Question

How do we name alkanes?

Answer 1

This will be explained MUCH better, and much more systematically in your organic text.........

Explanation:

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.