Bond Length and Resonance

[Icantplayspades]

I started my MCAT study plan and did my first TBR chapter! I got most of the practice questions correct but one is kinda puzzling to me:

The compound on the left is a “hybrid” of a single and double bond due to resonance(i.e. has single and double character because lone pair on nitrogen donates). If the bond is a “hybrid” of single and double, it must be weaker than a double bond. Because shorter bonds are stronger (more orbital overlap) and the bond on the left is weakest, it must be longer. I’m just making sure my logic is sound.

Thanks guys!

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[aldol16]

I would think about it this way - try it out and see if it works better for you. Just from drawing that resonance structure above, you know that the C-O bond is somewhere between a single bond and a double bond. So you can think of it as a "1.5" bond since it's more than a single bond but less than a double bond. You know that as bond order (number of bonds) increases, length decreases. For example, you know that the C-O bond in carbon monoxide is shorter than that in a ketone, which is shorter than that in an alcohol. So a 1.5 bond will have a length that's longer than that found with a double bond but shorter than that found in a single bond.

 

[PlsLetMeIn21]

Not that it impacts the way you answered this question, but your arrows in the left resonance structure are going backwards from convention.