Question

The most deactivating group for Electrophilic substitution reaction in benzene ring is
(a)- $-C{F}_3$
(b)- $-N{O}_2$
(c)- $-OC{H}_3$
(d)- $-CHO$

Answer 1

Hint: The deactivating group is known as the electron-withdrawing group, so the group which can attract electrons towards itself is known as a deactivating group. The group that has the most electronegative atoms will be the most deactivating group.

Complete answer:
The deactivating groups are those groups which can attract a shared pair of electrons towards itself from the system to which it is attached. So when a deactivating group is attached to benzene then the deactivating group will attract electrons from the benzene.
The deactivating group is also known as the electron-withdrawing group. The groups in which the electronegative atoms are present will act as an electron-withdrawing group. As the electronegativity of the group increases the ability of the deactivating group also increases.
So in the given option (a)- $-C{F}_3$, there are three fluorine atoms and we know that fluorine is the most electronegative atom in the periodic table.
In option (b)- $-N{O}_2$, there are two oxygen atoms and oxygen is also an electronegative atom but its electronegativity is less than that of fluorine.
In option (c)- $-OC{H}_3$, there is one oxygen atom and oxygen is also an electronegative atom but its electronegativity is less than that of fluorine.
In option (d)- $-CHO$, there is one oxygen atom and oxygen is also an electronegative atom but its electronegativity is less than that of fluorine.
So the $-C{F}_3$ is the most deactivating group due to the presence of three fluorine atoms.

Therefore, the correct answer is option (a).

Note:
Like the electron-withdrawing group we have an electron-donating group that is rich in electrons and they donate electrons to the system to which they are attached. Example $-C{H}_3$ is an electron-donating group.