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Each of the following structures is a resonance form of the molecule shown below EXCEPT one. Which one is the EXCEPTION?
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Correct Answer: C.
First, let’s number the carbons in the ring. Oxygen is attached to C1 and so N is attached to C3. So in the original molecule, we see that there are double bonds in the ring between C2=C3 and C5=C6. All of the answer choices follow the rules of drawing resonance structures except answer choice C. Let’s carefully follow the electrons for answer choice C and thus we can understand what went wrong.
Our starting material:
1) Keep in mind that oxygen is the most electronegative atom in the molecule and thus ‘wishes’ to withdraw electrons to itself.
2) The nitrogen is positively charged because it contributed its lone pair of electrons to carbon. Carbon’s double bond with the 2nd nitrogen breaks and thus nitrogen has a negative charge and an extra lone pair (note: this describes answer choice B).
3) Nitrogen’s lone pair bonds with C3 which breaks C3=C2 so now C2 has a lone pair and a formal negative charge (C2 is secondary carbanion and this describes answer choice D).
4) Remember that oxygen wants the electrons more than any other atom in this molecule. C2’s lone pair bonds with C1 as C=O breaks so now oxygen has the lone pair and the formal negative charge (this describes the following structure which is not an answer choice but to make it easy to refer to, we will call it answer choice E).
5) We just worked out why answer choice E is correct. Well, the only difference between E and C is that E correctly has the double bond C5=C6 like the starting material but answer choice C incorrectly places the double bond at C4=C5. That would only be possible if C6 had a + charge and C4 had a – charge.
6) And finally, answer choice A is like the starting material except, instead of oxygen pulling electrons from nitrogen along ‘the top’ (!!) of the molecule, oxygen pulls electrons from the C5=C6 double bond creating a C6=C1 double bond, a lone pair on oxygen and a secondary carbocation at C5 (because it lost its bond).
In case you are wondering, the current MCAT requires an understanding of resonance and Lewis structures but does NOT require specific knowledge of aromatic chemistry (i.e. EWG, EDG, addition reactions, etc.).
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