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Why is the preferred IUPAC name of acetone "propan-2-one" and why not simply propanone? It doesn't make sense to specify the position of the carbonyl group as it can occupy only one position in acetone, i.e. on the 2nd carbon.

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The IUPAC Blue Book defines rules where locants can be ommitted in IUPAC preferred names.

Unlike terminal locants in names for aldehydes that are omitted, there is no rule for homogeneous chains with three or more identical atoms that would allow for the ommission of the locant $2$ in propan-2-one.

This leads to the result that propanal is a IUPAC preferred name while propanone isn't.

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  • $\begingroup$ You might also be concerned why ACETone sounds like it has two carbons and not three (2-PROPanone). It is because acetone was prepared early on from 1-1/2 acetic acids by dry distillation of the calcium salts via a Claisen condensation. The missing carbon is CO2. $\endgroup$
    – user55119
    Nov 27 '17 at 20:16

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