Study Notes on Alkenes

Learning Outcome: Analyze the Electrophilic Addition of Halogens

• Alkenes are unsaturated hydrocarbons characterized by at least one carbon-carbon double bond ($C=C$), which acts as a nucleophilic site due to high pi-electron density.

• Halogens like chlorine ($Cl<em>{2}$) and bromine ($Br</em>{2}$) react with alkenes through electrophilic addition.

• Chemical Testing: A common test for unsaturation involves using bromine in carbon tetrachloride ($CCl_{4}$). The dark red-brown solution decolorizes instantaneously when added to an alkene, indicating the formation of a dihaloalkane. Alkanes do not undergo this reaction under similar conditions.

Learning Outcome: Evaluate Reactivity and Substitution Effects

• The rate of halogenation is significantly influenced by the degree of substitution around the double bond.

• Electron-donating alkyl groups increase the electron density of the $C=C$ bond, making it more reactive toward electrophiles.

• Reactivity Scale: The progression from ethene (rate = 1) to propene (61) to 2-methylprop-1-ene (5,400) and finally to 2,3-dimethyl-2-butene (920,000) demonstrates how increasing substitution accelerates bromination.

Learning Outcome: Describe the Mechanism of Halogen Addition

• The addition involves a two-step electrophilic mechanism:

  • Step 1 (Rate-Determining): Pi ($\pi$) electrons from the alkene attack a halogen molecule, leading to the formation of a cyclic bromonium (or chloronium) ion intermediate.

  • Step 2 (Fast): A halide ion attacks the cyclic intermediate, opening the ring to produce the final trans-dihalide product.

Learning Outcome: Detail Catalytic Hydrogenation and Industrial Uses

• Hydrogenation is the reduction of an alkene to an alkane by adding molecular hydrogen ($H_{2}$) across the double bond.

• Heterogeneous Catalysis: This process requires finely powdered metal catalysts such as nickel ($Ni$), palladium ($Pd$), or platinum ($Pt$). The reactants adsorb onto the metal surface, allowing the $H-H$ bond to break and hydrogen atoms to add to the same face of the alkene (syn-addition).

• Application: Partial hydrogenation is used in the food industry to produce margarine from plant oils by reducing the number of $C=C$ bonds, thereby raising the melting point.

Learning Outcome: Understand Geometric Isomerism and Physicochemical Trends

• Geometric (cis/trans) isomerism is a byproduct of restricted rotation around the $C=C$ bond. Any rotation would require breaking the pi bond, which requires substantial energy.

• Criteria: This isomerism occurs only if each carbon in the double bond is attached to two different substituent groups.

• Physical Differences: Geometric isomers possess different physical properties. For example, cis-1,2-dichloroethene has a boiling point of 60 °C and a melting point of -80 °C, while the trans isomer has a boiling point of 47 °C and a melting point of -50 °C.

Learning Outcome: Relate Alkene Isomerisation to Biological Systems

• In the chemistry of vision, the chromophore 11-cis-retinal (a penta-ene aldehyde) binds with the protein opsin to form rhodopsin.

• Light absorption causes the 11-cis-retinal to isomerize into its trans form at the C(11)-C(12) double bond.

• This geometric transition changes the shape of the molecule and triggers the nerve impulses translated by the brain as visual perception.