Question

Compare the stabilities of the following pairs of resonance contributing structures and explain how the hybrid structure is related to the structures.
(a) `R-underset((I))overset(o+)(C)= ddotO: harrR-C=overset(o+)ddotO:underset((II))(("acylium ion"))`
(b) `underset((III))(Me-overset(o+)underset( underset(..)( :O)^(ddotTheta))underset(|) ddot(S)-Me)harrunderset((IV))(Me-underset(underset(..)( :O))underset(||)(ddotS)-Me)`
(c) `underset((V))(Me_(2)C=underset(Me)underset(|)(C)-overset(o+)ddotO:harr)underset((VI))(Me_(2)overset(o+)(C)-underset(Me)underset(|)(C)=overset(o+)ddot(O: )`
(d) `underset((VII))(H_(2)C=ddotO:harr)underset((VII))(H_(2)overset(o+)C)-ddotO:^(Theta)`
(e) `underset((IX))(Me-overset(ddotTheta)(CH)-underset(underset(..)O: )underset(||)(C)-Me harr )underset((X))(Me-CH=underset( underset(..):O:^(Theta))underset(|)(C)-Me)`
(f). `underset((XI))(CH_(2)-CH_(2))harroverset(o+)underset((XII))(CH_(2))-overset(ddotTheta)CH_(2)` (g). `underset((XIII))(R-underset(underset(..):O:^(Theta))underset(|)(C)=overset(o+)underset(..)(OH)harr)underset((XIV))(R-underset(underset(..)( :O))underset(||)(C)-overset(..)underset(..)OH)`

Answer 1

(a) `(II)` is more stable and hybrid structure more resembles `(II)`. In `(II)`, both `C` and `O` have an octet and with one more covalent bond than `(I)`. Althrough there is positive charge on electronegative `O` atom, atoms with octet is preferred `(I)` is possible but has high energy because `C` has only six `overline e ^, s`.
(b) `(IV)` is more stable (neutral), but hybrid structure resembles both. Structure `(III)` has charge separation which is overcome in `(IV)` when `S` atom expands its octet by `(p pi - d pi)` bonding formed by the overlap of vacant `3d` `AO^, s` of `S` with filled `p-AO` of the `O` atom.
( c) `(VI)` is more stable and hybrid resembles `(VI)` more, althrough positive charge on `C` atom with incomplete octet in `(VI)` is more preferred than positive charge on more electronegative `O` atom in `(V)`
(d) `(VII)` is more stable (neutral) than `(VIII)` (charge separation), but hybrid `overset(+delta)(CH_(2))-overset(-delta)ddotunderset(..)O: ` is a mixture of both, since `O` is much more `EN` than `C`, and strongly attracts `pi-overline e^, s` to give changed structure, with positive charge on electopositive `C` and negative charge on `EN` `O` atom.
( e) `(X)` is more stable. Althrough both have same number of convalent bonds, the negative charge is more on electronegative `O` atom in `(X)` So, `(X)` is a major contributer of hybrid structure.
(f) `(XI)` is more stable (neutral) and is the only contributor of the hybrid structure. `(XII)` has high energy and unstable due to less number of convalent bonds. incomplete octet on `C`, and charge separation.
(g) `(XIV)` is more stable (neutral) and is a majpr contributor of the hybrid structure. Althrough both have the same number of covalent bonds all make some contribution to the hybrid, `(XIII)` has high energy due to charge separation.