How to Easily Distinguish Ortho, Meta, and Para Directors in EAS Reactions

Now we need to focus on benzene substituents and how they affect the location of subsequent additions. Here’s a list of the ones you would most likely see:

Electron donators / Activators (strongest to weakest): -O-, -NR2, -NH2, -OH, -OR, -R Electron Withdrawers / Deactivators (strongest to weakest): -NO2, -NR3+, -NH3+, -SO3H, -CN, -CO2H, -CO2R, -COH

Generally speaking, electron donators / activators have a lone pair of electrons or an electron density that “pushes” into the benzene. Electron withdrawers / deactivators have a positive charge on the substituent or a very electronegative atom attached to it, which “pulls” electrons out of the benzene.

All activators AND halogens are ortho-para directors; Deactivators (not halogens) are meta-directing.

Therefore, depending on the character of the initial substituent (R), a subsequent substituent would be placed at the ortho or para position if R is an activator/halogen or at the meta position if it is a deactivator (but not a halogen).

Other facts to know:

  • The more electron donating groups a benzene ring has initially, the faster an EAS reaction will occur (due to increased electron density to make benzene a better nucleophile).
  • If there are already two or more substituents on the ring, the strongest donating group gets priority when choosing the location of the added substituent.
  • When given an ortho / para choice, substituents will go to the location with the least steric strain.
Ari Rezaei
Ari Rezaei

My name is Ari Rezaei and I'm a dental graduate from Columbia University. My team and I built to provide challenging prep material to get you ready to conquer your classes and exams.