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Common Dimer - Responsible for the Trends in pKaHs of Both Pyridazenium Cations and Di-cations: An Educational Perspective

Sanjeev Rachuru, Pandiri Sreedhar, D. A. Padmavathi, V. Jagannadham*

Department of Chemistry, Osmania University, Hyderabad 500007, India

Abstract: A species formed by dimerization from monoprotonated 1,2-pyridazine i.e., 1,2-pyridazenium cation that was optimized by DFT calculations is identified as a common dimer to explain the systematic changes in the pKaHs of both Pyridazenium cations and Pyridazenium di-cations.

Keywords: Pyridazenium cation dimer, pKaHs, 1,2-pyridazine, 1,3-pyridazine, 1,4-pyridazine

Received December 14, 2022; Revised January 22, 2023; Accepted February 05, 2023

Citation: Sanjeev Rachuru, Pandiri Sreedhar, D. A. Padmavathi, V. Jagannadham, Common Dimer - Responsible for the Trends in pKaHs of Both Pyridazenium Cations and Di-cations: An Educational Perspective. World Journal of Chemical Education, 11 (1). 4-6. January 2023. doi:10.12691/wjce-11-1-2

1. Introduction

The acid-base properties of the three 1,2-, 1,3- and 1,4-pyridazenes and their cations are well studied by DFT calculations by our group [1]. In the present work, we have taken up the study of the trends in pKaHs of both Pyridazenium cations and Pyridazenium di-cations. A common dimeric species is identified to explain the systematic changes in the pKaHs of Pyridazenium cations and Pyridazenium di-cations. The present study constitutes an excellent one-hour classroom lecture for graduate students and young researchers.

2. Methods

All the chemical structures are drawn using ChemDraw. The pKaH values of the monoprotonated and di-protonated conjugate acids of the three pyridazines are from reference [2].

3. Discussion

The structures of the three pyridazines, equilibrium dissociation of their conjugate acids of both cations and di-cations with pKaH values are given in Table 1. Looking at the pKaH of these compounds, position of the second nitrogen changes the pKaH of the conjugate acids of mono-protonated species systematically. They decrease nearly by a factor of half each time (Table 1) [2].

It is known that there will be a small loss of basic strength or gain of acidic strength upon introduction of second nitrogen in nitrogen containing compounds. This is due to high electronegativity of nitrogen and doesn’t tend to easily donate its lone pair of electrons which is a characteristic property of a Lewis base. The fact that the pKa of NH₃ falls from 9.3 to 8.5 in hydrazine (NH₂=NH₂) is a glaring example [3]. However, the loss of base strength upon introduction of second nitrogen in six membered heterocycles like pyridazines is appreciable. Again, pyridine is an example whose pKaH is 5.23 [4] falls to 2.24 in 1,2-pyridazine. This is primarily due to the high electronegativity of nitrogen and weakens the base strength. And another striking factor is that pyridazinium ion with two adjacent nitrogens can form hydrogen-bonded dimer as shown in Figure 1 (structure X). The dimeric form is optimized and well established from our DFT studies [1]. This dimeric form makes the pyridazinium ion more stable than the neutral parent pyridazine or mono-protonated pyridazine hence cannot lose the proton easily. Consequently, the species becomes a weak acid. This in turn results in higher pKaH. This kind of dimer formation is not possible for 1,3- and 1,4-pyridazine.

The di-protonated species however became even stronger acid than the mono-protonated species with high negative pKaH values (Table 1) [2]. This is due to loss of electron accepting capacity of second nitrogen on protonation and the di-protonated species becomes less stable and more acidic compared to the mono-protonated species. The two positive charges on two adjacent nitrogens in 1,2-pyridazenium di-cation may experience strong mutual repulsion. This makes the di-cation less stable compared to the mono-cation and tend to lose a proton easily. Therefore, the di-cation becomes a stronger acid with lower pKaH. In the case of di-cations of 1,3 and 1,4-pyridazenes there will be an increase in distance between two positive charges and may attenuate the repulsive forces. Thus, making these di-cations little weaker acids with higher pKaH compared to the di-cation of 1,2-pyridazene (Table 1).

Table 1. Dissociation Equilibriums of pyridazenium cations and di-cations

Figure 1.

The common dimmer (Figure 1, X) that is responsible for explaining the higher acidic property (lower pKaH) of 1,2-pyridazenium cation and di-cation compared to their other structural analogues of 1,3- and 1,4-pyridazenium cations (Table 1). From our earlier work on DFT studies on the structure X [1] of Figure 1 that the species is well characterized in terms of the difference in the H-bond energy of the monomer cation and dimer cation which is found to be -6.50 kcals/mole. And as such the 1,2-pyridazenium di-cation (structure B, Figure 1) cannot form any kind of dimer like the one X, due to lack of free nitrogen. It can only lose two protons to give 1,2-pyridazenium cation (structure A, Figure 1) which in turn can form the dimer X.

4. Conclusions

Theoretically well characterized structure X is the common dimer for both the 1,2-pyridazenium cation and di-cation to explain the changes in pKaH.

References

[1]   | Sanjeev Rachuru, D. A. Padmavathi, R. Ravi, V. Jagannadham and Adam A. Skelton, Z. Physikal. Chem., Vol. 236, 2022, page 1617-1626.
[2]   | D. D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution, IUPAC, Butterworths, London, 1965 and 1972.
[3]   | G. Bredig, Z. Physikal. Chem., 1894, 13, 289.
[4]   | Adrien Albert, Reginald Goldacre and John Phillips, J. Chem. Soc., 1948, 2240-2249.