Addition Reactions of Alkenes and Markovnikov's Rule

Addition Reactions of Alkenes

General form:

 

For a cyclic compound:

 

Catalytic Hydrogenation

  • hydrogenationhydrogenation- the process of adding hydrogen to carbon-carbon double bonds to make the molecule more saturated in hydrogens
  • This can be done by passing hydrogen gas through a solution of an alkene in the presence of a metal catalyst such as Pd, Pt, or Ni.
  • The product is an alkane.
  • having either the cis- or trans- isomer of the molecule gives you the same product
  • The carbons in the double bond go from sp2 hybridized to sp3 hybridized
  • This reaction occurs because sigma bonds tend to be stronger than pi bonds, so the pi bond in the double bond is broken easily.
  • This process is enthalpy favored because the alkane’s single bonds are at a lower energy level than the alkene’s double bond(s).
  • This process is not entropy favored because you are going from 2 reactants to 1 product.
  • This reaction is enthalpy driven

 

Addition of HCl or HBr to form a Haloalkane

  • The carbons in the double bond go from sp2 hybridized to sp3 hybridized
  • This reaction occurs because sigma bonds tend to be stronger than pi bonds, so the pi bond in the double bond is broken easily.
  • This process is enthalpy favored because the alkane’s single bonds are at a lower energy level than the alkene’s double bond(s).
  • This process is not entropy favored because you are going from 2 reactants to 1 product.
  • This reaction is enthalpy driven

 

Addition of Water to an Acid Catalyst to form an Alcohol

  • the catalyst is the sulfuric acid
  • H corresponds to A and OH corresponds to B in the general form

 

Markovnikov’s Rule

  • The formation of haloalkane and alcohol are regioselectiveregioselective reactions- meaning the reaction favors the formation/breaking of a bond at a specific location over another
      * In other words- “Hydrogen goes where hydrogen is”
  • For example, the below reaction has two possible products:

   

  • The product at the top is favored because it is more substituted
  • why is the more substituted product favored?- there’s a carbocation intermediate in the reaction mechanism

  ## Carbocation Stability

  Order of carbocation stability:

  
  1. tertiary carbocations- carbocations that are attached to three carbon atoms/groups
  2. secondary carbocations- carbocations that are attached to two carbon atoms/groups
  3. primary carbocations- carbocations that are attached to one carbon atom/group
  4. methyl carbocations- a free-standing methyl group where the carbon has a positive charge

  • Knowing this, we can see that the top product is favored because it allows for the formation of an intermediate tertiary carbocation that is more stable.
      * because it’s more stable, this intermediate tertiary carbocation forms more rapidly

Mechanism of addition of HCl to an Alkene

 

  • step 1 is the slower step and determines the rate of reaction
  • the addition of water to form an alcohol has a similar mechanism, and the more substituted alcohol is the major product