Lesson 3: Alkenes Naming and Hydroboration-Oxidation Mechanism with CL2/CCl4

Hydroboration-Oxidation Mechanism:

1. Hydroboration:

   • Step 1: Hydroboration: Alkenes react with borane (BH3) in the presence of a base such as tetrahydrofuran (THF). The pi electrons of the double bond attack the boron atom in BH3, forming a three-membered cyclic transition state.

   • Step 2: Addition: This results in the formation of an intermediate called trialkylborane (R2B-H). The addition of BH2 group to the less substituted carbon of the double bond is a characteristic of this mechanism.

2. Oxidation:

   • Step 3: Oxidation: The intermediate trialkylborane undergoes oxidation with hydrogen peroxide (H2O2) and a base (such as NaOH). This process results in the replacement of the BH2 group with a hydroxyl group (OH), yielding an alcohol (R-OH).

   • Regioselectivity: This reaction is regioselective, meaning that the hydroxyl group (OH) attaches to the more substituted carbon of the former double bond, following Markovnikov's rule.

Reaction with Cl2/CCl4:

1. Chlorination:

   • Step 1: Formation of Chloronium Ion: Alkenes react with chlorine (Cl2) in the presence of a solvent like carbon tetrachloride (CCl4). The pi electrons of the double bond attack one chlorine atom, forming a chloronium ion (a cyclic three-membered ring with a positive charge on the chlorine).

   • Step 2: Nucleophilic Attack: A chloride ion (Cl-) from the solvent attacks the carbon with a positive charge in the chloronium ion. This results in the formation of a dihalide product (R-CHCl-CHCl-R').

2. Stereochemistry:

   • Anti Addition: Chlorination in the presence of a solvent like carbon tetrachloride typically proceeds via an anti addition mechanism, where the two chlorine atoms add to opposite faces of the double bond.

3. Regioselectivity:

   • Markovnikov's Rule: The chlorine atoms add to the double bond in such a way that the chlorine atom ends up on the more substituted carbon, following Markovnikov's rule.