Consider the heptatrienyl cation, $CH X_{2} = CH - CH = CH - CH = CH - CH X_{2} X^{+}$ (seven carbons in an open chain). Counting every resonance structure in which the positive charge sits on a different carbon atom as a separate structure (one structure per carbon that can carry the charge), how many resonance structures does it have?

Numerical answer type, tolerance 0.5.

Answer: 4

The positive charge can sit only on alternate carbons of the chain: C1, C3, C5 and C7 (numbering from one end). That gives 4 structures, confirmed by a Python count of the atoms in the same alternant class as the terminal carbon. The answer is 4.

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Answer: 4

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