Question

How many molar equivalents of hydride does sodium borohydride contain?

Answer 1

1 mol of sodium borohydride contains 4 mol of hydride.

Explanation:

The borohydride reduction of an aldehyde or ketone involves the addition of two hydrogen atoms.

`"RCHO"color(white)(l) stackrelcolor(blue)("1. BH"_4^"-"color(white)(m)) (→) "RCH"_2"OH"`
`color(white)(mmmmll)stackrelcolor(blue)("2. H"^+color(white)(m))(color(white)(m))`

However, only one of the `"H"` atoms comes from the borohydride.

We can write the major steps as

`"4RCHO" + "BH"_4^"-" → ("RCH"_2"O")_4"B"^"-"`

This is where 1 mol of `"BH"_4^"-"` provides the 4 mol of `"H"^"-"`.

The hydrolysis with aqueous acid provides the proton on the `"OH"` group.

`("RCH"_2"O")_4"B"^"-" + "H"_3"O"^+ + "2H"_2"O" → "4RCH"_2"OH" + "H"_3"BO"_3`