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Which of the following will be more basic, hydrazine $\ce{H2N-NH2}$ or ammonia $\ce{NH3}$?

My chemistry teacher said that $\ce{NH3}$ is more basic since after giving $\ce{H+}$ to hydrazine results in $\ce{H3N+-NH2}$, and accommodation of the lone pair of $\ce{-NH2}$ is not possible by $\ce{-N+H3}$, since it doesn't have the space. So instead, $\ce{-NH2}$ will pull electrons from it making it unstable. Ammonia has no such problem so it must be more basic.

I am not so pleased with this argument. In $\ce{H3N+-NH2}$, although the lone pair cannot be accommodated, but the positive charge present on its sides , to an extent, should neutralize the intensity of the lone pair, making it somewhat stable. And also, not to forget, hydrazine has two spots where we can get the electrons, therefore, its ambident nature should also support it's basicity.

I need a bit of clarity on this.

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4 Answers 4

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Obligatory link to Evans' pKa table.

Ammonia is more basic than hydrazine, by about one order of magnitude. This is expected, because the -NH2 group is more electronegative than -H or -CH3.

The second lone pair is not involved in the acid-base reaction, it does not point towards the -NH4+ group. You can, however, force two lone pairs into close proximity. This destabilizes the unprotonated form. The keyword is "proton sponge".

1,8-Bis(dimethylamino)naphthalene has a pKa of 12.3, it's one of the strongest known amine bases. Compare that to the pKa of aniline, which is something like 4.5.

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    $\begingroup$ Ok, I get yours and my teachers point, but please elaborate on why I am wrong. $\endgroup$
    – Rohinb97
    Oct 2, 2014 at 7:46
  • $\begingroup$ How exactly were enol pKas determined? $\endgroup$
    – Dissenter
    Feb 7, 2015 at 0:08
  • $\begingroup$ What about the alpha effect? Will that not enhance the basicity of hydrazine? $\endgroup$ Feb 11, 2017 at 19:05
  • $\begingroup$ Please dont give wrong pka values. The pka of the conjugate base of acid is 4.5, and not that of aniline. $\endgroup$ Jul 1, 2021 at 9:07
  • $\begingroup$ What about nucleophilicity? I guess hydrazine is better. $\endgroup$ Jul 19, 2022 at 11:39
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When protonated, ammonia and hydrazine give their conjugated acids:

$$\ce{NH3 + H3O+ <=> H4N+ + H2O} \tag1$$ $$\ce{H2N-NH2 + H3O+ <=> H3N^+-NH2 + H2O} \tag2$$

Let's rewrite these conjugate acids: $\ce{H3N^+-H}$ and $\ce{H3N^+-NH2}$ . We all know that electran withdrawing ability ($-I$ effect) of $\ce{-NH2}$ group is higher than that of $\ce{-H}$ group. Therefore, $\ce{-NH2}$ group in $\ce{H3N^+-NH2}$ destabilizes the positive charge more than $\ce{-H}$ group in $\ce{H3N^+-H}$. In other words, conjugate acid of $\ce{NH3}$ is more stable than that of $\ce{H2N-NH2}$. As a consequence, forward reaction of equation $(1)$ is favor than that in equation $(2)$. This means basicity of ammonia is greater compared to that of hydrazine.

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  • $\begingroup$ We didnt consider the M effect of NH2 here because its not possible to donate right @MathewMahindaratne ? $\endgroup$
    – Orion_Pax
    Mar 29, 2022 at 11:15
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I think these could be the reason-

For the first part,

...positive charge present on its sides , to an extent, should neutralize the intensity of the lone pair, making it somewhat stable

This is not possible because $\ce{NH_3^+}$(no vacant orbital) doesn't have any space at all to get involved with the lone pair.

...and the second part

...hydrazine has two spots where we can get the electrons, therefore, its ambident nature should also support it's basicity.

Abel already answered that at one time only one $\ce{-NH_2}$ takes part when we determine basicity and the second $\ce{-NH_2}$ plays no role.

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  • $\begingroup$ To clarify the first part, I am not saying that the electrons will jump to the protonated nitrogen. I'm saying that the presence of a positive charge near the electrons will try to reduce its intensity and make it somewhat stable. For the second point you made, more number of nucleophilic sites would mean more chances of attack of an $H^+$, which adds to the basicity of Hydrazine. $\endgroup$
    – Rohinb97
    Apr 1, 2015 at 8:46
  • $\begingroup$ You shouldn't compare the basicity of Hydrazine as a molecule. Just because it has two basic sites, it will not be more basic. It is akin to saying that just because Sulphuric acid has two acidic hydrogens, it is a stronger acid than Perchloro-acid, which is untrue. Remember, in any case, there will be only ONE protonation at a time. You should compare either Ka1, or Ka2, with the corresponding values for Ammonia. $\endgroup$
    – Lexicon
    Apr 1, 2015 at 14:17
  • $\begingroup$ I'm just saying that the probability of attack, and did not mean that it decreases it's $pK_b$ value. $\endgroup$
    – Rohinb97
    Apr 1, 2015 at 23:13
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Ammonia is more basic than hydrazine if you look at the neighbors you will see $NH_3$, and $NH_2-NH_2$ where Ammonia has hydrogen as third neighbor where hydrazine have N as neighbors which gives more strong - I effect, after protonation.

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