Title 04. Aromatic Hydrocarbon Flow Charts.pdf ✅

TEACH HIRE DOWNLOAD APP FROM PLAY STORE CLICK HERE AROMATIC HYDROCARBON GENERAL INTRODUCTION • These are also called arenes. • General formula is CnH2n–6y. Where y is the number of benzene ring in molecule. Illustrate example : Benzene Toluene CH3 Ethyl benzene CH2CH3 CH3 CH3 CH3 CH3 CH3 CH3 o/m/p-xylene • Following reactions are important in the determination of the structure of benzene. (i) C6H6 Pt H / ⎯⎯2⎯→  (ii) C6H6 2. H O/ Zn 1. O 2 ⎯⎯ ⎯3 → H–C–C–H || O || O (iii) C6H6 cold & dil. ⎯ KMnO ⎯ ⎯4→ No reaction (no decolourisation) (iv) C6H6 ⎯ Br ⎯2⎯ /CCl⎯4→ No reaction (v) C6H6 ⎯HCl ⎯→ No reaction On the basis of (i) and (ii) reactions Kekule in 1865 suggested that benzene is equilibrium mixture of cyclohexatrienes [(I) and (II)] as follows (I) (II) But reaction (iii), (iv) and (v) can not be explained by Kekule's structure. THE RESONANCE HYBRID STRUCTURE According to the resonance theory, benzene is a resonance hybrid of the following canonical forms. KEKULE structures   DEWAR'S structures  Contribution : Total 80% Total 20% 40% each i.e., 3 20 % contribution by each
TEACH HIRE DOWNLOAD APP FROM PLAY STORE CLICK HERE It should be noted that the concept of resonance is imaginary and the canonical forms mentioned above actually do not exist. It is the hybrid structure which is a reality. RESONANCE THEORY CAN EXPLAIN : (i) Stability of -electrons. (ii) Equal bond energies of all C–C bond. (iii) Equal bond lengths of all C–C bond. (iv) Lower value of heat of combustion. (v) Lower value of heat of hydrogenation. THE MOLECULAR ORBITAL STRUCTURE OF BENZENE : According to this theory, all of the six carbon atoms in benzene ring are in sp2 hybridised state. By overlapping of hybrid orbitals these six carbon atoms form a planar hexagonal ring. Now each of 'p' orbital on the six carbon atoms can overlap on either side with adjacent 'p' orbitals. These result in a molecular orbital which is actually made of two continuous rings, one ring above and another below the plane of hexagon. sidewise overlaping PHYSICAL PROPERTIES • Benzene is a colourless volatile liquid. It has characteristic smell. • Its boiling point is 80oC and freezing point is 5.5o C. • It is highly inflammable and burns with sooty flame. • It is lighter than water. Its specific gravity at 20oC is 0.8788. • It is immiscible with water but miscible with organic solvents such as alcohol and ether. • It is a non - polar compound and its dipole moment is zero. • It is extremely poisonous substance. Inhalation of vapours or absorption though skin has a toxic effect.
TEACH HIRE DOWNLOAD APP FROM PLAY STORE CLICK HERE PREPARATION METHODS OF BENZENE COOH SO3H Mg – Br OH HC  CH NaOH + CaO /  Sodalime H2O /  Zn reduction Cr2O3 + Al2O3 500oC red hot Fe tube (i) CH2 = CH2 (ii) Pt/ H2O HC HC CH2 CH2 Polymerisation Benzene
TEACH HIRE DOWNLOAD APP FROM PLAY STORE CLICK HERE CHEMICAL PROPERTIES OF BENZENE Cl2/AlCl3 Halogenation Cl HNO3+ H2SO4 Nitration NO2 H2SO4 Sulphonation SO3H CH3CH2–Cl/AlCl3 Friedal-Craft alkylation CH2CH3 CH3–C–Cl/AlCl3 Friedal-Craft acetylation C–CH3 O O (CO + HCl)/AlCl3 Gattermann-Koch reaction CHO (HCN +HCl)/AlCl3 Gattermann reaction CH=NH H3O  CHO (HCHO + HCl)/ZnCl2 Blace reaction CH2OH HCl ZnCl2 CH2Cl O3/H2O/Zn Ozonolysis CHO CHO Cl2/sunlight Cl Cl Cl Cl Cl Cl (Gammexane or BHC or 666) H2/Pd or Pt or Ni H.T.P. Na/liq. NH3 Birch reduction (i) CrCl3 + AlCl3 Cr V2O5/300oC OH V2O5/400o-500oC O O O Combustion 6 CO2 + 3 H2O + H C (exothermic) Benzene (Glyoxal) (Phenol) (maleic anhydride) (ii) Na2S2O4 (reduction) Sandwitch Compound Oxidation Oxidation O O OH OH –H2O