IR Spectra of Some 4-Aminoantipyrine Derivatives

M. M. Kaganskii, V. E. Chistyakov, G. M. Stepanova, E. V. Schmidt,
I. V. Schmidt

(Presented by the scientific-methodological seminar of the Faculty of Chemical Technology)

A large number of works have been devoted to the spectral study of 5-pyrazolones [1–9]; however, we have found no data in the literature on the infrared spectra of acyl derivatives of 4-aminoantipyrine.

The present work is devoted to consideration of the IR spectra of acyl derivatives of 4-amino- and 4-methylaminoantipyrine, synthesized earlier by some of us [10–12], with the general formula

\[ \begin{array}{c} \text{4-aminoantipyrine derivative with substituents } R_1 \text{ and } \mathrm{COR'} \end{array} \]

where \(R_1 = \mathrm{H};\ \mathrm{CH_3};\ \mathrm{COR'}\)—aliphatic or aromatic acid residues with various substituents in the radical or in the ring.

Experimental Part

The work was carried out on a two-beam UR-10 spectrophotometer. Samples were prepared as suspensions in vaseline oil. Recording was performed with NaCl and LiF prisms.

Discussion of Results

The characteristic frequencies of the IR absorption bands of the compounds studied are presented in the table.

The most complex spectral pattern, due to the presence in the molecules of antipyrilamides of an exocyclic carbonyl group

Table 1

Characteristic absorption bands of 4-aminoantipyrine derivatives (frequency in cm\(^{-1}\))

and amide groupings, is observed in the absorption regions of stretching vibrations of C–O and \(NH\).

In the region \(1710\text{–}1620\ \mathrm{cm}^{-1}\), antipyrilamides give two intense absorption bands caused by vibrations of the keto group of the ring and the carbonyl of the secondary amide (Amide I). The keto group of the heterocycle of antipyrilamides of fatty acids absorbs at \(1695 \pm 8\ \mathrm{cm}^{-1}\), while the Amide I band is found in the region \(1652 \pm 10\ \mathrm{cm}^{-1}\) (Nos. 1–5). In antipyrilamides of substituted aromatic acids (Nos. 6–37), the absorption band of the keto group shifts to \(1671 \pm 12\ \mathrm{cm}^{-1}\).

The position of the carbonyl absorption bands of substituted benzantipyrilamides is affected to a certain extent by the character and position of the substituent in the phenyl radical of the acid residue. The keto group of unsubstituted benzantipyrilamide (No. 6) absorbs at \(1680\ \mathrm{cm}^{-1}\). A noticeable shift to the low-frequency region (by \(10\text{–}15\ \mathrm{cm}^{-1}\)) is observed when halogen atoms are introduced into the benzoyl radical in the meta and para positions (Nos. 11, 15), and when methoxy and amino groups are introduced in the para position (Nos. 31, 35).

An especially strong shift of both carbonyl absorption bands to the low-frequency region occurs when hydrogens in the aroyl radical are replaced by hydroxy groups. For compounds Nos. 19, 25–28, coalescence of these bands into one broad band with a maximum in the region \(1639 \pm 8\ \mathrm{cm}^{-1}\) is characteristic. The exceptions are compounds Nos. 14, 21, 24, in which only the Amide I band undergoes a shift to \(1635\ \mathrm{cm}^{-1}\), and two distinct maxima are indicated in the spectrum. This fact apparently can be explained by the preferential formation in these compounds of an intramolecular hydrogen bond between the hydroxyl group in the ortho position and the carbonyl of the amide group. The occurrence of this bond prevents the formation of an intermolecular C–O...HO bond of the heterocycle.

In the absorption region \(3500\text{–}2800\ \mathrm{cm}^{-1}\) are located absorption bands associated with stretching vibrations of the amide group.

For secondary amides, depending on the type of hydrogen bond, trans and cis associates are possible. The trans associate is assigned an absorption band at \(3330\text{–}3280\ \mathrm{cm}^{-1}\), and the cis associate—\(3200\text{–}3160\ \mathrm{cm}^{-1}\) [13–18].

For antipyrilamides of fatty acids, in which the NH absorption bands are observed at \(3240\ \mathrm{cm}^{-1}\), a trans-associated state may be assumed.

For most antipyrilamides of aromatic acids, on the contrary, absorption in the region \(3157 \pm 22\ \mathrm{cm}^{-1}\) [cis associate] is more characteristic. In the spectra of some benzantipyrilamides with meta and para substituents in the aroyl radical, two bands are found in this region at \(3157 \pm 26\ \mathrm{cm}^{-1}\) and \(3277 \pm 48\ \mathrm{cm}^{-1}\). It may be assumed that in the latter case there is an equilibrium mixture of trans and cis associates.

Since the stretching vibrations of the bonded OH group lie in the same region (\(3317 \pm 24\ \mathrm{cm}^{-1}\)), interpretation of the nature of the association of oxybenzantipyrilamides is difficult.

Conclusions

1. IR spectra are presented for 38 acyl and acyl-\(N\)-methyl derivatives of 4-aminoantipyrine.

2. The behavior of the absorption bands belonging to the stretching vibrations of C–O and NH bonds is discussed as a function of the character of the acyl residue.

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