Resonance involved in the benzene ring makes the delocalized electron span effectively over the carbon atoms in the benzene ring. It partially stabilizes the arenium ion too. Partial stability of arenium ion makes benzene highly prone to electrophilic substitution reactions. Electrophilic substitution of benzene is the one where an electrophile substitutes the hydrogen atom of benzene. As the aromaticity of benzene is not disturbed in the reaction, these reactions are highly spontaneous in nature. Basic examples of electrophilic substitution reaction of benzene are nitration, sulfonation, halogenation, Friedel Craft’s alkylation and acylation, etc.Table of Contents
What is Electrophilic Substitution of Benzene?
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The Mechanism for Electrophilic Substitution of Benzene
An electrophilic substitution reaction generally involves three steps:
1. Generation of electrophile: In the presence of Lewis acid, generation of electrophile takes place. As the Lewis acid accepts the electron pair from the attacking reagent.
2. Formation of arenium ion: The electrophile generated attacks on the benzene ring to form a positively charged cyclohexadienyl cation better called an arenium ion containing one sp3 hybridized carbon atom. The positive charge is effectively distributed over three carbon atoms by resonance which makes it partially stable.
As the delocalization of electrons stops at an sp3 hybridized carbon atom, the arenium ion is not aromatic in nature.
3. Removal of positive charge from the carbocation intermediate: The arenium ion finally loses its proton from sp3 hybridized carbon to a Lewis base restoring the aromaticity.
Few examples of electrophilic aromatic substitution
1. Nitration of Benzene
Benzene reacts with nitric acid at 323-333k in presence of sulphuric acid to form nitrobenzene. This reaction is known as nitration of Benzene.
2. Sulphonation of Benzene
Sulphonation of benzene is a process of heating benzene with fuming sulphuric acid (H2SO4 +SO3) to produce benzenesulphonic acid. The reaction is reversible in nature.
3. Halogenation of Benzene
Benzene reacts with halogens in the presence of Lewis acid like FeCl3, FeBr3 to form aryl halides. This reaction is termed halogenation of benzene.
Benzene is a planar molecule which has delocalized electrons above and below the plane of the ring. Being electron-rich, it is highly attractive to electron-deficient species i.e., electrophiles.
Step 1: Generation of electrophile. Step 2: Formation of the carbocation.
Step 3: Removal of the proton.
In electrophilic aromatic substitution reactions, an atom attached to an aromatic ring is replaced with an electrophile.
Examples of such reactions include aromatic nitrations, aromatic sulphonation, and Friedel-Crafts reactions
Nitrobenzene, chlorobenzene and methyl benzene respectively are the products of nitration, chlorination and Friedel Craft alkylation of benzene.
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considering electrophilic aromatic substitution is chlorobenzene or benzene more reactive?
Let's expand the question to include all of the halobenzenes.
All of the halogens ($\ce{F, Cl, Br, I}$) are more electronegative than hydrogen. Therefore they will inductively remove electron density from a benzene ring. However, you can also draw resonance structures for the various halobenzenes where the halogen substituent is donating electron density to the ortho and para positions of the benzene ring through resonance.
Therefore, in the case of the halobenzenes these two effects are working in opposite directions. In order to determine which effect controls the reaction, we need some data.
The following table compares the relative rates of electrophilic aromatic nitration of the halobenzenes to the rate for benzene itself. If we assign benzene a relative rate of 1, we see that all of the halobenzenes react slower than benzene itself. This tells us that the inductive effect is stronger than the resonance effect in the halobenzene series.
\begin{array} \hline \text{Relative rates of aromatic electrophilic nitration} \\ \hline
\end{array}
\begin{array}{|c|c|c|c|} \hline
\ce{Ar-X} & \text{Relative rate} \\ \hline
\ce{Ar-H} & 1.0 \\ \hline
\ce{Ar-F} & 0.11 \\ \hline
\ce{Ar-Cl} & 0.02 \\ \hline
\ce{Ar-Br} & 0.06 \\ \hline
\ce{Ar-I} & 0.13 \\ \hline
\end{array}
(data source: see p. 26 here)
However, when the halobenzenes do undergo electrophilic aromatic substitution, they react preferentially at the ortho and para positions, because, as explained above, the resonance effect donates electron density to the ortho and para positions. Therefore, these two positions are the least deactivated and react preferentially.