13.1b Hydrocarbons - Alkenes

• Chemistry
• Secondary Education
• Age 12,13

ALKENES

Organic Chemistry

06 March 2024

Expectations

• Intro to alkenes
• Physical properties of alkenes
• Reactions of alkenes
• Practice Work

ALKENES

Organic Chemistry

06 March 2024

What are alkenes?

• unsaturated hydrocarbons
• contain at least one carbon-carbon double bond (C=C)
• form the homologous series 
• general formula: CnH2n

Physical Properties of Alkenes

• similar to that of alkanes
• can exhibit both structural and stereoisomerism

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

Nucleophiles and Electrophiles

Reactivity of Alkenes

• more reactive because they are unsaturated
• C=C bond contains a σ bond and a π bond 
• π bond = region of high electron density, hence, open to be attacked by electrophiles

Reactivity of Alkenes

• undergo electrophilic addition reaction = addition of an electrophile across C=C bond

General Mechanism of Electrophilic Addition in Symmetrical Alkenes

(carbocation)

General Mechanism of Electrophilic Addition in Symmetrical Alkenes

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

General Mechanism of Electrophilic Addition in Unsymmetrical Alkenes

"Markovnikov's rule"

• " In adding H-X to a C=C bond of an unsymmetrical alkene, the hydrogen atom attaches itself to the carbon atom with the greatest number of hydrogen atoms."

Classifications of Hydrocarbon according to the number of R-groups attached to the carbocation

• primary carbocation
• secondary carbocation
• tertiary carbocation

The Rs are alkyl groups; they can be the same or different. Carbocation is formed as an intermediate when the electrophile bonds with the carbon atom.

most stable

Stability

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

Reactions that Alkenes undergo

• Addition of hydrogen, H2 (Hydrogenation)
• Addition of steam, H2O (Hydration)
• Addition of halogen, X2 (Halogenation), where X is a halogen
• Addition of hydrogen halide, HX, where X is a halogen
• Oxidation by cold and hot potassium manganate (VII), KMnO4
• Addition polymerisation

Hydrogenation

• Reagent: hydrogen gas, H2 
• Condition: heat in the presence of nickel, Ni catalyst at 150oC
• Product: Alkanes

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

Hydration

• Reagent: steam, H2O 
• Condition: heat the gaseous alkene at 330oC and 60 atm in the presence of phosphoric 
  (V) acid, H3PO4, as a catalyst
• Product: Alcohols

Hydration (Mechanism of the reaction)

Hydration of Propene

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

Halogenation

• Reagent: halogen, X2 (in tetrachloromethane or aqueous)
  
• Condition: heat the gaseous alkene at 330oC and 60 atm in the presence of phosphoric 
  (V) acid, H3PO4, as a catalyst
• Product: Alcohols

Halogenation (Mechanism of the reaction)

Halogenation of ethene (bubbling into Aq Br2)

Two products are obtained when ethene is bubbled into aqueous Br2 at room temperature in the dark.

Br2 in CCl4 is used as a test to differentiate alkanes and alkenes as they are colourless. Alkene will decolourise the brown colour of bromine whereas alkane will not.

REACTIONS OF ALKENES

Organic Chemistry

22 February 2024

Practice Work

• Name the first member of the homologous series of alkenes.
• What is the molecular formula of the alkene containing 18 carbon atoms and one C=C bond?
• State the conditions used in the reaction between an alkene and hydrogen.
• Name the product formed when propene reacts with chlorine.