Benzene Structure and Azo Dyes

Draw the kekule’s structure of benzene and the accepted structure and list the simularities and differences

Both

• All bond angle are 120

• Planar structure

• 3 sets of electrons repelled

Kekule Structure

• Alternating single and double bonds (suggesting different bond lengths)

• Two dimethyl isomers expected

• Hydrogenation enthalpy 3x that on cyclohexene due 3 double bonds instead of 1

• Undergoes addition reactions

• Should decolourise bromine water

Actual Benzene

• C-C all the same length

• Only 1 isomer of dimethyl

• Hydrogenation enthalpy is less than 3x that of cyclohexene - less energy released so more stable due to delocalisation

• Undergoes substitution reactions

• Won’t decolourise bromine water

Explain, using a diagram, how benzene is delocalised

• Each carbon atom forms 3 sigma bonds

• This leaves the 4th electron in the p orbital of each carbon

• The 6 p orbitals overlap forming a pi cloud of electron density

• This is two rings of electrons one above and below the plane of the carbon atoms

• The electrons move freely within this pi system. They are delocalised

What is the difference between an aromatic compound and an arene

Aromatic compounds - contain a ring of delocalised electrons

Arenes - contain a benzene ring

Show the general mechanism for the electrophilic subsitution of benzene with elctrophile, E⁺

Give the reagents, conditions, electrophile, equations (generation of electrophile and regeneration at catalyst) and mechanism for the halogenation of benzene

Reagents - Cl₂ or Br₂

Conditions - Anhydrous AlFe₃/AlBr₃/FeCl₃/FeBr₃ Catalyst

Electrophile - Cl⁺ or Br⁺

Generation of electrophile - AlCl₃ + Cl₂ → AlCl₄^- + Cl⁺

Regeneration of catalyst - AlCl₄^- + H⁺ → AlCl₃ + HCl

Give the reagents, conditions, electrophile, equations (generation of electrophile and regeneration at catalyst) and mechanism for the nitration of benzene

Reagents - Concentrated HNO₃

Conditions - Concentrated H₂SO₄ (Catalyst and dehydrating agent) and less than 55C

Electrophile - NO2⁺

Give the reagents, conditions, electrophile and equation for the sulphonation of benzene

Reagents - Concentrated H₂SO₄

Conditions - Heat under reflux

Electrophile - SO₃H⁺

Give the reagents, conditions, electrophile and equation (including the generation of electrophile) for the Friedel-Crafts Reaction (adding carbon chains to a benzene ring)

Reagents - haloalkane with correct R group

Conditions - Anhydrous AlCl₃ catalyst, reflux

Electrophile - CH3⁺ (R⁺)

Generation of electrophile - AlCl₃ + CH₃Cl → CH₃⁺ + AlCl₄^-

Give the reagents, conditions, electrophile and equation (including the generation of electrophile) for the Friedel-Crafts Acetylation Reaction

Reagents - Acyl chloride

Conditions - Anhydrous AlCl₃ catalyst

Electrophile - e.g. CH₃CO⁺

Generation of electrophile - AlCl₃ + CH₃COCl → CH₃CO⁺ + AlCl₄^-