The topic of aromatic compound comes under the chapter of hydrocarbon. This chapter is covered in ncert class 11thand 12thchemistry book. Students appearing for competitive exams like JEE OR NEET or any other state exams are advised to finish the first reading of the chapter, thoroughly from the textbook and then switch to the prescribed books for in depth study from their respective competitive exams point of view. Also, the textbook questions should be solved properly. In this way students will be able to build a clear fundamental concept which will further simplify or make it easier to grasp interlinked topics.
Organic compounds can be classified into two types, acyclic and cyclic. Acyclic are the ones that have open chain of carbon atoms while cyclic compounds are the ones that have close chain of carbon atoms. It is often observed that students define aromatic compounds as cyclical structure that contains at least one benzene ring. This statement may not be entirely correct. The simplest way to define aromatic compound is a chemical compound (mostly organic) which containing one or more rings with pi electrons delocalized around it. These have relatively stable bonding and are resistant to many reactions. It is less dense than water at 20 degrees centigrade and is not soluble in water. It is however soluble in organic solutions such as hexane and carbon tetrachloride. It has a pleasant fragrance, but it should be noted that aromatic compounds cannot be identified based on their odor. So how exactly can it be identified? To be classified as aromatic organic compound following criterion must be met. First, it should have planar geometry. Secondly it should be cyclic in nature. Third criterion is there should be in continuous conjugation and lastly it should have (4n+2) electrons. This is called huckle’s rule.
Now what does it mean to fulfill the criteria of cyclic compound being planar geometry in nature? The aromatic compound should be planar in nature implies p-orbitals are more or less parallel and interact with each other. The cyclic organic compound is planar in nature when all the carbon atoms present in the ring are sp2 hybridized and they lie in the same plane.
What is hybridization? Hybridization can be defined as mixing of the atomic orbitals belonging to the same atom. Since the energies are slightly different the redistribution of energies takes place which results in formation of new orbitals having equal energies and identical shapes. This results in formation of new orbitals are called as hybrid orbitals.
Now what is sp2 hybridization? Mixing of one ‘s’ orbital and two ‘p’ orbital of equal energy results in the formation of new hybrid orbital known as sp2. This process is called as Sp2 hybridization. This mixture formed of s and p orbital is maintained at 1200and is trigonal in symmetry. Thus, this is also called as a trigonal hybridization
We also need to understand what is localized and non-localized pi bonds. Now the pi bonds are between two unhybridized p orbitals that are adjacent to each other (from hybridization model). These p orbitalsaligned with respect to each other will overlap. To understand this more consider you have these unhybridized orbitals in more than 2 adjacent atoms. All of these unhybridized p orbitals which are next to each other will overlap. The electrons within these p orbitals will be delocalized as they can extend out to more than 2 atoms rather than being localized to 2 atoms from 2 p orbitals that are overlapping with each other. Example for this is benzene.
There are many uses of aromatic compounds in everyday life. They are used as solvents for fats, rubber etc. They are used in preparation of pesticides, dry cleaning of woolen clothes. These are also the raw materials for preparation of several drugs, perfumes, explosives. Petroleum and coal tar contain many aromatic hydrocarbons. The parent compound benzene was first obtained from the gas obtained by pyrolysis of whale oil by Faraday.
This is the basic idea regarding aromatic compounds. Students must study the principal reaction of benzene. It a can be studied under the following category of reaction namely oxidation, reduction, addition and substitution reactions.
Learn how to solve tricky question:
(A) PhF (B) PhCl (C) PhBr(D) PhI
- A) A<B<C<D
- B) A=B=C=D
- C) D<C<B<A
- D) D<B<A<C