands according to changes in intermolecular association, with evidence for cis and trans associates and possible dimers or trimers involving NH and NH2 groups. Dilute-solution spectra indicate free NH, NH2, and CO absorptions, supporting the conclusion that solid-state association occurs through intermolecular hydrogen bonding.
Full Text
INFRARED ABSORPTION SPECTRA AND THE STRUCTURE OF UREA DERIVATIVES
L. G. Tignibidina, A. G. Pechenkin, V. E. Chistyakov, M. M. Kaganskii
(Presented by the scientific-methodological seminar of KhTF)
To clarify the dependence of the anticonvulsant action of urea derivatives on their structure, we have obtained and described [1, 2] a series of compounds of the general formula $\mathrm{R—NH—CO—NH_2}$.
In the present work, the IR spectra of the indicated compounds have been recorded and systematized.
The experiments were carried out on a UR-10 spectrometer with NaCl and LiF prisms in the ranges 700–1800 and 2700–3600 $\mathrm{cm^{-1}}$. The spectra of pure substances were recorded both in Vaseline oil and in dilute solutions. Samples were prepared at the rate of 30 mg per 4 drops of oil. Solutions in benzene and chloroform were taken in the concentration range 0.1–0.5 M, at a temperature of 25°C; the cuvette thickness was 0.62 mm.
Our results, together with data from other authors, are presented in Table 1.
Discussion of Results
The IR absorption spectra and structures of urea and its derivatives [3] are analogous to the corresponding characteristics of simple amides [4].
Davis and Hallam [5] showed that the chain length of the alkyl radical and its nature do not affect the position and intensity of the absorption bands of the $\mathrm{NH_2}$, $\mathrm{NH}$, and $\mathrm{C–O}$ groups of primary amides. We established that a similar regularity also holds for urea and its derivatives of the type $\mathrm{RNHCONH_2}$, where $\mathrm{R = Alk—CH—Alk}$ (I), $\mathrm{R = Ar—CH—Alk}$ (II), and $\mathrm{R = ArAlk—CH—ArAlk}$ (III).
We recorded the IR spectra of twelve compounds of type (I), six compounds of type (II), and six compounds of type (III). The spectra of these types of compounds are so similar that they can be represented by generalized absorption bands in the form of three classes (Table 1).
The region 3600–3000 $\mathrm{cm^{-1}}$. According to Stewart [6] and Urbanski [7], the band at $3438 \pm 18\ \mathrm{cm^{-1}}$ {in our experiment, $3426 \pm 16$ for (I, II) and $3470 \pm 10$ for (III)} in urea is associated with asymmetric NH stretching vibrations. Two other bands, $3343 \pm 20$ and $3247 \pm 20\ \mathrm{cm^{-1}}$ {in our experiment, $3334 \pm 15$ (I), $3327 \pm 18$ (II, III), and $3210 \pm 10$ (I, II, III)}, are formed by symmetric NH stretching vibrations.
From comparison of the cited results obtained for urea and its derivatives, it follows that introduction into urea of alkyl and arylalkyl radicals in place of one hydrogen atom shifts the $\nu \mathrm{NH}$ bands into the low-frequency region. This may be explained by strengthening
Table 1
IR absorption spectra of \( \mathrm{R{-}NHCONH_2} \) in vaseline oil
| \( \mathrm{R=Alk{-}CH{-}Alk} \) | \( \mathrm{Ar{-}CH{-}Alk} \) | \( \begin{matrix}\mathrm{Ar\ Alk\ AlkA}\\ \backslash\\ \mathrm{CH}\end{matrix} \) | \( \mathrm{R=H} \) | Identification |
|---|---|---|---|---|
| \(3426\pm16\) | \(3426\pm16\) | \(3470\pm10\) | \(3438\pm18\) | \(\nu_{as}\mathrm{NH_2}\) [6, 7] |
| \(3334\pm15\) | \(3328\pm18\) | \(3327\pm15\) | \(3343\pm20\) | \(\nu_s\mathrm{NH}\) [6, 7] |
| \(3214\pm5\) | \(3210\pm10\) | \(3210\pm10\) | \(3247\pm20\) | \(\nu_s\mathrm{NH}\) [6, 7] |
| \(3100\pm0\) | \(3080\pm10\) | \(3080\pm10\) | — | |
| — | \(3030\pm0\) | \(3030\pm\) | — | \(\nu_s\mathrm{CH}\) of benzene |
