Topic 18: Organic Chemistry — Arenes

Describe the structure of benzene

an arene consisting of a ring of 6 carbon atoms with 6 hydrogen toms. The outer electron from the p-orbital of each carbon atom is delocalized into the center to form a ring making it very stable. The overlap of electrons results in the formation of π bonds.

why was Kekule model for benzene dis-proven

• all C—C bonds are the same length

• less reactive than alkenes

• ΔH of hydrogenation is lower than expected

• resistance to bromination

reaction mechanism of benzene

why does benzene undergo electrophilic substitution

The high electron density of the ring makes it open to attacks by electrophiles. Adding to the ring disturbs the delocalized electron system, so we substitute a hydrogen atom on the ring. Because the mechanism involves the initial disruption to the ring, electrophiles must be more powerful than those which react with alkenes.

nitration of benzene

reactant: HNO3

product: nitrobenzene + H2O

reagent: conc. nitric acid, conc. sulphuric acid catalyst

condition: reflux at 55

electrophile generation: 2H2SO4 + HNO3 ↔ 2HSO4¯ + H3O+ + NO2+

halogenation of benzene

reactant: Cl2

product: C6H5Cl + HCl

reagent: chlorine, halogen carrier catalyst

condition: reflux in presence of a halogen carrier

electrophile generation: Cl2 + FeCl3 + FeCl4¯ + Cl+

Friedel-Crafts alkylation of benzene

reactant: C2H5Cl

product: C6H5C2H5 +HCl

reagent: halogenoalkane + anhydrous AlCl3 catalyst

condition: room temp, ether solvent

electrophile generation: C2H5Cl + AlCl3 + AlCl4¯ + C2H5+

Friedel-Crafts acylation of benzene

reactant: RCOCl

product: C6H5COR + HCl

reagent: acyl chloride, anhydrous AlCl3

condition: reflux 50ºC, ether solvent

bromination of benzene

reactant: Br+

product: C6H5Br + H+

reagent: bromine, AlBr3 catalyst

condition: heat under reflux

electrophile generation: Br2 + AlBr3 + AlBr4¯ + Br+

combustion

2benzene + 15O2 → 12CO2 + 6H2O

hydrogenation

reactant: 3H2

product: C6H12

reagent: Ni catalyst

condition: heat under pressure

sulfonation of benzene

reactant: H2SO4

product: C6H5SO3H + H2O

reagent: fuming sulfuric acid

condition: warm at 40ºC

electrophile generation: fuming sulfuric acid is made by dissolving sulfur(VI)oxide in concentrated sulfuric acid

why does benzene resist bromination?

Benzene’s delocalized electron system is very stable. The electrons are distributed evenly throughout the delocalized electron system and therefore benzene is unable to polarize a non-polar molecule like bromine.

Why does phenol react with bromine water more easily than benzene?

Phenol has an alcohol group attached to the carbon rings. the lone pair of electrons on the oxygen atom is delocalized into the benzene ring, activating it. This causes an increase in the electron density and therefore induces a dipole in bromine molecules.