Here's why:
* The bulky chlorine atom: The chlorine atom at the meta position (3) is large and creates significant steric hindrance. This crowding makes it very difficult for other groups to approach and attack the ortho or para positions.
* The methyl group: The methyl group at the ortho position (1) also contributes to the steric hindrance, further blocking access to the ortho and para positions.
Consequences of Steric Hindrance:
* Low reactivity: The steric hindrance makes the ortho and para positions significantly less reactive towards electrophilic aromatic substitution reactions.
* Meta selectivity: Because of the steric hindrance, the meta position becomes the most accessible and thus, the preferred site for electrophilic attack.
Exceptions:
While substitutions at the ortho and para positions are generally not possible, there might be some exceptions under specific conditions. These exceptions can be due to:
* Highly reactive electrophiles: Very reactive electrophiles, like those generated under extremely acidic conditions, might be able to overcome the steric hindrance.
* Specific catalysts: Certain catalysts might facilitate the reaction by providing a pathway to overcome the steric hindrance.
Key Takeaway:
The combination of the bulky chlorine and the methyl group in 3-chlorotoluene creates a significant steric hindrance, making substitutions at the ortho and para positions very difficult. This generally leads to preferential meta substitution.
