I understand that adding borane to an alkene will produce a hydroborane. Addition of -OOH will form an alcohol.

My lecture notes state that addition of borane to:

  • A carboxylic acid will form an alcohol
  • An amide will form an amine
  • An aldehyde/ketone will form an alcohol
  • A nitrile will form an amine
  • An epoxide will form an alcohol
  • An alkene will form an alkane
  • An alkyne will form an alkene

I can't find the mechanisms for these anywhere. I'd appreciate seeing a few of them, namely carboxylic acid to alcohol, amide to amine, epoxide to alcohol and alkene to alkane. I should then be able to have a good crack at working out the rest.


2 Answers 2


Henry Rzepa gives a mechanism in which two borane molecules attach to the carboxyl group of a carboxylic acid. The boron atoms are attached to the electronegative but electron-rich portions of the carboxyl group (meaning the oxygen atoms) and then transfer hydride ion moieties to the carbon while taking away one of the oxygens. This exchange of hydrogen for oxygen reduces the carboxyl carbon. Other carbonyl compounds may be assumed to react similarly.


I highly recommend Organic Chemistry by Clayden, Greeves and Warren. They do an unusually good job of drawing correct arrow-pushing mechanisms.

  • Ch 24, p.619 has the mechanism for acid and amide reductions and a table comparing reactivity of various carbonyl groups.

  • Ch 47, p. 1281 has the mechanism for hydroboration of alkenes and alkynes

The un-catalyzed reduction ketones, aldehydes, and epoxides with simple diborane (B2H6) are not synthetically useful. However, catalyzed versions like the Corey-Bakshi-Shibata Reduction serve to illustrate the mechanistic concept.


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