In a sn2 reaction why do we need sp3 hybrid orbital and not sp2 for the substrate
In an Sn2 (nucleophilic substitution) reaction, the substrate typically needs to have an sp3 hybrid orbital rather than an sp2 hybrid orbital. This is because the Sn2 reaction occurs through a concerted mechanism, where the nucleophile attacks the substrate's "backside" resulting in the displacement of the leaving group and the formation of a new bond.
The sp3 hybrid orbital provides a more suitable geometry for this backside attack. It has a spherical shape with maximum electron density directed away from the central atom, creating an open region for the nucleophile to approach.
On the other hand, the sp2 hybridized carbon atom has a trigonal planar geometry with a p orbital perpendicular to the plane. This p orbital overlaps with the nucleophile's orbital, hindering the backside attack.
To determine the hybridization of an atom, you need to examine its bonding and lone pair electrons. The number of electron groups (bonds and lone pairs) around an atom determines its hybridization. For example, if an atom has three bonding groups and one lone pair, it is sp3 hybridized.
In summary, the presence of an sp3 hybrid orbital in the substrate allows for the proper geometry and electron density for the Sn2 reaction to occur effectively.