The reaction between CH3OH (methanol) and an alkyl halide can undergo different mechanisms depending on the specific conditions and reactants involved. However, in general, the reaction between CH3OH and an alkyl halide is more likely to proceed via an E2 (elimination) mechanism rather than an SN2 (substitution) mechanism. Here's why:
Nucleophilicity of Methoxide (CH3O-): Methanol (CH3OH) can act as a nucleophile when it is deprotonated to form methoxide ion (CH3O-). In an SN2 mechanism, the nucleophile directly attacks the alkyl halide's electrophilic carbon, leading to substitution. However, the nucleophilic strength of methoxide is relatively weak compared to other stronger nucleophiles, making SN2 less favorable.
Steric Hindrance: SN2 reactions are favored when the alkyl halide has a primary (1°) or, in some cases, secondary (2°) carbon, as there is less steric hindrance around the electrophilic carbon. However, when dealing with tertiary (3°) alkyl halides, the steric hindrance becomes significant, and SN2 reactions become highly unfavorable due to the difficulty of nucleophiles attacking the crowded carbon.
Basicity of Methoxide: In an E2 mechanism, the reaction involves the removal of a proton (hydrogen) from a carbon adjacent to the leaving group (alkyl halide) and the simultaneous elimination of the halide ion. Methoxide ion (CH3O-) is a relatively strong base, and it can readily abstract a proton from the carbon adjacent to the halide, especially in the case of tertiary alkyl halides. This promotes the E2 pathway.
Elimination vs. Substitution: E2 reactions are favored over SN2 reactions when dealing with tertiary alkyl halides due to the reasons mentioned above. In this case, the stronger base, methoxide (CH3O-), will likely prefer to participate in an E2 elimination to form an alkene rather than a substitution reaction to replace the halide.
It's important to note that the specific reaction conditions and the nature of the alkyl halide and nucleophile can influence the outcome of the reaction. For example, SN2 reactions can be more prevalent in cases where the alkyl halide is primary or secondary and when using a stronger nucleophile. Nonetheless, the factors mentioned above generally make E2 elimination more favorable when methoxide (CH3O-) is involved as a nucleophile.