Predicting the Effect of a Substituent on the Reactivity of an Aromatic Ring toward Electrophilic Substitution The pKa of p-(trifluoromethyl)benzoic acid is 3.6. Is the trifluoromethyl substituent an activating or deactivating group in electrophilic aromatic substitution?

Question

Predicting the Effect of a Substituent on the Reactivity of an Aromatic Ring toward Electrophilic Substitution

The [latex]pK_{a} [/latex]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.

Accepted Answer

A [latex]pK_{a} [/latex] of 3.6 means that p-(trifluoromethyl)benzoic acid is stronger than benzoic acid, whose [latex]pK_{a} [/latex] is 4.19. Thus, the trifluoromethyl substituent favors dissociation by helping stabilize the negative charge. Trifluoromethyl must therefore be an electron withdrawing, deactivating group.

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