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Topic: Why isn't NaH used for carbonyl reduction?  (Read 13198 times)

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Offline chiddler

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Why isn't NaH used for carbonyl reduction?
« on: February 28, 2012, 04:24:19 PM »
Rather than LiAlH4 or NaBH4, why not use NaH?

Offline Wastrel

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Re: Why isn't NaH used for carbonyl reduction?
« Reply #1 on: February 28, 2012, 06:34:03 PM »
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.

Offline g-bones

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Re: Why isn't NaH used for carbonyl reduction?
« Reply #2 on: February 28, 2012, 07:57:10 PM »
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?

Offline Arctic-Nation

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Re: Why isn't NaH used for carbonyl reduction?
« Reply #3 on: February 29, 2012, 12:39:25 PM »
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.

Offline chiddler

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Re: Why isn't NaH used for carbonyl reduction?
« Reply #4 on: February 29, 2012, 09:38:54 PM »
well i'm satisfied with the responses.

thanks very much!

Offline opsomath

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Re: Why isn't NaH used for carbonyl reduction?
« Reply #5 on: March 02, 2012, 01:45:49 PM »
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.

Offline Mobius1988

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Re: Why isn't NaH used for carbonyl reduction?
« Reply #6 on: March 02, 2012, 08:42:49 PM »
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"??

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