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Compare the basic strengths of above two compounds.

In the first compound (1,2-dihydropyrazine), the lone pair of one N will participate in resonance but the lone pair of other N will not, similarly in pyridine, lone pair will not participate in resonance. I think that since the first compound atleast has $2$ nitrogens (and only one in pyridine), it should be slightly more basic than pyridine, but I came to know that it's the other way round. So how do we compare their basic strengths and what are their $\mathrm{p}K_\mathrm{b}$ values?

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    $\begingroup$ The additional nitrogen in 1,2-dihydropyrazine rather pulls the electrons towards itself since it is a electronegative atom causing decrease in basic nature. $\endgroup$
    – Desai
    Commented Jan 6, 2021 at 21:31
  • $\begingroup$ pKb(pyridine) = 8.77 at 25°C [CRC HANDBOOK for Physics & Chemistry; 2004 - Edition 8] $\endgroup$
    – Desai
    Commented Jan 6, 2021 at 21:36
  • $\begingroup$ Related : alfa.com/en/pyrazines/?page=2 $\endgroup$
    – Desai
    Commented Jan 6, 2021 at 21:43

2 Answers 2

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What I would do is I would compare their acid strengths. Due to inductive forces, deprotonating the 1,2-dihydropyrazine would result in a more stable compound than that in pyridine--the nearby nitrogen (electronegative) will share the burden of the negative charge. Therefore, 1,2-dihydropyrazine is more acidic and thus is less basic.

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  • $\begingroup$ is more acidic and thus is less basic. This linking is valid for conjugate acid/base pair, not for eventually amphoteric components. $\endgroup$
    – Poutnik
    Commented Apr 21, 2023 at 4:12
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First of all the first compound has two nitrogen atom so we need to know which one is more basic . One of the nitrogen atom has its lone pair involved in resonance due to which they are not fully available for donation so it is less basic nitrogen. Now since we know the more basic nitrogen in first compound we can compare the basicity between the two compounds in first compound there is a -I effect off nitrogen on the basic nitrogen atom and in the second compound there is only + I effect on the basic nitrogen atom due to which the nitrogen of first compound is slightly electron deficient for donation and second one is electron rich so the second compound is a better base.

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