The compound in question, and the name generated by ChemSketch:

enter image description here

I think this compound should be named as "cyclopentylmethanal" because the principal functional group (carbonyl) lies in the methane chain and not in the ring. So, the ring should be treated as a substituent for methanal, but apparently this is not the right answer.

I also noted that the following compounds validate my logic (ring is being treated as a substituent): enter image description here

So, what is the exceptional rule with the first compound?



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