How phosphorus tribromide and phosphorus triiodide can form by using only two molecules of red phosphorus of bromine and iodine respectively?
Dear Student,
Chloroalkane can be formed easily with the help of PCl3.For example when ethanol reacts with PCl3 ,it forms chloroethane.The reaction is as follows:
3CH3CH2OH+PCl3------>3CH3CH2Cl+H3PO3
But we cannot form Bromoalkane or Iodoalkane with the above method because PBr3 and PI3 are not stable compounds. Hence this reaction takes place in the presence of red phosphorus where this red phosphorus forms freshly prepared PBr3 and PI3 by reacting with bromine or iodine.
2P + 3I2 → 2PI3
R--OH reacts in the presence red phosphorus/Br2 to form R-Br(bromoalkane).
CH3CH2OH+red P/Br2------> CH3CH2Br
R--OH reacts in the presence red phosphorus/I2 to form R-I(Iodoalkane).
CH3CH2OH+red P/I2------> CH3CH2I
R in the above examples is alkyl group
Regards
Chloroalkane can be formed easily with the help of PCl3.For example when ethanol reacts with PCl3 ,it forms chloroethane.The reaction is as follows:
3CH3CH2OH+PCl3------>3CH3CH2Cl+H3PO3
But we cannot form Bromoalkane or Iodoalkane with the above method because PBr3 and PI3 are not stable compounds. Hence this reaction takes place in the presence of red phosphorus where this red phosphorus forms freshly prepared PBr3 and PI3 by reacting with bromine or iodine.
PBr3 is prepared by treating red phosphorus with bromine. An excess of phosphorus is used in order to prevent formation of PBr5.Equation is as follows where 2 molecules of red phosphorus is being used.
2P + 3Br2 → 2PBr3
2P + 3I2 → 2PI3
R--OH reacts in the presence red phosphorus/Br2 to form R-Br(bromoalkane).
CH3CH2OH+red P/Br2------> CH3CH2Br
R--OH reacts in the presence red phosphorus/I2 to form R-I(Iodoalkane).
CH3CH2OH+red P/I2------> CH3CH2I
R in the above examples is alkyl group
Regards