2-ethyl-4-methylhexane

[ALEXAME]

Hello, I am trying to understand whether this is expected behavior or a bug.

When I enter:

2-ethyl-4-methylhexane

OPSIN returns a structure corresponding exactly to that name (hexane backbone with ethyl at C2 and methyl at C4).

However, if I manually apply longest-chain IUPAC naming, I would expect the preferred parent chain to become heptane, leading to a preferred name such as:

3,5-dimethylheptane

So my question is:

Does OPSIN intentionally preserve the input nomenclature exactly as written, even if the entered name is not the preferred longest-chain IUPAC name?

Or is there an option / expected behavior for normalization toward preferred IUPAC naming?

Thank you.

[dan2097]

Does OPSIN intentionally preserve the input nomenclature exactly as written, even if the entered name is not the preferred longest-chain IUPAC name?

Yes, it uses the numbering based on the name as entered, regardless of whether that would be the numbering if a preferred IUPAC name was instead used.

Or is there an option / expected behavior for normalization toward preferred IUPAC naming?

There is not. Pragmatically this would be very difficult to implement as OPSIN would need to implement a chemical structure to name algorithm to know what was the preferred nomenclature to describe the structure such that it could be numbered accordingly.