[Rutherford]

How to determine from these two, what is the stronger base: NH₂^- or CH₃^-?

[XGen]

I believe basicity increases with decreasing electronegativity.

[AWK]

Think about reactions
NaNH₂ + CH₄
and
CH₃Na and NH₃

Which one is possible?



printing errors corrected

[Rutherford]

I am not sure, both of them seem little possible. Maybe the second one?

[gritch]

What you need to do is look at the pKa values of the resulting acids these would make, NH₃ and CH₄.
A stronger base should reaction to form a weaker acid, that is an acid with a very high pKa value.
The pKa value of ammonia, NH³⁺ is 36.
The pKa value of methane, CH₄ is ~60.


Source: http://chemweb.unp.ac.za/chemistry/Physical_Data/pKa_values.htm

[Rutherford]

In the answer NH₂^- is stronger (but the answer is not completely reliable). From the data you gave, it should be CH₃^-, but in the problem no data was given, so I think that it should be concluded by some other way. Still, thanks for helping.

[rrr_asd]

I think, CH₃^- is stronger base, because CH₄ is more stable than NH₃.

[Rutherford]

Yes, that was my first thought when attempting to solve this problem, but I saw in the answer NH₂^-. I thought maybe it is a weaker acid than CH₃^- but it isn't (as gritch show). Mistake in the answer.

[rrr_asd]

For two processes:
NH₂^- + H⁺ = NH₃  ΔG₁
CH₃^- + H⁺ = CH₄  ΔG₂

ΔG₁>ΔG₂
so CH₃^- is stronger.

[Rutherford]

Yes, now I am sure about it. It's only that I wasn't given any info about pKa of ΔG.