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

[MechRocket]

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?

 

[sjb-2812]

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?

 

[pseudophonist]

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]

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.

 

[LudusRex]

LiAlH4 (lithium aluminum hydride) is a versatile reagent that can act as both a base and a nucleophile, depending on the reaction conditions and reactants involved.

In the presence of a carboxylic acid, LiAlH4 typically acts as a nucleophile, attacking the carbonyl group and reducing it to an alcohol. This is because the carbonyl carbon is more electrophilic than the hydroxyl proton, making it a more favorable target for nucleophilic attack.

However, in the absence of a carbonyl group, LiAlH4 can act as a base and deprotonate an alcohol or other acidic functional groups. This is because the hydride ion (H-) is a strong base and can readily abstract a proton from a sufficiently acidic hydrogen.

It is also possible for LiAlH4 to deprotonate alpha carbons of carbonyl functions, as these carbons can be quite acidic. However, this reaction is typically slower and less favorable compared to nucleophilic attack on the carbonyl group.

In general, the reactivity of LiAlH4 as a base or a nucleophile depends on the relative electrophilicity of the reactants and the reaction conditions. In many cases, nucleophilic attack on the carbonyl group is favored due to the higher electrophilicity of the carbonyl carbon. However, in certain cases where the carbonyl group is not present or is less electrophilic, deprotonation by LiAlH4 may occur instead.