chem 4

• Reactions of Alkenes and Alkynes

  
• Alkenes


  Organic compounds containing a carbon-carbon double bond

  
• Alkynes


  Organic compounds containing a carbon-carbon triple bond

  
• Addition to C=C and C≡C multiple bonds


  Various reagents X-Y added to yield saturated products

  
• Addition of HX to alkenes

  1. Step 1: Electrophilic attack on double bond

  2. Step 2: Carbocation formation and reaction with halide anion

  
• Addition of HX to alkenes

  • Regiospecific - only one product formed
  • Markovnikov's rule - H attaches to carbon with fewer alkyl substituents, X attaches to carbon with more alkyl substituents

  
• Carbocation


  Trivalent carbon atom with an unoccupied p orbital, sp2-hybridized

  
• Carbocation stability

  • Increases with increasing substitution
  • More highly substituted carbocations are more stable
• Addition of water to alkenes (hydration)


  Yields alcohols, requires acid catalyst and high temperatures
• Addition of halogens to alkenes (halogenation)


  1. Yields 1,2-dihaloalkanes

  2. Rapid test for unsaturation - Br2 decolorization

  
• Halogenation of cycloalkenes

  • Occurs with anti stereochemistry - bromine atoms come from opposite faces

  
• Oxidation


  Addition of oxygen or removal of hydrogen from a molecule
• Reduction


  Addition of hydrogen or removal of oxygen from a molecule
• Reduction of alkenes (hydrogenation)


  Addition of H2 to C=C bond, using metal catalyst like Pd or Pt

  
• Alkene hydrogenation in food industry


  • Unsaturated vegetable oils hydrogenated to produce saturated fats for margarine and cooking products
• Oxidation of alkenes (epoxidation)


  1. Reaction with peroxyacid to form epoxides

  2. Epoxide ring-opening with water to form diols

  
• Oxidation of alkenes (hydroxylation)


  Addition of -OH group to each carbon of C=C, using KMnO4 in basic solution
• Oxidation of alkenes (cleavage)


  KMnO4 in acidic solution cleaves the double bond, yielding carbonyl products

  
• Hydroxylation


  Addition of an -OH group to each of the -C=C-

  
• Hydroxylation reaction

  1. Reaction of an alkene with KMnO4 (oxidizing agent) in basic solution (mild NaOH)

  2. Yields a 1,2-diol (glycol)

  3. A syn-addition reaction

  
• Oxidation of alkenes in acidic KMnO4 solution

  1. Cleavage of the double bond

  2. Carbonyl-containing products are obtained

  
• Oxidation products from tetrasubstituted alkenes

  • Two carbonyl-containing products are ketones

  
• Oxidation products from alkenes with one hydrogen on the double bond

  • One of the carbonyl-containing products is a carboxylic acid

  
• Oxidation products from alkenes with two hydrogens on one carbon

  • CO2 is formed

  
• What alkene gives a mixture of acetone and propanoic acid on reaction with acidic KMnO4?

  
• Polymer


  A large—sometimes very large—molecule built up by repetitive bonding together of many smaller molecules, called monomers
• Biological polymers

  • Cellulose
  • Proteins
  • Nucleic acids
• Radical polymerization of an alkene

  Involves three kinds of steps: initiation, propagation, and termination

  
• Conjugated dienes


  Compounds that have alternating single and double bonds
• Conjugated dienes behave quite differently from nonconjugated dienes
• Conjugated dienes


  • There is an electronic interaction between the two double bonds because of p orbital overlap across the central single bond
• Electrophilic addition reactions of conjugated dienes with HX and X2


  • Mixtures of products are often obtained, unlike isolated alkenes which usually follow Markovnikov's rule

  
• Alkenes


  Organic compounds containing a carbon-carbon double bond

  
• Alkynes


  Organic compounds containing a carbon-carbon triple bond

  
• Addition to C=C and C≡C multiple bonds


  Various reagents X-Y added to yield saturated products

  
• Addition of HX to alkenes

  1. Step 1: Electrophilic attack on double bond

  2. Step 2: Carbocation formation and reaction with halide anion

  
• Addition of HX to alkenes

  • Regiospecific - only one product formed
  • Markovnikov's rule - H attaches to carbon with fewer alkyl substituents, X attaches to carbon with more alkyl substituents

  
• Carbocation


  Trivalent carbon that is sp2-hybridized with an unoccupied p orbital

  
• Carbocation stability

  • Increases with increasing substitution
  • More highly substituted carbocations are more stable
• Addition of water to alkenes (hydration)


  Yields alcohols, requires acid catalyst and high temperatures