Which of the following functional groups is activating in nuc substitution? A. N

Question

Which of the following functional groups is activating in nuc substitution? A. Nitro group meta to the leaving group B. Nitro group ortho or para to the leaving group C. Alkoxy group ortho or para to the leaving group D. Alkoxy group meta to the leaving group Which of the following is not true of halogen substituents on a benzene ring? A. They direct meta in electrophilic aromatic substitution. B. They deactivate in electrophilic aromatic substitution. C. They are leaving groups in nucleophilic aromatic substitution. D. More electronegative halogen substituents react faster in SNAr reactions.

Explanation / Answer

1. OPTION B. Nucleophilic substitutions of aromatic systems have increased rate if the electrophilic nature of the aromatic ring is increased. To do this, the ring-electron density is to be brought down by electron withdrawing groups like nitro group while electron-donating groups like alkoxy groups increase the electron density on the ring giving it more negative character thus preventing the attack of the nucleophile. If the nitro group is meta to the nucleophile, resonance contributors show that there is not accumulation of negative charge on the meta carbon but on the ortho and para carbons. Therefore, it a nitro group is in ortho/para positions, it tends to activate the aromatic system for nucleophilic substitution reactions.

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