Aromatic Chemistry (1)

Aromatic Chemistry (1)

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(d) 

The electrophilic substitution of aromatic compounds with: 

(i) concentrated nitric acid in the presence of concentrated sulfuric acid
(ii) a halogen in the presence of a halogen carrier
(iii) a haloalkane or acyl chloride in the presence of a halogen carrier (Friedel–Crafts reaction) and its importance to synthesis by formation of a C–C bond to an aromatic ring.

Edexcel

18A/4. understand the reactions of benzene with: 

i  oxygen in air (combustion with a smoky flame)
ii  bromine, in the presence of a catalyst
iii  a mixture of concentrated nitric and sulfuric acids
iv  halogenoalkanes and acyl chlorides with aluminium chloride as catalyst (Friedel-Crafts reaction) 

AQA

3.3.10.2

Electrophilic attack on benzene rings results in substitution, limited to monosubstitutions. 

Nitration is an important step in synthesis, including the manufacture of explosives and formation of amines. 

Friedel–Crafts acylation reactions are also important steps in synthesis. 

Aromatic Chemistry (I)

In this first of several blog posts on aromatic chemistry i.e. the chemistry of aromatic molecules, (benzene (C₆H₆)being the simplest), I’m just looking simply at the reactions of benzene.   I’ll present the significant reactions from the different A level specs together with the reaction conditions, reagents and equations.  

But before I go any further I have to say that aromatic chemistry is my kind of chemistry.  As a trained dyestuffs chemist back in the day, it was all aromatic-based all dyes are molecules with aromatic ring structures: sometimes quite simple like some of the indicators you use or sometimes incredibly complex like some deeply coloured vat dyes on cotton.  I show some of the dye structures below at the end of this post.   

Reactions of benzene

1,. Nitration

This is an electrophilic substitution reaction.  

Reagents are concentrated nitric and sulphuric acids.  

Resultant product is nitrobenzene

Typical class practical is the nitration of methyl benzoate or nitrobenzene itself.

Equation:

The product is a mono substituted benzene molecule with the nitro–  group NO₂

Nitrated aromatic molecules form the basis of explosives.  For example nitration of methyl benzene (called toluene) creates the unstable molecule tri nitro toluene TNT).

Nitrated aromatic molecules can be reduced using tin and hydrochloric acid to form amines the precursors of dyestuffs and drugs. 

The reaction scheme below shows the formation of an azo dye from the reduction of TNT.

2. Halogenation

This is also an electrophilic substitution reaction.

Reagents are the halogen in the presence of a halogen carrier such as iron(III) chloride (FeCl₃) or aluminium chloride (AlCl₃).  These two chlorides are often referred to as Halogen Carriers.  Clearly you would use a chloride for the introduction of chlorine and a bromide for the introduction of bromine into the  benzene ring.  Other methods beyond the scope of the A level course are used to introduce iodine and fluorine into the benzene ring.

The product is the halogenobenzene such as chlorobenzene.

Note how this reaction differs from the bromination of an alkene.  Bromine does not added across the π delocalised ring system of 6 electrons in benzene like it does across the double bond in an alkene.  The ring system requires too much energy to be broken into the activation energy for the reception of a bromine atom is too high in the order of 150 kJ per mole.  

Equation:

The product here is bromobenzene.

3. Friedl-Crafts reaction

The Friedl-Crafts reaction takes its name from the two chemists who first carried this reaction.  

The significance of this reaction is that it creates a carbon carbon bond (–C—C–)between the benzene ring and the substituted group. 

There are two types mentioned above in the extracts from the A level specifications.

Type 1 is alkylation:

This where an alkyl group is attached to the benzene ring.

Reagents are a typical haloalkane (e.g. chloroethane C₂H₅Cl ) and aluminium chloride (AlCl₃).

Equation:

The product here would be ethyl benzene:

Type 2 is acylation

This is where an acyl group is attached to the benzene ring.

Reagents are a typical acylhalide (e.g. ethanol chloride CH₃ COCl ) and aluminium chloride (AlCl₃).

Equation:

The product here would be a ketone in this case: phenyl methyl ether.

4. Combustion

Benzene burns in air with a very yellow smoky flame.  

This flame indicates the high carbon content of the molecule.  Benzene is burning incompletely. 

Equation:

C₆H₆  +  4 1/2 O₂  →  2C  + 2CO  +   2CO₂    +   3H₂O

This is not the only possible equation for this incomplete combustion – there are many other.

Some aromatic structures

Methyl orange indicator

Several large vat dye structures