p K aH of the Two Conjugate Acids (CH 2 N 5+ ) and (HN 6+ ) of Two Hypothetical Molecules the Pentazine (CHN 5 ) and the Hexazine (N 6 ) of Azabenzene Series: A Chemical Education Perspective

Extrapolation of a simple straight-line graph is used to calculate the p K aH of two cations (CH 2 N 5+ ) and (HN 6 + ) (eqn. 1 and 2) of two hypothetical molecules pentazine (CHN 5 ) and hexazine (N 6 ). This is achieved by simply an extrapolation of the locus of the plot of p K aH 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 p K aH values for these two species were found from the extrapolation of the locus of the plot of p K aH versus average ionization potential (I v /eV) of the neutral azines. This article is useful in graduate research classroom to explain the acid-base properties and to determine the p K aH values.


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 pK aH of their conjugate acids are known [6,7]. The last two compounds the pentazine (XI, CHN 5 ) and the hexazine (XII, N 6 ) are two hypothetical molecules [8,9]. They were neither yet synthesized nor yet are pK aH values of their cations known. In the present work the estimation of their pK aH values is taken up.

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.

Results and Discussion
This paper substantiates the determination of pK aH of the two conjugate acids (CH 2 N 5 + ) and ( HN 6 + ) of two hypothetical molecules of azabenzene series the pentazine (CHN 5 ) and the hexazine (N 6 ). This is in continuation of our earlier work on the determination of pK a of pentazole (of N(1)-H acidity) molecule [10] by extrapolation method of the locus of the plot of pK a versus number of nitrogen atoms and by DFT calculations again a hypothetical molecule not yet synthesized and theoretical determination of pK a s 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 pK a values of azoles decrease systematically with number of nitrogen atoms.
In the present work we tried the estimation of pK aH of the two conjugate acids (CH 2 N 5 + ) and ( HN 6 + ) in two different methods. It is noteworthy that for every nitrogen added to the ring system decreases the pK aH systematically by 3-4 units ignoring 1,2-and 1,4-pyridazines in the sequence (Table 1). Even using pK aH of 1,2-and 1,4-pyridazines with 1,3-pyridazine gives an average pK aH of 1.33 (Table 1). This is close to the pK aH of 1,3-pyridazine (1.23) which is originally used to see the systematic change of pK aH 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 CHN 5 and N 6 more stable than the cations CH 2 N 5 + and HN 6 + . Hence is the decrease in pK aH . A plot of pK aH 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 pK aH of -9.49 and -13.14 for the two conjugate acids (CH 2 N 5 + ) and (HN 6 + ) respectively. From the Table 1 for every nitrogen added to the ring system the mean ionization potential (I V /eV) increases systematically by a factor of 0.80 eV. Or taking the average I V (10.43 eV) of 1,2-, 1,3-and 1,4-pyridazines, the systematic change in I V is 0.88 eV ( Table 1). A plot of mean ionization potential (I V /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 (I V /eV) 12.85 and 13.66 for pentazin (CHN 5 ) and hexazin (N 6 ) respectively. A plot of pK aH versus mean ionization potentials (I V /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 (I V /eV) 12.85 and 13.66 gave pK aH of -8.84 and -12.35 for pentazinium cation (CH 2 N 5 + ) and hexazinium cation (HN 6 + ) respectively. The difference of less than one pK aH unit in pK aH 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 pK a values of various carbon acids [13]. Probably to our knowledge HN 6 + is the worlds strongest positively charged nitrogen acid. Average pKaH of the three compounds with two nitrogen atoms in the ring system is 1.

Notes
The authors don't have any competing financial interest.