CH610B Sophomore Organic II

Dr. Brian Pagenkopf

Hydrides, Reducing agents, and Sources of Nucleophilic H:¯

Above are electrostatic potential surfaces that explain a great deal about the reactivity of NaBH₄. The best way to think about NaBH₄ is as an H:¯ atom (a so-called hydride ion) bonded to the Lewis acid BH₃, as indicated above on the right. This way of thinking helps explain why the NaBH₄ reagent reacts by donating a nucleophilic H:¯ atom to electrophiles such as the carbon atom of carbonyls. On the left is shown the actual structure of BH₄¯; notice the overall negative charge (red color). On the right for comparison is BH₃, showing a partial positive charge (blue color) on the boron atom and a partial negative charge (red color) on the hydrogen atoms, consistent with the electronegativies of those types of atoms. The partial positive charge on boron explains its Lewis acid (electron pair seeking) properties, i.e. why it forms such a nice complex with the negatively-charged oxygen atom of the tetrahedral intermediate.

However, a surprising point to remember is that BH₄¯ does not behave chemically much like BH3 at all! For example, BH₄¯ does not readily react with double bonds (hydroboration).

Sources include http://www.cm.utexas.edu/academic/courses/Spring2001/CH610B/Iverson/index.html