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From my textbook, the reduction of carboxylic acids by LiAlH4 "has to be in anhydrous conditions, such as dry ether, followed by aqueous acid." I read from the internet that the reduction reaction has to be in anhydrous conditions since LiAlH4 reacts violently with H2O. But why does the reduction reaction need to be 'followed by aqueous acid'? Isn't that contradictory to the fact that LiAlH4 needs to be in anhydrous conditions?

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When the reaction is done, you need to quench it to (1) destroy the remaining LiAlH4 (2) protonate the lithium alkoxide to give you the alcohol. The aluminium salts formed can be separated from the alcohol. If you don't quench the reaction it makes separation essentially impossible.

Of course, if you add the water before the reaction is done, then that defeats the purpose of using an anhydrous environment.

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  • $\begingroup$ Saturated aqueous Na2SO4 is better than aqueous acid for decomposition of excess LiAlH4 in that all of the aluminum salts are coagulated and a clear ether solution of the alcohol is formed. $\endgroup$ – user55119 Aug 31 '18 at 18:28
  • $\begingroup$ @user55119 indeed, one can do much better than just adding something like HCl (the Fieser workup seems to be fairly popular, too, although I've no personal experience with it). $\endgroup$ – orthocresol Aug 31 '18 at 18:33

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