I read about perchloride in a talk and after trying to search for its chemical formula, the search results imply that perchloride is actually named perchlorate. Is this correct?


Short answer: Yes, sometimes "perchlorate" can be called "perchloride" in older literature, but because of the classical definition of a "perchloride", it's so confusing and technically wrong that the $\ce{ClO_4^-}$ anion should be called either "perchlorate" or "chlorate(VII)".

In older naming systems, pre-IUPAC, a "per-ide" is quite simply "any substance containing an unusually high proportion of the named element". So, a "perchloride" is any substance with more chlorine than is "normal" for that mixture of elements. This name was typically given to metal halide salts where the metal has multiple possible oxidation states; the highest oxidation state of the metal forms the metal "per-X-ide". for instance, titanium perchloride aka titanium tetrachloride, $\ce{TiCl_4}$ (IUPAC name titanium(IV) chloride), as differentiated from the trichloride and dichloride salts of titanium, and iron perchloride aka ferric chloride (that name's rarely used as iron only has two stable chlorides, and so the traditional "ferric" and "ferrous" descriptors work fine).

However, the perchlorate anion isn't actually a "perchloride" by that definition; it's more a "peroxide", as there's only one chlorine (which is relatively "normal") that has been "superoxidized" by four oxygens (not normal).

This use of "perchloride" as a synonym for "perchlorate" is more the result of a bad simplification of the traditional naming system, where salts of "-ic" acids are "-ides". While the rule follows for the hydrohalic acids (HF, HCl, HBr, HI) and most oxoacids (which, when they end in "-ous", form "-ites", as in hypofluorous acid, chlorous acid and sulfurous acid), the highly oxygenated oxoacids like chloric, perchloric, sulfuric, nitric, phosphoric and boric acids form "-ates", not "-ides", to mirror the low-oxygen "-ous" acids and their "-ite" anions/salts.

When not talking about the perchlorate anion, you sometimes hear about organic molecules that have had all their hydrogens replaced by halogens. Perchlorocarbons and perfluorocarbons are the chlorine and fluorine derivatives of hydrocarbons (for instance, the dry-cleaning chemical perchloroethylene, "Perc", and the perfluoroethylene monomer of polytetrafluoroethylene, aka Teflon).

"Peroxide" has found a home in the IUPAC system as the official name of the $\ce{(O_2)^{2-}}$ anion, composed of two single-bonded oxygens (often as the related "hydroperoxide", which is an OOH functional group bound to a carbon or to hydrogen). However, because each chlorine anion in a "perchloride" is typically bound to whatever the chlorine is oxidizing, the multiple chlorides are not a single anion and so aren't named as such in IUPAC.


-ide means the lowest oxidation state. For $\ce{Cl}$, if it gains 1 e$^-$, oxidation state is -1 (i.e. $\ce{Cl-}$); in case of $\ce{S}$ (16th group) it's 2 e$^-$ (i.e. $\ce{S^{2-}}$)

per- means higher than usual oxidation state. For $\ce{Cl}$ it's +7 ($\ce{ClO4-}$); for $\ce{S}$ look at compounds with O-O bond (peroxomonosulfate $\ce{SO5^{2-}}$)

So per*ide like your perchloride is an oxymoron.

Compare http://en.wikipedia.org/wiki/Chlorite


per- -ide

Refers, as far as I know, to double anions. For example: $\ce{HgCl2, H2O2}$ are named "Mercury dichloride" and "hydrogen peroxide" respectively. I believe that "per- -ide" has been dropped for most cations (e.g. $\ce{HgCl2}$ was previously called "mercury perchloride"). "Peroxide" is the only one I've still see a lot around.

On the other hand,

per- -ate

Refers, as far as I know, to anions derived from 'per- -ic' acids. Perchloric acid is $\ce{HClO4}$ and perchlorate is $\ce{ClO4-}$

This system of naming isn't so common now. We generally refer to 'chlorate' or 'chloride' and indicate the oxidation number.

E.g. $\ce{NaClO4}$ is now often referred to as "sodium chlorate (VII)" instead of "sodium perchlorate".

You might want to look a bit into the wikipedia article.

  • $\begingroup$ "Peroxide" stuck around because IUPAC adopted the name as the official term for the $\ce{O_2^2-}$ anion. $\endgroup$
    – KeithS
    May 7 '13 at 21:42

Perchloride is a word that has been used 100 years ago in the expression Iron perchloride to describe the Iron(III) chloride $FeCl_3$, so as to avoid any risk of confusion with the more usual Iron(II) chloride $FeCl_2$ which is obtained in the reaction of Hydrochloric acid with metallic iron. This prefix shows that there is more $Cl$ atom in $FeCl_3$ than in $FeCl_2$.

The prefix per- is used for translating more. Today it is only used in expressions like perchloric acid or perchlorate ion. It points out that the Chlorine atom is more oxidized in these substances than in any other chlorine compounds.

Today nobody uses the prefix per- before chloride any more when speaking of $FeCl_3$, because it is not the $Cl$ atom that is oxidized, but the $Fe$ atom.


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