Page 191 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

5B.6 ALKALI-INDUCED DISPROPORTIONATION OF PHOSPHORUS 171

REVIEW PROBLEM 5B.9
Using SAM as the methyl donor, use arrow pushing to rationalize a methylation step
in the Challenger mechanism.

REVIEW PROBLEM 5B.10
Explain the final step of the Challenger mechanism, namely the reduction of tri-
methylarsine oxide to trimethylarsine, using arrow pushing:

Me AsO + 2RSH → Me As + RS−SR + H O
2
3
3

5B.6 ALKALI-INDUCED DISPROPORTIONATION OF PHOSPHORUS

Switching gears, we’ll now turn to a purely chemical topic, with no obvious biological
connection. White phosphorus, which consists of P tetrahedra, is the most commonly
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encountered and used form of phosphorus. It’s also the most reactive, and is protected
from air by storing under water. Like many nonmetals, white phosphorus reacts with hot
alkali, disproportionating to phosphine (PH ) and sodium hypophosphite (NaH PO ). The
3 2 2
reaction is thus useful for the synthesis of these two compounds:

P + 3NaOH + 3H O → PH + 3NaH PO (5B.26)
4 2 3 2 2
Just by looking at the balanced equation, it’s difficult to get an insight into the mechanism.
But recall the strategy we recommended in Chapter 1: Think like a lone pair! Think about
what the likely nucleophile is and where it should attack. You’ll quickly see that there aren’t
many choices. Hydroxide appears to be the only reasonably good nucleophile around and
P is a logical electrophile, with strained P–P bonds as promising leaving groups:
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O H
H H
P P
− (5B.27)
− +
HO P P P P HO
HO
P P

–
On ring opening, the departing P center picks up a proton from water, producing the first
P–H bond. The process repeats itself to create phosphine, PH .
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