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pKaH of the Two Conjugate Acids (CH₂N₅⁺) and (HN₆⁺) of Two Hypothetical Molecules the Pentazine (CHN₅) and the Hexazine (N₆) of Azabenzene Series: A Chemical Education Perspective

R. Sanjeev, R. Ravi, D. A. Padmavathi, V. Jagannadham*

Department of Chemistry, Osmania University, Hyderabad-500007, India

Abstract: Extrapolation of a simple straight-line graph is used to calculate the pKaH of two cations (CH₂N₅⁺) and (HN₆⁺) (eqn. 1 and 2) of two hypothetical molecules pentazine (CHN₅) and hexazine (N₆). This is achieved by simply an extrapolation of the locus of the plot of pKaH of protonated pyridine, pyridazines, s-triazine and 1,2,4,5-tetrazine versus the number of nitrogen atoms of the cyclic azines. Even well matched pKaH values for these two species were found from the extrapolation of the locus of the plot of pKaH versus average ionization potential (Iv/eV) of the neutral azines. This article is useful in graduate research classroom to explain the acid-base properties and to determine the pKaH values.

Keywords: pKaH, azines

Received June 20, 2022; Revised July 25, 2022; Accepted August 03, 2022

Citation: R. Sanjeev, R. Ravi, D. A. Padmavathi, V. Jagannadham, pKaH of the Two Conjugate Acids (CH₂N₅⁺) and (HN₆⁺) of Two Hypothetical Molecules the Pentazine (CHN₅) and the Hexazine (N₆) of Azabenzene Series: A Chemical Education Perspective. World Journal of Chemical Education, 10 (3). 105-109. January 2022. doi:10.12691/wjce-10-3-3

1. Introduction

A total of 12 azines can be formulated on successive substitution of each sp2 carbon (=CH-) of benzene by nitrogen atoms (scheme 1). Several reviews appeared in literature about synthesis of these azines [1,2,3,4,5]. One of the two meanings of azines is, in heterocyclic chemistry a class of six-membered aromatic ring compounds. And the other class is N-N linked diimines. The aromatic azines are the compounds containing one nitrogen (pyridine) to six nitrogen atoms (hexazine) [3]. The compounds with one to four nitrogen atoms, pyridine (I), pyridazines (II), (III), (IV), s-triazine (VII) and 1,2,4,5-tetrazine (X) and the pKaH of their conjugate acids are known [6,7]. The last two compounds the pentazine (XI, CHN₅) and the hexazine (XII, N₆) are two hypothetical molecules [8,9]. They were neither yet synthesized nor yet are pKaH values of their cations known. In the present work the estimation of their pKaH values is taken up.

2. Methods

All the linear correlations were done using the Kaleida Graph software, Version 4.1 for windows, Reading, PA, USA. The chemical structures are drawn using chemdraw.

Scheme 1.

3. Results and Discussion

This paper substantiates the determination of pKaH of the two conjugate acids and of two hypothetical molecules of azabenzene series the pentazine (CHN₅) and the hexazine (N₆). This is in continuation of our earlier work on the determination of pKa of pentazole (of N(1)-H acidity) molecule [10] by extrapolation method of the locus of the plot of pKa versus number of nitrogen atoms and by DFT calculations again a hypothetical molecule not yet synthesized and theoretical determination of pKas of P(1)-H Phospholes [11]. This is due to the zeal we got from a small note in the Hand Book of Heterocyclic Chemistry by Katrizsky et.al [12] in the context of the explanation of the effect of aza substitution on the N(1)-H acidity which shows how the pKa values of azoles decrease systematically with number of nitrogen atoms.

