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$\mathrm{p}K_\mathrm{a}$ of methane is $\sim 50$ and that of ammonia is $\sim 37,$ thus ammonia is a better acid than methane, which implies methane is a better base than ammonia.

But I don't see how: methane doesn't have any lone pair of electron, neither it has any $\ce{OH-}.$ How is it possible?

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    $\begingroup$ What you have stated implies that H3C- is a stronger base than HN2- $\endgroup$
    – PCK
    Feb 20, 2020 at 11:48
  • $\begingroup$ @Ivan can you explain "Acidic and basic strength of a compound are unrelated". I thought a strong acid is a weak base and vice versa, do you have some explanation article, data, examples, anything to elaborate. $\endgroup$ Feb 20, 2020 at 17:39
  • $\begingroup$ better is an adjective that is very subjective and therefore inappropriate here $\endgroup$
    – Karl
    Feb 20, 2020 at 18:57
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    $\begingroup$ It is generally also true that strong acids are weak bases. HCl loves to give its proton, and hates to take up a second one. But that is not the crucial point of that elementary undergrad lecture about acids and bases which you seem to remember partially. ;) $\endgroup$
    – Karl
    Feb 20, 2020 at 19:17
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    $\begingroup$ @KarstenTheis For all practical purposes, your "spectator ion" and a very weak base are the same thing. Saying that a strong acid has no conjugate base is however just nonsense. $\endgroup$
    – Karl
    Feb 20, 2020 at 22:29

2 Answers 2

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You got it wrong. Acidic and basic strength of a compound are unrelated, about as much so as the man's name and his weight. Within the definition you and I are currently using (Brønsted–Lowry), acidity is about having a proton on a polar bond ready to ionize, while basicity is about having electron pairs ready for accepting a proton. A compound can have one and not the other, or vice versa, or both, or neither. Why should the two be related? They shouldn't and aren't. Well, there is one obvious exception: a strong acid can't be a strong base at the same time, otherwise it would turn upon itself. Except that, everything is possible.

Indeed, methane is a very weak acid, about 13 orders of magnitude weaker than ammonia (as attested by the difference in their pKa). In fact, it is one of the weakest acids among all compounds. This is not unexpected, given its almost non-polar bonds and the lack of stabilization in the anion. Also, methane is a very weak base. Forcing it to accept a proton was no small achievement in itself (see Methanium). This is not unexpected either, given its lack of lone pairs or negative charge.

Now you must be thinking that I am pulling your leg, because every kid knows that strong acid means weak base and vice versa. It is written in every textbook, said by every teacher, and that with great confidence, too. Can they all be wrong? No, they are right. How so?

Here's the trick: strong acid does indeed mean weak base, but not in the same compound. Look again at that equilibrium: $$\ce{HA <=> H+ + A-}$$

See that? It is $\ce{HA}$ that acts as an acid here. And now look at the reverse reaction: it is $\ce{A-}$ acting as a base. Now if the equilibrium is shifted to the right, then $\ce{HA}$ is a strong acid, which inevitably means that its conjugate base $\ce{A-}$ is a weak base.

So it goes.

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  • $\begingroup$ "strong acid means weak base" should be "stronger weak acid means weaker conjugate base", which is a mouthful. It is easier to say (once these are introduced) larger Ka means smaller Kb of the conjugate base. $\endgroup$
    – Karsten
    Feb 20, 2020 at 22:20
  • $\begingroup$ Also, one must be careful about the vice-versas. A strong acid has a weak conjugate base by simple mathematics and a strong base has a weak conjugate acid also by mathematics. But the conjugate acid of a weak base can be either weak or strong, as is the case for the conjugate base of a weak acid. $\endgroup$
    – Jan
    Feb 21, 2020 at 12:26
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When A is considered a stronger acid than B, it does not follow that B is a stronger base than A. Rather, the appropriate base comparison is between the conjugate bases formed from acids A and B. Properly, the conjugate base of B is a stronger base than the conjugate base of A.

In terms of the compounds given in the question, the conjugate base of ammonia is amide ion ($\ce{NH3<=>H^+ + \color{blue}{NH2^-}}$) and the conjugate base of methane is methide ion ($\ce{CH4<=>H^+ + \color{blue}{CH3^-}}$). Therefore, if ammonia is a stronger acid than methane then methide ion is a stronger base than amide ion, regardless of how basic the initial compounds methane and ammonia might be. Which is true, as anyone knows who has tried to combine a Grignard reagent with a compound having hydrogen bonded to nitrogen.

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