resonance and bond strength related question in TBR

[bored]

Hello, I have come across something that doesnt make sense to me and an explaination will be appreciated!

its TBR's discussion on resonance, example 1.8 of organic chemistry (if anyone has book)

basically, they give me Drawing-A and Drawing-B and ask where the C--O bond is the longest.
I think the longest bond is the weakest bond and the weakest bond is the one that has no resonance possible. but apparently the longer bond is the one with resonance according to TBR ex. 1.8 in organic chemistry volume 1...

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[ingramw1202]

Can you take a picture of the drawings (a and b) and post it please

 

[member232]

Because of resonance due to the electrons donated by Nitrogen, the carbonyl double bond turns into a single bond in a resonance structure. Thus, for the left picture, with the nitrogen, the double bond is between bond order 1 and 2, while the right picture has a double bond of order 2. The left one is weaker, and thus longer as it is in between a single and double bond.