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Consider the two contributing resonance structures I and II drawn for nitromethane (CH3-NO2). In structure I both N-O bonds are equivalent (one single, one double in canonical form); in structure II one oxygen is shown carrying a negative charge while the nitrogen bears a positive charge. Pick the correct statement about these structures.

1. Structure II is an unlikely contributor because the electrons have moved onto oxygen
2. Structure II is an unlikely contributor because nitrogen would have only a sextet of electrons
3. Structure II is an acceptable and significant contributor
4. None of these

Correct answer: Structure II is an acceptable and significant contributor

Solution

In the nitro group, the two equivalent resonance structures differ only in which oxygen carries the formal double bond and which carries the negative charge. In every such canonical form nitrogen retains a full octet (it is positively charged but not electron-deficient) and the negative charge sits on the highly electronegative oxygen. These are exactly the conditions for a good, important resonance contributor, so structure II is acceptable and important.

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