4.2 - Aromaticity

Kekule structure of benzene

Six membered ring containing single and double bonds

Why the Kekule structure of benzene is incorrect

• benzene does not undergo addition reactions unlike compounds with double bonds

• Each C-C bond length is equal (less than C-C and greater than C=C)

• Enthalpy of hydration is less than expected to more thermodynamically

molecular formula of benzene

C6H6

geometry of benzene

Planar

Bond angle in benzene?

120 between 3 adj carbon atoms

Kinds of bonds benzene contains

Sigma and pi

How sigma bonds are formed in benzene

Each C atom is bonded to two other carbon atoms and a hydrogen atom by sigma bonds

How pi bonds are formed in benzene

Fourth outer shell electron is in a 2p orbital above and below the plane of the carbon ring. These p orbitals overlap sideways, forming pi delocalised electron system structure above and below the plane of the carbon ring

Stability of benzene

The more electrons delocalised, the more stable the compound

Name for extra stability in benzene

Delocalisation/resonance energy

Kind of mechanism benzene undergoes

Electrophilic substitution

Why benzene does not undergo addition reactions

This would disrupt the stable delocalised pi electron system and the resulting product would be less stable

Why benzene is more susceptible to electrophilic attack

• Delocalised ring of electrons is an area of high electron density

• Makes it susceptible to attack by an electrophile

• If a H atom is replaced by an electrophole, the delocalisation must be maintained

Three reactions that benzene undergoes which involve electrophilic substitution

• Nitration

• Halogenation

• Friedel-Crafts Alkylation

Reagent used in nitration of benzene

Conc. HNO3 (aq)

Catalyst used in nitration of benzene

Conc. H2SO4 (aq)

Electrophile in nitration of benzene

NO2^+

Equation for formation of the electrophile in nitration of benzene

Mechanism for nitration of benzene

Overall equation for nitration of benzene

Equation for reforming the catalyst in nitration of benzene

Reagents and conditions in nitration of benzene

Conc. H2SO4 catalyst

50•C

Reagent used in halogenation of benzene

Br2 or Cl2

Catalyst used for halogenation of benzene?

FeBr3 or AlCl3

Equation for formation of the electrophile in halogenation of benzene?

Mechanism for halogenation of benzene

Overall equation for halogenation of benzene

Equation for reforming the catalyst in halogenation of benzene

Reagents and conditions for halogenation of benzene

FeBr3 or AlCl3

Room temp

Reagent for alkylation of benzene

Chloroalkane (R-Cl)

Catalyst for alkylation of benzene

AlCl3

Equation for formation of the electrophile in benzene

Mechanism for alkylation of benzene

Electrophile in alkylation of benzene

CH₃CH₂⁺ (ethyl carbocation)

Overall equation for alkylation of benzene

Equation for reformation of the catalyst in alkylation of benzene

Reagents and conditions for alkylation of benzene

Compare C-Cl bond strength in chlorobenzene to that of a chloroalkane

• greater bond strength in chlorobenzene

• Overlap between non bonding p electron pairs on the chlorine and the delocalised pi electron ring system

• Gives pi character to the bond

• Strengthens the bond

• Much more energy is needed to break it

Compare the reaction of chlorobenzene with aqueous sodium hydroxide with that of a chloroalkane

• chloroalkanes react with aqueous sodium hydroxide in a nucleophilic substitution reaction to give alcohols

• Chlorobenzene does not react with aq NaOH

  • Resistant to nucleophilic substitution because nucleophiles would be repelled by the stable pi system of electrons