Benzene and Aromaticity | Organic Chemistry 2
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
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
1) Activation of the Electrophile (Formation of a 'super-electrophile' E+).
2) Electrophilic Attack:
3) Proton Loss:
Halogenation of Benzene
1) Activation of Bromine by the Lewis Acid FeBr3:
2) Electrophilic Attack on Benzene by Activated Bromine:
3) Bromobenzene Formation:
Nitration of Benzene
1) Activation of Nitric Acid by Sulfuric Acid:
2) Aromatic Nitration:
Sulfonation of Benzene
1) Haloalkane Activation (with Lewis Acid):
2) Electrophilic Alkylation:
Limitations: overalkylation + carbocation rearrangements
1) Acylium Ions Formation:
2) Electrophilic Acylation: