Resonace structure

[durden]

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I'm slightly confused by the bootcamp's answer choice for this question but even more so by the explanation.

The question is which is the best resonance structure.

Answer: There are several important factors to consider when choosing the best resonance form. First of all, make sure that you have as many full octets as possible. Next, minimize your formal charges. After that think about charge location. Place negative charges on the most electronegative atoms and positive charges on the least electronegative atoms. Also, avoid separation of opposite charges ( A). Choice C has placed the negative charge on Oxygen (most electronegative) and positive on the adjacent carbon (least electronegative. B is a tautomer.

It sounds like it's making a case for C.

 

[Vistabane]

Carbon may be less electronegative than Nitrogen, but choice B is still a better resonance structure because all atoms have an octet in choice B. Choice C does not give carbon a full octet. Also, the location of the double bonds in choice B gives a conjugated pi system, which will be considerably more stable causing it to be a resonance form that contributes more to the actual structure of the compound.

 

[jHoon]

Octet rule takes the cake here.The positive charge resting on Nitrogen is a very very very common thing you'll see when dealing with amines/amides (especially in biochem). The Nitrogen can stablalize that positive charge quite well and you get the bonus stability with by having resonance with oxygen bearing the negative charge.

Positive charge on nitrogon > positive charge on carbon (in terms of stability). I think answer choice C was trying to trick you into thinking that carbon is tertiary, but due to resonance its not. taht's an sp2 hybridized orbital.