Chemistry of Benzene Substituents | Organic Chemistry 3

The Chemistry of Benzene Substituents is studied in this chapter: the reactivity of methylbenzene, the benzylic oxidation and reduction, the nucleophilic aromatic substitution, the properties and reactivities of phenol, the Claisen rearrangement, the arenediazonium salts.

Reactivity of the Methylbenzene

Benzylic radicals, cations and anions are stabilized by resonance with the benzene ring:

 

 

Same process for benzylic radicals and cations.

Consequences:

relatively easy radical halogenations
SN1 and SN2 reactions
benzylic anion formation

 


Mechanism: Radical Process (Initiation step to activate Br2 and then propagation step)

 

Benzylic cation is a good electrophile

SN1 and SN2 reactions

Benzylic anion is a good nucleophile

 

Benzylic Oxidation and Reduction

Oxidation:
 

 

Selective oxidation of a benzylic alcohol:
 

 

 

Reduction:
 


 

Benzylic substituent can be used as a protecting group for the hydroxy function.

Nucleophilic Aromatic Substitution


Nucleophile attacks at ipso position of 2 electron withdrawing substituents
Mechanism: Addition-Elimination

 

Aromatic Substitution through benzyne intermediate:
 


Mechanism: Elimination-Addition

Properties and Reactivities of Phenol


Enol form is favored.
Phenols are acidic because the corresponding anions are resonance stabilized.

 

Reactivity:

- 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: