Lewis acid

[chiddler]

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Which is not a lewis acid?

A. Cl3CH (Ka = 8.0 x 10^-9)
B. H3CNO2 (Ka = 8.2 x 10^4)
C. NaCH3
D. BF3

Answer: C.

I need help with A and B. I think my understanding of what a lewis acid is is off. They accept electron pairs, yes? So why are A and B lewis acids? They donate hydrogens easily (though A is a weak acid) but that doesn't say much about electron acceptance or donation.

Thanks very much.

 

[syoung]

Gotta draw it out i think.

EDIT:
NaCH3 has no lone pairs nor can it accept lone pairs. hence it is not a lewis acid

Cl3CH is an awesome lewis acid, because H+ will dissociate and H+ is a Lewis acid

BF3 is easy, The B is deficient and can accept another lone pair

not sure for CH3NO2, but i'm assuming since it gave you a Ka, it is a weak acid, therefore lewis acid.

 

[SaintJude]

Hmm, I actually have trouble with the definition as well. Is it legitimate to solve the answer by noting that A, B, D are all composed of very electronegative atoms & EWgs that will thus readily accept electrons? That's how I solved it, but I wonder if that's just coincidence.

 

[cloak25]

In A and B the central atom is attached to electronegative terminal atoms and therefore carries a partial positive charge. This makes them more susceptible to attack by a lewis base, making them lewis acids. Syoung is right on why C is not a lewis acid.

http://www2.chemistry.msu.edu/courses/CEM812/HSAB_Theory.pdf

 

[chiddler]

Thanks very much for the responses. I like that pdf.