CHM030 4.1: Reaction of Alkenes

• The most important reaction of these two functional groups, alkenes and alkynes, is the addition of the C=C and C≡C multiple bonds of various reagents X—Y to yield saturated products.
• Addition of HX to alkenes

  hydrohalogenation
• General reaction mechanism of Hydrohalogenation

  electrophilic addition
• Examples of halogen acids (HX)


  HCl, HBR, HI
• Attack of the π bond from an alkene to an electrophile produces a carbocation and the halide anion
• regiospecific 


  only one product is formed

  only one of the two possible orientations of addition occurs
• Markovnikov's Rule 


  H will go to the carbon with more H
• Markovnikov's Rule in the addition of HX to an alkene:

  • the H attaches to the carbon with fewer alkyl substituents (H will go to the C with more H)
  • the X attaches to the carbon with more alkyl substituents
• TRUE OR FALSE (An explanation of Markovnikov's Rule)

  TRUE: more highly substituted carbocations are more stable than less highly substituted ones because alkyl groups tend to donate electrons to the positively charged carbon atom.
• TRUE OR FALSE (An explanation of Markovnikov's Rule)

  TRUE: the more alkyl groups there are, the more electron donation there is and the more stable the carbocation.

  
  

  methyl -> primary -> secondary -> tertiary
• TRUE OR FALSE (An explanation of Markovnikov's Rule)

  TRUE: carbocation stability increases with increasing substitution (directly proportional)
• Markovnikov's Rule in the addition of HX to an alkene:

  When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results.
• Hydration


  the addition of water to alkenes to yield alcohols (ROH)
• The addition of water to alkenes (hydration) requires a phosphoric acid catalyst (H3PO4) and reaction temperatures of up to 250 °C which can destroy sensitive molecules
• Addition of Water to Alkenes: Hydration

  In the case of simple alkenes, hydration follows Markovnikov’s rule:

  H+ of H2O adds to the carbon of the double bond with the greater number of hydrogens, and OH- of H2O adds to the carbon with the smaller number of hydrogens.
• Halogenation


  the addition of halogens such as Br and Cl to alkenes to yield 1,2-dihaloalkenes
• Addition of Halogens to Alkenes: Halogenation

  When the halogenation reaction is carried out on a cycloalkene, such as cyclopentene, only trans -1,2-dibromocyclopentane is formed rather than the mixture of cis and trans products that might have been expected if a planar carbocation intermediate were involved
• What type of stereochemistry is halogenation

  anti-stereochemistry: the two bromine atoms come from

  opposite faces of the double bond approximately 180° apart
• Reduction


  in organic chemistry, it usually refers to the addition of hydrogen or removal of oxygen from a molecule
• Hydrogenation


  Addition of H2 to the C=C bond occurs when an alkene is exposed to an atmosphere of hydrogen gas in the presence of a metal catalyst to yield an alkane.
• Reduction of Alkenes: Hydrogenation

  For most alkene hydrogenations, either palladium metal or platinum (as PtO2 ) is used as the catalyst.
• What type of stereochemistry is hydrogenation

  syn stereochemistry: both hydrogens add to the double bond from the same side
• TRUE OR FALSE

  TRUE: Catalytic hydrogenation is a heterogenous process:
• TRUE OR FALSE

  TRUE: The hydrogenation reaction occurs on the surface of solid catalyst particles rather than in solution.
• Oxidation


  in organic chemistry, it usually refers to the addition of oxygen or removal of hydrogen from a molecule
• Oxidation of Alkenes: Epoxidation

  Epoxides undergo an acid-catalyzed ring-opening reaction with water (hydrolysis) to give the corresponding dialcohol, or diol, also called a glycol.
• Oxidation of Alkenes: Epoxidation

  The net result of the two-step alkene epoxidation/hydrolysis is thus a hydroxylation — the addition of an ‒OH group to each of the two double-bond carbons.
• Hydroxylation


  addition of an – OH group to each of the –C=C–
• Oxidation of Alkenes: Hydroxylation

  • carried out in a single step by reaction of an alkene with KMnO4 (oxidizing agent) in basic solution (mild NaOH)
  • yields a 1,2-diol (glycol)
  • a syn-addition reaction
• When oxidation of the alkene is carried out with KMnO4 in acidic rather than basic solution, cleavage of the double bond occurs and carbonyl-containing products are obtained.
• Oxidation of Alkenes: Cleavage

  If the double bond is tetrasubstituted, the two carbonyl containing products are ketones (R−C(=O)−R')
• Oxidation of Alkenes: Cleavage

  If a hydrogen is present on the double bond, one of the carbonyl-containing products is a carboxylic acid (R−COOH)
• Oxidation of Alkenes: Cleavage

  If two hydrogens are present on one carbon, carbon dioxide (CO2) is formed