Preparation of Benzene - Pharmaceutical Organic Chemistry-II PDF

Preparation of benzene Pharmaceutical Organic Chemistry-II Datta Meghe College of Pharmacy, DMIHER (DU), Wardha  Course Outcomes Sr. Learning Objective CO PO No. 1. Understand the methods for BP301-CO3 PO1...

Preparation of benzene Pharmaceutical Organic Chemistry-II Datta Meghe College of Pharmacy, DMIHER (DU), Wardha  Course Outcomes Sr. Learning Objective CO PO No. 1. Understand the methods for BP301-CO3 PO1 preparation of benzene. PO2 PO3 PO4 PO6 PO9 PO10  Specific Learning Objective After completion of this lecture student should be able to Understand methods for preparation of benzene.  Small scale preparation of benzene 1) By passing acetylene through red-hot tube at 500 °C Red hot tube _ 3 HC =CH Acetylene Benzene CaO 2) By heating benzoic acid or heat its sodium salt with soda lime + NaOH  Small scale preparation of benzene 3) Heating phenol with Zinc dust + Zn + ZnO Phenol Benzene 4) Treating chlorobenzene with magnesium followed by treatment with dil HCl. Chlorobenzene Phenylmagnesium  Large scale preparation of benzene 1) From petroleum: Aromatization of C6 to C8 fraction of ptroleum naptha yields mixture of benzene,toulene and xylenes. CH3(CH2)4CH3 Benzene + 4H2 CH3(CH2)5CH3 Toulene + 4H2 CH3(CH2)6CH3 Xylene +  Large scale preparation of benzene 2) From liquid oil fraction of coal tar : Distillation of coal tar gives light oil fraction. This is then washed with sulphuric acid to remove basic impurities. and with sodium hydroxide to remove acidic impurities. This oil is then washed with water dried and distilled. The fraction passing upto 110°C contains benzene toulene and thiophene. fraction collected between 80-82°C is pure benzene. Traces of thiophene impurities removed by heating it with hydrogen under pressure at 400°C and redistillation.  Physical properties of benzene Colourless liquid, Boiling point 80- 82°C, Melting point 5.5°C. Insoluble in water, Soluble in alcohol, ether and chloroform. Good solvent for organic and inorganic compounds. Vapours are toxic, cause unconsciousness after inhalation. Burn with luminous sooty flame. Benzene and its derivatives shows characteristic IR spectrum Two bands near 1600 cm-1 and 1500 cm-1. (aromatic C-C bonds)  Resonance energy of benzene Stability of benzene is due to delocalized π molecular orbital. This stability can be measured using heat of hydrogenation. Hydrogenation of Cyclohexene evolves 28.6 Kcal/mol. Hydrogenation of two double bonds in 1,3 cyclohexadiene evolves 55.4 Kcal/mol. 1,3,5 cyclohexatriene with three ordinary double bonds should evolve 85.8 Kcal/mol. But hydrogenation of benzene yields only 49.8 Kcal/mol.  + H2 + 28.6Kcal Cyclohexene Cyclohex ane + 2H2 + 55.4 Kcal Cyclohexadiene Cyclohexane + 3H2 + 85.8 Kcal 1,3,5 Cyclohexatriene Cyclohexane + 3H2 + 49.8 Kcal Benzene Cyclohexane  Resonance energy of benzene Stability of benzene is due to the difference of 36 Kcal between the heat of hydrogenation of benzene and estimated heat of hydrogenation of 1,3,5 cyclohexatriene (Hypothetical). This added stability if called resonance energy. Resonance energy is measure of how much more stable a resonance hybrid structure is than its  SUMMARY Small scale preparation of benzene 1) By passing acetylene through red-hot tube 2) By heating benzoic acid with soda lime 3) Heating phenol with Zinc dust 4) Treating chlorobenzene Large scale preparation of benzene 4) From petroleum 5) From liquid oil fraction of coal tar  Bibliography 1) Organic chemistry by Morisson & boyd. 2) Textbook of Organic chemistry by B. S. Bahl & Arun Bahl.  QUESTIONS 1) Discuss the structure of benzene. 2) Explain large scale preparation of benzene. Thank you