"Structure of benzene"

Chemistry, B.Sc. Sem-Ist
Session-2 Chapter-4 Aromatic Hydrocarbon

1. 1 Late Ku. Durga K. Banmeru Science College , LONAR DIST. BULDANA (Maharashtra), India . Section –II Unit-4 Aromatic Hydrocarbons “ Structure of Benzene ” B. Sc. Ist year Sem-Ist Subject :- Chemistry

2. 2 Dr. Suryakant B. Borul (M.Sc., M.Phil. , Ph.D.) Head Of Department Department of Chemistry Late Ku. Durga K. Banmeru Science College, Lonar Teacher Profile

3. Introduction- The word “ aroma ” means fragrance that is pleasant smell . Benzene is the parent hydrocarbon of aromatic compounds and aromatic hydrocarbons are those which contain one or more benzene rings. These are also called as arenes . “AROMATIC HYDROCARBONS” For Example-

4. Structure of Benzene The structures benzene suggested by German chemist, August Kekule is the most accepted. According to Kekule , structure of Benzene molecule is made up of hexagone of six carbon atoms joined alternately by single and double with one hydrogen atom attached to each carbon. Kekule structure of Benzene

5. Evidence in support of structure of Benzene proposed by Kekule Molecular Formula- F rom elemental analysis, percentage composition and molecular weight determination, the molecular formula of benzene is C 6 H 6 . 2) Cyclic Structure- on catalytic hydrogenation, benzene yield cyclohexane . Formation of cyclohexane inducts that all the carbon atoms are present in cyclic form i.e. benzene must be six membered ring compound.

6. 3) Position of Hydrogen- B enzene gives only one mono-substituted product and three di -substituted products. This indicated that all the six hydrogen are equivalent. Keeping apart the fourth valency of each carbon, structure of benzene, mono-substituted product and di -substituted products are as follows;

7. 4) Presence of three double bonds- B enzene adds on three molecules of chlorine to give benzene hexachloride (BHC). It is used as powerful insecticide.

8. 5) Position of double bonds- Presence of three alternate double bonds can be proved by ozonolysis reaction. When Benzene reacts with ozone which to form triozonide , which on hydrolysis gives three molecules of glyoxal .

9. 6) Conformation of structure by synthesis- Benzene molecule can be further confirmed by its synthesis from acetylene. Benzene is obtained by passing acetylene gas through red hot tube.

10. Objections to Kekule’s Structure S tructure of benzene as proposed by Kekule give two possible ortho disubstituted products as shown; H owever, in actual practice, only one ortho disubstituted product is obtained . Kekule himself discarded this objection by saysing that the position of double bonds is not fixed but double bonds are mobiles.

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