Chemistry

• Tetravalence of carbon


  • Shapes of organic molecules
• Organic compounds are vital for sustaining life on earth and include complex molecules like genetic information bearing deoxyribonucleic acid (DNA) and proteins that constitute essential compounds of our blood, muscles and skin
• Organic chemicals appear in materials like clothing, fuels, polymers, dyes and medicines
• Chemists began to distinguish between organic compounds obtained from plants and animals and inorganic compounds prepared from mineral sources


  Around the year 1780
• Berzilius, a Swedish chemist proposed that a 'vital force' was responsible for the formation of organic compounds
• This notion was rejected in 1828 when F. Wohler synthesised an organic compound, urea from an inorganic compound, ammonium cyanate
• The pioneering synthesis of acetic acid by Kolbe (1845) and that of methane by Berthelot (1856) showed conclusively that organic compounds could be synthesised from inorganic sources in a laboratory
• The development of electronic theory of covalent bonding ushered organic chemistry into its modern shape
• Tetravalence of carbon


  Explained in terms of its electronic configuration and the hybridisation of s and p orbitals
• Hybridisation


  • Influences the bond length and bond enthalpy (strength) in organic compounds
  • The greater the s character of the hybrid orbitals, the greater is the electronegativity
• π (pi) bond formation

  • Parallel orientation of the two p orbitals on adjacent atoms is necessary for a proper sideways overlap
  • Rotation about carbon-carbon double bond (C=C) is restricted
  • The electron charge cloud of the π bond is located above and below the plane of bonding atoms
  • π bonds provide the most reactive centres in the molecules containing multiple bonds
• Structural representations of organic compounds

  1. Lewis structure or dot structure

  2. Dash structure

  3. Condensed structure

  4. Bond line structural formulas
• Complete structural formulas


  • Represent the two-electron covalent bond by a dash (-)
  • Single dash represents a single bond, double dash for double bond, triple dash for triple bond
  • Lone-pairs of electrons on heteroatoms may or may not be shown
• Condensed structural formulas


  Omit some or all of the dashes representing covalent bonds and indicate the number of identical groups attached to an atom by a subscript
• Bond-line structural formulas


  • Carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zig-zag fashion
  • The only atoms specifically written are oxygen, chlorine, nitrogen etc.
  • The terminals denote methyl (-CH3) groups (unless indicated otherwise by a functional group)
  • The line junctions denote carbon atoms bonded to appropriate number of hydrogens required to satisfy the valency of the carbon atoms
• Wedge-and-dash representation


  • Solid-wedge is used to indicate a bond projecting out of the plane of paper, towards the observer
  • Dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer
  • Bonds lying in plane of the paper are depicted by using a normal line (-)
• Molecular models


  • Framework model
  • Ball-and-stick model
  • Space filling model
• Classification of organic compounds

  • Acyclic or open chain compounds (aliphatic compounds)
  • Alicyclic or closed chain or ring compounds (homocyclic and heterocyclic)
  • Aromatic compounds (benzenoid and non-benzenoid)
• Alicyclic compounds


  • Cyclopropane
  • Cyclohexane
  • Cyclohexene
  • Tetrahydrofuran
• Alicyclic compounds exhibit some of the properties similar to those of aliphatic compounds
• Aromatic compounds


  Special types of compounds
• Types of aromatic compounds

  • Benzenoid aromatic compounds
  • Non-benzenoid compounds
  • Heterocyclic aromatic compounds
• Benzenoid aromatic compounds


  • Benzene
  • Aniline
  • Naphthalene
• Non-benzenoid aromatic compounds


  • Tropolone
• Heterocyclic aromatic compounds


  • Furan
  • Thiophene
  • Pyridine
• Organic compounds can also be classified on the basis of functional groups, into families or homologous series
• Functional Group


  An atom or group of atoms joined in a specific manner which is responsible for the characteristic chemical properties of the organic compounds
• Homologous Series


  A group or a series of organic compounds each containing a characteristic functional group
• Examples of homologous series

  • Alkanes
  • Alkenes
  • Alkynes
  • Haloalkanes
  • Alkanols
  • Alkanals
  • Alkanones
  • Alkanoic acids
  • Amines
• Organic chemistry deals with millions of compounds, so a systematic method of naming has been developed (IUPAC system of nomenclature)
• The IUPAC system of nomenclature correlates the names with the structure such that the reader or listener can deduce the structure from the name
• Before the IUPAC system, organic compounds were assigned names based on their origin or certain properties (trivial or common names)
• Common names are useful and in many cases indispensable, particularly when the alternative systematic names are lengthy and complicated
• Compounds containing carbon and hydrogen only are called hydrocarbons
• Saturated hydrocarbon


  Contains only carbon-carbon single bonds
• Unsaturated hydrocarbon


  Contains at least one carbon-carbon double or triple bond
• The IUPAC name for a homologous series of saturated hydrocarbons is alkane
• IUPAC names of some unbranched saturated hydrocarbons

  • Methane
  • Ethane
  • Propane
  • Butane
  • Pentane
  • Hexane
  • Heptane
  • Octane
  • Nonane
  • Decane
• Alkyl group


  Derived from a saturated hydrocarbon by removing a hydrogen atom from carbon
• Some alkyl groups


  • Methyl
  • Ethyl
  • Propyl
  • Butyl