Regioselectivity in electrophilic aromatic substitution
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص562-563
2025-06-24
400
We start with electrophilic aromatic substitution. It was established that an electron-donating substituent favours ortho/para and an electron-withdrawing substituent favours meta substitution. Although meta substitution is usually slower than ortho/para substitution (because electron-withdrawing groups deactivate the ring), it usually gives the meta product alone.
Most reactions of benzene rings with electron-donating substituents give ortho/para mixtures and, if the substituent is very electron-donating, may lead to both ortho and para substitution in the same molecule. Control in favour of the para product can usually be achieved by reducing the reactivity of the substituent and increasing its size.
Of course, if the para position is blocked, ortho substitution is the only option, and we will come back to the idea of blocking substituents shortly. But there is a general way of directing electrophiles to the ortho position using activation by metallation.
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