But let us suppose that you are asked to name the following molecule (i):
"H"_3"CCH"_2"CH"_2"CH(CH"_3")CH"_3
Now the molecular formula is "C"_6"H"_14 (is it? perhaps you should check), but the LONGEST carbon chain is CLEARLY 5 carbons long.
On the other hand, for the molecule (ii), "H"_3"CCH"_2"CH"_2"CH(CH"_2"CH"_3")CH"_3, the longest carbon chain is 6 carbons long.
So (i) is a pentane derivative, H_3C^(v)C^(iv)H_2C^(iii)H_2C^(ii)H(CH_3)C^(i)H_3, and here I have superscripted the carbons to give their position on the chain. I would come up with the name "2-methylpentane". Had I numbered from the other end, to give "4-methylpentane", this would contravene the rule that substituent numbers be minimized, i.e. be as low as feasible.
And (ii) is a hexane derivative, "2-methylhexane", because clearly I could rewrite it to give the equivalent structure, H_3C^(vi)-C^(v)H_2C^(iv)H_2C^(iii)H(CH_3)C^(ii)H_2C^(i)H_3, but here it is correctly named.
Anyway, at the moment I do not know what level you are learning. There is a lot of expertise available on these boards, so if there is a more specific question fire away.
