Which type of halides are optimal for SN2 reactions?

In SN2 reactions, the optimal substrates are those that offer less steric hindrance to the nucleophile as it approaches the electrophilic carbon to perform the backside attack characteristic of this mechanism. Methyl halides and 1° halides are particularly well-suited for SN2 reactions because their structures allow for an unhindered approach of the nucleophile.

Methyl halides consist of a carbon atom bonded to three hydrogens and a halogen, resulting in minimal steric hindrance. This allows the nucleophile to attack the carbon atom easily, facilitating the SN2 reaction pathway.

1° halides also provide low steric hindrance, as they have only one carbon atom attached to the carbon bonded to the halide. This accessibility is crucial for the SN2 mechanism, which requires a direct approach to the carbon center.

In contrast, 2° halides experience moderate hindrance, making them less favorable for SN2 reactions compared to 1° halides and methyl halides. 3° halides, having three alkyl groups attached to the carbon, create significant steric hindrance, which typically makes them unsuitable for SN2 reactions. Instead, they tend to favor elimination reactions or SN1 mechanisms due to the relative stability