lewis acid and base

[604boo]

Hi guys
I am little bit confused. I know this can be a stupid question but, how do you identify which one is lewis acid or base?
I know that lewis acid is "acceptor" and base is "donor" but how do you know if AlCl3 or BF3 is acid or not?
Thank you for your help~!

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[poc91nc]

Hi guys
I am little bit confused. I know this can be a stupid question but, how do you identify which one is lewis acid or base?
I know that lewis acid is "acceptor" and base is "donor" but how do you know if AlCl3 or BF3 is acid or not?
Thank you for your help~!

In the case of the above mentioned lewis acids...VSEPR works pretty well

AlCl3...Al has an electron config of 1s2 2s2 2p6 3s2 3p1
according to VSEPR...the compound is sp2...therefore there is an empty p orbital that can accept electrons, or hybridize to give an sp3 config

In the case of AlCl4- (which is what happens if R-Cl: is present, think benzene alkylation) ....the compound would become sp3

The same line of reasoning works for BF3....another lewis acid is FeCl3 (recognize this from ochem?)

For periods 1-3...VSEPR seems to work nicely...although there are acceptions like H-S-H...according to VSEPR the bond angles should be slightly lower than 109.5...but they're 90 degrees (I think)...therefore the model doesn't work.

That is the way I look at it...if anything I said is incorrect please correct me.

 

[poc91nc]

Also...notice that boron and aluminum are in the same group. Both will have an empty p orbital.

The same goes for Gallium. Halides of aluminum, boron, and gallium are lewis acids of the form BX3, AlX3, GaX3.