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In Cannizaro reaction when concentration of OH- is less in the rds step hydride shift occurs, though it is very difficult for hydride ion to shift yet the reaction proceeds but what is the driving force here??

My teacher told me that this mechanism was proposed because the hydride shift occurs but there must be some reason as to why the hydride shift?

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Hydride shift occurs because in the intermediate dianion there is no other suitable leaving group. It is either $\ce{H-}$, $\ce{R-}$ or $\ce{O^{2-}}$. Of those $\ce{H-}$ is the best leaving group. Note though that even under these conditions $\ce{H-}$ will of course not be solvated on its own (it is way too unstable for that). It is directly transferred from the dianion to the aldehyde.

This reaction will go "to the right" since the hydride shift is irreversible. There is simply no mechanism to form a dianion from the alcohol and carboxylic acid.

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