Chemistry - Oxygen or carbon with a positive charge? (comparison of canonical structures)

Solution 1:

Which of the contributing structures of the resonance below is more stable?

Technically, neither. Both structures A and B are resonance contributors to the same true structure of the ion. Structures A and B, because they both represent the same species, cannot have different energies, and therefor they cannot have different stabilities.

What the question is asking is "Which contributor is more important in describing the structure and behavior of the hybrid?" Contributor B probably approximates the structure better (i.e. the positive charge is more on the oxygen than on the carbon), but structure A represents the reactivity better (i.e. nucleophiles want to attack the carbon and not the oxygen).

Solution 2:

The difference between the two structures is just the amount of space available to the lone pair. If you think about it in terms of the particle in a box approximation, the path length for that lone pair is much longer when in the double bond state, so it will be a lower energy state.

Solution 3:

Structure A does NOT have on octet on the carbon that has the plus charge. Structure B has an octet on every atom. Thus, structure be is more stable.