Benzene and Aromaticity | Organic Chemistry 2

The benzene and the aromaticity are studied in this chapter: the naming and properties of benzene, the Hückel’s rule: aromatic, antiaromatic, non-aromatic systems, the electrophilic aromatic substitution, the halogenation of benzene.

Naming Benzene

Monosubstituted benzene: place the substituent name before 'benzene'.
More highly substituted systems:
- use 1,2-, 1,3-, and 1,4- (or ortho-, meta-, and para-) to indicate the positions
- the ring is numbered and substituents labeled in aphabetical order

 

Properties of Benzene

Benzene is particularly stable ⇒ not the same reactivities as alkenes 
(except the hydrogenation with Ni at high pressure and high temperature).
 

sp2 orbitals in the plane
p orb. perpendicular to the plane

 

Benzene is stabilized by delocalization of the π electrons.
C-C bonds are between a single and a double bond.
The electrons of the p orbitals form a π cloud above and below the plane of the ring.


Benzene and aromatic rings are characteristic in NMR spectroscopy:
low-field resonances ⇒ 1H NMR: δ ~ 6.5-8.5 ppm, 13C NMR: δ ~ 120-140 ppm

Hückel's Rule

Aromatic: cyclic conjugated polyenes, 4n + 2 π electrons, planar
⇒ stable system

 

All of the molecules below have 6 π electrons (4n + 2 with n = 1):

 

 

Antiaromatic: cyclic conjugated polyenes, 4n π electrons, planar 
⇒ unstable system

 

The molecules below have 4n π electrons (n = 1 and n = 3):

 

Nonaromatic: nonplanar or noncyclically delocalized system

 

The first molecule is non planar and the π electrons of the two others are noncyclically delocalized ⇒ nonaromatic systems:

Electrophilic Aromatic Substitution

Typical reactions of benzene: electrophilic aromatic substitutions

 

 

Mechanism:

1) Activation of the Electrophile (Formation of a 'super-electrophile' E+).

2) Electrophilic Attack:

3) Proton Loss:

Halogenation of Benzene


Mechanism:

1) Activation of Bromine by the Lewis Acid FeBr3:
 


2) Electrophilic Attack on Benzene by Activated Bromine:
 


3) Bromobenzene Formation:
 

Nitration of Benzene

 

Mechanism:

1) Activation of Nitric Acid by Sulfuric Acid:
 

 

2) Aromatic Nitration:
 

Sulfonation of Benzene

 

Mechanism:

Friedel-Crafts Alkylation

 

Mechanism:

1) Haloalkane Activation (with Lewis Acid):
 

 

2) Electrophilic Alkylation:
 

 

Limitations: overalkylation + carbocation rearrangements

Friedel-Crafts Acylation

 

Mechanism:

1) Acylium Ions Formation:
 

 

2) Electrophilic Acylation: