[tashkent]

Greetings everyone!

I've encountered a confusing problem in my GRE Chemistry exam recently.  The question asks for : Which of the following substances is uncommonly used as a Lewis acid?  The choices are: Aluminum chloride, aluminum bromide, ammonia and AlH3.  I think the answer should be ammonia, coz its a Lewis base.  But I fell for this question, thinking that ammonia could accept an electron, forming an amide (NH2-).  So I answered aluminum bromide, since I'm not familiar with the reaction it catalyzes.  I'm also having doubts about LiAlH3, since it can accept an electron, forming LiAlH4.

Are my observations correct?  I would appreciate if someone can shed light on this problem.  I believe I mixed up everything...

Regards,
Tashkent

[Mitch]

All are very good lewis acids except ammonia.

[jdurg]

How can LiAlH₃ accept an electron and become LiAlH₄?  In order to make that conversion, it would have to accept a hydrogen atom which is NOT an electron.  Also, if ammonia accepts an electron it would become NH₃- and NOT NH₂-.  In order to become NH₂- it would need to lose a positively charged hydrogen atom.  If you're getting these things confused, then you'll have a really tough time on the GRE.  

[maxyoung]

ammonia can accept an electron? there will be nine e's around nitrogen
it will violate the octect rule

[Fat Elvis]

If you wrote it the way it came in the gre, "which is uncommonly used as a Lewis acid?" Uncommonly is kind of a weird adverb. Aluminum chloride and bromide are both commonly used as Lewis acids. Alane (AlH3) isn't used for much of anything, much less as a Lewis acid (it is a LA, though). It's normally used as a reducing agent or hydride transfer agent. So I'd say it's uncommonly used as a LA. Ammonia, on the other hand, is never used as a LA, because it's in fact a Lewis base. But I'm sure that the answer they're looking for is NH3.

NH3 can act as a hydrogen bond donor (and also as an acceptor), so in that respect it sort of has Lewis acidic character. But that's a bit of a stretch. The vacant orbital in that case is the N-H anti-bonding orbital.

[Donaldson Tan]

a lewis acid is an electron pair acceptor.

[agrobert]

what about ammonia in a lewis adduct, for example with iodine, I2

[AWK]

what about ammonia in a lewis adduct, for example with iodine, I2

Ammonia reacts with iodine. The so called ammonia adduct is in fact actuct NI3 with NH3

http://en.wikipedia.org/wiki/Nitrogen_triiodide