Numbering Scheme for Ozone-Depleting Substances and their Substitutes
Various ozone-depleting substances (CFCs, HCFCs, HBFCs, and halons) and their substitutes (HFCs, HCs, and PFCs) are numbered according to a system devised several decades ago and now used worldwide. Although it may seem confusing, in fact it provides very complex information about molecular structure and also easily distinguishes among various classes of chemicals. For example, it is not intuitive that 1,1-dichloro-1-fluoroethane (CFCl2-CH3) is an ozone-depleting substance. The designation HCFC-141b, in contrast, immediately conveys its ozone-depleting nature while concisely describing its structure.
- Decoding the Number (Other than Halons)
- Decoding the Number (Halons)
The first step, and one that will provide a valuable way to check the results, is to understand the prefixes CFC, HCFC, HFC, PFC, and Halon. In CFCs and HCFCs, the first "C" is for chlorine (atomic symbol: Cl), and in all of them, "F" is for fluorine (atomic symbol: F), "H" is for hydrogen (atomic symbol: H) and the final "C" is for carbon (atomic symbol: C). It is the chlorine that makes a substance ozone-depleting; CFCs and HCFCs are a threat to the ozone layer but HFCs are not. PFC is a special prefix meaning "perfluorocarbon". "Per" means "all," so perfluorocarbons have all bonds occupied by fluorine atoms. Finally, halons are a general term for compounds that contain C, F, Cl, H, and bromine (atomic symbol: Br). Halon numbers are different from the others and will be discussed separately. As you decode the structure, double-check the results against the prefix. For example, an HFC contains no chlorine, so your results should not show any Cl atoms.
|Prefix||Meaning||Atoms in the Molecule|
|CFC||chlorofluorocarbon||Cl, F, C|
|HCFC||hydrochlorofluorocarbon||H, Cl, F, C|
|HBFC||hydrobromofluorocarbon||H, Br, F, C|
|HFC||hydrofluorocarbon||H, F, C|
|Halon||N/A||Br, Cl (in some but not all), F, H (in some but not all), C|
Compounds used as refrigerants may be described using either the appropriate prefix above or with the prefixes "R-" or "Refrigerant." Thus, CFC-12 may also be written as R-12 or Refrigerant 12.
Blends of refrigerants are assigned numbers serially, with the first zeotropic blend numbered R-400 and the first azeotropic blend numbered R-500. Blends that contain the same components but in differing percentages are distinguished by capital letters. For example, R-401A contains 53% HCFC-22, 13% HFC-152a, and 34% HCFC-124, but R-401B contains 61% HCFC-22, 11% HFC-152a, and 28% HCFC-124.
Now that we understand that the prefix describes what kinds of atoms are in a particular molecule, the next step is to calculate the number of each type of atom. The key to the code is to add 90 to the number; the result shows the number of C, H, and F atoms. For HCFC-141b:
|141 + 90||=||2||3||1|
One more piece of information is needed to decipher the number of Cl atoms. All of these chemicals are saturated; that is, they contain only single bonds. The number of bonds available in a carbon-based molecule is 2C + 2. Thus, for HCFC-141b, which has 2 carbon atoms, there are 6 bonds. Cl atoms occupy bonds remaining after the F and H atoms. So HCFC-141b has 2C, 3H, 1F, and 2Cl:
|HCFC-141b||=||C 2H 3FCl 2|
Notice that the HCFC designation ( h ydro c hloro f luoro c arbon) is a good double-check on the decoding; this molecule does, indeed, contain H, Cl, F, and C. The "b" at the end describes how these atoms are arranged; different "isomers" contain the same atoms, but they are arranged differently. The letter designation for isomers is discussed below.
Let's look at another example: HFC-134a.
|134 + 90||=||2||2||4|
Again, there are 6 bonds. But in this case, there are no bonds left over after F and H, so there are no chlorine atoms. Thus:
|HFC-134a||=||C 2H 2F 4|
In this case, too, the prefix is accurate: this is an HFC ( h ydro f luoro c arbon), so it contains only H, F, and C, but no chlorine.
One final example: PFC-218.
|218 + 90||=||3||0||8|
This time, there are 2 x 3 + 2 = 8 bonds. However, there are no bonds left over after F, so there are no chlorine atoms or H atoms. Thus:
|PFC-218||=||C 3F 8|
Once again, the prefix is accurate: this is a PFC ( p er f luoro c arbon), so it contains only F, and C.Note that any molecule with only 1C (e.g., CFC-12) will have a 2-digit number, while those with 2C or 3C will have a 3-digit number.
