What characteristic of reactants can facilitate backside attack in nucleophilic reactions?

In nucleophilic reactions, backside attack occurs when a nucleophile approaches the electrophilic carbon from the side opposite to the leaving group. This approach is particularly important in mechanisms like SN2 reactions, where the nucleophile directly displaces the leaving group.

The characteristic of reactants that facilitates backside attack is when the reactants are less hindered. Less steric hindrance around the electrophilic carbon allows the nucleophile to approach more easily, enabling an effective overlap of orbitals necessary for the nucleophilic attack. In contrast, higher steric hindrance would block or slow down this approach, making it more difficult for the nucleophile to effectively collide with the electrophile from the appropriate angle.

Overall, reactants that are less hindered allow for a more favorable nucleophilic attack, which is why this characteristic is essential in facilitating the reaction. This is especially true in the case of primary alkyl halides being more reactive in SN2 reactions compared to secondary or tertiary alkyl halides, which experience increased steric hindrance.