Question

For each of the following substituted benzenes: (1) \({{\bf{C}}_{\bf{6}}}{{\bf{H}}_{\bf{5}}}{\bf{Br}}\); (2) \({{\bf{C}}_{\bf{6}}}{{\bf{H}}_{\bf{5}}}{\bf{CN}}\) ; (3) \({{\bf{C}}_{\bf{6}}}{{\bf{H}}_{\bf{5}}}{\bf{OCOC}}{{\bf{H}}_{\bf{3}}}\):

1. Does the substituent donate or withdraw electron density by an inductive effect?
2. Does the substituent donate or withdraw electron density by a resonance effect?
3. On balance, does the substituent make a benzene ring more or less electron-rich than benzene itself?
4. Does the substituent activate or deactivate the benzene ring in electrophilic aromatic substitution?

Short answer

\(\begin{aligned}{*{20}{c}}{}&{{C_6}{H_5}Br}&{{C_6}{H_5}CN}&{{C_6}{H_5}OCOC{H_3}}\\\hlinea&{withdraw}&{withdraw}&{withdraw}\\b&{donate}&{withdraw}&{donate}\\c&{less}&{less}&{more}\\d&{deactivate}&{deactivate}&{activate}\end{aligned}\)

Step-by-step solution

Inductive effect

The phenomenon of unequal sharing of electrons of a sigma bond between two atoms in a compound resulting in a dipole is called the inductive effect.

Electron-withdrawing substituents show a negative inductive effect (-I), and electron-donating groups show a positive inductive effect (+I).

Electronegative atoms in a compound show -I effect.

Resonance effect

The generation of polarity on a molecule due to the delocalization of pi electrons is called the resonance effect.

Electron-withdrawing substituents pull the electron density from the pi bonded system, while electron-donating substituents donate their electrons to the system.

Electron-rich species or the species containing lone pair of electrons attached to a pi bonded system participate in resonance, i.e., they show resonance effect).

Effect of substituents on the electron density of the benzene ring

All the three substituents (Br, CN, and \(OCOC{H_3}\)) are electron-withdrawing in nature. Hence, they withdraw the electron density from the benzene ring by the inductive effect.

Br and \(OCOC{H_3}\) consist of lone pair of electrons that can delocalize over the benzene ring. Hence, \({C_6}{H_5}Br\) and \({C_6}{H_5}OCOC{H_3}\) donate their electron densities by resonance effect, while \({C_6}{H_5}CN\) withdraws its electron density from the ring.

c. \({C_6}{H_5}OCOC{H_3}\)donates its electron density to the ring, making it more electron-rich than benzene.

\({C_6}{H_5}CN\)and \({C_6}{H_5}Br\) withdraw electron density from the ring, making it less electron-dense than benzene.

d. \({C_6}{H_5}CN\) and \({C_6}{H_5}Br\) withdraw electron density from the ring, thereby deactivating the ring towards electrophilic aromatic substitution. And \({C_6}{H_5}OCOC{H_3}\) activates the ring by increasing its electron density.