# What is the meaning of the prefix "per-"?

The prefix "per-" appears in the names of many compounds, such as in perchlorate, persulfate, peroxide. What does it mean?

• See my comment in the thread under my answer here. Nov 9 '15 at 14:12

Per- can mean a number of different things, depending on the context. It can mean that there is a peroxi bond(oxygen-oxygen bond) in the system, persulfate is a good example of this.

More generally it tends to mean that the compound is the most completely altered product of a certain(possibly hypothetical) reaction. For example perchlorates and permanganates are the most completely oxygenated compounds of chlorine and manganese, respectively. This also describes another common use of per-, perhalogenated compounds. In that context, it usually means that all hydrogens present in the parent compound, that were bonded to a carbon, have been replaced with a halogen, for example perfluoroethylene (C2F4, also known as tetrafluoroethylene, the monomer of Teflon(PTFE))

• According to current IUPAC recommendations, the prefix "per-" is no longer recommended for organic compounds in which all substitutable hydrogen atoms are completely substituted (e.g. perfluoroheptane).
– user7951
Nov 9 '15 at 1:45
• @Loong are there substituting recommendations or must one say octadecafluoroheptane?
– Jan
Nov 9 '15 at 13:15
• @Jan The preferred IUPAC name for perfluoroheptane $(\ce{C7F16})$ is hexadecafluoroheptane. Note that all locants are omitted, i.e. the name is not 1,1,1,2,2,3,3,4,4,5,5,6,6,7,7,7-hexadecafluoroheptane.
– user7951
Nov 9 '15 at 13:27
• I came here looking for this compound - perfluorobenzene - which has all its hydrogens replaced by a halogen, am commenting if anyone else googles this as well. Jan 7 '18 at 16:17