Nitration of Methyl Benzoate

Theoretical Background

• Electrophilic reactions are important class of aromatic substitution organic reaction through which an atom bonded to the aromatic ring, most probably hydrogen atom, is replaced by an electrophile as shown below:

Schematic illustration of Electrophilic aromatic Substitution reaction

• Benzene rings are highly reactive towards electrophilic substitution reactions owing to the abundance of delocalized Π – electrons.
• Electrophilic substitution reactions can be classified into: aromatic nitration , aromatic halogenation, aromatic sulphonation , as well as acylation and alkylating Friedel-Crafts reactions .
• In nitration reactions, a nitro group (-NO2) will act as an electrophile that can attack the benzene ring in an electrophilic substitution reaction.
• A mixture of concentrated nitric and sulfuric acids is used as the nitrating agent as this will generate the nitronium ion, NO2 + which is a very strong electrophile that stimulates the electrophilic nitration reaction as follows:

Electrophilic nitration reaction of benzene ring

• Electrophilic nitration reaction is a multi-step reaction whose mechanism includes three main steps or stages as follows:

Mechanism of Action:

• Step 1: Generation of the Electrophile:

• Step 2: Electrophilic attack on the aromatic ring:

• Step 3: Re-aromatization:

-COOCH3 group on the benzene ring possesses two effects on the precession of the reaction. 

• First, the partial positive charge of the carbonyl carbon acts as a ring deactivator, thus the NO2+ ion as a strong electrophile can lead to the formation of dinitro-methyl benzoate. That is why the reaction is always kept in an ice bath in order to allow formation of the mono nitro derivative only. 
• Second, the -COOCH3 has a directing effect on the attacking NO2+ ion towards the meta-position.

The overall reaction of nitration of methyl benzoate experiment can be demonstrated as follows:

Principle Of Work

• The nitration of methyl benzoate lab involves introducing a nitro group at the meta position of the aromatic ring through an electrophilic substitution reaction. 
• An acidic mixture of equal volumes of nitric acid and sulfuric acid serves as the nitrating agent, generating a nitronium ion that acts as the electrophile. 
• This ion attacks the aromatic ring, adding a nitro group at the meta position relative to the ester group, resulting in the formation of m-nitro methyl benzoate, as shown below:

Mechanism Nitration of methyl benzoate forming nitro methyl benzoate