Benzene

enzene was discovered by Michael Faraday an isolated by Hoffman from coal tar. The structure of benzene was proposed by kekule

According to kekule:

" Benzene has regular cyclic hexagonal structure in which Three double bonds alternate three single bonds"

X- ray study of benzene structure:

1-Benzene has regular cyclic hexagonal structure

2-All the carbon and hydrogen atoms lie in same plane they are coplanar all the angles are of 120 degree

3-All carbon- carbon Bond length is 1.397 angstrom and carbon- hydrogen bond length is 1.09 angstrom

Draw back of kekule structure:

Benzene is less reactive. Benzene show addition reaction as well as substitution reaction. Benzene has less heat of formation.

Nomenclature of benzene:

1-Mono substituted benzenes are named by prefixing benzene with the name of substituent. The whole name is written as one word

2-If there are two substituent then their relative position is indicated by prefix Ortho, meta and para in common system of naming and by numerals in IUPAC

3- If two or more substituents are different then the substituent which treated as high priority group is given the number one position on benzene ring. Other substituents are number by counting from position 1 in the manner to give them lowest number

The order of priority of group

  • COOH,-CN,aldehyde,ketone,-OH,

-NH2,-OR,-R

4- If two substituent are different and they are not present in priority order list then they are named in alphabetical order and last name substituent is on position 1 on benzene ring

5-If there is a substitute which gives a special name to the molecule then this name use as parent name

Atomic orbital treatment if benzene:

According to atomic orbital hybridization each carbon in Benzene is SP2 hybridized mean each carbon has three SP2 hybrid orbital and one unhybridized 2pz orbital

These SP2 hybrid orbital arranged them self in triangular planar structure angle is 120 degree

Unhybridized 2pz orbital lie perpendicular to this triangular planar structure

SP2 hybrid orbital form sigma bond by SP2-SP2 overlap with adjacent carbon and third SP2 orbital overlap with 1S orbital of hydrogen

All these sigma bond lie in same plane they are coplanar

The UNhybridized 2pz orbital sideway to way to form a pi bond

Pi electrons in benzene ring which are six in number are delocalized

When there is delocalization there is stability. Therefore benzene has extra stability due to extensive delocalization

Resonance:

“The possibility of different pairing scheme of valence electron of atom is called resonance”.

Resonance is intramolecular phenomena. Structures which we obtained are known as RESONATING STRUCTURE. These resonating structure are hypothetical by combination of these resonating structure hybrid structure form which is real

Stability:

Stability of molecule is directly proportional to number of resonating structure. Benzene has 5 resonating structure due to which it has extra stability

Resonating structures of benzene:

Benzene has 5 resonating structure. Two of them proposed by which has 80% contribution in overall benzene structure and remaining
three was proposed by Dewar which has 20% contribution because dewar structures are unstable

Difference between resonance and tautomerism:

In case of resonance shifting of atom does not take place while in tautomerism shifting of atom take place.

Condition for resonance:

The most important condition for resonance is conjugation if there is no conjugation there will be no resonance

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