What do these numbers mean, exactly?

May 23, 2015 5:55 AM

Sometime, in front of chemical names, you will see three numbers, like this: 1,1,1 Triclorotriflouroethane. (to quote one example with which I am familiar.) What do they denote, and where can I find out more? (I realize that this may be simply a lack of Google-fu but I do have an underlying reason for needing to know.) Mefi chemists, help!
posted by pjern to Science & Nature (6 answers total)

The term you need to search for this is IUPAC Nomenclature (in this case organic). It will explain what chemical names mean.
posted by Thing at 6:07 AM on May 23, 2015 [1 favorite]

They're about which carbon atoms are bonded to what.
posted by Etrigan at 6:08 AM on May 23, 2015

B.Sc. Chemistry here, unused and old - my recollection is that the number (e.g. 1,1, 1) indicate where functional groups are located on the main body of the molecule. Functional groups are smaller molecules that occur over and over, and the numbers indicate where these are attached.

You can have two molecules with the same number of atoms (so same formula) but the physical layout of the molecules may be different. These are isomers, and may behave very differently.

For example, common sugar comes in two forms which are effectively mirror images of each other.

Look up functional groups, isomers and organic chemistry for more info.
posted by parki at 6:12 AM on May 23, 2015

IUPAC organic nomenclature: it's all about indicating the structure of isomers, where the components of a molecule are the same but the properties can be very different.
posted by holgate at 6:14 AM on May 23, 2015

Best answer: There's a pretty thorough explanation of the numbering system here. In the case of 1,1,1 Triclorotriflouroethane, the three chlorine atoms are all bonded to the first carbon atom on the chain. The fluorines, therefore, are all bonded to the second carbon (since ethane means it is a two-carbon chain). In pure ethane, all six of the atoms bonding to the carbon chain are hydrogen. So as the chemical name gets more complex it is reflecting other elements taking the place of some or all of the hydrogens. Here's a page about 1,1,2-Trichloro-1,2,2- Trifluoroethane, which has the same number of carbon, chlorine and fluorine atoms, but in a different configuration. You could express 1,1,1 Triclorotriflouroethane as 1,1,1 Tricloro-2,2,2-triflouroethane, but the 2,2,2 is not really necessary since there's only one other carbon for all three Fluorines to be.
posted by beagle at 6:17 AM on May 23, 2015 [6 favorites]

Best answer: ethane
1,1,1-trichloroethane
1,1,2-trichloroethane
1,1,1-trichlorotrifluoroethane
1,1,2-trichlorotrifluoroethane

In addition to what's been mentioned, the general rule is that systematic names should be a simple as possible. 1,2,2-trichloroethane is an invalid name because it would be identical to 1,1,2-trichloroethane, but the latter has smaller numbers. For ethane, which position is 1 and which is 2 is arbitrary.
posted by grouse at 8:06 AM on May 23, 2015 [1 favorite]