[azmanam]

I don't know how I feel about the mechanism of LAH reduction of an amide generally proposed in undergrad textbooks.  I've always lived by the mantra 'never draw a positive charge in a basic mechanism/negative charge in acidic mechanism' (except for the counter ion... but we don't care about that part).  We don't kick out RO- under acidic conditions, nor have an alcohol directly attack an unactivated carbonyl because we'd draw O- in the intermediate. 

The step I have problems with is the expulsion of the aluminum hydroxide in the first reduction step.  That generates a carbocation/iminium ion intermediate.  I'm real uncomfortable drawing a carbocation in a basic mechanism.  I'm also uncomfortable with ^-O-Al^-H₃ as a leaving group with a 2^- charge.

Am I being too meticulous or do others share my concerns?  I'm teaching this mechanism in class in a few days and don't want to sound like a doofus.  Please advise!

[azmanam]

I like this mechanism much better, which completely avoids positive charges... but I've never seen it drawn this way.  Not in my textbook, not in another textbook, not on the MSU e-text[1], not on the UCalgary site[2]...

[1] http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/crbacid2.htm
[2] http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch20/ch20-3-4-2.html

[Doc Oc]

I like the 2nd one better as well, but I doubt that many of the undergrads will care one way or the other.  Most of them will groan about having to learn another mechanism.

[stewie griffin]

I don't have the correct answer for you but I too was always bothered by the mechanism typically taught here. I like the second mech better.
Perhaps if it were a more advanced class you could actually explain the merits/drawbacks of each mechanism, but for a typical 2nd yr orgo class I assume most won't ever notice the positive charge in basic media problem.

[stewie griffin]

Azmanam, what happens though when you have two alkyl groups on the amide nitrogen? Your second mechanism wouldn't be applicable then.
Is there a reason why can't we just postulate a direct S_N2 by hydride on the intermediate aminal.

[azmanam]

Yeah, I thought about that after my op, too.  Secondary amides couldn't be deprotonated.  But I agree, I like an S_N2-like attack of hydride displacing the ^-OAlH₂.

Thanks for the thoughts, all. 

[movies]

Don't forget about the Li in your mechanism.  The amide is almost certainly coordinated to Li before the first hydride addition (not the reactivity difference between LiAlH₄ and NaAlH₄).  You could then consider that the leaving group might be the aggregate of Li–O–AlH₃.

I am skeptical of the direct S_N2 displacement.  It's pretty sterically demanding.