Addition Reactions of Alkenes

Section 12.4 Pages 332-339

Addition Reaction

Addition of two atoms or groups of atoms to a double bond.

R can be carbon or hydrogen.

R

R R

R

+A

B

C

C

A

R

R

R

R

B

Types of Addition Reactions

The major alkene addition reactions include Addition of hydrogen (H2) – hydrogenation Addition of halogens (Cl2 or Br2) –

halogenation Addition of water (H2O) – hydration Addition of hydrogen halides (HBr or HCl) –

hydrohalogenation Note: Double bond becomes a single bond.

Hydrogenation: Adding H2

Double bond is broken H atom added to both sides of double bond Platinum, palladium, or nickel catalyst

required Heat and/or pressure may also be needed

R

R R

R

+H

H

C

C

H

R

R

R

R

H

Pt, Pd, or Ni

heat or pressure

Examples of Hydrogenation

Hydrogenation of 1-propene to propane

Hydrogenation of trans-2-butene

C C

H H

CH3H

+ H HPd

C C CH3

H

H

H

H

H

C C

H3C H

CH3H

+ H HPd

C C CH3

H

H

H

H

H3C

YOU TRY THESE!

Write hydrogenation reactions for: 1-pentene 2-methyl-2-butene ethene

Halogenation: Adding Br2 or Cl2

This reaction occurs easily and does not require a catalyst.

R

R R

R

+Br

Br

C

C

Br

R

R

R

R

Br

Examples of Halogenation

Bromination of 1-propene to form 1,2-dibromopropane

Chlorination of trans-2-butene to form 2,3-dichlorobutane

C C

H H

CH3H

+ Br Br C C CH3

Br

H

Br

H

H

C C

H3C H

CH3H

+ Cl Cl C C CH3

Cl

H

Cl

H

H3C

YOU TRY THESE!

Write halogenation reactions for: 1-pentene with bromine 2-methyl-2-butene with chlorine Ethene with chlorine

Hydration: Addition of H2O

Requires a trace of acid (H+) as a catalyst The product is an alcohol Markovnikov’s rule applies

R

R R

R

+H

OH

C

C

H

R

R

R

R

OH

Markovnikov’s Rule

A mixture of products is formed when an unsymmetrical alkene is hydrated.

One product is always favored over the other. Vladimir Markovnikov came up with a rule to

determine the major product formed. The carbon of the carbon-carbon double

bond that originally has more hydrogen atoms receives the hydrogen atom being added.

“The rich get richer.”

Example of Markovnikov’s Rule

Hydration of 1-propene to form 2-propanol (major) and 1-propanol (minor)

C C

H H

CH3H

+ H OH

C C CH3

OH

H

H

H

H

C C CH3

H

H

OH

H

H

Major product

Minor product

H+

YOU TRY THESE!

Write the hydration equation for each of the following molecules. Indicate the major and minor products for the reactions where applicable. 1-pentene 2-methyl-2-butene ethene

Hydrohalogenation: Adding HCl or HBr

Follows Markovnikov’s rule

R

R R

R

+H

Cl

C

C

H

R

R

R

R

Cl

Examples of Hydrohalogenation

Hydrobromination of 1-propene to form 2-bromopropane (major) and 1-bromopropane (minor)

C C

H H

CH3H

+ H Br

C C CH3

Br

H

H

H

H

C C CH3

H

H

Br

H

H

Major product

Minor product

YOU TRY THESE!

Write the hydrohalogenation equation for each of the following molecules. Indicate the major and minor products for the reactions where applicable. 1-pentene with HCl 2-methyl-2-butene with HBr Ethene with HCl