CH610B Sophomore Organic II

Dr. Brian Pagenkopf

Benzene Substituted with Electron Donating Groups and Electron Withdrawing Groups

Activating, deactivating, donating and accepting groups all exert their strongest influence on carbons 2, 4, and 6 (the ortho and para positions). If this is true, then why do activating donor groups cause reaction at the ortho and para positions, whereas electron accepting groups cause reaction at the meta position? The key to this puzzle is understanding where the electron density is greatest. (Reactions occur where the electrons are).

Often to show electron density we turn to electrostatic potential surfaces, but as the following images show, there doesn’t appear to be much difference between the ortho, meta and para carbons.

To explain the reactivity resonance structures provide a much more useful picture of where the electron density is. With an electron donor, such as methoxy, three important resonance structures show a negative charge on the ortho and para carbons, but not on the meta carbon.

Electron withdrawing groups also exert their greatest influence on the ortho and para carbons, but by removing electron density the from the ortho and para carbons only the meta carbon has much electron density left.

Phenol Acidity: Substituent Effects

The effect of electron donating groups on a phenol is to make it less acidic. For example consider the resonance structures for the following phenoxide:

However, if an electron withdrawing group on the ring can further delocalize the negative charge then the anion is more stable and the phenol more acidic.

The relative positions of groups on the ring determines how much they can interact with each other. The greatest interaction is when groups are ortho or para, and the interactions are less when the groups are meta. The following comparisons illustrate this point.