Which is more basic, hydrazine or ammonia?

Question

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.

Answer 1

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.

Answer 2

Obligatory link to Evans' pK_a 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 pK_a of 12.3, it's one of the strongest known amine bases. Compare that to the pK_a of aniline, which is something like 4.5.

Answer 3

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.

Answer 4

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.