Question

What is the role of lewis acid in preparation of aryl bromides and chlorides in dark?

Answer 1

Direct chlorination or bromination of benzene or other aromatic hydrocarbon gives chloroarenes and bromoarenes. These reactions are carried out in the presence of Lewis acids such as ferric or aluminium halides (FeCl, FeBr,AlCl3) in the dark, at ordinary temperatures (310-320 K).The reaction is electrophilic aromatic substitution.The halogen itself is not a stronger electrophile for the attack of benzene ring.The Lewis acid promotes the electrophilic aromatic substitution by polarizing the halogen molecule. Actually it generates a better electrophile for the attack of benzene ring .The reaction mechanism is shown below.
The AlCl3 is lewis acid (electron deficient) . It attracts the lone pair electrons of halogen and acquire -ve charge.The donating halogen acquire +ve charge and inturn it pull the electron of terminal halogen make it electron deficient (electrophile) then pi electron attack from benzene ring takes place.Activated electrophile acts at normal temperature.

Answer 2

Direct chlorination or bromination of benzene or other aromatic hydrocarbon gives chloroarenes and bromoarenes. These reactions are carried out in the presence of Lewis acids such as ferric or aluminium halides (FeCl, FeBr,AlCl3) in the dark, at ordinary temperatures (310-320 K).The reaction is electrophilic aromatic substitution.The halogen itself is not  a stronger electrophile for the attack of benzene ring.The Lewis acid promotes the electrophilic aromatic substitution by polarizing the halogen molecule. Actually it generates a better electrophile for the attack of benzene ring .The reaction mechanism is shown below.
The AlCl3 is lewis acid (electron deficient) . It attracts the lone pair electrons of halogen and acquire -ve charge.The donating halogen acquire +ve charge and inturn it pull the electron of terminal halogen make it electron deficient (electrophile) then pi electron attack from benzene ring takes place.Activated electrophile acts at normal temperature.