Second Trimester
Lecture 5: Alkenes: E vs Z Isomerism and Reactions with HBr, Cl2, H2/Pd
I. E vs Z Isomerism:
Definition: E (trans) and Z (cis) isomers refer to the relative spatial arrangement of substituents around a double bond.
Determining E vs Z:
Priority Rule: Assign priorities to substituents based on atomic number or molecular weight.
Geometry: E (trans) if higher priority groups are on opposite sides; Z (cis) if they are on the same side.
II. Reactions with HBr:
Mechanism of Addition:
Electrophilic Addition: HBr adds to the double bond of an alkene.
Formation of Bromonium Ion: Br2 approaches the alkene, and one bromine atom is attacked by the pi electrons of the double bond, forming a bromonium ion.
Nucleophilic Attack: A bromide ion (Br-) attacks the bromonium ion, resulting in a product where the bromine adds to the more substituted carbon (Markovnikov addition).
III. Reactions with Cl2:
Mechanism of Chlorination:
Formation of Chloronium Ion: Cl2 approaches the alkene, and one chlorine atom is attacked by the pi electrons of the double bond, forming a chloronium ion.
Nucleophilic Attack: A chloride ion (Cl-) attacks the chloronium ion, resulting in a product where the chlorine adds to the more substituted carbon (Markovnikov addition).
IV. Hydrogenation with H2/Pd:
Hydrogenation Mechanism:
Catalytic Hydrogenation: Alkenes react with hydrogen (H2) in the presence of a palladium (Pd) catalyst.
Adsorption: H2 molecules adsorb onto the Pd surface.
Addition: Hydrogen atoms add across the double bond in a syn addition manner, resulting in the formation of an alkane.