Page 234 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

GROUP 16 ELEMENTS: THE CHALCOGENS
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−
Se Se
Se + Se Se
Ph − +
Se CI P Ph
Ph − Cl CI P Ph P (6.43)
Ph Ph Ph
CI P Ph Ph
Ph Ph
Ph

For the Se chain to grow, we need to somehow clip off one of the P’s. Observing that the
final product is Ph PCl , a chloride ion seems clearly indicated as the nucleophile needed
2
3
for this purpose:
Se Se
Se Se Se Se
+ +
Ph P Ph CI Ph Ph
CI P Ph P P Ph
− Ph Ph CI − Ph Ph
CI Ph Ph
(6.44)
− Se
Se Se
+
− Ph PCl 2 P Ph
3
Ph
Ph

The Se chain now has an anionic Se atom at one end, which can carry out a nucleo-
3
philic attack on SeCl . Proceeding in this manner, we can build up a Se chain, as shown
2
8
below:
− Se
Se Se Se Se
+ Cl Se Se
Cl Se Ph − + Ph
P
Ph Cl P Ph
Cl
Ph Ph
Se
Se Cl
Se
Se Se Se Ph
Se Se P
Ph
Cl Ph
(6.45)
Closure of the Se ring clearly has to involve a Se-on-Se attack, the last step being a
8
chloride-induced cleavage of Ph PCl :
3 2