Organic Chemistry

• Hydrocarbons are compounds of carbon and hydrogen only
• Hydrocarbons play a key role in daily life, e.g., LPG, CNG, LNG are used as fuels
• Petrol, diesel, and kerosene oil are obtained by fractional distillation of petroleum
• Coal gas is obtained by destructive distillation of coal
• Natural gas is found in upper strata during drilling of oil wells
• Alkanes are saturated open chain hydrocarbons containing carbon-carbon single bonds
• General formula for alkanes is CnH2n+2
• Methane (CH4) is the first member of the alkane family
• Alkanes are inert under normal conditions and were earlier known as paraffins
• Alkanes have tetrahedral structures with H-C-H bond angles of 109.5°
• Alkanes have σ bonds formed by head-on overlapping of sp3 hybrid orbitals of carbon and 1s orbitals of hydrogen atoms
• Alkanes exhibit nomenclature and isomerism
• Structural isomers of alkanes can have continuous or branched chains
• Alkyl groups are derived from alkanes by removal of one hydrogen atom
• General formula for alkyl groups is CnH2n+1
• Nomenclature of substituted alkanes follows specific rules
• Different isomeric alkyl groups can be formed based on molecular formulas
• Alcohols can be obtained by attaching -OH groups at different carbons of the chain
• Correct structure must be written from the given IUPAC name
• Structure and IUPAC Name:

  • 4-Ethyl-2-methylhexane
  • Structure: 1 CH3 - 2 CH - 3 CH2 - 4 CH - 5 CH2 - 6 CH3
• 3,3-Diethyl-5-isopropyl-4-methyloctane

  • Structure: 8 CH3 - 7 CH2 - 6 CH2 - 5 CH - 4 CH - 3 C - 2 CH2 - 1 CH3
• 1. sec-Butyl-4-isopropyldecane

  • Structure: 1 CH3 - 2 CH2 - 3 CH2 - 4 CH - 5 CH - 6 CH2 - 7 CH2 - 8 CH2 - 9 CH2 - 10 CH3
• 1. (2,2-Dimethylpropyl)nonane

  • Structure: 1 CH3 - 2 CH2 - 3 CH2 - 4 CH2 - 5 CH - 6 CH2 - 7 CH2 - 8 CH2 - 9 CH3
• 1. Ethyl-5-methylheptane

  • Structure: 1 CH3 - 2 CH2 - 3 CH - 4 CH2 - 5 CH - 6 CH2 - 7 CH3
• Halogens in order of rate of replacement of hydrogens in alkanes: F2 > Cl2 > Br2 > I2
• Rate of replacement of hydrogens in alkanes: 3° > 2° > 1°
• Fluorination is too violent to be controlled
• Iodination is slow and reversible, can be carried out in the presence of oxidizing agents like HIO3 or HNO3
• Halogenation proceeds via a free radical chain mechanism involving initiation, propagation, and termination steps
• Initiation: Homolysis of chlorine molecule in the presence of light or heat
• Propagation: Chlorine free radical attacks methane molecule, forming methyl free radical and H-Cl
• Termination: Reaction stops due to consumption of reactants or side reactions
• Alkanes on combustion in the presence of air or dioxygen are completely oxidized to CO2 and H2O with the evolution of heat
• Controlled oxidation of alkanes with regulated supply of dioxygen or air at high pressure and suitable catalysts gives various oxidation products
• Isomerisation of n-alkanes to branched chain alkanes using anhydrous aluminium chloride and hydrogen chloride gas
• Aromatization of n-alkanes with six or more carbon atoms to benzene and its homologues in the presence of oxides of vanadium, molybdenum, or chromium supported over alumina
• Reaction of methane with steam in the presence of nickel catalyst to form CO and H2
• Pyrolysis of higher alkanes at high temperature to decompose into smaller alkanes, alkenes, etc.
• Conformations of alkanes involve free rotation around C-C single bonds, resulting in different spatial arrangements called conformations or conformers
• Rotation around C-C single bond is hindered by a small energy barrier called torsional strain