Benzene and Aromaticity | Organic Chemistry 2
Nomenclature of Benzene Derivatives
Monosubstituted benzenes: place the name of the substituent before 'benzene'
Disubstituted benzenes:
- use 1,2-, 1,3-, and 1,4- (or ortho-, meta-, and para-) to indicate the positions
- add substituents as prefixes in alphabetical order. Carbon 1 is carbon with the hydroxy functional group or the substituent first in alphabetical order
Properties of Benzene
Stability:
Benzene is particularly stable ⇒ not the same reactivities as alkenes
This molecule is stabilized by delocalization of its 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
sp2 orbitals in the plane
p orb. perpendicular to the plane
NMR:
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 system:
Cyclic conjugated polyenes with 4n + 2 π electrons and a plane geometry. This system is stable
All of the molecules below have 6 π electrons (4n + 2 with n = 1) and are planar:
Antiaromatic system:
Cyclic conjugated polyenes with 4n π electrons and a plane geometry. This system is unstable and wants to gain or lose 2 π electrons
The molecules below have 4n π electrons (n = 1 and n = 3):
Nonaromatic:
Non-planar or non-cyclically delocalized system
The first molecule is non-planar and the π electrons of the other two are non-cyclically delocalized ⇒ these are nonaromatic systems:
Electrophilic Aromatic Substitution
Typical reactions of benzene are electrophilic aromatic substitutions. The halogenation, nitration and sulfonation of benzene as well as the Friedel-Crafts alkylation and acylation work in the same way:
Mechanism:
- Activation of the electrophile ⇒ formation of a 'super-electrophile' E+
- Electrophilic attack of benzene
- Proton loss:
Halogenation of Benzene
Mechanism:
- Activation of bromine by Lewis acid FeBr3:
- Electrophilic attack of benzene by activated bromine:
- Proton loss:
Nitration of Benzene
Mechanism:
- Activation of nitric acid by sulfuric acid:
- Aromatic nitration:
Sulfonation of Benzene
Mechanism:
Friedel-Crafts Alkylation
Mechanism:
- Activation of haloalkane with Lewis acid:
- Electrophilic alkylation:
Limitations: overalkylation + carbocation rearrangements
Friedel-Crafts Acylation
Mechanism:
- Formation of acylium ions:
- Electrophilic acylation: