Decoding HFO Numbering

 Undoubtedly you have seen news articles mentioning HFO refrigerants with names like 1234yf, 1234ze(Z), or 1234ze(E). Although these names look like a secret code, there is method in the madness. The good news is that technicians probably don’t need to know exactly how to read this secret code to do their job. However, telling me I don’t need to know what’s behind the curtain just encourages me to pull the curtain back. So here goes.

What is an HFO

First, you need to understand what an HFO is. An HFO is essentially an HFC with a double bond between two carbon atoms. You might remember from high school chemistry that carbon has a valence of 4. Think of the valence as the number of Velcro tabs on the atom. The carbon atoms in a normal hydrocarbon molecule are joined by single bonds, just one set of Velcro tabs joined between each carbon atom in the chain. They are called saturated because they are connected to largest number of atoms possible. Unsaturated hydrocarbon molecules, like HFOs, have a double bond between two of the carbon atoms. They use two sets of Velcro tabs between two of the carbon atoms. The double bond means there is one less atom in the molecule since two bonds are used between a pair of carbon atoms. Thus the designation as unsaturated.

Secret Code

The first four numbers of the secret refrigerant numbering code identify, in order: the number of double bonds, the number of carbon atoms, the number of hydrogen atoms, and the number of fluorine atoms in the molecule. However, there are many ways those atoms can be arranged, and different arrangements of the same components create different refrigerants with different physical properties. The last two or three letters describe how the atoms are arranged in the molecule.

First Number

The first number in the HFO numbering system describes the number of double bonds. At present, I am not aware of any HFO refrigerants that have more than one double bond. Currently all HFO refrigerants start with the number 1.  The 1 at the start of R1234ze(Z) indicates that the molecule has one double bond.  

Second Number

The second number is equal to the number of carbon atoms minus one.  The 2 in R1234ze(Z) indicates that the molecule has three carbon atoms: (#Carbons (3) - 1 = 2).

Third Number

The third number is equal to the number of hydrogen atoms plus one. The 3 in R1234ze(Z) indicates that the molecule has two hydrogen atoms (#Hydrogens (2) + 1 = 3).

Fourth Number

The fourth number is equal to the number of Fluorine atoms. The 4 in R1234ze(Z) indicates that the molecule has four Fluorine atoms.

First Lower Case Letter

HFO refrigerants are based on propylene, which has three carbon atoms. The first lower case letter identifies the atom connected to the middle carbon atom: x for chlorine, y for fluorine, and z for hydrogen. The lower case z in R1234ze(Z) indicates that the atom bonded to the middle carbon is hydrogen.

Second Lower Case Letter

The way the atoms are arranged on the ends of the molecule can vary. The second lower case letter describes the arrangement of the atoms on the end carbon containing the double bond. The letters are defined as 

a: 2 chlorine atoms

b: 1 chlorine atom and 1 fluorine atom

c: 2 fluorine atoms

d: 1 hydrogen atom and 1 chlorine atom

e: 1 hydrogen atom and 1 fluorine atom

f: 2 hydrogen atoms

The lower case e in R1234ze(Z) indicates that the end carbon with the double bond is connected to 1 hydrogen atom and 1 fluorine atom.

Upper Case Letter in Parenthesis

In some instances, there are two ways to connect the remaining hydrogen atoms. (Z) indicates the hydrogen atoms are on the same side of the double carbon bond. Z stands for zusammen: German for together. (E) indicates the hydrogen atoms are on opposite sides of the double carbon bond. E stands for entgegen: German for opposite. The (Z) on the end of R1234ze(Z) indicates that the two hydrogen atoms are located on the same side of the carbon double bond.

Although R1234yf, R1234ze(E), and R1234ze(Z) are all built out of the exact same type and number of atoms, the difference in how the atoms are arranged makes them three different refrigerants with different physical properties.