Second Trimester
Lecture 9 Alkenes - Final Exam Rapid Review and Comprehensive Reaction Guide
1. Electrophilic Addition of HBr:
Step 1: Protonation: The pi electrons of the alkene attack the hydrogen of HBr, forming a carbocation intermediate on the more substituted carbon (Markovnikov's rule).
Step 2: Nucleophilic Attack: The bromide ion (Br-) attacks the carbocation, resulting in the formation of the alkyl bromide product.
2. Bromination (Br2):
Step 1: Formation of Bromonium Ion: The pi electrons of the alkene attack one bromine atom, leading to the formation of a cyclic bromonium ion intermediate.
Step 2: Nucleophilic Attack: A bromide ion (Br-) attacks the more substituted carbon of the bromonium ion from the opposite side, leading to anti addition of bromine atoms across the double bond.
3. Hydroboration-Oxidation (BH3 followed by H2O2/NaOH):
Step 1: Hydroboration: BH3 adds to the alkene in a syn addition manner, with boron attaching to the less substituted carbon.
Step 2: Oxidation: The boron-alkyl intermediate is oxidized by hydrogen peroxide (H2O2) in a basic medium, converting the boron group to a hydroxyl group (OH) with anti-Markovnikov regioselectivity.
4. Ozonolysis (O3):
Step 1: Formation of Ozonide: Ozone reacts with the alkene to form an unstable ozonide intermediate.
Step 2: Cleavage: The ozonide cleaves to form two carbonyl compounds (aldehydes or ketones), depending on the substituents on the original alkene.
5. Hydrogenation (H2 with Pd/C catalyst):
Step 1: Adsorption: Hydrogen molecules adsorb onto the palladium catalyst surface.
Step 2: Addition: Hydrogen atoms add syn to the alkene, reducing the double bond to a single bond, forming an alkane.
6. Epoxidation (mCPBA):
Step 1: Peracid Reaction: The alkene reacts with a peracid such as mCPBA (meta-chloroperoxybenzoic acid), forming an epoxide.
Step 2: Ring Formation: The oxygen from the peracid forms a three-membered ring with the carbons of the double bond, creating an epoxide (oxirane).
7. Syn Dihydroxylation (OsO4 followed by NaHSO3):
Step 1: Formation of Osmate Ester: OsO4 reacts with the alkene to form a cyclic osmate ester intermediate.
Step 2: Hydrolysis: The osmate ester is hydrolyzed to form a vicinal diol, with both hydroxyl groups adding to the same face of the double bond (syn addition).