Benzene – Discovery, Structure, X-Ray Research Studies, Properties & Uses of Benzene

Discovery of Benzene

The word benzene obtains historically from gum benzoin, often called ‘Benjamin’. Gum benzoin was called a fragrant resin. Michael Faraday, an English scientist first discovered Benzene in illuminating gas. The name benzene was given by German Chemist Mitscherich in 1833. The cyclic structure of benzene stayed a mystery until 1865 when German professor August Kekule illuminated it when he dreamt of a snake biting its tail.

However, Kekule did not find the existence of interactions between the double bonds. American teacher Linus Pauling proposed that benzene displayed a hybrid structure composed of delocalized electrons. This was the refinement of Kekule’s discovery. Benzene has a rather enjoyable, sweet odor; however, it is carcinogenic.

Structure of Benzene

Molecular Formula

1. The empirical formula of benzene is identified by the elemental analysis.
2. Its molecular mass figured out by the vapor density method is 78.108. This is six times the empirical formula mass (CH = 12 + 1 = 13). For that reason, the molecular formula of benzene is C₆H₆.
3. The molecular formula of benzene suggests that it is a highly unsaturated substance.

Kekule’s Structure

The structure of benzene continued to be a severe problem for chemists for about 40 years. A German chemist, Kekule at last solved the problem in 1865. Kekule proposed a cyclic routine hexagonal structure for benzene, which includes three double bonds alternating with 3 single bonds. He supported his theory by the following arguments.

(i) Benzene gives only one monosubstituted product.

(ii) Benzene gives only three disubstituted products.

These points validate the regular hexagonal structure of benzene in which all the carbon atoms are occupying identical positions in the particle. Therefore, benzene forms only one toluene, one phenol, and one nitrobenzene.

 

(iii) Benzene adds three hydrogen particles in the existence of a catalyst.

(iv) Benzene adds three particles of chlorine in the presence of sunlight.

These 2 reactions verify the presence of 3 double bonds alternating with 3 single bonds.

X-Ray Research Studies of Benzene Structure

The X-ray studies of benzene have verified the hexagonal structure for it. These studies have likewise revealed that all the carbon and hydrogen atoms remain in the exact same plane. All the angles are 120 °. All C – C and C -H bond lengths are 1.397 Å and 1.09 Å, respectively.

Modern Concepts About the Structure of Benzene

The hexagonal framework of benzene can be easily described using the hybridization method. According to this, each carbon in benzene is sp² hybridized. The 3 sp² hybrid orbitals on each carbon are used to form three σ-bonds, 2 with surrounding carbon atoms and one with hydrogen. The unhybridized 2pz orbitals remain at a right angle to these sp² orbitals. Given that all the sp² orbitals are in the very same plane for that reason all the carbon and hydrogen atoms are coplanar.

All the angles are 120 ° which validates the regular hexagonal structure of benzene. The unhybridized 2pz orbitals partially overlap to form a constant sheath of the electron cloud, covering, above and below, the six-carbon sigma bonds of the ring. Given that each 2pz orbital is overlapped by the 2pz orbitals of adjacent carbon atoms, for that reason, this overlapping provides, ‘diffused’ or ‘delocalized’ electron cloud.

Resonance of Benzene

The possibility of different pairing schemes of valence electrons of atoms is called resonance” and the different structures hence organized are called “Resonance structures”. The resonance is represented by a double-headed arrow (↔) e.g., the following different pairing schemes of the fourth valence (the p-electrons) of carbon atoms are possible in benzene.

• In alkanes the C-C bond length is 1.54Å.
• In alkenes the C = C bond length is 1.34Å.
• In alkynes, the C ≡ C bond length is 1.20Å.
• In benzene, the C-C bond length is 1.397Å.

The C-C bond length in benzene is intermediate in between those in alkanes and alkenes. The resonating single and double bonds in benzene can better be represented as a total circle inside the ring.

Properties of Benzene

The different properties of benzene are pointed out below:

• Benzene is immiscible in water but soluble in organic solvents.
• It is a colorless liquid and has an aromatic odor.
• It has a density of 0.87 g cm-3. It is lighter than water.
• Benzene has a moderate boiling point and a high melting point. (Boiling point: 80.5 ° C, Melting point: 5.5 ° C).
• Benzene shows resonance.
• It is extremely ignitable and burns with a sooty flame.

Uses of Benzene

1. Benzene is used in the production of tires and rubber.
2. Benzene is used as a solvent for organic substances.
3. To prepare a lot of items with essential uses:

(i) Benzene hexachloride – used as insecticides.

(ii) Phenylethene (styrene) – polymerized into polystyrene (a plastic product).

(iii) Phenol – polymerized into substances used in dyes and as a disinfectant.

(iv) Cyclohexane – polymerized to provide nylon – used in carpet.

4. The adhesives used to attach soles to shoes contain benzene.
5. Benzene is included in items utilized in the printing market for cleaning and preserving printing devices. In addition, the ink that is used in printing regularly consists of benzene. Benzene is an active ingredient of a range of painting products, such as base and topcoat paints, lacquers, spray paints, sealers, and stains.
6. Petroleum and oil refineries produce items that contain benzene, such as gasoline, fuel oils, and kerosene.
7. Benzene is also utilized in the production of asphalt that is utilized by the roofing and paving business.
8. Vehicle repair work facilities use solvents that contain benzene to clean parts such as hydraulic systems, fuel system elements, and brakes.

MCQs with Answers: Benzene