26.2 Reactions of Benzene and Alkylbenzene
Reactivity of benzene
- Unlike alkenes, benzene is resistant to addition reactions. This is because it involves breaking the delocalised electron system and thus losing its stability
- Instead, benzene undergoes substitution This involves replacing one or more hydrogen atoms with another group of atoms.
- Benzene is particularly attractive to electrophiles due to its high electron Therefore benzene mainly undergoes electrophilic substitution.
- The reactivity of benzene is also affected by the presence of other group(s) in the benzene This is known as substituent group effect, and it will be explained later.
Electrophilic substitution
- Electrophilic substitution is the replacement of an atom or a group of atom by an electrophile. In benzene, the electrophile replaces the hydrogen atom(s).
- The general mechanism of electrophilic substitution:
- Suppose, an electrophile X⁺ is generated from a compound The electrophile is attracted to the electron-rich region of the benzene ring.
- Two electrons from the delocalised electron system are used to form a co-ordinate bond with the electrophile
- An intermediate with the delocalised electron system partially broken is formed.
- Y⁻ ion, from XY has a lone pair of electrons on it and it forms a bond with the hydrogen The hydrogen loses its electrons to the delocalised electron system to bond with Y⁻. The delocalised electron system is now restored.
- Summary of reactions that benzene and alkylbenzene undergoes:
- Halogenation(addition of halogen) into the ring and side chain
- Nitration(addition of nitro group, NO2).
- Oxidation of side chain
- Hydrogenation
Reaction of benzene – halogenation
- Reagent : Chlorine/bromine gas, Cl2/Br2
Condition : Aluminium chloride, AlCl3 or iron(III) chloride, FeCl3
Product : Halobenzene
- Benzene reacts with chlorine/bromine in the presence of halogen carriers such as aluminium chloride, AlCl3 to form chloro- or bromobenzene, One hydrogen atom in the benzene ring is substituted by the halogen.
- Use chlorine as an example of halogen:
C6H6 + Cl2 → C6H5Cl + HCl
- The mechanism of this reaction – electrophilic substitution
- Aluminium chloride polarises the Cl-Cl bond to create the electrophile Cl⁺.
Cl2 + AlCl3 → Cl⁺ + AlCl4⁻ - The electrophile is attracted to the benzene, it then bonds with the benzene using two electrons from the delocalised electron system. Note that the delocalised electron system is partially broken in the intermediate
- A hydrogen ion is expelled from the ring by AlCl4⁻ and leaving its electrons in the The delocalised electron system is now restored. Steamy fumes of hydrogen chloride is also observed.
- Aluminium chloride polarises the Cl-Cl bond to create the electrophile Cl⁺.
Reaction of benzene – nitration
- Reagent : Concentrated nitric acid, HNO3
Condition : Reflux at a temperature lower than 50 ºC and the presence of concentrated sulfuric acid, H2SO4 as catalyst
Product : Nitrobenzene
- Nitration happens when one or more hydrogen atoms in benzene is replaced by a nitro group, NO2.
- Benzene reacts with nitric acid in the presence of concentrated sulfuric acid to give nitrobenzene
C6H6 + HNO3 → C6H5NO2 + H2O
- The mechanism of this reaction – electrophilic
- The electrophile, nitronium ion, NO2⁺ is formed by the reaction of nitric acid and sulfuric acid
HNO3 + 2H2SO4 → NO2⁺ + 2HSO4⁻ + H3O⁺ - As before, the electrophile is attracted to the benzene ring and forms a bond with it. The delocalised electron system is partially broken
- Hydrogen ion is expelled and it bonds with HSO4⁻ to regenerate the catalyst. The delocalised electron system is restored
- The electrophile, nitronium ion, NO2⁺ is formed by the reaction of nitric acid and sulfuric acid
- If the temperature exceeds 50 ºC, 1,3-dinitrobenzene will be formed as Notice that the second nitro group is added to the 3 position of the ring.
Substituent group effect
- If a benzene already has a first substituent in it, second incoming substituent is affected by the nature of the first substituent
- The first substituent has an effect on:
- the position of the incoming second substituent.
– If X is an electron-withdrawing group, it deactivates the ring and it is 3-directing.
– Examples are -NO2, -CN, -COOH, -CHO and -COR.
– If X is an electron-donating group, it activates the ring and it is 2,4-directing.
– Examples are -OH, -NH2, -C6H5 and alkyl groups
– The only exception are halogens, they are electron-withdrawing but they are 2,4-directing. - the rate at which the second substituent is introduced.
– If X is an electron-withdrawing group, it deactivates the ring and the rate of reaction decreases
– If X is an electron-donating group, it activates the ring and the rate of reaction increases.
Reaction of alkylbenzene – halogenation
- In the presence of halogen carriers and the absence of UV light at room temperature, the halogen is substituted into the ring via electrophilic substitution.
- Take methylbenzene as an example, since methyl group is electron-donating and 2,4-directing, the incoming halogen is substituted at the 2 or 4 position. 2-chloro and 4-chloromethylbenzene are produced
C6H5CH3 + Cl2 → C6H4ClCH3 + HCl
- In the presence of UV light and the absence of halogen carriers, the halogen is substituted into the alkyl group via free-radical substitution. Take methylbenzene as an example, (chloromethyl)benzene is produced
C6H5CH3 + Cl2 → C6H4(CH3Cl) + HCl
- Further substitutions are possible, producing (dichloromethyl)benzene and (trichloromethyl)benzene.
Reaction of alkylbenzene – nitration
- Since methyl group activates the ring, making the ring more reactive, the temperature has to be lowered to 30 ºC to prevent multiple substitutions
- Methyl group is an electron-donating group, it activates the ring and is 2,4-directing, therefore the nitro group is substituted at the 2 or 4 positions positions
- Take methylbenzene as an example, 2-nitro and 4-nitromethylbenzene is produced.
- If the temperature exceeds 30 ºC, multiple substitutions by nitro groups is possible. However, each time a nitro group substitutes, the rate of reaction of the next substitution decreases because nitro group is electron-withdrawing. Therefore, 2,4,6-trinitromethylbenzene is rare
Reaction of alkylbenzene – oxidation
- When methylbenzene is heated under reflux with acidified potassium manganate(VII), side-chain oxidation occurs. Benzoic acid is produced
- Any carbon side-chain group is oxidised to -COOH group under these conditions.
Reaction of alkylbenzene – hydrogenation
- Reagent : Hydrogen gas, H2
Condition : Heat in the presence of nickel, Ni catalyst at 150 °C
Product : Cyclohexanes - In hydrogenation, hydrogen atoms are added to the benzene The delocalised electron system is permanently broken.
- With benzene:
- With methylbenzene: