Chemistry - Benzene, Aromatic & Carbonyl Compounds

Kekule Model

Hexagonal ring with alternating single and double C-C bonds
pi electrons in benzene are localised in the double bonds

Delocalised Model

p-orbitals of all six carbons overlap sideways above and below the plane of benzene
This overlap forms a continuous ring of electron density, creating a delocalised pi electron system with 6 electrons spread across it

Evidence for Delocalised Model

1. All six Carbon-Carbon bonds are the same length, contradicting Kekule where bonds would alternate in length due to the alternating single and double C-C bond

2. Benzene does not decolourise bromine water as its pi electron system is too weak to induce a dipole

3. Benzene has a slightly less exothermic enthalpy of hydrogenation than the Kekule model would suggest, this is due to the delocalised pi electrons

What mechanism does Benzene use

Electrophilic Substitution
Electrophile must be generated in situ using metal halide carrier
e.g AlBr3 + Br2 → AlBr4- + Br+
Br+ attacks benzene ring, replaces an H+
Regeneration: H+ + AlBr4- → AlBr3 + HBr

Nitration of Benzene Reagent

Conc H2SO4 and Conc HNO3
H2SO4 + HNO3 → NO2+ + HSO4- + H2O
Regeneration: H+ + HSO4- → H2SO4

Friedel Crafts Acylation

Acyl group is added to Benzene in the presence of a Metal Halide catalyst
Benzene is reacted with an acyl chloride using AlCl3 catalyst
Generation: CH3CH2COCl + AlCl3 → [AlCl4]- + CH3CH2CO+
Regeneration: H+ + [AlCl4]- → HCl + AlCl3

Phenol

Can act as a very weak acid as H+ breaks off OH group and one of the electron pairs on oxygen delocalises into pi electron system, making the phenoxide ion stable
Weak acid as oxygen is still the most electronegative element, so attracts a lot of the electron density to it, meaning often it re-attracts the H+ to it
Phenol reacts with Alkaline solns to produce soluble salt + water
Phenol reacts vigorously with metals like Na to produce soluble salt + H2(g)

Reactions of Phenol

Phenol reacts more readily with electrophiles than benzene because one of the lone pairs on oxygen overlaps with pi electron system, increasing electron density making it more susceptible to electrophilic attack.
Phenol decolourises Bromine water to form 2,4,6 tribromophenol
Phenol reacts with dilute HNO3 to substitute one group
Phenol reacts with conc HNO3 to substitute 3 groups

Directing Effects in Electrophilic Substitution

Electron Donating Groups: -CH3, -OH, NH2
Donate e- density into pi system, direct electrophiles to 2,4,6
Electron Withdrawing Groups: -NO2
Remove e- density from pi electron system, making it less reactive and directing electrophiles to 3

Carbonyl Reaction Mechanism

Nucleophilic Addition (C=O is polarised)

Reduction of Aldehydes and Ketones

Can be reduced to alcohols using NaBH4 (Sodium borohydride)
It will reduce C=O but not C=C as high e- density in C=C repels H- generated by NaBH4

Testing for Carbonyls

Using 2,4 DNPH: Condensation reaction to form deep orange ppt
Using Tollen’s Reagent: Contains Ammoniacal Silver Nitrate [Ag(NH3)2]+ which is reduced to form a silver mirror (Aldehyde only)