# How do we name alkanes?

Apr 3, 2017

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):

${\text{H"_3"CCH"_2"CH"_2"CH(CH"_3")CH}}_{3}$

Now the molecular formula is ${\text{C"_6"H}}_{14}$ (is it? perhaps you should check), but the LONGEST carbon chain is CLEARLY 5 carbons long.

On the other hand, for the molecule (ii), ${\text{H"_3"CCH"_2"CH"_2"CH(CH"_2"CH"_3")CH}}_{3}$, the longest carbon chain is 6 carbons long.

So (i) is a pentane derivative, ${H}_{3} {C}^{v} {C}^{i v} {H}_{2} {C}^{i i i} {H}_{2} {C}^{i i} H \left(C {H}_{3}\right) {C}^{i} {H}_{3}$, and here I have superscripted the carbons to give their position on the chain. I would come up with the name $\text{2-methylpentane}$. Had I numbered from the other end, to give $\text{4-methylpentane}$, this would contravene the rule that substituent numbers be minimized, i.e. be as low as feasible.

And (ii) is a hexane derivative, $\text{2-methylhexane}$, because clearly I could rewrite it to give the equivalent structure, ${H}_{3} {C}^{v i} - {C}^{v} {H}_{2} {C}^{i v} {H}_{2} {C}^{i i i} H \left(C {H}_{3}\right) {C}^{i i} {H}_{2} {C}^{i} {H}_{3}$, 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.