Aromatic Chemistry (2) Electrophilic Substitution in Benzene

Aromatic Chemistry (2) Electrophilic substitution

Specification extracts:

OCR

Electrophilic substitution 

6.1.1.(e) 

the mechanism of electrophilic substitution in arenes for nitration and halogenation
For nitration mechanism, learners should include equations for formation of NO2+.
Halogen carriers include iron, iron halides and aluminium halides. 

For the halogenation mechanism, the electrophile can be assumed to be X+. 

Edexcel

18A/5. 

understand the mechanism of the electrophilic substitution reactions of benzene (halogenation, nitration and Friedel-Crafts reactions), including the generation of the electrophile 

AQA

3.3.10.2

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

Students should be able to outline the electrophilic substitution mechanisms of: nitration, including the generation of the nitronium ion and acylation using AlCl3 as a catalyst.
Students could carry out the preparation of methyl 3-nitrobenzoate by nitration of methyl benzoate, purification by recrystallisation and determination of melting point. 

Aromatic Chemistry (2) Electrophilic substitution

In this second blog post on aromatic chemistry, I’m looking simply at the reaction mechanism of the electrophilic substitution of benzene.  

I’m going to discuss a general mechanism for the electrophilic substitution first then focus in on the peculiarities of the different examples quoted in the above A level specifications.  

The mechanism for the electrophilic substitution reactions of benzene

1,. General mechanism

What is an electrophile? 

An electrophile is essentially an electron deficient species be it molecule, ion or atom.  Let’s call it E+

Why should it be that substitution in benzene and other arena/aromatic molecules is facilitated using electrophiles?

Benzene is electron rich.  It has an accessible molecular orbital above and below the plane of the 6 carbon atom ring.  This molecular orbital is often called a π system (it forms in orbital theory from the side on overlap of 6 p orbitals that each contain one electron.)

So above and below the plane of the benzene molecule is this electron rich area open to attack from an electron deficient species E+. 

You can see in stage one of the mechanism above that an electron pair from the delocalised ring system attacks the electrophile.

This attack disrupts the π system and a bond forms between a carbon atom in the ring and the electrophile.  The ring itself now carries a positive charge.

In stage 2 loss of a proton results in the reformation of the π system and the expulsion of a proton from the benzene molecule.  

The overall effect is for the electrophile to take the place off the expelled proton. 

This is generally what happens in electrophilic substitution but let’s now look at some peculiarities of the mechanism in different contexts and we’ll start with the nitration of benzene using concentrated sulphuric and nitric acids.  

2. Mechanism of the nitration of benzene

In nitration the electrophile is the nitronium ion NO₂+

But how does this positive electrophile form?

That can be explained if we look at the role of the concentrated sulphuric acid.

Sulphuric acid is a stronger acid than nitric acid and one in the presence of the other results in sulphuric acid protonating the nitric acid as in the equation below. 

The resultant unstable protonated nitric acid molecule then loses water to form the nitronium ion NO₂+.

The mechanism then follows the general route outlined above now that the electrophile has been formed.

The only other thing to say is that the reaction temperature is kept below 55℃ to prevent the formation of the disubstituted 1,3 dinitrobenzene.

3. Mechanism for the Halogenation of Benzene

Here we need to discuss the role of the halogen carrier.

Iron, iron (III) bromide or chloride and aluminium chloride can act as halogen carriers. They are Lewis acids i.e. they are electron deficient and electron pair acceptors.

Question is how do they function in the mechanism?

For iron the answer is simple, it reacts with the halogen to form the respective higher oxidation number halide.

Iron(III)bromide, iron(III)chloride or aluminium chloride are all three electron deficient Lewis acids.  In the mix with the halogen they form the particular complex as shown below:

The electron deficient aluminium chloride forms the tetrahedral complex AlCl₄^— since it accepts a pair of electrons from the chlorine molecule 

The molecule of chlorine becomes the electrophile (Cl+ )and forms a bond with a carbon atom in the ring.

At this carbon atom, the bonding is no longer planar sp2 but tetrahedral sp3.  The benzene π system has been disrupted.

A second stage follows on from this in which the positive ion loses a proton and reforms the benzene π system.

The aluminium chloride is regenerated — in this sense it is a catalyst in the process.

You observe fumes of hydrogen chloride given off and a temperature rise in the reaction mixture in the lab.

4. Mechanism for the acylation of benzene.

In this example aluminium chloride is used in conjunction with an acid chloride here ethanol chloride to generate the electrophile.

AlCl₃  +  CH₃COCl       →      CH₃CO+          +          AlCl₄^—

The electrophile then can attack the electron rich ring of benzene and form a bond with one of its carbon atoms.

Then the resultant carbocation deprotonates interacting with aluminium chloride complex.

Substitute the acyl chloride for a chloroalkane under similar conditions gives the mechanism for the formation of an alkyl benzene.

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