Kekule structure and resonance in benzene

You will learn about Kekule structure of benzene with its obejections and resonance in benzene.
This will explain the unusual stability and equality of carbon carbon bond length.
Explanation for the lack of reactivity of benzene towards addition reaction.

1. PRESENTATION ON OF BACHELOR OF PHARMACY PREPARED BY: PALAK KHETAN K.P.SINGH MEMORIAL INSTITUTE OF PHARMACY KEKULE’s STRUCTURE OF BENZENE AND IT’s OBJECTIONS RESONANCE IN BENZENE

2. KEKULE's STRUCTURE OF BENZENE In 1865, August Kekule suggested that benzene consists of a cyclic planar structure of six carbons with alternate single and double bond. Each carbon was attached to one hydrogen. Benzene according to this proposal was simple 1,3,5-cyclohexatriene. Atoms are gambling like snake(twisting) and twinning But look at One of the snake has seized its own tail

3. OBJECTION’s FOR KEKULE’S STRUCTURE FOR BENZENE If Kekule’s structure was correct EX- Two isomers of dibromobenzene should be formed

4. BUT, Only one ortho – dibromobenzene could be prepared. To overcome this objection , Kekule further suggested that benzene was a mixture of two forms (I and II ) in rapid equilibrium and the position of single and double bonds are not fixed but they oscillate back and forth. .

5. Kekule’s structure could not explain the stability of benzene towards oxidising agents (all unsaturated compounds undergo oxidation but benzene resists oxidation like all saturated compounds). This structure could not explain the ease of benzene to undergo substitution reactions rather then additon reactions like other alkenes after having three double bonds ( being unsaturated in nature benzene does not reacts with HBr or Br 2 and does not decolourize bromine in CCl 4 ). Kekule’s formula for benzene contains two kinds of bonds i.e. single and double bonds but X- ray diffraction shows that benzene is a regular hexagon with an angle of 120˚and all the C–C bonds lengths are equal. So, Kekule’s structure failed to explain the UNUSAUL STABILITY AND EQUALITY OF C–C BOND LENGTHS in benzene.

6. RESONANCE IN BENZENE The phenomenon in which two or more structures can be written for a substance which involves the identical position of atoms is referred to as resonance. The actual structure of the molecule is referred to as resonance hybrid. The alternative structures are known as resonating structures . To represent the resonance a double headed arrow ( )will be used. In case of benzene, the Kekule’s structure I and II represent the resonating structures.

7. The structure A represent the resonance hybrid or a hybrid of these two resonating structures. To explain all the limitations of Kekule’s structure , it has been proposed that benzene is a resonance hybrid of the Kekule’s structure I and II. These two structures are the canonical forms of benzene. In actual, benzene is a resonance hybrid of these two structures (A) The hybrid structure is represented by inserting a circle or a dotted circle in the hexagon. This circle represents the π – electrons which are delocalised between the six carbon atoms of benzene ring. \

8. It means that the any two adjacent carbons atoms of the benzene molecule are neither joined by a pure single(–) nor by a pure double(=) bond. Spectroscopic measurements show that benzene is planar and all the carbon – carbon (C—C) bonds are of equal length, 1.40Å. carbon – carbon single bond( C–C) length is 1.54Å. carbon –carbon double bond(C C) length is 1.34Å.

9. This value lies between the carbon–carbon single bond (C–C) and carbon–carbon double bond (C=C) length. The resonance hybrid is more stable than any of it’s contributing structures. This explains that why benzene is more stable then either of it’s kekule’s structures. It also explains the unusual stability and equality of C–C bond length in benzene. For benzene, the stability due to resonance is so great that π – bonds of the molecule will normally resists breaking. THIS EXPLAINS LACK OF REACTIVITY OF BENZENE TOWARDS ADDITION REACTIONS.