As a reminder, both mesomeric and induc ve effects play a role in how halogens (F, Cl, Br, I) influence
benzene resonance.
Mesomeric Effect: This involves the delocaliza on of electrons through π-bonds. Halogens can
donate their lone pair of electrons into the benzene ring's π-system, affec ng its aroma city.
Induc ve Effect: This is based on the electronega vity difference between the halogen and
carbon. Halogens pull electron density away from the benzene ring due to their higher
electronega vity.
Comparing the Halogens
Halogen Mesomeric Effect Induc ve Effect Overall Effect
Fluorine (F) Weak Strong Net electron-withdrawing
Chlorine (Cl) Moderate Moderate Net electron-withdrawing
Bromine (Br) Strong Moderate Net electron-dona ng
Iodine (I) Strong Weak Net electron-dona ng
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Key Observa ons
Mesomeric Effect Strength: As we move down the halogen group (F to I), the mesomeric effect
becomes stronger. This is due to the increasing size of the halogen's atomic orbitals, which
allows for be er overlap with the benzene ring's π-system.
Induc ve Effect Strength: While the induc ve effect is generally stronger for smaller halogens
(F), it decreases as we move down the group. This is because the electron cloud of larger
halogens is more diffuse, making it less effec ve at pulling electron density away from the
benzene ring.
Overall Effect: The combined effect of mesomeric and induc ve effects determines the overall
impact of the halogen on benzene resonance.
o Fluorine: The strong induc ve effect dominates, making fluorine a net electron-
withdrawing group.
o Chlorine and Bromine: These halogens have a more balanced effect, with both
mesomeric and induc ve effects contribu ng. However, the net effect is typically
electron-withdrawing.
o Iodine: The strong mesomeric effect outweighs the weak induc ve effect, making iodine
a net electron-dona ng group.
Implica ons
The varying effects of halogens on benzene resonance can influence the reac vity and proper es of
halogenated benzenes. For example:
Electrophilic Aroma c Subs tu on: Electron-withdrawing halogens (like F and Cl) make the
benzene ring less reac ve towards electrophilic aroma c subs tu on, while electron-dona ng
halogens (like I) make it more reac ve.
Acid-Base Proper es: Halogens can affect the acidity of benzoic acid deriva ves. Electron-
withdrawing halogens increase acidity, while electron-dona ng halogens decrease acidity.
In most of the cases, resonance effect is stronger and outweighs induc ve effect.
For example, the -OH and -NH2 groups withdraw electrons by induc ve effect (-I). However they also
release electrons by delocaliza on of lone pairs (+R effect). Since the resonance effect is more stronger
than induc ve effect the net result is electron releasing to rest of the molecule. This is clearly observed
in phenol and aniline, which are more reac ng than benzene towards electrophilic subs tu on
reac ons.
Whereas the induc ve effect is stronger than the resonance effect in case of halogen atoms. These are
electronega ve and hence exhibit -I effect. However at the same me they also release electrons by
delocaliza on (+R effect) of lone pair.
This is evident in case of reac vity of halobenzenes, which are less reac ve than benzene towards
electrophilic subs tu on due to -I effect of halogens.
But it interes ng to note that the subs tu on is directed at ortho and para posi ons rather than meta
posi on. It can be ascribed to the fact that the electron density is increased at ortho and para posi ons
due to +R effect of halogens as shown below.
