[Ilaria]

Dear all,

I would like to understand why after the reduction of an amide with LiAlH₄ in THF the reaction is quenched with 5% aqueous NaOH and not just with water. Does this have to do with the dangerous reactivity of LiAlH₄ towards water or with the formation of emulsions? What happens and what is formed when you quench with water and when you quench with NaOH?
Thank you in advance!

[kriggy]

AFAIK by quenching with just water you get some nasty aluminium residues. By using hydroxide you convert them to soluble stuff

[clarkstill]

Not quite to make them soluble, they generally crash out as a nice white powder you can filter off when you use the Feiser workup:

http://chem.chem.rochester.edu/~nvd/pages/magic_formulas.php?page=aluminum_hydride_reduction

Otherwise, on anything but a tiny scale you get all sorts of horrible aluminium emulsions that make layer separation in the workup virtually impossible.

[Dan]

I generally workup with Rochelle's salt (even though it can take a long time for the phases to separate) because I always end up with a mountain of MgSO₄ with the Feiser workup.

http://www.ochemonline.com/Quenching_aluminum_based_reactions

[rolnor]

When using NaOH in the workup, it forms sodium aluminate that is easy to filter of probably?

[Reddart]

Personally, I prefer a mixture of 1:1 Glaubler's salt : celite. A majority of the time I can just filter the mixture over celite and get pure, quantitative product after solvent removal.