Steric hindrance refers to the crowding around a reactive site in a molecule, usually due to bulky groups attached near a double or triple bond. It can significantly affect addition reactions in the following ways:
1. Slows down the reaction
- Bulky groups near the double bond make it harder for the electrophile or nucleophile to approach the bond.
- As a result, the reaction occurs more slowly or may require harsher conditions.
2. Influences the pathway
- In reactions like hydrohalogenation or halogenation, steric hindrance can favor attack on the less hindered carbon of the double bond.
- This can change the major product compared to an unhindered alkene.
3. Affects stereochemistry
- Steric hindrance can prevent syn addition (same side) if there isn’t enough space.
- In such cases, anti addition (opposite side) may become more favorable.
4. Selective addition
- Highly hindered alkenes or alkynes may react only partially, stopping at the alkene stage instead of forming the fully saturated product.
- This is useful in synthesis when selective products are desired.
In short: Steric hindrance makes addition reactions slower, affects which carbon is attacked, influences stereochemistry, and can lead to selective reactions.