organic 2

• Hydrocarbons are organic compounds containing only carbon and hydrogen
• Hydrocarbons play a key role in daily life as a source of energy
• Different hydrocarbons used as fuels include:

  • LPG (liquefied petroleum gas) majorly composed of butane
  • CNG (compressed natural gas) majorly composed of methane
  • LNG (liquefied natural gas) obtained by liquefaction of natural gas
  • Petrol, diesel, and kerosene oil obtained by fractional distillation of crude oil
  • Coal gas obtained by destructive distillation of coal
• Hydrocarbons are used in the manufacture of polymers, solvents for paints, dyes, and drugs
• Aliphatic hydrocarbons are classified into saturated (Alkanes) and unsaturated (Alkenes and Alkynes) compounds
• Alkanes are saturated hydrocarbons with only carbon-carbon single bonds (C-C)
• Alkenes are unsaturated hydrocarbons with carbon-carbon double bonds (C=C)
• Alkynes are unsaturated hydrocarbons with carbon-carbon triple bonds (C≡C)
• Alicyclic hydrocarbons are aliphatic compounds forming ring-like structures (cycloalkanes)
• Aromatic hydrocarbons are cyclic hydrocarbons with at least one benzene ring
• Methane has a tetrahedral structure with carbon at the center and hydrogen atoms at the corners
• Alkanes exhibit structural and stereo isomerism, such as chain and position isomerism
• Types of carbon atoms in alkanes include primary, secondary, tertiary, and quaternary
• Alkanes can be prepared from unsaturated hydrocarbons through hydrogenation or from alkyl halides using zinc and HCl or Wurtz reaction
• Alkanes can also be prepared from sodium salts of carboxylic acids through decarboxylation or Kolbe's electrolytic method
• General reaction:

  • Sodium carboxylate + Alkane -> Sodium carboxylate + Alkane
  • Mechanism:
  • R-COO- + 2 H2O -> OH- + H+ + R-COONa
  • Electrolysis of carboxylate ion Sodium carboxylate:
  • At Anode: 2 R-COO- + 2 H2O -> 2 e- + H2 + 2 OH- + 2 R-COO.
  • At Cathode: 2 R-COO- + 2 H2O -> 2 e- + H2 + 2 OH- + 2 R-COO.
  • Sodium acetate (Sodium ethanoate) + 2 CH3-COONa -> 2 CH3-COONa + 2 CO2 + R-R
• Physical Properties:

  • Physical state: C1 to C4 are gases, C5 to C17 are liquids, and those with 18 carbon atoms or more are solids at 298 K due to weak van der Waals forces
  • Colour and odour: Colourless and odourless
  • Polarity: Alkanes are almost non-polar molecules due to covalent C-C and C-H bonds
  • Solubility: Insoluble in H2O but soluble in organic solvents
  • Boiling point: Lower boiling point compared to other derivatives, increases with molecular mass, decreases with branching
• • Oxidation to carboxylic acids: Ethane oxidized to ethanoic acid
• Chemical Properties:

  • Substitution reaction: Chlorination of methane
  • Combustion: Alkanes completely oxidized to CO2 and H2O with evolution of heat
  • Isomerisation: n-Alkanes isomerize to branched chain alkanes
  • Pyrolysis/Cracking: Higher alkanes decompose into lower alkanes, alkenes, etc.
  • Reaction with steam: Methane reacts with steam to form CO and H2
  • Aromatization/Reforming: n-Alkanes form benzene and its homologues
  • Controlled oxidation:
  • Oxidation to alcohols: Methane oxidized to methanol
  • Oxidation to aldehydes: Methane oxidized to methanal
• Conformations:

  • Alkanes contain carbon-carbon sigma (σ) bonds allowing free rotation
  • Conformations/Conformers/Rotamers: Spatial arrangements of atoms that can be converted by rotation around a C-C single bond
  • Conformations of Ethane: Infinite conformations by rotation around C-C single bonds, hindered by torsional strain
• In ethane, there are infinite numbers of conformations due to the rotation of one carbon atom around the C-C axis
• Conformational isomers of ethane are also known as conformers, rotamers, or conformations
• Extreme cases of ethane conformations are eclipsed conformation and staggered conformation
• Eclipsed conformation: hydrogen atoms attached to two carbons are as close together as possible
• Staggered conformation: hydrogen atoms are as far apart as possible
• Skew conformation: any other intermediate conformation between eclipsed and staggered conformations
• Sawhorse projections are used to represent eclipsed and staggered conformations of ethane
• Newman projections are used to represent eclipsed and staggered conformations of ethane
• In all conformations of ethane, bond angles and bond lengths remain the same
• Alkenes have at least one double bond and the general formula is CnH2n
• Alkenes are also known as olefins
• The structure of ethene includes a strong sigma (σ) bond and a weak pi (π) bond
• The C=C double bond in alkenes is shorter in bond length than the C-C single bond
• Alkenes behave as sources of loosely held mobile electrons due to the presence of the pi (π) bond
• Alkenes are easily attacked by electrophilic reagents
• In IUPAC nomenclature of alkenes, the longest chain containing the double bond is selected
• The suffix 'ene' replaces 'ane' of alkanes in the nomenclature of alkenes
• Alkenes exhibit both structural and stereo isomerism
• Structural isomerism in alkenes includes chain and position isomerism
• Stereo isomerism in alkenes includes geometrical isomerism