Sciencemadness Discussion Board - How to number carbon rings

How to number carbon rings

Atrum - 27-12-2014 at 20:42

I am still pretty foggy on a lot in chemistry. One thing that causes me some confusion is how carbon ring based compounds are named.

For example. 3-Aminophthalhydrazide (Luminol).

Why is the amine group position 3, and what designates positon 1 in the ring?

Furboffle - 27-12-2014 at 21:12

common naming becomes convoluted. think of 3-Aminophthalhydrazide as a shortened version of 3-aminophthalic hydrazide, which is an alternative common name. if you look at phthalic acid, the root compound, the reasoning becomes more logical, in phthalic acid you have your 2 carboxylic acid groups attached to benzene. these would be positions 1 and 2. if you add that amine adjacent to one of the carboxyls that would become position 3 giving you 3-aminophthalic acid. the carboxyl groups take priority in the naming which is why they are deemed position 1 and 2. but in this case the OH groups on the carboxyls have been substituted by hydrazine

forgottenpassword - 28-12-2014 at 03:16

If you look on the IUPAC website I'm sure that you'll find lots of documentation dealing with naming and numbering conventions for organic structures. If you find that to be too much of a good thing, then all introductory organic chemistry books will give you an overview of the convensions.

In practice, numbering is usually assigned by the person who is talking about a structure, along with a diagram, simply for the purposes of referring to structural features. Whether or not it conforms with IUPAC recommendations is beside the point.

[Edited on 28-12-2014 by forgottenpassword]

Atrum - 28-12-2014 at 18:48

Thank you for your help. It makes a little more sense now.
Also I did try to locate the information on the iupac website but I found my self overwelmed with the large amount of information found there.

If I understand correctly, the numbering begins at the largest carbon branch. In this case the two carboxyl groups are equally large so they are 1 and 2 with the amine group being the 3rd.

I suppose I will understand better with practice.

Thanks again.

[Edited on 12-29-2014 by Atrum]

forgottenpassword - 29-12-2014 at 03:15

It depends on why it is that you are trying to learn it. If it's simply to answer questions in an exam, which is the reason that most people learn it, then you are perhaps best-off just reading through the recommended course textbooks. As I mentioned, in practice, and as you will see for yourself if you look at some scholarly papers, numbering is often non-IUPAC and simply for the convenience of discussing certain structural features by using prose. Someone will draw and number a structure and refer to various substituents on the 3 or 4 position, for instance. A person is not expected to know WHICH is the 3 or 4 position beforehand, the author should make it plain if they hope to be understood.

It seems to me that your confusion stems from trying to rationalize non-IUPAC naming conventions with IUPAC naming conventions. The latter are taught in schools largely for the purposes of assigning marks in exams. The former are used in journal papers and may or may not overlap with IUPAC naming conventions. But in reality it hardly matters at all, because a clearly labeled picture is provided, and the purpose of numbering is simply as an aid to discuss the chemistry of the molecule in question, which is of more interest to people reading it than the correct assigning of IUPAC recommended nomenclature. Although, having said that, very large numbers of compounds have common ring-systems which are fairly-consistently labeled in the same fashion as they were since before IUPAC was even invented. I don't want to give the impression that most authors just make them up on the spot, because they don't, although they could do and it would make no difference. Then there are things like the Cahn-Ingold-Prelog rules which conform to a different set of numbering standards! Not entirely different, just slightly different.

[Edited on 29-12-2014 by forgottenpassword]

Atrum - 31-12-2014 at 12:15

So at what point should I really pay attention to the iupac naming practices. It sounds to me from what you have said that it is not all that important.

My reasons for wanting to learn about the numbering is because I figured that it was important to keep things to a standard everybody can follow.

j_sum1 - 31-12-2014 at 17:10

Quote: Originally posted by Atrum

So at what point should I really pay attention to the iupac naming practices. It sounds to me from what you have said that it is not all that important.

My reasons for wanting to learn about the numbering is because I figured that it was important to keep things to a standard everybody can follow.

Then you have the right goal: communication.
If you speak IUPAC, everyone will understand you if you say 2-propanone. However, people normally refer to acetone and it communicates well. (In spite of being one of the most counter-intuitive nomenclatures out there. Ace- looks like one if you play cards, means two if you speak of acetate or acetylene, but denotes three carbons in the case of acetone.)

Atrum - 31-12-2014 at 17:46

Quote:

I see what you mean. If there is a common name then use it. Such as (methyl ethyl ketone) people may not know what I am talking about if I use it's other name. 2-butanone.

Thanks.

[Edited on 1-1-2015 by Atrum]

[Edited on 1-1-2015 by Atrum]

careysub - 31-12-2014 at 18:26

I'll also point out the ambiguity in single ring aromatic naming, based on how you view the attached groups. Add a methyl to benzene it is toluene, add an amino group and it is aniline, add a hydroxy and it is phenol, add an aldehyde and it is benzaldehyde.

But if it has a methyl and hydroxy group is is a hydroxytoluene or a methylphenol; similarly aminotoluene or methylaniline; etc.

There are established preferences on which is what, but from basic principles it is confusing.