Question 15.1: Predicting the Effect of a Substituent on the Reactivity of ...
Predicting the Effect of a Substituent on the Reactivity of an Aromatic Ring toward Electrophilic Substitution
The pK_{a} of p-(trifluoromethyl)benzoic acid is 3.6. Is the trifluoromethyl substituent an activating or deactivating group in electrophilic aromatic substitution?
Strategy
Decide whether p-(trifluoromethyl)benzoic acid is stronger or weaker than benzoic acid. A substituent that strengthens the acid is a deactivating group because it withdraws electrons, and a substituent that weakens the acid is an activating group because it donates electrons.
A pK_{a} of 3.6 means that p-(trifluoromethyl)benzoic acid is stronger than benzoic acid, whose pK_{a} is 4.19. Thus, the trifluoromethyl substituent favors dissociation by helping stabilize the negative charge. Trifluoromethyl must therefore be an electron withdrawing, deactivating group.