[INeedSerotonin]

Which one of these cannot be considered a Lewis acid?

(A) Fe3+
(B) BF3
(C) HCN
(D) CH3+
(E) CH4 <---- this is the answer

Hello

Could you guys please help me solve this? I know that a Lewis base donates electrons, and a Lewis acid receives it, but I'm stuck here.

I know that the options with a + sign can be acids, because they can receive electrons, and HCN is an acid according to Arrhenius--and therefore according to Lewis too--, so we have BF3 and CH4.

Any hints?

[Babcock_Hall]

Think about the Lewis octet rule.

[INeedSerotonin]

Think about the Lewis octet rule.

Can I say that CH4 is already stable, while BF3 can still receive electrons?

For example, boron has three electrons on its valence shell. It makes 3 covalent bonds with fluorine, so boron still needs 2 electrons to become stable like a noble gas. So it can receive electrons, which makes it possible for it to be an acid.

Is this correct?

[Babcock_Hall]

That is pretty much it, but you could identify the orbital in BF₃ that is empty if you like.  Can CH₄ receive any more electrons?

[INeedSerotonin]

That is pretty much it, but you could identify the orbital in BF₃ that is empty if you like.  Can CH₄ receive any more electrons?

I think that I should use the Pauling's rule to find that orbital. Linus Pauling's diagram goes like 1s2 2s2 2p6, and boron has 3 electrons on its valence shell, so its configuration is 1s² 2s² 2p¹. Sharing three electrons with fluorine, it becomes 1s2 2s2 2p4. So I think that the empty orbital is the orbital "p", and it still needs two electrons.

I think that CH4 cannot receive any more electrons.

[Babcock_Hall]

If we use hybrid orbitals, we would say that boron has three sp² orbitals that are involved with bonds to fluorine.  This leaves one unhybridized p orbital that is empty.  You are correct that CH₄ cannot receive more electrons.