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The three structures of diazomethane shown below differ only in the placement of electrons, while the connectivity of the atoms (H2C-N-N) stays the same in each: (A) H2C(-) - N (triple bond) N (B) H2C = N = N(-) (C) H2C = N(+) = N(2-) [a minor, charge-separated structure] What is the correct relationship between structures A, B and C?

1. They are resonance (canonical) structures of the same neutral molecule
2. They are constitutional (structural) isomers
3. They are geometrical (cis-trans) isomers
4. They are three completely different compounds

Correct answer: They are resonance (canonical) structures of the same neutral molecule

Solution

In all three depictions the skeleton is H2C-N-N with the same atom-to-atom connectivity. What changes from one drawing to the next is only where the bonding and lone-pair electrons are placed, which in turn shifts the formal charges around. Structures related in this way are resonance (canonical) forms of a single species, not separate molecules. Diazomethane is overall neutral, so the net charge on every resonance form is zero; the positive and negative formal charges within any one form simply cancel. Isomers, by contrast, would require a genuine difference in connectivity (constitutional) or spatial arrangement (geometric).

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