TBR: O CHem Chapter Passage 2 Q12

[moose45]

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If the carbonyl bond in caffeine can have a single and double bond through resonance, why isn't it considered a 1.5 bond (a bond shorter than a double bond). I seen it on the AAMC exam 3, where they consider it as 1.5

 

[knightstale4]

If the carbonyl bond in caffeine can have a single and double bond through resonance, why isn't it considered a 1.5 bond (a bond shorter than a double bond). I seen it on the AAMC exam 3, where they consider it as 1.5

Hey, can you give the page # and book #?

 

[moose45]

O Chem Part 1 Page 59 Question 12

 

[PiBond]

If the carbonyl bond in caffeine can have a single and double bond through resonance, why isn't it considered a 1.5 bond (a bond shorter than a double bond). I seen it on the AAMC exam 3, where they consider it as 1.5

The fact that we can draw a viable resonance structure where it has a single bond leads us to believe that the bond is in fact longer than another carbonyl compound (ie cyclohexanone) that doesn't have a resonance structure that represents the bond as a single bond. Remember that double bonds are shorter than single bonds because they contain more s-character and that bonds exist as a hybrid of their resonance structures. Thus, we can take this to mean that the both compounds have longer carbonyl bonds than our generic carbonyl that can't be represented as such.