Tertiary alkyl halides undergoing elimination reactions in the presence of a strong base like LDA (Lithium diisopropylamide) in THF (tetrahydrofuran) tend to favor the E2 pathway over E1 and SN1 reactions.
This preference is primarily due to the characteristics of LDA as a strong, non-nucleophilic base that promotes bimolecular elimination (E2). In E2 reactions, the base abstracts a proton from a β-carbon of the alkyl halide while the leaving group departs simultaneously, resulting in the formation of an alkene. The concerted nature of this mechanism is particularly favorable for tertiary substrates due to steric hindrance; there are fewer steric barriers for the base to access β-hydrogens compared to approaches that would occur in E1 or SN1 mechanisms.
In contrast, E1 and SN1 mechanisms involve the initial formation of a carbocation, which can be more favorable for secondary and primary substrates but less so for tertiary ones when a strong base is used. Even though tertiary carbocations are stable, the presence of LDA and the reaction conditions (like the polar aprotic solvent THF) still favor the E2 mechanism because the rate of an E1 or SN1 process would require the formation of a carbocation, which is more pronounced in polar protic solvents.
Moreover, since LDA is a strong base and THF is a polar aprotic solvent, it helps facilitate the E2 mechanism by efficiently removing the proton while avoiding carbocation rearrangements or competing nucleophilic substitutions.
In summary, the strong basicity of LDA and the characteristics of the reaction environment in THF favor the E2 elimination pathway for tertiary alkyl halides, leading to the formation of alkenes over substitution or unimolecular elimination pathways.