Molar heat capacities and an infrared study of 2-amino-5-methylpyridine

Introduction

2-Amino-5-methylpyridine (AMP; molecular formula: C₆H₈N₂; CAS Registry No. 1603-41-4) is an important intermediate compound which is widely used in medicine, dye and pesticide industries.  As early as 1978, 2-amino-5-methylpyridine (Figure 1) was reported as initial materials, from which a new kind of anticoccidiostat was prepared. At present, 2-amino-5-methylpyridine is used widely since it has been successfully employed as a basic organic compound in a wide range of synthetic processes. In medicine area, 2-amino-5-methylpyridine was used to synthesize medicine for treating or preventing diseases associated with e.g. anxiety, sleep disorders, insomnia and fibrosis. In chemical synthesis, 2-amino-5-methylpyridine was regarded as a kind of reactant, from which researchers can get some new compounds for further study. However, the properties of 2-amino-5-methylpyridine itself have received little attention, probably because it is common and even seems to count for little. During our literature search, only one paper was found to be associated with its property-photo induced aminoimino tautomerism. Therefore, in order to improve the process of chemical synthesis and increase understanding of the properties of this common but important compound, the study on thermodynamic properties of the substance is necessary. Heat capacity is one of the most fundamental thermodynamic properties of substances.

1.Molar heat capacities and standard molar enthalpy 

In this paper, the low-temperature heat capacity of the organic compound over the temperature range (80 to 398 K) was measured by an automated adiabatic calorimeter. A solid–liquid phase transition was observed and the enthalpy and the entropy of the phase transition were determined based on the heat capacity measurements. The standard molar enthalpy of formation was determined by oxygen-bomb combustion calorimetry. The heat capacities (Cp,m) of 2-amino-5-methylpyridine (AMP) were measured by a precision automated adiabatic calorimeter over the temperature range from 80 to 398 K. A solid–liquid phase transition was found in the range from 336 to 351 K with the peak heat capacity at 350.426 K. The melting temperature (Tm), the molar enthalpy (△_fusH⁰_m ), and the molar entropy (△_fusS⁰_m) of fusion were determined to be 350.431±0.018 K, 18.108 kJ/mol and 51.676 KJ/mol, respectively. The mole fraction purity of the sample used was determined to be 0.99734 through the Van't Hoff equation. The thermodynamic functions (HT–H298.15 and ST–S298.15) were calculated. The molar energy of combustion and the standard molar enthalpy of combustion were determined,△Uc(C₆H₈N₂,cr)= –3500.15±1.51 kJ/mol and △cHm0 (C₆H₈N₂,cr)= –3502.64±1.51kJ/mol, by means of a precision oxygen-bomb combustion calorimeter at T=298.15K. The standard molar enthalpy of formation of the crystalline compound was derived, △_fH0_m(C₆H₈N₂,cr)= –1.74±0.57 kJ/mol.[1]

2.An Infrared Study of 2-Amino-5-methylpyridine

The photoreaction of 2-amino-5-methylpyridine was investigated by matrix-isolation infrared spectroscopy and densityfunctional theory (DFT) calculation. Photoinduced reversible amino (N=C-NH₂)-imino (NH-C=NH) tautomerism was found between 2-amino-5-methylpyridine and 5-methyl-2(1H)-pyridinimine; the amino tautomer changes to the imino tautomer by UV irradiation (340>λ≥300 nm) and the reverse change occurs by longer-wavelength light irradiation (420>λ≥340 nm). The results of the CASSCF calculation revealed that the amino-imino tautomerism proceeds in vibrational relaxation process from electronic excited state to the ground state. The IR spectra of 2-amino-5-methylpyridine in the T1 state and 5-methyl-2-pyridinamino radical were also obtained by UV irradiation (λ≥300 nm).

The tautomerism occurs in vibrational relaxation process from electronic excited state to the ground state, because the UV excitation energy is approximately comparable to the S1-S0 transition energy and is lower than the tautomerism barriers in the S1 and T1 states. In the UV irradiation for the amino tautomer, the tautomerism being a proton (hydrogen atom)migration assisted by the intramolecular hydrogen bond proceeds, whereas the non-hydrogen bonded hydrogen-atom dissociation produces a 5-methyl-2-pyridinamino radical that reverts to the initial amino tautomer by annealing procedure at 28 K. The reverse tautomerism from the imino to amino tautomer accompanying the ring cleavage of the imino to produce the ketenimine compound occurs by longer-wavelength light irradiation (420 >λ≥340 nm). In addition, a photoinduced transient species was found during the UV excitation and was identified as the amino tautomer in the T1 state.[2]

References

[1]ZHANG, J. N., TAN, Z. C., MENG, Q. F., et al. Molar heat capacities and standard molar enthalpy of formation of 2-amino-5-methylpyridine[J]. Journal of thermal analysis and calorimetry,2009,95(2):461-467. DOI:10.1007/s10973-008-9267-9.

[2]Akai N, Harada T, Shin-Ya K, Ohno K, Aida M. Photoinduced amino-imino tautomerism: an infrared study of 2-amino-5-methylpyridine in a low-temperature argon matrix. J Phys Chem A. 2006;110(18):6016-6022. doi:10.1021/jp056290t