THEORY OF BENZENE

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THEORY OF BENZENE
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Monosubstituted benzenes are named by ending the name of the substituent in benzene.

benzene nomenclature1

[1] Bromobenzene

[2] Nitrobenzene

[3] Methylbenzene (toluene)

[4] Ethylbenzene

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THEORY OF BENZENE
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Benzene acts as a nucleophile, attacking a large and varied number of electrophiles.

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THEORY OF BENZENE
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The benzene reacts with the nitric-sulfuric mixture adding nitro groups.

nitracion-benceno

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The reaction of benzene with a solution of sulfur trioxide in sulfuric acid produces benzenesulfonic acids.

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Benzene reacts with halogens in the presence of Lewis acids to form halogenated derivatives.

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The amino group is a strong activator, orienting to ortho/para. However, in acid media it is protonated, transforming into a strong deactivator (ammonium salt) that orients to the meta position.
Protonation of the amino can be avoided by protecting it with ethanoyl chloride in pyridine.
 
Aniline nitration without amino protection
 
protección del grupo amino

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The amino group is introduced into the aromatic ring by reduction of the nitro.

reduction nitro to amino

The reagents used in the reduction can be:

  • Sn, HCl
  • H2 , Ni, EtOH
  • Fe, HCl

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The reversibility of sulfonation allows it to be used to protect activated positions of benzene. Let's see an example:

protection for 01

To obtain o-bromotoluene, we perform the following steps:

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The reaction of 1-chloro-2,4-dinitrobenzene with nucleophiles (hydroxide, ammonia, methoxide, etc.) produces the substitution of chlorine by the corresponding nucleophile. It is called ipso (same place), to indicate that the nucleophile occupies the same position as the starting chlorine.

aromatic nucleophilic substitution 01

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Halogenated benzenes react with dilute soda under conditions of high pressure and temperature to form phenols. This reaction does not require deactivating groups in the ortho/para position and follows a different mechanism than aromatic nucleophilic substitution by addition-elimination.

bencino-01.png

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The carbon attached directly to benzene is known as the benzylic position. In this position, highly stable carbocations, carbanions and radicals are formed due to the possibility of delocalizing the charge on the aromatic ring.

benzylic position 01

SN1 in benzylic positions

Primary haloalkanes on benzylic positions give SN1 since the primary carbocation delocalizes by resonance within benzene.
benzylic position 02

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Chain oxidation with permanganate and dichromate

Hot potassium permanganate and dichromate oxidize alkylbenzenes to benzoic acids. This reaction is only possible if there is at least one hydrogen in the benzylic position. The length of the chains does not matter, or if they are branched, they all break through the benzylic position, generating the carboxylic group.
oxidation side chains 01

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The Birch reduction uses sodium or lithium in solution as reagents, its mechanism is radical and reduces benzene to 1,4-cyclohexadiene.

birch reduction 01

 

Birch with activating substituents

The double bonds of the final cyclohexadiene lie close to the substituents that activate the ring.
birch reduction 02
 
Birch with deactivating substituents
The double bonds of the final cyclohexadiene lie away from the substituents that deactivate the ring.
birch reduction 03

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Allyl phenyl ethers undergo a concerted reaction when heated, involving the movement of six electrons, called the Claisen rearrangement. The intermediate formed in the reaction is of high energy and rapidly tautomerizes to give the final product.

claisen transposition 01

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The benzenediazonium salts are attacked by nucleophiles in the presence of copper (I) salts that act as a catalyst, obtaining a wide variety of products.

sandmeyer reactions

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Formation of azo compounds

Diazonium salts are electrophilic in nature and can be attacked by activated benzenes (phenol, aniline). This reaction is known as azo coupling and generates products of industrial interest called azo dyes.
 
Step 1 . Formation of the diazonium salt
azo coupling 01