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Recently, I was looking at the chapter on chemoselectivity in Clayden et al.'s Organic Chemistry (2nd ed.). On p. 530, it is mentioned that sodium borohydride reduction only occurs in protic solvents or in the presence of electrophilic metal ions, such as $\ce {Li+}$ and $\ce {Mg^2+}$ while the more powerful reducing agent lithium aluminium hydride does not require protic solvents. Why is that the case?

Reference

Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.

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    $\begingroup$ Principally solubility. Sodium borohydride is essentially insoluble in aprotic solvents. $\endgroup$ – Waylander Aug 10 '18 at 17:51
  • $\begingroup$ @Waylander What about the need for the ions? These ions help to increase polarity of solvent, increasing the solubility of NaBH4? $\endgroup$ – Tan Yong Boon Aug 11 '18 at 1:28
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    $\begingroup$ Li and Mg cations act as Lewis acids coordinating to carbonyl oxygen, Na is much weaker in this respect. $\endgroup$ – Mithoron Aug 11 '18 at 17:54
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There are two reasons:- 1. LAH is too reactive. So it can't exist in protic solvent whereaa NaBH4 can. 2. Protic solvent hydrolyse Na+ so that BH4- can show reducing action(as it a weak reducing agent).

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  • $\begingroup$ What about the presence of metal cations? It is an "either... or... " condition. Thus, NaBH4 can be used in an aprotic solvent with metal cations. Why is that so? $\endgroup$ – Tan Yong Boon Aug 13 '18 at 13:59

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