Organic Chemistry, nomanclature - where to put the number?

Hi everyone,

I think I know the answer, but I'd be really grateful if someone could either correct me or confirm that I'm right!

When naming a molecule, I understand we start the name with the number of the carbon that has a branch eg 2-methylpropane.

But when we put the number in the middle of the word, i.e. propan-2-ol is it correct that the number in the middle of the word always refers to the carbon that has the functional group attached to it (and we never use a number in the middle of the word for just a branched alkyl group)?

thanks in advance!
Gav

Reply 1

gavinlee

OP

also, on the same subject, how do we formulate the name of a molecule when the functional group is bonded to a alkyl brank? For example, if we have 2-methylchlorobut-?-ane , what number would we put for we the Cl is bonded to the carbon in the methyl group?

Reply 2

Sinuhe

8

Do you mean something like CH3-CH(CH2Cl)-CH2-CH3? This is 1-chloro-2-methylbutane - simply start counting at Cl, and you have a branch at the 2- position. Or if it were a substituent which alters the ending rather than used as a prefix, so e.g. something like -OH instead of -Cl, you could call it 2-methylbutan-1-ol

If the situation is more complicated, you can use brackets, so e.g. 1-bromo-3-(hydroxymethyl)pentane for BrCH2-CH2-CH2(CH2OH)-CH2-CH3. But these names are awkward, so it's best just to draw the structure and assign it a name (e.g. 'compound 1' or something) which you can use in your text later on to refer to it.

Reply 3

gavinlee

OP

Sinuhe - thanks once again for your reply & help ;-)

To take your example, if we started counting at the Cl atom, wouldn't the structure be CH2Cl-CH(CH3)-CH2-CH3 which would be 1-chloro-2-methylbutane? Wouldn't CH3-CH(CH2Cl)-CH2-CH3 be called 2-chloro-2methylbutane as the chlorine in bonded to the methyl group on which is bonded to the second carbon?

Reply 4

Sinuhe

8

gavinlee

Sinuhe - thanks once again for your reply & help ;-)

To take your example, if we started counting at the Cl atom, wouldn't the structure be CH2Cl-CH(CH3)-CH2-CH3 which would be 1-chloro-2-methylbutane? Wouldn't CH3-CH(CH2Cl)-CH2-CH3 be called 2-chloro-2methylbutane as the chlorine in bonded to the methyl group on which is bonded to the second carbon?

Well, the two formulae you've written are exactly the same thing. Let's rewrite it using the following nomenclature (just to avoid emphasising the unimportant bits):
Et = ethyl = CH2CH3
Me = methyl = CH3

So, in the picture I've attached, the carbon marked * is the central carbon (the one that isn't explicitly drawn):
CH2Cl-*CH(CH3)-CH2-CH3 == CH3-*CH(CH2Cl)-CH2-CH3

As you can see, both the structures are actually the same thing: it's just that in one case you've written CH3 to the left, and in the other to the top (i.e. in brackets) of THE SAME CARBON. Just picture the world as you would see it if you were the carbon marked * above. You'd see that the following groups are attached to you:
- CH2Cl
- CH3
- H
- CH2CH3
Since the same groups are attached in both cases, it must be the same molecule. (This isn't, strictly speaking, completely true - the carbon here is chiral, and the arrangement of substituents in space is important: but this would not alter its name, except insofar that you would write (S) or (R) (or (+)/(-) or equivalent) in front of the name if you wanted to refer specifically to one of the enantiomers, as determined by Cahn-Ingold-Prelog rules.)

Since the formulae represent the same structure, it obviously makes sense to use just one name for both.

So how could you draw 2-chloro-2-methylbutane to see what the difference is? Easy: first draw butane. Then add Cl to position 2, and then add Me to position 2. You then get CH3-CH(Cl)(Me)-CH2-CH3 (or as in the second picture attached, to make it easier to visualise). This is clearly not the molecule we're talking about! :smile:

I think what you WANT to get to is 2-(chloromethyl)butane - i.e. a methyl group has been substituted with chlorine (to give you -CH2Cl), and attached to butane on position 2. This would indeed be equivalent to 1-chloro-2-methylbutane, but is more complicated, and I would guess it isn't 'correct' in the strict IUPAC sense (there are some odd rules regarding this which no one but secondary school teachers actually cares about). It WOULD be unambiguous, but I'm not sure you could get away with it in secondary school. :smile:

OP