Chemistry > Aromatic Compounds > 2.0 Electrophilic Aromatic Substitution Reactions
Aromatic Compounds
1.0 The Structure of Benzene
1.1 A Resonance Picture of Benzene
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
2.0 Electrophilic Aromatic Substitution Reactions
3.0 Nitration
4.0 Sulphonation
5.0 Halogenation
6.0 Friedel-Crafts Alkylation
7.0 Friedel-Crafts Acylation
8.0 Orientation and Reactivity in Electrophilic Aromatic Substitution
8.1 Donation of electrons into a benzene ring by resonance
8.2 Withdrawal of electrons from a benzene ring by resonance
9.0 Ortho / Para Ratio
9.1 Directive influence of the groups during substitutions in benzene ring
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
10.0 Reactions of Alkyl Benzenes
2.1 A General Mechanism for Electrophilic Aromatic Substitution
1.2 The Stability of Benzene
1.3 The Resonance Explanation of the Structure of Benzene
1.4 Bond lengths and angles in benzene
1.5 Hückle’s Rule: The $\left( {4n{\text{ }} + {\text{ }}2} \right)\pi $ Electron Rule
8.2 Withdrawal of electrons from a benzene ring by resonance
9.2 Mechanism of o and p-directing groups
9.3 Mechanism of o- and p-directing groups not have unshared pair of electrons
9.4 Mechanism of o- and p-directing gps having unshared pair of electron(s)
9.5 Mechanism of m-directing groups
9.6 Competitive orienting effect of two substituents
Benzene is susceptible to electrophilic attack primarily because of its exposed $\pi $ electrons. In this respect benzene resembles an alkene, for in the reaction of an alkene with an electrophile the site of attack is the exposed $\pi $ bond.
We see however, that benzene differs from an alkene in a very significant way. Benzene’s closed shell of six $\pi $ electrons gives it a special stability. So although benzene is susceptible to electrophilic attack, it undergoes substitution reactions rather than addition reactions. Substitution reactions allow the aromatic sextet of $\pi $ electrons to be regenerated after attack by the electrophile has occurred. We can see how this happens if we examine a general mechanism for electrophilic aromatic substitution.
Once the electrophile, ${E^ + }$ is generated in the reaction, it enters into some kind of a weak interaction with the $\pi $ cloud of benzene ring leading to the formation of a $\pi $ -complex. this $\pi $ -complex is a donor-acceptor type of a complex, benzene being the donor and electrophile, the acceptor. These adducts are known as change transfer complexes. In the complex that benzene forms with bromine, it has been shown that the halogen molecule is located centrally and at right angles to the plane of the benzene ring.
In step 1 the electrophile takes two electrons of the six-electron $\pi $ system to form a $\sigma $ bond to one carbon atom of the benzene ring. Formation of this bond interrupts the cyclic system of $\pi $ electrons, because in the formation of the arenium ion the carbon that forms a bond to the electrophile becomes $s{p^3}$ hybridized and, therefore, no longer has an available p-orbital. Now only five carbon atoms of the ring are still $s{p^2}$ hybridized and still have p-orbitals. A calculated electrostatic potential map for the arenium ion formed by electrophilic addition of bromine to benzene indicates that positive charge is distributed in the arenium ion ring (figure) just as was shown in the contributing resonance structures.
In step 2 a proton is removed from the carbon atom of the arenium ion that bears the electrophile. The two electrons that bonded this proton to carbon becomes a part of the $\pi $ system. The carbon atom that bears the electrophile becomes $s{p^2}$ hybridized again, and a benzene derivative with six fully delocalized $\pi $ electrons is formed. We can represent step 2 with any one of the resonance structures for the arenium ion.
(The proton is removed by any of the bases present, for example, by the anion derived from the electrophile).
Kekulé structure are more appropriate for writing mechanisms such as electrophilic aromatic substitution because they permit the use of resonance theory, it can be described however, using the modern formula for benzene in the following way
Where ${k_1}\;and\;{k_{-1}}$ is the rate constant of the forward and backward reactions in step-1, ${k_2}$ is the rate constant of the step-2.
