Question

The most stable canonical structure among the given structure is:


A. I
B. II
C. III
D. All are equally stable

Answer 1

Hint: There are many factors for deciding stability of canonical structures like resonance is the major factor, inductive effect, number of \[\pi \] bonds, \[\pi \]-lone pair conjugation, position of charges etc. So, by using these effects we can check the stability order of canonical structures.

Complete step-by-step answer:
Canonical structures are a way of representing or describing the delocalized electrons within certain molecules or polyatomic ions like in phosphate ions. Here all three are resonance structures. In this example many effects work like inductive effect, number of \[\pi \]bonds, positive charge - lone pair conjugation, position of charge etc.
In the 1st structure \[-I\] effect of oxygen, incomplete octet of cation.
In the 2nd structure \[-I\] effect of oxygen, incomplete octet of cation.
In the 3rd structure \[\pi \] bonds are more than 1st and 2nd, octet is complete, and positive charge is present on the most electronegative element which makes it highly unstable.
Conclusion from the above three factors is that the 3rd structure is unstable than other 2 structures because of positive charge on the most electronegative atom. In the 1st part \[-I\] effect is less than the 2nd part which makes cation less stable than the 1st part.
So, the stability order is:
$III < II < I$
So, “A” is the correct answer.

Note: You should know which effect is dominating where. In the 3rd structure there is positive charge on the most electronegative atom.