Toluene Structure Properties Reactions and Uses in Organic Chemistry

What is Toluene?

Toluene is a transparent, colourless liquid with an odour similar to benzene. Toluene's chemical formula is C6H5CH3.

The chemical compound toluene is naturally occurring and mainly derived from petroleum or petrochemical processes. The toluene chemical is present in gasoline, glues, and paints. The liquid toluene smells like paint thinners, is colourless and insoluble in water. It's a mono-substituted colourless liquid that has a CH3 group attached to a phenyl group. 

The Properties of Toluene

In comparison with benzene, toluene is more electrophilic. It reacts in the same position with normal fragrance due to the greater percentage of methyl group than electron-releasing properties. Chlorotoluene undergoes sulfonation to produce p-toluene sulfonic acid, which then undergoes chlorination by Cl2 in the presence of FeCl3 to yield ortho and para isomers.

Structure of Toluene

Paints and glues are both manufactured with toluene as a solvent. Toluene is used extensively as a chemical raw material.

Production of Toluene

As a by-product of gasoline production, toluene can also be found naturally in crude oil. Toluene is also produced as a by-product of coal cooking.

Toluene can be produced at a low cost at industrial levels. There are several ways to synthesise it. Toluene is obtained from the reaction between benzene and methyl chloride in the presence of Lewis acid.

C6H5H + CH3Cl → C6H5CH3 + HCl

Chemical Properties

Toluene exhibits electrophilic aromatic substitution reactions similar to those of normal aromatic hydrocarbons. Toluene has a greater capacity for releasing electrons than hydrogen atoms in the same position because of the methyl group present in it. Compared to benzene, methyl is more electrophilic. In the presence of FeCl2, it is chlorinated by Cl2 with sulfonation to give chlorotoluene sulfonic acid, and by sulfonation to give para- and ortho-isomers of chlorotoluene.

A significant factor affecting toluene's oxidative capacity is its methyl side chain. Benzaldehyde is produced by combining the compound with potassium permanganate and chromyl chloride. This is known as the Étard reaction.

The methyl group is halogenated in the presence of free radicals. As an example, N-Bromosuccinimide (NBS) yields benzyl bromide in the presence of AIBN when heated with toluene. Bromination of toluene with HBr and H2O2 is also possible by using light and HBr.

C6H5CH3 + Br2 → C6H5CH2Br + HBr 

C6H5CH2Br + Br2 → C6H 5CHBr2 + HBr

A strong base will deprotonate the methyl group; the pKa is estimated to be around 41. The methylcyclohexane is formed by hydrogenation. Hydrogen pressure and a catalyst are required.

Uses of Toluene

Benzene can be synthesised from toluene. Toluene reacts with hydrogen gas according to the chemical equation below.

C6H5CH3 + H2 → C6H6 + CH4

In contrast, benzene and xylene are used for the second most common application.

Chemicals that are Produced from Toluene

Benzene and xylene are synthesised using toluene as well as the following chemical reactions:

• Foam polyurethane

• Trinitrotoluene – Explosive

• TNT

• Synthetic Drugs.

The Use of Toluene as a Solvent

Solvents commonly made with toluene include:

• Glues

• Paints

• Paint Thinners

• Printing Ink

• Rubber

• Leather Tanners

• Silicone Sealants

• Chemical Reactants

• Lacquers

• Disinfectants.

Toluene's Other Applications

Internal combustion engines can run on it as gasoline fuel.

Toluene's Niche Applications

Carbon nanomaterials, nanotubes, and whole rods are dissolved in it.

Commercial Preparation of Toluene

From Coal Tar:

Light oil fraction of coal tar is the primary source of commercial production of toluene. The light oil fraction is washed with conc. H2SO4 in order to remove the bases present in it, then with NaOH to remove acidic substances and finally with water.

It is subjected to fractional distillation. The vapours collected between 80 – 110oC is 90% benzol it contains 70 - 80% benzene and 14 - 24% toluene. 90% benzol is again distilled, and the part distilling between 108 – 1100C is collected as toluene.

Preparation of Toluene from Methylcyclohexane and N-Heptane

It is also obtained by cyclization of n-heptane followed by an aromatization.

Reactions of Toluene

1. Oxidation of Toluene

As toluene is an aromatic compound, it is less susceptible to an oxidation reaction. The methyl group of toluene is a side chain in the aromatic ring structure and is oxidised to the carboxyl group in the presence of a strong oxidising agent.

The oxidation of toluene forms benzaldehyde which can further be oxidised to form benzoic acid. There are many oxidising agents like potassium permanganate.

2. Bromination of Toluene

The reaction of toluene with bromine is known as bromination of toluene. The bromination of it can take place either on the side chain or an aromatic ring. 

Both bromination reactions occur with a different mechanism. Generally, side chain by free radical mechanism and aromatic bromination follows electrophilic substitution mechanism.

3. Nitration of Toluene

Introduction of a nitro group into toluene forms ortho-toluene & para-toluene and the reaction is called nitration of toluene.

The reaction follows the electrophilic substitution mechanism, and the mixture of concentrated sulfuric and nitric acid behaves as a nitrating agent.

In this case, concentrated sulfuric acid acts as a catalyst and generates a nitronium ion which behaves as an electrophile.

Nitronium ions attack on aromatic rings, majorly at ortho and para positions which further form ortho and para-products.

Due to the presence of a methyl group on the ring of toluene, the nitration of toluene is around twenty-five times faster than benzene. As the methyl group is activating towards the -ortho and -para directing groups, hence the nitration of toluene gives poly substituted nitro-products. 

However, the use of low temperature can prevent the substitution of more than one nitro group on the aromatic ring.

Note:

Under normal conditions, toluene gives all three isomers, out of which ortho-derivative forms around 63 % and 34% of para-product and 3% of meta-product is formed.

High yield of ortho products can be explained by the resonating structure of the arenium ion which forms as an intermediate.