Arrange the following resonating structures in the order of decreasing stability :
(a) `underset(("Vinyl chloride"))underset((I))(CH_(2)=CH-ddotunderset(..)CI: )harrunderset((II))( :overset(Theta)(CH_(2))-CH=overset(o+)underset(..)CI: )harrunderset((III))(overset(o+)CH_(2))-CH=underset(..)ddotCI^(Theta)`
b. `underset(("Formic acid"))underset((IV))(H-overset(ddotO: )overset(||)C)-underset(..)ddotOH harrunderset((V))(H-overset( :ddotO:^(Theta))overset(|)(C)=underset(o+)ddotO)-H harrunderset((VI))(H-overset( :ddotO:^(Theta))overset(|)underset(o+)(C)-underset(..)ddotO-Hharr) underset((VII))(H-underset(ddotTheta)overset( overset(o+):O: )overset(|)C-underset(..)ddotO-H)`
( c) Write the resonating structure of phenol in order of decreasing stability.

Explanation

(a) (i) is most stable because it has no formal charges.
(ii) (III) is least stable because it has an `overline e-`deficient carbon. In (III), `C1` uses an empty `3d` orbital to accommodate a pair of `overline e^, s` which is not possible in the `C` atom. Thus, the order of stability is `(I) gt (II) gt (III)`
(b) (i) `(IV)` and `(V)` have greater number of convalent bonds , hence, these are more stable than either `(VI)` or `(VII)` . Furthermore between `(IV)` and `(V),(IV)` has no formal charges and hence is more stable than the `(V)`
(ii) `(VII)` is less stable than `(VI)` because in `(VII)` the positive charge is on oxygen, which is a more `EN` atom than carbon on which positive charge is present in `(VI)`. Thus, the order of stability is `(IV) gt (V) gt (VI) gt (VII)`
( c) In the resonating structure of phenol as shown below, structure `(I)` and `(II)` are more stable than the others.
Structures `(III),(IV)`, and `(V)` are less stable due to charge separation and positive charge on oxygen atom.

Order of stability `rArr (I)=(II)gt(III)=(V) gt (IV)`
(I)and (II) `rArr` Unchanged structures
`(IV) rArr` More charge separation
Order of energy `rArr (IV) gt (III)=(V)gt (I)=(II)`
(III)and (V) rArr Less charge separation.