Benzene and Aromaticity | Organic Chemistry 2

Benzene derivatives and aromaticity are studied in this chapter: name and properties of benzene derivatives, Hückel’s rule - aromatic, antiaromatic, non-aromatic systems, electrophilic aromatic substitution, halogenation of benzene

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:

  1. Activation of the electrophile ⇒ formation of a 'super-electrophile' E+
  2. Electrophilic attack of benzene


     
  3. Proton loss:

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Halogenation of Benzene


Mechanism:

  1. Activation of bromine by Lewis acid FeBr3:


     
  2. Electrophilic attack of benzene by activated bromine:


     
  3. Proton loss:

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Nitration of Benzene


Mechanism:

  1. Activation of nitric acid by sulfuric acid:


     
  2. Aromatic nitration:

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Sulfonation of Benzene


Mechanism:

Friedel-Crafts Alkylation


Mechanism:

  1. Activation of haloalkane with Lewis acid:


     
  2. Electrophilic alkylation:


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Limitations: overalkylation + carbocation rearrangements

Friedel-Crafts Acylation


Mechanism:

  1. Formation of acylium ions:


     
  2. Electrophilic acylation:

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