Second Trimester
Lecture 4: Alkenes: Reactions with Br2, HBr, Epoxides, Syn vs. Anti Addition
I. Bromination of Alkenes:
Mechanism of Bromination (Br2):
Step 1: Electrophilic Attack: Bromine (Br2) approaches the alkene and the pi electrons of the double bond attack one bromine atom, forming a bromonium ion intermediate.
Step 2: Nucleophilic Attack: A bromide ion (Br-) from the solvent attacks the carbon with a positive charge in the bromonium ion. This results in the formation of a dibromide product (R-CHBr-CHBr-R').
Regioselectivity and Stereochemistry:
Anti Addition: Bromination typically proceeds via an anti addition mechanism, where the two bromine atoms add to opposite faces of the double bond.
Regioselectivity: Bromine atoms add to the double bond in such a way that the bromine ends up on the more substituted carbon, following Markovnikov's rule.
II. Hydrobromination:
Addition of HBr to Alkenes:
Mechanism: HBr adds to the alkene in a similar fashion to bromination, forming a bromonium ion intermediate.
Markovnikov vs. Anti-Markovnikov Addition: The bromide ion can attack either carbon of the bromonium ion, resulting in Markovnikov (more substituted carbon) or Anti-Markovnikov (less substituted carbon) addition, depending on the reaction conditions.
III. Epoxide Formation:
Synthesis and Mechanism:
Mechanism of Epoxidation: Alkenes can be converted into epoxides (three-membered cyclic ethers) using peracids or mCPBA (meta-chloroperoxybenzoic acid).
Reaction: The alkene reacts with the peracid, resulting in the formation of an oxirane ring structure.
IV. Syn vs. Anti Addition:
Definition and Examples:
Syn Addition: When two substituents add to the same face of the double bond.
Anti Addition: When two substituents add to opposite faces of the double bond.
Comparison:
Mechanisms: Syn addition typically involves concerted addition where both groups add simultaneously from the same direction, while anti addition proceeds through a stepwise mechanism involving intermediate formation.