Addition Reactions of Alkenes

General form:

For a cyclic compound:

Catalytic Hydrogenation

• hydrogenation- 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 regioselective 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