| — | — | \(1660\pm10\) | \(1685\pm5\) | \(\nu(\mathrm{CO+NH})\) [10], \(\nu\mathrm{CO}\) [12, 13] |
| \(1655\pm5\) | \(1655\pm5\) | — | \(1625\pm5\) | \(\delta\mathrm{NH}\) [10, 12], \(\delta\mathrm{NH_2}\) [6] |
| \(1603\pm7\) | \(1598\pm8\) | \(1605\pm4\) | \(1600\pm5\) | \((\mathrm{CO+\delta NH})\) [10], \(\delta\mathrm{NH_2}\) [12] |
| \(1555\pm10\) | \(1560\pm10\) | \(1550\pm10\) | — | \(\nu_{as}\mathrm{CH_2}\) [3] |
| — | \(1490\pm5\) | \(1495\pm5\) | — | C–C of benzene |
| \(1463\pm5\) | \(1460\pm5\) | \(1460\pm5\) | \(1466\pm4\) | \(\nu(\mathrm{C{-}N})\) [10, 12], \(\nu\mathrm{CN_2}\) [13] |
| \(1380\pm0\) | \(1380\pm0\) | \(1380\pm0\) | — | \(\delta_s\mathrm{CH_3}\) |
| \(1330\pm10\) | \(1330\pm10\) | \(1330\pm10\) | — | \(\gamma\mathrm{CO}\) [13], \(\gamma\mathrm{NH_2}\) [6] |
| — | \(1200\pm10\) | \(1260\pm10\) | — | \(\delta\mathrm{CH}\) of benzene |
| \(1170\pm10\) | \(1152\pm7\) | \(1140\pm5\) | \(1155\pm5\) | \(\rho\mathrm{NH_2}\) [10, 12, 13] |
| \(1120\pm10\) | \(1030\pm0\) | \(1030\pm0\) | \(1055\pm5\) | \(\rho\mathrm{NH_2}\) [12], \(\gamma\mathrm{NH_2}\) [10] |
| — | — | — | \(1000\pm10\) | \(\nu(\mathrm{C{-}N})\) [10, 12], \(\nu\mathrm{CN_2}\) [13] |
| \(855\pm10\) | \(850\pm15\) | \(850\pm16\) | — | |
| \(780\pm0\) | \(757\pm8\) | \(770\pm20\) | \(785\pm5\) | \(\nu\mathrm{CO}\) [13], \(\gamma\mathrm{NH_2}\) [6] |
| \(720\pm0\) | — | — | — | |
| — | \(700\pm0\) | \(700\pm0\) | — | \(\delta\mathrm{CH}\) of benzene |
association of the compounds upon introduction of the indicated substituents into the urea molecule. The introduction of two phenyl groups linked to one another by an alkyl chain (III), however, leads to the opposite phenomenon—hindrance of association through the primary amino group and a shift of the absorption band to the high-frequency region by \(40\text{–}50\ \mathrm{cm^{-1}}\) (\(3438\pm18\) and \(3470\pm10\ \mathrm{cm^{-1}}\)). It should be noted that in some cases, in compounds of type (III), such as
\[ \begin{array}{cc} (\mathrm{C_6H_5})_2\mathrm{CHCH_2CHC_6H_5} & \mathrm{C_6H_5CH_2CH_2CHC_6H_5} \\[-2pt] \phantom{(\mathrm{C_6H_5})_2\mathrm{CHCH_2}\,}\big| & \phantom{\mathrm{C_6H_5CH_2CH_2}\,}\big| \\[-2pt] \mathrm{NHCONH_2} & \mathrm{NHCONH_2}, \end{array} \]
\(\nu\mathrm{NH}\) become completely nonassociated and have a value of \(3505\ \mathrm{cm^{-1}}\). Association in these compounds occurs through secondary NH, which has absorption bands at \(3380\pm10\ \mathrm{cm^{-1}}\) and \(3300\ \mathrm{cm^{-1}}\). This phenomenon is apparently due to the strong shielding effect of the benzene rings on the \(\mathrm{NH_2}\) group.
In compounds (I, II, III), in addition to absorption bands common with urea, there are individual bands at 3100 (I) and 3080 (II, III) \(\mathrm{cm^{-1}}\). Bassel [8] noted that for secondary amides the bands \(3100\text{–}3060\ \mathrm{cm^{-1}}\) belong to the cyclic cis-dimer, while the band \(3330\text{–}3280\ \mathrm{cm^{-1}}\) belongs to the trans-associate. Shūbī and Mizushima [9] investigated the associations of cyclic amides of the cis form with the possibility of formation of the trans form excluded. In the spectra of these compounds, instead of the \(3380\text{–}3280\) band, they found bands at \(3200\text{–}3160\) and \(3100\text{–}3060\ \mathrm{cm^{-1}}\). These results confirm the correctness of assigning the \(3330\text{–}3280\) band to the trans-associate and the \(3200\text{–}3160\ \mathrm{cm^{-1}}\) band to the cis-associate. In all types (I, II, III) of urea derivatives studied by us, bands have been recorded that are associated both with cis-associates (—A) (\(3100\text{–}3080\) and \(3210\pm10\ \mathrm{cm^{-1}}\)) and with trans-associates (—B) (\(3334\pm15\) and \(3328\pm18\ \mathrm{cm^{-1}}\)), to which, by analogy with amides [9], we assign structures A and B.
