- Addition of halogen to the double bond takes place when propene reacts with
bromine or chlorine at low temperatures.
+
lowtemperature
Propene
H 2 C CH CH 3 X 2
XX
CCl H^2 C CH CH^3
(addition reaction)^4
- Substitution occurs when propene reacts with bromine or chlorine at very high
temperatures or under very low halogen concentration.
+ or low cohightemnc. of Xperature HX
H 2 CCPropeneCH CH 3 2 H 2 C HCH 2 +
(substitution reaction)
X 2 X
- Allylic substitution: any reaction in which an allylic hydrogen atom is
replaced.
Allylic hydrogen atoms
CC
H H
HCH
H H
13.2A ALLYLIC CHLORINATION (HIGH TEMPERATURE)
- Propene undergoes allylic chlorination when propene and chlorine react in the gas
phase at 400°C ––– the “Shell Process”.
+ Cl 2 400 Cl HCl
oC
gas phase +
Propene
H 2 C CH CH 3 H 2 C CH CH 2
3-Chloropropene
(allyl chloride)
- The mechanism for allylic substitution:
- Chain-initiating Step
Cl Cl hν 2 Cl
2) First Chain-propagating Step
