This document provides an overview of aromatic electrophilic substitution reactions (AES). It defines important terms like arenium ions, electrophiles, nucleophiles and discusses the effects of substituents on reactivity. The mechanisms of common AES reactions like nitration, sulfonation, Friedel-Crafts alkylation and acylation are covered. The document also discusses topics like the mesomeric and inductive effects of substituents, the synthesis of tribromobenzene, and the relative reactivities of benzene and substituted benzenes in bromination. Examples of AES on phenols, xylenes, cresols and other aromatic compounds are provided.
9. MESOMERIC EFFECT
• The mesomeric effect in chemistry is a property of substituents or functional groups in a
chemical compound.
• The effect is used in a qualitative way and describes the electron withdrawing or releasing
properties of substituents based on relevant resonance structures and is symbolized by the
letter M.
• The mesomeric effect is negative (–M) when the substituent is an electron-withdrawing
group and the effect is positive (+M) when the substituent is an electron releasing group.
• +M EFFECT ORDER :
–O− > –NH2 > –NHR > –OR > –NHCOR > –OCOR > –Ph > –F > –Cl > –Br > –I
• -M EFFECT ORDER :
–NO2 > –CN > --S(=O)2−OH > –CHO > –C=O > –COOCOR > –COOR > –COOH > –CONH2 >
–COO−
10. INDUCTIVE EFFECT
• The inductive effect is an experimentally observed effect of the transmission of
charge through a chain of atoms in a molecule, giving rise to a permanent dipole
in a bond.
• It is present in a σ bond as opposed to electromeric effect which is present on a
π bond.
• All halides are electron withdrawing groups, and all alkyl are electron donating.
• If the electronegative atom is then joined to a chain of atoms, usually carbon, the
positive charge is relayed to the other atoms in the chain. This is the electron-
withdrawing inductive effect, also known as the
• However, some groups, such as the alkyl group, are less electron-withdrawing
than hydrogen and are therefore considered as electron-releasing. This is
electron-releasing character and is indicated by the