Alkenes

• Alkenes


  Also called olefins, contain a carbon-carbon double bond
• Types of alkenes


  • Terminal alkenes (double bond at end of chain)
  • Internal alkenes (at least one carbon atom bonded to each end of double bond)
  • Cycloalkenes (double bond in a ring)
• General formula of alkenes

  CnH2n (unsaturated hydrocarbons)
• Degree of saturation


  Maximum number of hydrogen - Actual number of hydrogen
• Structure and bonding of alkenes

  • Double bond consists of a σ bond and a π bond
  • Each carbon is sp2 hybridized and trigonal planar, with bond angles of approximately 120°
• Geometrical isomerism in alkenes

  Restricted rotation of C=C bonds leads to cis (Z) and trans (E) forms
• Cis (Z)


  Groups/atoms are on the same side of the double bond
• Trans (E)


  Groups/atoms are on opposite sides across the double bond
• Nomenclature of alkenes


  1. Identify the longest carbon chain containing the double bond

  2. Number the carbon chain starting from the end closest to the double bond

  3. Name the alkene based on the number and position of the double bond
• Physical properties of alkenes

  • Exhibit weak van der Waals interactions, similar to alkanes of comparable molecular weight
  • Have low melting and boiling points, increasing with number of carbons
  • Soluble in organic solvents, insoluble in water
  • C-C single bond between alkyl group and double bond carbon is slightly polar
• Cis and trans isomers


  Often have different physical properties due to the dipole moment
• Stability of alkenes


  Tetrasubstituted > Trisubstituted > Disubstituted > Monosubstituted
• Interesting alkenes


  • Ethylene, an industrial starting material
  • Five naturally occurring alkenes
• Preparation of alkenes


  1. From alkyl halides and alcohols via elimination reactions

  2. Elimination reactions are stereoselective and regioselective
• Reactions of alkenes


  • Characteristic reaction is addition, where the π bond is broken and two new σ bonds are formed
  • Alkenes are electron-rich and react with electrophiles
• Addition reactions of alkenes

  1. Hydrohalogenation (electrophilic addition of HX)

  2. Hydration (electrophilic addition of water)

  3. Hydroboration-oxidation
• Markovnikov's rule


  In the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom
• Exceptions to Markovnikov's rule can occur, such as with peroxides
• Rearrangements can occur in some hydrohalogenation reactions
• Hydroboration-oxidation


  A two-step reaction sequence that converts an alkene into an alcohol
• Hydrohalogenation—Electrophilic Addition of HX

  1. H2O + NaCl

  2. CH3CHCH2 + CH3CHCH2 + CH3CHCH2

  3. Br Br

  4. OH Br

  5. Cl Br
• Hydration


  Addition of water to an alkene to form an alcohol
• Hydration—Electrophilic Addition of Alcohols

  Alcohols add to alkenes, forming ethers
• Consequences of carbocation intermediates


  • Markovnikov's rule holds
  • Addition of H and OH occurs in both syn and anti fashion
  • Carbocation rearrangements can occur
• Hydroboration—Oxidation


  Converts an alkene into an alcohol
• Hydroboration—oxidation results in the addition of H2O to an alkene
• Oxymercuration-demercuration


  Addition of water to an alkene to form an alcohol
• Oxymercuration-demercuration


  • Markovnikov orientation
  • 100% yields
  • No rearrangements
• Halogenation—Addition of Halogen


  Addition of X2 (X = Cl or Br) to an alkene to form a vicinal dihalide
• Halogenation


  Addition of halogens to alkenes
• • No rearrangements occur in halogenation
  • Only anti addition of X2 is observed
  • Carbocations are not intermediates</b>
• Halohydrin Formation


  Addition of the elements of X and OH to the double bond
• Addition of hydrogen (reduction)

  1. Requires catalyst

  2. Synthesis of alkanes
• Hydroxylation of Alkene with KMnO4

  Alkenes reacts with cold alkaline solution of KMnO4 or OsO4 to form a diol
• Ozonolysis


  Cleavage of alkenes to form carbonyl compounds
• Polymerization


  Process whereby many small and simple molecules join together to produce very large molecules called polymers
• Polymer


  Large molecules made up of many monomers
• Monomer


  Simpler substance of which polymer is made
• Addition Polymerization


  Monomers' double-bonds open up to form continuous chain
• Condensation Polymerization


  Elimination of smaller molecule when functional groups react