Hydroboration of alkenes
Overview:
The general form of the hydroboration of alkenes mechanism is as follows:
First step is the attack of the alkene on BH3, which then forms a four membered ring intermediate of partial bonds. It is because of this intermediate that hydroboration forms the anti-Markovnikov product. The boron atom is highly electrophilic because of its empty p orbital (ie. it wants electrons), and forms a slight bonding interaction with the pi bond. Since some electron density from the double bond is going towards bonding with the boron, the carbon opposite the boron is slightly electron deficient, left with a slightly positive charge. Positive charges are best stabilized by more highly substituted carbons, so the carbon opposite the boron tends to be the most highly substituted. Once the transition state breaks down, BH2 is attached to the least substituted carbon.
Peroxide then removes the borane and replaces it with the alcohol to form the anti-markovnikov product.
An example of the hydroboration mechanism:
Nucleophilic addition to carbonyl groups
Overview:
The general form of the nucleophilic addition to carbonyl group mechanism is as follows:
First step is the attack of the nucleophile on the partially positive carbon to make the tetrahedral intermediate with the full negatively charged oxygen. The oxygen then becomes protonated to yield the alcohol.
Variety of nucleophiles:
- Grignard Reagents
- Alcohols
- Amines
- Alkyl Lithium Reagents
- Acetylide Ions
Example of nucleophilic addition to carbonyl groups:
In this case, acetylide anion is acting as the nucleophile
Alcohol dehydration
Overview:
The general form of alcohol dehydrations is as follows:
The first step involves the protonation of the alcohol by an acid, followed by loss of water to give a carbocation.
Elimination occurs when the acid conjugate base plucks off a hydrogen. Alcohol dehydrations generally go by the E1 mechanism.
Example of alcohol dehydration:
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