Periodic Table of Elements - Organic Chemistry

Organic Chemistry - Alkenes (Unsaturated)

Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond C=C between two adjacent cabon atoms.

  • The general formula for alkenes is CnH2n.
  • The simplest alkene is ethylene (C2H4), which has the IUPAC name ethene.

    • Alkenes Structure:
    ethene (commony known as ethylene):

    HH
    ||
    H-C=C-H

    propene (commony known as propylene):

    HHH
    |||
    H-C=C-C-H

  • Alkenes are also called as olefins.
  • The acidity levels of alkenes are much higher than the ones in alkanes.
  • The physical state depends on molecular mass. The simplest alkenes, ethene, propene and butene are gases. Linear alkenes of approximately five to sixteen carbons are liquids, and higher alkenes are waxy solids.
  • For alkenes the distinctive reaction type is Addition.
  • Alkenes are generally more reactive than the related alkanes.
  • The alkenes are insoluble in water, but are soluble in organic solvents.
  • Alkenes in which carbon atoms are arranged in ring (cyclic) structure are called cycloalkenes. Ex. cyclopentene
  • Boiling points increase with increasing molecular mass i.e. number of carbon atoms.

    • Alkenes Preparation:
    Alkenes are generally prepared through:

  • Alcohol dehydration
  • Alkyl halide dehydrohalogenation
  • Vicinal dihalide dehalogenation
  • Cracking: is the breakdown of a large alkane into smaller, more useful alkanes and alkenes by using high pressure and temperature in red hot tube.
    • CH3CH2CH3 C2H4 + CH4

  • Hydration: Addition of water to alkanes (hydration) forms aliphatic alcohols.
    • ethylene + water ethanol
    H2C=CH2 + H2O C2H5OH
  • Combustion of Alkenes : The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water.
    • ethylene + oxygen carbon dioxide + water
    H2C=CH2 + 3O2 2CO2 + 2H2O
  • Hydrogenation (Addition of Hydrogen): Hydrogenation of alkenes produces the corresponding alkanes. The reaction is carried out under pressure at a temperature of 200 °C in the presence of a metallic catalyst. Usually catalysts are based on nickel, platinum or palladium. For example, the catalytic hydrogenation of ethylene produces ethane.
    • ethylene/ethene + oxygen ethane
    H2C=CH2 + H2 CH3CH3
  • Halogenation (Addition of Halogen): When alkenes reacts with halogens, halogens readily add across the double bond of the alkenes to form dihalides. For example, the addition of elemental bromine or chlorine to alkenes produces vicinal dibromo- and dichloroalkanes respectively.
    • ethane + bromine dibromoethane
    H2C=CH2 + Br2 CH2BrCH2Br
    ethane + chlorine dichloroethane
    H2C=CH2 + Cl2 CH2ClCH2Cl

    Oxidation reactions of alkenes form carbon-oxygen bonds.
    Types of oxidation reactions of alkenes:

  • epoxidation of alkenes:
    • alkene + peroxyacid epoxide (oxyrane) + acid
  • hydroxylation of alkenes:

    • Reaction with concentrated, hot potassium permangante KMnO4 or other oxidizing salts like osmium tetroxide OsO4 in an acidic solution will yield ketones or carboxylic acids.

  • ozonolysis of alkenes:

    • Reaction with ozone leads to the breaking of the double bond, yielding two aldehydes or ketones.

    Alkene nomenclature:

  • The naming convention of the alkenes is same as alkanes except that the suffix -ene is used instead of -ane. The suffix "ene" in organic chemistry always refers to a carbon-carbon double bond.
  • The root name is based on the longest continuous chain containing both ends of the alkene unit, the C=C..
  • The chain is numbered so as to give the alkene unit the lowest possible numbers.
  • The locant for the first carbon of the alkene is used in the name.