Question

Relative stabilities of the following structures of are in this decreasing order:

A. $\text{II}>\text{I}>\text{III}$
B. $\text{I}>\text{II}>\text{III}$
C. $\text{III}>\text{II}>\text{I}$
D. $\text{I}>\text{III}>\text{II}$

Answer 1

Hint:. The relative stabilities of the three compounds will be checked on the basis of the rules of stability. The rules include whether the compound with ions is stable more without ions. And which rule has more priority than another.

Complete step by step answer:
Let us see the rules first to judge the structures on the basis of stability. The rules are written in ascending order of priority.
Rule (1)- Non-polar compounds are more stable than polar compounds. This is because covalent bonds are more stable and strong than ionic bonds.

Rule (2)- The resonating structures where opposite charges are close are more stable and favourable because negative and positive are opposite charges on coming closer, they release a great amount of energy.

Rule (3)- The compound in which all elements have their octet complete are more stable. This is because elements achieve inert configuration by their octet completion.

Rule (4)- The resonating structures in which electronegative elements bear negative charge and electropositive elements bear positive charge are more favourable.

Let us now compare the three resonating structures given in the question rule-wise:

Compounds	Structures	Rule 1	Rule 2	Rule 3	Rule 4
I		Yes, the compound is nonpolar.	No charges are present	Octets of all the elements are complete. Carbon made four, oxygen made two and hydrogen made one bond.	No charges are present.
II		The compound is polar.	Charges are present. The charges are present far away from each other.	Octets of all elements are complete except the first carbon atom from left. It has only 6 electrons.	The rule is followed as the more electronegative element ‘oxygen’ has negative charge and ‘electropositive’ element carbon has positive charge.
III		The compound is polar.	Charges are present. The charges are very close to each other as they are present on adjacent carbons.	Octets of all elements are complete except the second carbon atom from left. It has only 6 electrons.	It follows this rule as the negative charge is present on less electronegative element. As, carbon is less electronegative than oxygen.

Hence, the conclusion of all the rules is that it is clear that compound I is most stable among other compounds as only it follows Rule (1). Then, among II and III, compound III is more stable as it follows rule (2) but compound II does not.
So, the correct answer is “Option D”.

Note: We while checking stability tend to forget rule (1) and rule (3) and their priorities we consider rule (4) prior rule (3). That’s where we make mistakes and the answer gets wrong. Remember that Rule (3) needs to be checked before checking Rule (4).