Aromatic Hydrocarbons

Hydrocarbons are organic compounds made up of carbon and hydrogen atoms. They have 3 structural varieties: aliphatic, aromatic, and halogenated. Aliphatic hydrocarbons contain either straight or branched chained carbon arrangements while aromatic compounds are purely ring. The halogenated hydrocarbons are distinguished by their carbons, hydrogens and a halogen atom. 

Aliphatic and aromatic compounds

In chemistry, aliphatic compounds refer to compounds that do not have the ring whereas those that contain rings mostly belong to the aromatic compounds. Furthermore, aliphatic compounds include both compounds with the long and short carbon chains. Arenes are special types of unsaturated cyclic hydrocarbons contains either single or group of rings. The arena was originally known as aromatic compounds because of their pleasant aroma. The simplest arene is benzene, C6H6In the case of aromatic compounds, the two ends of a carbon chain are attached to form a ring, therefore they are also cyclic compounds. Rings in these cyclic compounds contain 3-20 carbon atoms, however, five and six-membered rings are the most common.  

Benzene

The word benzene is derived from gum benzoin, which is otherwise known as  ‘Benjamin’. Gum benzoin, an aromatic resin, first introduced by Michael Faraday, an English scientist while the name benzene was given by German Chemist Mitscherich in 1833. Benzene is the simplest arene and one of the most common aromatic hydrocarbon. Its molecule is made of 6 carbon atoms connected in a planar ring with one hydrogen atom attached to each. It is a widely used industrial chemical as it is richly found in products such as paint, lacquer, and varnish removers, industrial solvents, gasoline, glues,  paints, furniture wax,  detergents, thinners. I tis It's used to manufacture large scale plastics, detergents, drugs and pesticides, resins, synthetic fibers, rubber lubricants, and dyes. Benzene can be observed during natural incidents such as volcanoes and forest fires. It is slightly sweet in taste, aromatic, gasoline-like odour and the area can be sensed when the benzene in the air reaches 1.5 to 4.7 ppm.

 

Resonance of benzene

Resonance helps to explain the structure and reactivity of organic molecules. The benzene molecule exhibits resonance hence it is more stable. The oscillating double bonds in the benzene ring are explained according to the valence bond theory with the help of a resonance structure. Each carbon atoms in the benzene ring is sp2 hybridized. One of the 2 sp2 hybridized orbitals of one atom overlaps with the sp2 orbital of the adjacent carbon atom that results in a six C-C sigma bonds. While the leftover sp2 hybridized orbitals merge with s orbital of hydrogen to form6 C-H sigma bonds. Remaining unhybridized p orbitals of carbon atoms form π bonds with adjacent carbon atoms by lateral overlap. This forms  C1 –C2, C3 – C4, C5 – C6 π bonds or C2 – C3, C4 – C5, C6-C1 π bonds. The above structural formula shows the extremes in electron sharing between any two adjacent carbons in benzene. One extreme is a normal double bond. When two or more equivalent structures are drawn for the benzene molecule, resonance occurs.  alkenes.

Aromaticity of benzene

Benzene is a pure aromatic compound because the Carbon-Carbon bonds formed in the ring are neither completely single nor double, but they are intermediate. Aromatic compounds are divided into benzenoids that containing benzene ring and non-benzenoids that are devoid of the benzene ring, provided they comply with the  Huckel rule. According to him, for a ring to be aromatic it should have Planarity, complete delocalization of the pi electrons in the ring and presence of (4n+2) π electrons in the ring where n is a positive integer

Preparation of benzene

A number of chemical reactions yield benzene. The most common reactions that help in the preparation of benzene are explained below:

1. Decarboxylation of aromatic acids

The term "decarboxylation" refers to the replacement of a carboxyl group (-COOH) with a hydrogen atom: RCO2H → RH + CO. Benzene can be prepared from aromatic acids by decarboxylation reaction where sodium salt of the benzoic acid  is heated with soda lime to produce benzene along with sodium carbonate.

 

 

2. Reduction of phenols.

Phenol can be transformed into benzene with the help of a strong reducing agent like Zinc dust with strong heating. When Zn dust is strongly heated, the phenol gets converted into phenoxide ion and proton thus released accepts an electron from Zn forming H radical.

Chemical properties of benzene

1. Electrophilic substitution

In the electrophilic substitution of benzene, an electrophile replaces the hydrogen atom of benzene. The reaction is very spontaneous as the aromaticity of benzene is not disturbed. Some examples of electrophilic substitution reaction of benzene are nitration, sulfonation, halogenation, Friedel Craft’s alkylation and acylation, etc. Nitration, the most common method of electrophilic substitution happens when one or more of the hydrogen atoms on the benzene ring is substituted by a nitro group- NO2. For this, benzene is treated with a mixture of concentrated nitric acid and concentrated sulphuric acid at a temperature not exceeding 50°C and the reaction is as below.

 

 

2. Sulphonation of benzene

Sulfonation is a reversible reaction that generates benzenesulfonic acid by combining sulfur trioxide with fuming sulfuric acid, followed by which the reaction is reversed by adding hot aqueous acid to benzenesulfonic acid to produce benzene.

 

 

 

3. Halogenation of benzene

Benzene reacts with chlorine or bromine in the presence of a catalyst such as aluminium chloride or aluminium bromide, replacing one of the hydrogen atoms on the ring by a chlorine or bromine atom. The reactions happen at room temperature

 

4. Friedel Craft’s alkylation

Friedel–Crafts alkylation requires the alkylation of an aromatic ring with an alkyl halide. The reaction occurs in the presence of a strong Lewis acid, such as aluminium chloride, ferric chloride. 

 

 

 

5.Directive influence of a functional group in monosubstituted benzene

This involves ortho and para directing groups and activating –OH, -NH2, - NHR, -NHCOCH3, -NH3

 

6. Meta directing group and deactivating groups

It is seen in groups such as –NO2, -CN, -COR. –COOH, -COOR, -SO3H

 

Carcinogenicity and toxicity

The carcinogenic property of benzene is harmful to health and is therefore recommended keeping off all substances containing benzene and its compounds. Combining benzene and other polynuclear hydrocarbons increase the cancerous particles in the environment.

Read More

  1.  Ionic bonds and ionic compounds  

  2. Chemical reactions and chemical equations

  3. Classification and IUPAC nomenclature of organic compounds

  4. Alkynes

Questions

  1. Distinguish between chained and cyclic hydrocarbons.

  2. Write the differences between aliphatic and aromatic compounds.

  3. Explain the methods of preparation of benzene.

  4. Describe the chemical properties of benzene.

 

 

 



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