Question

Arrange the following groups to decreasing –R (or –M) power.
1.$N{O_2}$
2.$S{O_3}H$
3.$C{F_3}$
4.$CHO$
A.1>3>2>4
B.1>2>3>4
C.1>4>3>2
D.4>3>2>1

Answer 1

Hint: The flow of electrons from one part of the conjugated system to the other creating the centers of low and high electron density due to the phenomena of resonance is called resonating effect or mesomeric effect.

Complete step by step answer:
A group is said to have –R or –M effect if they withdraw electrons from the conjugated system or double bond towards themselves due to resonance.
In the above case the order of decreasing -R or –M effect is given as:
$N{O_2}$>$C{F_3}$>$S{O_3}H$>$CHO$

Hence the correct answer is A. This effect is a permanent effect present in a molecule whether they are participating in the reaction or not.

Additional information: When a single lewis structure cannot describe all the property of a molecule accurately, several structures with similar energy, the position of nuclei, bonding and nonbonding pairs of electrons are drawn and collectively they describe all the properties of a molecule, this phenomenon is known as resonance. The structure drawn during the phenomena of resonance is known as the canonical structure. Due to the lower energy of resonance hybrid as compared to the energy of resonance hybrid, resonance stabilized the molecule. And the resonance hybrid is that structure which represents the molecule more accurately. It should be remembered that the canonical structure has no real existence.


Note:
The polarity produced in a molecule by the interaction between a lone pair of electrons and a pi bond is described by the resonance effect. This effect helps to understand the stability of the compound and its energy state.