[Chemistry Class Notes] on Preparation of Benzene Pdf for Exam

Benzene is basically just an Organic compound. Some of its characteristics are: –

It is an aromatic hydrocarbon; whose chemical formula is C₆H₆.

Where is It found?

Benzene is present in crude oil, which is unrefined petroleum. Also as a natural byproduct of oil refining.

Structure

As you know, carbon usually can form just 4 single bonds. And as benzene has six carbon molecules and six hydrogen molecules, no structure could account for all the bonds, the structure of Benzene has remained a mystery. It was the chemist Kekulé who finally found the answer to this mystery when he saw a dream about a snake eating itself, it gave him the idea of a ringed structure. Which led him to develop a six Carbon membered ring, each attached to one hydrogen atom. The benzene ring forms three delocalized π -orbitals shared with all six of the carbon atoms, with respect to the molecular orbital theory. Whereas, valence bond theory suggests two stable resonance structures for the ring.

Discovery of Benzene

Benzene was first discovered in illuminating gas, by Michael Faraday who was an English Scientist. The origin of the word Benzene was from gum benzoin which was known as an aromatic resin.

Preparation

Benzene can be prepared in many ways: –

1. Decarboxylation of Sodium Benzoate

This is the laboratory method to obtain Benzene from Sodium benzoate. In this process, Sodium benzoate and Soda-lime (Sodium Hydroxide, along with Calcium Oxide) is heated which causes decarboxylation i.e., removal of carbon dioxide, to produce Benzene and Sodium Carbonate as the by-product.

2. Heating Phenol with Zinc

To make Benzene from Phenol, Phenol reacts with Zinc dust at a higher temperature, the phenol is converted to a phenoxide ion and a proton, which accepts an electron from Zn forming an H radical. Which results in the formation of ZnO and the phenoxide ion that was formed, converts itself into Benzene.

3. Polymerization of Ethyne

To produce Benzene from Ethyne (acetylene), it has to undergo cyclic polymerization. For this, Ethyne is made to pass through a red hot tube at a temperature of 873K, which in turn, converts itself into Benzene.

4. Reduction of Benzenediazonium Chloride

Making Benzene from Benzene-Diazonium Chloride requires the reduction of Benzenediazonium chloride with hypophosphorous acid at room temperature, resulting in the formation of Benzene and the reagent will get oxidised to phosphorus.  

5. Hydrolysis of Sulfonic Acid

Hydrolysis of Sulfonic acid, accompanied by superheated steam produces Benzene from sulphonic acid.

Properties of Benzene

• Benzene is immiscible in water and cannot form a homogeneous mixture with it.    Whereas, it is soluble in organic solvents.

• Benzene is a liquid, colourless aromatic compound which has an aromatic odour.

• Benzene is highly inflammable and upon combustion, will produce a sooty flame.

• Benzene shows resonance and can exist in different forms depending upon the position of the double bond, making it extremely stable.

• Benzene is found to be lighter than water as the density of Benzene is 0.87g cm³

• Benzene has a moderate boiling point of 80.5oC and a high melting point of 5.5oC.
  

Resonance of Benzene

The usual representation of the structure of Benzene consists of 3 double bonds and three single bonds drawn as 1,3,5-cyclohexatriene or 2,4,6-cyclohexatriene However, the real structure of Benzene is like a hybrid of the two as all the electron density flows through all P-orbitals equally. Therefore, every side, in reality, forms a bond that is an intermediate of a single and a double bond, which keeps oscillating, inside the ring. All the carbon atoms that are present inside this ring have sp² hybridization. As there are two sp² hybridised orbitals, one of these, attaches itself to the sp² hybridised orbital of the Carbon atom lying next to it, forming a C-C bond. The next sp² hybridised orbital, attaches itself to the s orbital of Hydrogen, forming a C-H bond. Therefore, forming six C-C sigma bonds and six C-H sigma bonds. Now, there are unhybridized p orbitals remaining, they will form π bonds with the next carbon atom by lateral overlap.

Aromaticity of Benzene

What makes Benzene an Aromatic Compound?

In Benzene, the bond between two Carbon atoms (C-C) are neither single nor a double bond. Instead, it forms a bond that is of intermediate length.

Aromatic compounds are divided into two: –

Given that, they follow Huckel’s rule. According to this rule, for a given ring to be aromatic, it must have the following properties. 

1. The compound must be planar.

2. There should be complete delocalization of the π electrons in the ring.

3. Should have the presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .)

Uses of Benzene

Benzene is an industrial chemical that is widely used in the production of pesticides, resins, detergents, synthetic fibres, plastics, drugs, dyes. Benzene can be naturally produced from volcanoes and forest fires. It evaporates rapidly from soil and water, if it leaks from storage tanks it can lead to the contamination of water wells and water sources situated close by.

Benzene also has household uses too, but the extent of its use is limited due to its toxic and carcinogenic nature. In homes, Benzene is used in glue, adhesive, cleaning products, tobacco smoke, etc.

• It is also used to prepare phenol and aniline which is used in dyes.

•  It is used to manufacture nylon fibres.

•  Degreasing metals.

• One of the most important uses of benzene is to manufacture different chemicals such as ethylbenzene, cyclohexane, cumene, nitrobenzene, etc.