Halon numbers directly show the number of C, F, Cl, and Br atoms. The numbering scheme above does not give a direct number for the number of Cl atoms, but that can be calculated. Similarly, Halon numbers do not specify the number of H atoms directly. Note that you don't need to add anything to decode the number:
For this molecule, there are 2 x 1 + 2 = 4 bonds, all of which are taken by Cl, F, and Br, leaving no room for any H atoms. Thus:
|Halon 1211||=||CF 2ClBr|
Isomers of a given compound contain the same atoms but they are arranged differently. Isomers usually have different properties; only one isomer may be useful. So far, we've deciphered the "HCFC" and the "141" of something like HCFC-141b, and we now know the specific atoms in the molecule. The remaining piece of the puzzle is determine the arrangement for a particular isomer. Since all of the compounds under discussion are based on carbon chains (1-3 carbon atoms attached in a line of single bonds: e.g., C - C - C), the naming system is based on how H, F, Cl, and Br atoms are attached to that chain.
A single C atom can only bond with 4 other atoms in one way, so there are no isomers of those compounds. For two-carbon molecules, a single lower-case letter following the number designates the isomer. For three-carbon molecules, a lower-case two-letter code serves this purpose.
First, consider two-carbon molecules. For example, HCFC-141, HCFC-141a, and HCFC-141b all have the same atoms (2C, 3H, 1F, and 2Cl), but they are organized differently. To determine the letter, total the atomic weights of the atoms bonded to each of the carbon atoms. The arrangement that most evenly distributes atomic weights has no letter. The next most even distribution is the "a" isomer, the next is "b," etc. until no more isomers are possible.
A common way of writing isomers' structure is to group atoms according to the carbon atom with which they bond. Thus, the isomers of HCFC-141 are:
|HCFC-141:||CHFCl - CH 2Cl (atomic weights on the 2 carbons = 37.5 and 55.5)|
|HCFC-141a:||CHCl 2 - CH 2F (atomic weights on the 2 carbons = 21 and 72)|
|HCFC-141b:||CFCl 2 - CH 3 (atomic weights on the 2 carbons = 3 and 90)|
For HFC-134, the isomers are:
|HFC-134:||CHF 2 - CHF 2|
|HFC-134a:||CF 3 - CH 2F|
Another way of writing out chemical structures is to specify, for each of the Cl, F, and Br atoms, the ordinal number of the carbon to which they are bonded and to use numerical prefixes (2=di, 3=tri, 4=tetra, 5=penta, etc.) to specify the total number of each kind of atom. The suffix for the molecular name depends on the number of carbons. Molecules with 1C end in "methane" (since there are no isomers of methane-derived molecules, they have no letter designation), " 2C end in "ethane," and 3C end in "propane." It is assumed that any bonds not occupied by Cl, F, or Br are occupied by H, so H atoms are not specified. So, the isomers of HCFC-141 can be written as:
|HCFC-141:||CHFCl - CH 2Cl||1,2-dichloro-1-fluoroethane|
|HCFC-141a:||CHCl 2 - CH 2F||1,1-dichloro-2-fluoroethane|
|HCFC-141b:||CFCl 2 - CH 3||1,1-dichloro-1-fluoroethane|
For HFC-134, the isomers are:
|HFC-134:||CHF 2 - CHF 2||1,1,2,2-tetrafluoroethane|
|HFC-134a:||CF 3 - CH 2F||1,1,1,2-tetrafluoroethane|
CFC-12 does not have any isomers, since it contains only 1C. In addition, there is no need to number the carbons. Thus, its name is difluorodichloromethane.
Molecules with 3C atoms are more complicated to name. The first letter designates the atoms attached to the middle carbon atom, and the second letter designates decreasing symmetry in atomic weights of atoms attached to the outside carbon atoms. Unlike 2C chains, however, the most symmetric distribution is the "a" isomer, instead of omitting the letter entirely.
|Atoms on Middle Carbon||Code Letter|
For example, HCFC-225ca is:
|C 3HF 5Cl 2 (3C = 8 bonds),|
|CF 3 - CF 2 - CHCl 2, and|
When no isomers are possible, no letters are used. For example, there is only one way to arrange 3C and 8F, so it is written as PFC-218 and not PFC-218ca.