Which of the contributing structures of the resonance is more stable?
I'm watching a video lecture by a professor of my college where he puts this question to the class. The class unanimously says B. At first I thought they were wrong, but then the professor agreed with them.
Is this true? The reason he gave was that oxygen's octet would be complete in B, hence it contributes more. But I don't understand this.
1) Isn't the octet complete in A too? It has 2 covalent bonds - 4 electrons (2 from sharing) and 2 lone pairs. Adding up to 8.
2) Oxygen being more electronegative than carbon, would pull the electrons towards itself, leading me to think that it would be more comfortable with a lone pair rather than another bond. Also, won't it prefer to lose out that '+' charge which the bonding's burdened onto it?
Am I wrong?
Another thing which made me believe that Prof. may have made a mistake while talking about the more relevant contributor, is that when he added Br$^-$ to the compound, he added it to the canonical structure A. And don't you explain reactions with the most relevant contributor, i.e. B?
The presence of an extra bond in the canonical structure B, along with the completion of the carbon valency makes it more stable than A. The reaction with Br$^-$ is best explained looking at A, though, because the incoming nucleophile ends up bound to carbon, not oxygen.