Answer
Steric Hindrance Prevents the Addition of a Third t-Butyl Group
Alkylation reactions typically proceed by nucleophilic substitution, often involving a base and an alkyl halide. In the case of adding t-butyl groups (–C(CH3)3) to a molecule, steric effects play a dominant role in determining how many groups can be added.
🔴 Bulky Nature of the t-Butyl Group
- The t-butyl group is extremely large due to three methyl groups attached to a central carbon atom.
- Once one or two t-butyl groups are attached, they create significant steric hindrance — spatial crowding around the reactive site.
- This crowding makes it physically difficult for a third bulky group to approach and react with the molecule.
⚠️ Decreased Reactivity of the Substrate
- As more bulky groups are added, the electrophilic center becomes less accessible to further nucleophilic attack.
- The substrate’s reactivity drops significantly after two substitutions due to this shielding effect.
- Even strong bases or elevated temperatures may not overcome the spatial restriction imposed by two large t-butyl groups.
🧪 Electronic and Kinetic Factors
- Each added alkyl group contributes electron density, further destabilizing the transition state for additional alkylation.
- The reaction becomes both kinetically slower and thermodynamically less favorable after the second alkylation.
✅ Summary
The inability to add a third t-butyl group is primarily due to steric hindrance. After two bulky groups are attached, the reaction site becomes so crowded that it prevents further substitution. This spatial limitation, combined with reduced reactivity of the already substituted molecule, effectively halts the reaction after the second alkylation.