Halogenation of Benzene

Common reactions include chlorination and bromination, which are the most common halogenation reactions involving electrophilic substitution. Others that will react this way include those things that are electrophilic as well. Some of these are included as follows:

• Halogenation—the addition of heat and chlorine or bromine gas (plus a catalyst) to yield bromobenzene or chlorobenzene

• Nitration—this involves the addition of heat and HNO3 to yield nitrobenzene (C6H5NO2) plus water

• Sulfonation—this involves heat plus SO3 and H2SO4, leading to benzenesulfonic acid (C5H5SO3H).

• Alkylation (the Friedel-Crafts reaction)—this takes heat plus an R-Cl molecule to lead to an arene or alkyl side chain added to a benzene molecule plus

HCl.

• Acylation (the Friedel-Crafts reaction)—this takes heat plus RCO-Cl to lead to an aryl ketone, which is a benzene molecule plus a C-OR side chain.

How do these types of electrophilic substitution reactions occur? It is likely a two-step process in which the electrophile forms a sigma bond with the benzene ring first. Secondly, a proton is removed from the benzene molecule, leading to a substitution of an electrophile with hydrogen. There is a benzenonium intermediate that has both the hydrogen atom and the electrophile attached for a brief period of time. This will have a temporary higher energy state that will become lower when the hydrogen ion is lost.

HALOGENATION OF BENZENE

While halogenation works for bromine and chlorine gas, it does not work as well for iodine and fluorine. Iodine is unreactive with benzene unless it is mixed with copper (II) chloride, making iodine more electrophilic. Fluorine is too reactive, on the other hand, so it cannot be used to fluorinate benzene directly. There are chemicals containing fluorine that work better than fluorine alone.