Question

Which of the following groups can exert both +M and -I effect?
A. \[ - CHO\]
B. \[ - N{O_2}\]
C. \[ - Cl\]
D. \[ - C{H_3}\]

Answer 1

Hint: Mesmeric effect can be understood as the polarisation of a given molecule through the transfer of electrons or through the pi bonds present in the molecule. Inductive effect can be understood as the phenomenon which results in the formation of a permanent dipole due to the difference in electronegativities of the different chemical substituents in a given molecule.

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us first understand some important basic concepts. Mesmeric effect occurs due to the movement of pi electrons either towards or away from a particular substituent group in a conjugated orbital system. Mesmeric effect is of two types: + M and – M.
Positive Mesomeric Effect takes place when there is a transfer of pi electrons towards a given conjugate system by a particular group. This transfer of electrons increases the electron density of the conjugated system under consideration. For a substituent to exhibit + M effect, it should have a lone pair or a negative charge.
In inductive effect, the more electronegative substituents tend to pull the electron cloud towards themselves. This results in the formation of a partial positive and partial negative charge. There are 2 types of Inductive effects: + I and – I.
Negative Inductive Effect or - I effect is caused by more electronegative substituents which tend to pull the electrons towards themselves, thus developing the partial negative charge on them.
Now, let us discuss the options given to us:
A. \[ - CHO\]
It is an electron withdrawing group. Hence, it does not show +M effect.
B. \[ - N{O_2}\]
It is an electron withdrawing group. Hence, it does not show +M effect.
C. \[ - Cl\]
It is an electron withdrawing group. But it has lone pairs of electrons which it can donate in a molecule. So, it shows a + M effect. Also, it is highly electronegative, hence it exhibits – I effect as well.
D. \[ - C{H_3}\]
It is an electron donating group. Hence, it does not show - I effect.

Hence, Option C is the correct option

Note: On the other hand, Negative Mesomeric Effect can be understood as the transfer of pi electrons to a particular group by a conjugated system. This decreases the electron density over the conjugated system under consideration. For a substituent to exhibit – M effect, it must have either a positive charge on the molecule or must have a vacant orbital to accept the electrons.
Positive Inductive is caused by chemical substituents that are electropositive or relatively weaker electronegative in nature. They tend to allow the electron cloud to be moved away from them.