Reactions With Alkenes

Organic Reactions

** ALKENES **

General Formula: CnH2n


Reactivity:

* Non-polar molecules with Van Der Waals intermolecular forces, so not soluble in water

* Volatile (4 first alkenes are all gaseous at room temperature)

* Unsaturated (C=C), so more reactive than alkanes:

- second bond weaker, so easily broken

- C=C rich in electrons (4e) attracts electrophiles groups


Main Reactions

Addition Reactions + Polymerization

ADDITION (Halogenation)

Alkene + Halogen (F2, Cl2, Br2, I2) → Dihaloalkane


Halogenation:

Addition of a halogen yields to dihaloalkanes


Example:

C2H4 + Cl2 → C2H4Cl2


Uses:

*Makes starting material for PVC and indication of alkenes

* Bromine dissolves in dichloromethane is used to distinguish between alkanes and alkenes. If reddish-brown color of Br2 disappears when added to unknown, the unknown has alkenes in it.

Addition (Hydrogenation)

Alkene + Hydrogen → Platinum or Palladium Catalyst


(also called Reduction)


Hydrogenation:

Addition of Hydrogen with Pt or Pd catalyst yields to Alkanes


Example:

C2H4 + H2 → C2H6

(PtO2/Pd)


Uses:

Hydrogenation of oil for margarine


Addition (Hydrogen Hallides)

Alkene + Acid Hallide (HF, HCl, HBr, HI) → Haloalkane


Addition of Hydrogen Hallides yield to Haloalkane


Example:

C2H4 + HCl → C2H5Cl


Uses:

Haloalkanes are useful starting materials for organic syntheses.


Addition (Hydration)

Alkene + Water → Alcohol


Hydration:

Addition of water in acidic condition yields to alcohols.


Example:

C3H6 + H2O → C3H8O

(acid)


Uses:

Commercial manufacture of ethanol.


Polymerization

Polyethylene


a. Two kinds of polyethylene

HDPE = gallon milk cartons (more rigid)

LDPE = plastic bags, squeeze bottles (more flexible)


b. Reaction

nCH2=CH2 → [-CH2-CH2-]n


c. Monomer

Ethylene

CH2=CH2


d. Conditions

Temp: 200 C

Pressure: 2000 atm

Initiator: O2

Polymerization of Alkenes with Acid