In the present work we tried the estimation of pKaH of the two conjugate acids and in two different methods. It is noteworthy that for every nitrogen added to the ring system decreases the pKaH systematically by 3-4 units ignoring 1,2- and 1,4-pyridazines in the sequence (Table 1). Even using pKaH of 1,2- and 1,4-pyridazines with 1,3-pyridazine gives an average pKaH of 1.33 (Table 1). This is close to the pKaH of 1,3-pyridazine (1.23) which is originally used to see the systematic change of pKaH by 3-4 units for every nitrogen added to the ring system. This is due to the high electronegativity of nitrogen which decreases the N-H bond energy and makes the proton to dissociate easily. This makes the neutral base CHN₅ and N₆ more stable than the cations and Hence is the decrease in pKaH. A plot of pKaH versus number of nitrogen atoms yields a good straight line with a correlation coefficient of 0.9872 (Figure 1). On extrapolation of the locus of this plot to number of nitrogen atoms 5 and 6 gave pKaH of -9.49 and -13.14 for the two conjugate acids and respectively. From the Table 1 for every nitrogen added to the ring system the mean ionization potential (IV/eV) increases systematically by a factor of 0.80 eV. Or taking the average IV (10.43 eV) of 1,2-, 1,3- and 1,4-pyridazines, the systematic change in IV is 0.88 eV (Table 1). A plot of mean ionization potential (IV/eV) versus number of nitrogen atoms yields a good straight line with a correlation coefficient of 0.9953 (Figure 2). On extrapolation of the locus of this plot to number of nitrogen atoms 5 and 6 gave mean ionization potentials (IV/eV) 12.85 and 13.66 for pentazin (CHN₅) and hexazin (N₆) respectively. A plot of pKaH versus mean ionization potentials (IV/eV) gave a good straight line with a correlation coefficient of 0.9704 (Figure 3). On extrapolation of the locus of this plot to mean ionization potentials of (IV/eV) 12.85 and 13.66 gave pKaH of -8.84 and -12.35 for pentazinium cation and hexazinium cation respectively. The difference of less than one pKaH unit in pKaH values obtained in the present work (see rows 7 and 8 and last column of Table 1) from two independent methods is not unexpected. Even large differences were experienced between experimental and theoretical pKa values of various carbon acids [13]. Probably to our knowledge is the worlds strongest positively charged nitrogen acid.

Table 1. Mean Ionization Potentials (Iv/eV) and pKaH values of azinesa

Figure 1. pKa versus number of nitrogen atoms of azines

Figure 2. Plot of mean ionization potential (IV/eV) versus number of nitrogen atoms in the azines

Figure 3. Plot of pKa versus mean ionization potential (IV(n)/eV)

Notes

The authors don’t have any competing financial interest.

References

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[2]   | D. L. Boger, Chem. Rev. 1986, 86. 781-793.
[3]   | Anirban Panda, Croat. Chem. Acta 86 (4) (2013) 545-553.
[4]   | Nurullah Saracoglu, Tetrahedron, 63 (2007) 4199-4236.
[5]   | J. Safari and S. Gandomi-Ravandi, RSC Adv., 2014, 4, 46224.
[6]   | J. Spanget-Larsen, J. Chem. Soc. Perkin Trans. 11 1985, page 417.
[7]   | F. Brogli, E. Heilbronner and T. Kobayashi, Helvetica Chimica Acta - L701.55, Fasc. 1 (1972)- Nr. 30, page 274.
[8]   | Hurst, Derek T. (1996). “Other Tetrazines and Pentazines”. Comprehensive Heterocyclic Chemistry II. pp. 957-965. https://en.wikipedia.org/wiki/Pentazine.
[9]   | J. Fabian and E. Lewars, Can. J. Chem. 82: 50-69 (2004).
[10]   | R. Sanjeev, Jagannadham Vandanapu, and Adam A. Skelton, Australian Journal of Chemistry, 74(8) 584-590, (2021).
[11]   | R. Sanjeev, Jagannadham Vandanapu, and Adam A. Skelton, Australian Journal of Chemistry, https://www.publish.csiro.au/CH/justaccepted/CH21122.
[12]   | Handbook of Heterocyclic Chemistry, Elsevier, 3rd Edition, 2010, page 146 and 147, By Alan R. Katritzky, Christopher A. Ramsden, John A. Joule, Viktor V. Zhdankin.
[13]   | I. E. Charif, S.M. Mekelleche, D. Villemin, and N. Mora-Diez, Journal of Molecular Structure: THEOCHEM, 818 (2007) 1-6.