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Isomerism:
The existence of two or more compounds with same molecular formula but different properties (physical, chemical or both) is known as isomerism; and the compounds themselves are called isomers. The term was given by Berzelius. The difference in properties of two isomers is due to the difference in the arrangement of atoms within their molecules.
Isomerism may be of two types:-

1) Structural isomerism
When the isomers differ only in the arrangement of atoms or groups within the molecule, without any reference to space, these are known as structural isomers and the phenomenon asstructural isomerism.

2) Stereo isomerism
When isomers have the same structural formula but differ in relative arrangement of atoms or groups in space within the molecule, these are known as stereoisomers and the phenomenon as stereoisomerism. The spatial arrangement of atoms or groups is also referred to as configuration of the molecule and thus we can say that the stereoisomers have the same structural formula but different configuration.


Electron displacement effects:
Various effects have been distinguished in molecules in which electrons are displaced away from or towards a specified atom or group. The symbols used are as follows:

E – electromeric effect;
I – inductive effect (sometimes Is);
Id – inductomeric effect;
M – mesomeric effect;
R – resonance effect;
T – tautomeric effect, use mesomeric effect.

In addition, a plus sign before the symbol indicates that electron displacement is away from the atom or group (e.g. +I); a minus sign indicates displacement towards an atom or group (e.g. −I).


Benzene and its Properties:
Benzene, or benzol, is an organic chemical compound with the molecular formula C6H6. It is sometimes abbreviated Ph–H.  Because it is a known carcinogen, its use as an additive in gasoline is now limited, but it is an important industrial solvent and precursor in the production of drugs, plastics, synthetic rubber, and dyes. Benzene is a natural constituent of crude oil, and may be synthesized from other compounds present in petroleum. Benzene is an aromatic hydrocarbon and the second [n]-annulene ([6]-annulene), a cyclic hydrocarbon with a continuous pi bond.

Physical Properties of Benzene

* Benzene is a colorless liquid (melting point 5.5°C, boiling point 80.4°C) with a characteristic smell of petrol.

* It is immiscible with water, but dissolves in organic solvents.

* It is lighter than water: density 0.87 g cm-3

* Its vapors are toxic.

Chemical Properties of Benzene

Benzene does not give reactions characteristic of unsaturated hydrocarbons such as ethane, even though it has 6 electrons. On the other hand, it is quite stable and gives substitution reactions. Because of the high electron density on the ring, a tendency by the electron-deficient groups to attack it with ease gives it electrophilic substitution reactions. Some typical reactions given by benzene are described below:

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Amit - IT BHU