Consider the following organic compounds:
Compound $(1)$ has two double bonds, which I'll label $C1$ and $C4$ such that the methyl group is attached to $C1$ and thereby assign lowest numbers to all functional groups and substituents. Officially its name is 1-methylcyclohexa-1,4-diene.
Generally we try to give just enough information to be able to reconstruct the original compound, but not too much. With that in mind, I feel that compound $(1)$ should be named simply 1-methylcyclohexa-4-diene:
- Since it ends in -diene, it's understood that there are two double bonds.
- Since it ends in -4-diene, it's understood that one of them begins at $C4$.
- If the second double bond were located after $C4$, the numbering is backwards; instead of, say, -cyclohexa-4,6-diene, it should've been named -cyclohexa-1,3-diene.
- If the second double bond were located at $C2$ or $C3$, then priority was given to the methyl substituent over the alkene functional groups. Rather than calling it 1-methylcyclohexa-2,4-diene, it should've been named 5-methylcyclohexa-1,3-diene.
- Therefore, the only option left is for the second double bond to be at $C1$, and therefore specifying 1-methylcyclohexa-1,4-diene is extraneous.
Following a similar line of thought, compound $(2)$ ought to be named just 1-methylcyclohexa-3-diene, rather than its proper name of 1-methylcyclohexa-1,3-diene.
Taking this logic to an extreme, the third compound should be named 1,4-dimethylcyclohexadiene, instead of its proper name of 1,4-dimethylcyclohexa-1,4-diene, because the same logic used above to determine that the second double bond is located at $C1$ should be sufficient to determine the locations of both double bonds in this case, where they both have substituents!
Why in all of these cases, and others like them, do we specify both locants of the alkenes, when one (for compounds $(1)$ and $(2)$) or none (for compound $(3)$) is sufficient?