[chiddler]

Rather than LiAlH4 or NaBH4, why not use NaH?

[Wastrel]

I think part of the answer is that sodium hydride is a much better base than it is a reducing agent, so molecules tend to condense.

[g-bones]

I believe its because the hydrogen has its electrons in an S orbital which, by definition, is non directional, which makes it a terrible nucleophile.  LAH, on the other hand, has covalent bonds to the hydrogen, imparting directionality to the electrons of the Al-H bond, making them more nucleophilic.  Anybody else care to weigh in?

[Arctic-Nation]

Clayden's Organic Chemistry says it's because hydride's 1s orbital is too small to efficiently overlap with the carbonyl π* orbital. NaBH4 and LiAlH4 react through their σ orbitals, which are comparable in size to the π*.

The implication is that hydride can much more effectively overlap with low-energy C-H σ* orbitals.

[chiddler]

well i'm satisfied with the responses.

thanks very much!

[opsomath]

In more practical terms, NaH will simply deprotonate it at the alpha hydrogen - it does not reduce. I  have done this reaction, it is useful when you have a recalcitrant enolate.

[Mobius1988]

In more practical terms, NaH will simply deprotonate it at the alpha hydrogen - it does not reduce. I  have done this reaction, it is useful when you have a recalcitrant enolate.

I apologise for what is probably a really simple question but what do you mean by "recalcitrant enolate"??