What is the pKa value of the nitrogen atoms, specifically the amine groups?

How would you estimate the $\mathrm pK\_\mathrm a$ values of the nitrogen atoms in 1,4-dimethyl-1,2,3,4-tetrahydroquinoxaline?


[![1,4-dimethyl-1,2,3,4-tetrahydroquinoxaline](https://i.stack.imgur.com/vNaIE.png)](https://i.stack.imgur.com/vNaIE.png)


I struggle with it because I am unsure whether to classify them as tertiary amines (with a $\mathrm pK\_\mathrm a$ of around 9) or whether they can be considered as aromatic amine (similar to aniline), which would result in a lower $\mathrm pK\_\mathrm a$ value (around 5ish?) due to the delocalisation of the aromatic ring.


Can someone explain to me which is the correct idea? I really struggle with the concept.




The nitrogen atoms are attached to the aromatic ring, and this placement is the dominant factor. The conjugate acid formed by protonating either nitrogen atom would be expected to have a $\mathrm pK\_\mathrm a$ similar to that of protonated aniline, not a protonated aliphatic amine.


We might expect this constant to be modified by the second nitrogen atom. Let us explore this.


The second nitrogen atom actually has two conflicting influences:


* Electron-withdrawing inductive effect, which would decrease the conjugate acid $\mathrm pK\_\mathrm a$ (stronger conjugate acid, weaker base). We expect this to be strongest in the ortho position as in the structure given in the original problem.
* Electron-releasing mesomeric effect, which would increase the conjugate-acid $\mathrm pK\_\mathrm a$ (weaker conjugate acid, stronger base). As with similar effects acting through a benzenoid ring, this would be stronger in the *ortho* and *para* positions, weaker in the *meta* position.


Wikipedia gives the following conjugate-acid $\mathrm pK\_\mathrm a$ values:





| Amine | Structure | $\mathrm pK\_\mathrm a$ |
| --- | --- | --- |
| [Aniline](https://en.wikipedia.org/wiki/Aniline) | [aniline](https://i.stack.imgur.com/8ZgDf.png) | 4.63 |
| [*o*-Phenylenediamine](https://en.wikipedia.org/wiki/O-Phenylenediamine) | [o-phenylenediamine](https://i.stack.imgur.com/LxMi5.png) | 4.57 |
| [*m*-Phenylenediamine](https://en.wikipedia.org/wiki/M-Phenylenediamine) | [m-phenylenediamine](https://i.stack.imgur.com/wczxq.png) | 5.11 |
| [*p*-Phenylenediamine](https://en.wikipedia.org/wiki/P-Phenylenediamine) | [p-phenylenediamine](https://i.stack.imgur.com/VFE4V.png) | 6.31 |



Compared with the aniline base case, the *ortho*-diamine (matching the arrangement of nitrogen atoms in the original problem) shows very nearly the same acid-base balance, as the two effects described above appear to balance out. The *meta* arrangement gives a slightly stronger base, but it is the *para* isomer that shows a clearly stronger basic behavior: the mesomeric electron donation is relatively strong with this arrangement while the induction effect is minimal.


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