Chemistry of Benzene Substituents | Organic Chemistry 3
Reactivity of Methylbenzene
Benzylic radicals, cations and anions are stabilized by resonance with the benzene ring
Consequences:
relatively easy radical halogenations
SN1 and SN2 reactions
benzylic anion formation: H of the methyl substituent are slightly acidic (pKa = 41)
Reactivity of methylbenzene (or toluene):
Radical halogenation:
Mechanism:
Radical process via an initiation step to activate Br2 then propagation steps
Substitution nucleophilic:
Mechanism:
SN1 and SN2 reactions, the benzylic cation being a good electrophile
Deprotonation reaction:
A strong base as BuLi is needed
Benzylic Oxidation and Reduction
Oxidation:
Selective oxidation of a benzylic alcohol:
Reduction:
A benzylic substituent can be used as a protecting group for the hydroxy function
Nucleophilic Aromatic Substitution
Mechanism:
Addition-elimination. Nucleophilic attack will occur at the ipso position of 2 electron withdrawing substituents
Aromatic Substitution through benzyne intermediate:
Mechanism:
Elimination-addition
Properties and Reactivity of Phenol
There is a keto-enol equilibrium with phenol. The enol form is favored by aromaticity
The proton of the hydroxy function is acidic because the corresponding anion is stabilized by resonance
Reactivity of phenol:
- Electrophilic Aromatic Substitution
- SN reactions with the phenoxide ion:
Claisen Rearrangement
Mechanism:
Concerted reaction involving the movement of 6 electrons
Aliphatic Claisen rearrangement:
Arenediazonium Salts
Stabilized by resonance:
Synthesis:
Reactivity:
- Hydrolysis:
- Sandmeyer reaction:
with Nu = Nucleophile (Cl, Br, CN ...)
- Diazo coupling with strongly activated benzene:
