When does LiAlH4 act as a base and when does it act as a nucleophile?

  • Thread starter MechRocket
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Main Question or Discussion Point

When you throw in LiAlH4 with a carboxylic acid, you always see the reaction being written out as the Hydride ion attacking the carbonyl function.

But why can't it just de-protonate the OH?

Also, can't LAH also deprotonate alpha carbons of carbonyl functions that are pretty acidic???

Why do we always see it acting as a nucleophile rather than a base when we throw it in with carbonyls? De-protonation is much faster than nucleophilic attack isn't it?
 

Answers and Replies

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When you throw in LiAlH4 with a carboxylic acid, you always see the reaction being written out as the Hydride ion attacking the carbonyl function.

But why can't it just de-protonate the OH?

Also, can't LAH also deprotonate alpha carbons of carbonyl functions that are pretty acidic???

Why do we always see it acting as a nucleophile rather than a base when we throw it in with carbonyls? De-protonation is much faster than nucleophilic attack isn't it?
Where have you seen LAH acting as a nucleophile for acids?
 
the proper charge in AlH4 (-) is on the aluminium not the hydrogen. Essentially the negative charge density on the hydrogen is too low for them to act as a base. This isn't to say it won't, but that the nucleophilicity of the aluminium is high enough that the acid-base reaction is of low importance.
This is the reason we use reagents like Aluminium hydride and Borohydride: because they're allow a sort of nucleophilic addition of hydride.
 
SpectraCat
Science Advisor
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When you throw in LiAlH4 with a carboxylic acid, you always see the reaction being written out as the Hydride ion attacking the carbonyl function.

But why can't it just de-protonate the OH?

Also, can't LAH also deprotonate alpha carbons of carbonyl functions that are pretty acidic???

Why do we always see it acting as a nucleophile rather than a base when we throw it in with carbonyls? De-protonation is much faster than nucleophilic attack isn't it?
In addition to the other comments, I would add that you are right that proton exchange is usually (much) faster than other chemical processes, in cases where there is a clear proton donor and acceptor. However, in this case, the Al in the AlH4- is coordinatively saturated ... in other words, there is no place for another (5th) proton to coordinate, so although there may be proton donors around, there really isn't a proton acceptor. On the other hand, hydrides can form fairly strong "dihydrogen bonds" .. these are like normal H-bonds, except the partially negatively charged hydride interacts with the partial positively charged proton. I suspect the reason this doesn't happen to a significant extent for AlH4- is (as noted by a previous poster), the negative charge resides primarily on the aluminum center.
 

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