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.