How do we name alkanes?

1 Answer
Apr 3, 2017

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


But let us suppose that you are asked to name the following molecule (i):


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.