Reactions of arenes: Electrophilic aromatic substitution

1 Mar

Characteristically, the reagents that react with the aromatic ring of benzene and its derivatives are electrophiles. Electrophilic reagents add to alkenes. A different reaction takes place when electrophiles react with arenes. Substitution is observed instead of addition. If we represent an arene by the general formula ArH, where Ar stands for an aryl group, the electrophilic portion of the reagent replaces one of the hydrogens on the ring:

We call this reaction electrophilic aromatic substitution; it is one of the fundamental processes of organic chemistry.

The first step is rate-determining. In it a carbocation forms when the pair of  π-electrons of the alkene is used to form a bond with the electrophile. Following its formation, the carbocation undergoes rapid capture by some Lewis base present in the medium. The first step in the reaction of electrophilic reagents with benzene is similar. An electrophile accepts an electron pair from the π-system of benzene to form a carbocation:

The carbocation formed in this step is a cyclohexadienyl cation. Other commonly used terms include arenium ion and σ-complex. It is an allylic carbocation and is stabilized by electron delocalization which can be represented by resonance.

Most of the resonance stabilization of benzene is lost when it is converted to the cyclohexadienyl cation intermediate. In spite of being allylic, a cyclohexadienyl cation is not aromatic and possesses only a fraction of the resonance stabilization of benzene. Once formed, it rapidly loses a proton, restoring the aromaticity of the ring and giving the product of electrophilic aromatic substitution.

NITRATION OF BENZENE

Now that we’ve outlined the general mechanism for electrophilic aromatic substitution, we need only identify the specific electrophile in the nitration of benzene to have a fairly clear idea of how the reaction occurs. The electrophile (E+) in this reaction is nitronium ion.

SULFONATION OF BENZENE

The reaction of benzene with sulfuric acid to produce benzenesulfonic acid:

Among the variety of electrophilic species present in concentrated sulfuric acid, sulfur trioxide is probably the actual electrophile in aromatic sulfonation. We can represent the mechanism of sulfonation of benzene by sulfur trioxide by the sequence of steps:

HALOGENATION OF BENZENE

According to the usual procedure for preparing bromobenzene, bromine is added to benzene in the presence of metallic iron (customarily a few carpet tacks) and the reaction mixture is heated.

The active catalyst is not iron itself but iron(III) bromide, formed by reaction of iron and bromine. Iron(III) bromide is a weak Lewis acid. It combines with bromine to form a Lewis acid- Lewis base complex:

FRIEDEL-CRAFTS ALKYLATION OF BENZENE

Alkyl halides react with benzene in the presence of aluminum chloride to yield alkyl-benzenes.

Alkylation of benzene with alkyl halides in the presence of aluminum chloride was discovered by Charles Friedel and James M. Crafts in 1877. Crafts, who later became president of the Massachusetts Institute of Technology, collaborated with Friedel at the Sorbonne in Paris, and together they developed what we now call the Friedel-Crafts reaction into one of the most useful synthetic methods in organic chemistry. Alkyl halides by themselves are insufficiently electrophilic to react with benzene. Aluminum chloride serves as a Lewis acid catalyst to enhance the electrophilicity of the alkylating agent. With tertiary and secondary alkyl halides, the addition of aluminum chloride leads to the formation of carbocations, which then attack the aromatic ring.

FRIEDEL-CRAFTS ACYLATION OF BENZENE

Another version of the Friedel-Crafts reaction uses acyl halides instead of alkyl halides and yields aryl ketones.

The electrophile in a Friedel-Crafts acylation reaction is an acyl cation (also referred to as an acylium ion. Acyl cations form by coordination of an acyl chloride with aluminum chloride, followed by cleavage of the carbon-chlorine bond:


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