Davis and Hallam [5] suggest that in primary amides predominantly cyclic trimers are formed, although the possibility of dimer formation is not excluded.
Since urea derivatives contain a primary amide group, they probably can also form similar associates.
The region 1700–700 cm\(^{-1}\). The urea absorption band at \(1685 \pm 5\), associated with \(\nu\)C–O [7] and with the sum of \(\nu\)C–O and \(\delta\)NH\(_2\) vibrations [10], is shifted to the low-frequency region to \(1655 \pm 5\) for (I) and (II) and \(1660 \pm 10\) cm\(^{-1}\) for (III), which is probably due to an increase in the strength of the hydrogen bonds upon association through the C–O groups.
Fig. 1. Infrared absorption spectra and structure of urea derivatives
Although, according to [10], the contribution of the \(\delta\)NH\(_2\) vibrations to the \(1685 \pm 5\) cm\(^{-1}\) band is greater than the contribution of \(\nu\)C–O, this band was assigned by Urbanski [7] only to \(\nu\)C–O, since the O-alkylureas of type (IV) that he studied do not contain this band, despite the presence of NH\(_2\) groups,
\[ \begin{array}{c} \mathrm{NH}\\[-2mm] \diagup \\[-1mm] \mathrm{C{-}OR}\\[-1mm] \diagdown \\[-2mm] \mathrm{NH_2} \end{array} \qquad \mathrm{IV}. \]
In interpreting the 1685 cm\(^{-1}\) band in the urea derivatives (I, II, III), we adhere to the latter opinion.
The deformation vibrations of NH-urea are characterized by bands at \(1625 \pm 5\) and \(1600 \pm 5\) cm\(^{-1}\) [6, 10, 12, 13]; in urea derivatives the first disappears, the second is retained, and a new band appears at \(1555 \pm 10\), assigned by Becher [11] to \(\nu_{\mathrm{as}}\ \mathrm{C}<_{\mathrm{N}}^{\mathrm{N}}\). In the range 1460–700 cm\(^{-1}\) bands arise, most of which are absent from the spectrum of urea; they are due either to vibrations of the benzene ring or to alkyl groups. The band at \(1330 \pm 10\) cm\(^{-1}\) is also new and characterizes \(\delta\)NH, which is absent in urea [11]. Simultaneously with the appearance of this band in (I, II, III), the \(1000 \pm 10\) cm\(^{-1}\) band of urea disappears. It may be assumed that the latter passes into the band at \(1330 \pm 10\) cm\(^{-1}\) and characterizes not only \(\delta\)NH, but also \(\nu_s\) CN\(_2\) and \(\nu\)(C–N) [10, 12].
The spectra of compounds (I, II, III) in benzene and chloroform solutions indicate the presence of absorption of free NH\(_2\) and NH groups in the region 3505–3000 cm\(^{-1}\):
\[ \nu\mathrm{NH_2}\ 3505 \pm 10\ \mathrm{cm}^{-1}\quad \nu\mathrm{NH}\ 3435\ \mathrm{cm}^{-1}\quad \nu\mathrm{NH_2}\ 3405 \pm 10\ \mathrm{cm}^{-1}. \]
In the region below 1700 cm\(^{-1}\) the following absorption bands are observed: \(1695 \pm 5\) cm\(^{-1}\), \(\nu\)C–O of the non-associated form; \(1600 \pm 5\) cm\(^{-1}\), \(\delta\)NH\(_2\); the band at \(1555 \pm 5\) cm\(^{-1}\) is greatly reduced in size or disappears altogether. Dilute solutions absorb weakly at \(\nu < 1500\) cm\(^{-1}\), and therefore we do not dwell on this in detail. On the basis of the absorption bands obtained for NH\(_2\), NH, and CO in dilute solutions, it can be consid-
to assume that association in molecules of urea derivatives occurs only through intermolecular hydrogen bonds, which are broken in solutions with the formation of monomers.
Conclusion
The IR spectra of 24 urea derivatives in vaseline oil and in benzene and chloroform solutions have been recorded and systematized.
It has been noted that the spectra of urea derivatives (I, II, III) in vaseline oil have characteristic absorption bands similar to those of urea itself, but shifted into the low- or high-frequency region, respectively, as a result of strengthening or weakening of intermolecular association.
On the basis of the IR spectra obtained for urea derivatives (I, II, III), the formation of associates of various types has been proposed: cis and trans with respect to the NH group, as well as dimers and trimers with respect to the NH$_2$ group.
From the IR spectra of dilute solutions it has been established that urea derivatives in the solid state are associated through intermolecular hydrogen bonds.
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