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In reduction of carbonyls (aldehyde and ketones) to alcohols using NaBH4/Methanol, alkoxides are formed with the Boron which usually gets protonated on addition of water/acid.

My question is what is the function of methanol in this reaction as methanol reacts with NaBH4 to form sodiumtetramethoxyborate and hydrogen gas.

Some sources say methanol protonates the alkoxide ion and after doing series of reduction experiments I find that the alkoxide formed only gets protonated to an alcohol on addition of water.

So what does methanol do in this reaction/what is the effect of methanol?

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    $\begingroup$ I would say it is a solvent for the reaction. $\endgroup$ – Nilay Ghosh Aug 1 '17 at 14:21
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    $\begingroup$ MeOH does react with NaBH4 but that reaction is slow at ice-bath temperature and the reaction with carbonyls is faster. $\endgroup$ – Waylander Sep 20 '18 at 9:46
  • $\begingroup$ whenever we mix NaBH4 and MeOH gas bubbles come out due to formation of H2. So, it does react with MeOH $\endgroup$ – Suchi Ghosh Jan 11 at 4:16
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I believe methanol does protonate the alkoxide. The resulting methoxide coordinates to $\ce{BH3}$ forming $\ce{(MeO)BH3-}.$ The electron donating character of the methoxy group makes the remaining protons more hydride which results in a more reactive hydride source.

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  • $\begingroup$ Interesting, but a reference maybe? $\endgroup$ – Oscar Lanzi Jan 11 at 1:44
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You need methanol to solve NaBH4, other solvents are not very suitable. The active reducing species can be NaBH4 or NaOCHxHx (in which NaBH4 has first reacted with methanol)

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