notes on nomenclature
Univerriry of South Florida Tampa 33620
KURT LOENING ROY M. ADAMS
Chemical Abslrmts Service Columbus, Ohio 43210
G e n e w College Bearer Falls. Penn>ylronia I S O l O
Oxoacids and Their Salts
Nomenclature is very traditional in spirit. This is an advantage in some respects because the maintenance of a constant pattern makes it possible to read older literature without keeping in mind changes which have taken place over the years on the other hand, the perseverance of the traditional alongside of the improved or simplified, continues practices in wide usage which should have been dropped long before. Nowhere is the traditional more evident than in the naming of oxoacids and their salts. Limitations of the Conventional Pattern. Lavoisier considered that all acids contained oxygen but recognized that in several instances the same element might combine with different proportions of oxygen. He distinguished between these by the ending -eux (-ite for the salt) for the acid poorer in oxygen and -ique (-ate) for the acid richer in oxygen. With the recognition that there were often more than two oxygen acids of the same element and that hydrogen, not oxygen, was the element characteristic of acids, in English, the acids came to be distinguished in the following manner
Despite the great usefulness of the above series of names for acids and salts, the system has many limitations. Per L'ersus Peroxo. As knowledge increased, it became evident that there was an essential difference between peracids like perchloric and permanganic, on the one hand, and those like persulfuric and perphosphoric, on the other. The relation of the latter to hydrogen peroxide is indicated by the currently used peroxo prefix. Seueral Acid.%of the Same O.~idationState. There is not just one sulfuric acid but at least two: HzS04 and HzS20,. Salts of both acids are well established. In the case of phosphorus, there are a number of phosphoric acids. These are often distinguished as ortho, H3P04,meta, HPOs, and pyru, HaPz07. Even these prefixes are insufficient because there
are a variety of compounds of the formula HPO. representing varying degrees and modes of polymerization. Acids of Unusual Apparent Oxidation States. The conventional pattern for distinguishing oxygen acids of the same element breaks down when there are more than four different oxidation states. Also there is the problem of relating names for two elements showing common oxidation states. Consider the compound Na4PzOe.; superficially P has an apparent oxidation state of 4+, intermediate between NaaPOn and NazHP03 where the apparent oxidation states are 5+ and 3+, respectively. It has often been called a b p h o s p h e but might almost equally well have been called a perphosphi& Thereare a number of acids which resemble hypophosphoric acid, H4PZOe, in exhibiting a n apparent oxidation state not expected for the particular element involved. Sulfur forms several of these among which are: HzSz03, HzS206, and H z S Z O ~The . systematic namethiosulfnric acid has become standard for the first hut the trivial names dithionic and dithionous acids are still used for the latter two. Acids of Transition Elements. Consider the salts of the 0x0 acids of a transition element like rhenium: ,MRe04, MzReO,, MRe03, MzRe03, and MReOz (salts of more highly hydrated and of condensed acids are also known). Certainly, the first should be called a perrhenate in analogy to the perchlorates, e.g., MCIO4, and the third a rhenate in analogy to the chlorates, MC103. However, MzRe04 is the derivative of the state of oxidation just helow the per-ate acid and should be called a rhenate. Hence, MRe03 received the name hyporhenate and M2Re03 (with no analog among the 0x0 halogen acids)the name rhenite. Similarly. MRe02, the analog of the chlorites, received the name h y p o r h e n k (The situation among the salts of h a l o a c i d s o f e transition elements was even more confusing but now has heen systematized by coordination nomenclature as discussed below). The net result is that for names of the oxoacids and their salts we are left with a collection of hits and pieces, historical accidents, inconsistencies, apparent contradictions. and anything hut an orderly set. of systematic names as shown in Table 3. h e there any possible ways out of this dilemma? Coordination Type Names. Alfred Werner proposed a systematic nomenclature for coordination compounds (Tahle 1). This system applied to both acids and salts and completely distinguished among the various oxidation states of the same element. Somehow these names never caught on. Perhaps they were too long. Perhaps it was too difficult to remember the proper infix to designate the applicable oxidation state. The traditional names persistzd. Alfred Stock proposed a significant modification of Table 1.
Oxidn. State Formulas 3 4
ReOzReOa-
Coordination Type Names
----Names
Werner
dioxorheniate trionorhencate
According to Stock-Werner dioxorhenate(II1) trioxorhenate(1V)
Volume 50. Number 2. February 1973 / 123
Table 2. Oxidn.
Haloacids Names Accordlngto
Surk-Wernpra
'Tcday the Ewenr-Bassett system of giving the charge an the ion is equally acceptable to tho Staek system ofindicating the oridationatate.
Table 3. H.BO.
(HBO& H&O. HOCN
Names of Oxoacids
onhohorieaeid or boricacid mctaboric acid ~ ~ ~ b o ~ i ~ ~ ~ i d evsnmseid HAOs
H&Os
H&Os HzSOs H&OI H&02
H&O, HzSOs H&Os I1
=3.4..
H&0, H&OI
.I
HBT~OD H fro.
HCIIOI HClO.
~-
~
IHOl2OPPOIOH12 (HOhP-0HzPHOa H2PzHxOn HPHsOZ HIAIO* HaArOa
HSb(0Hl"
.
.
srld hypophosphoricecidor diphosphoric(IV1 m i d diphorphoodIl1. V l acid phoaphonic acid diphorphonicacid phoiphinicacid arrenx acid ameniaus acid horahvdroro-
HCIO. HC102 HClO HBrO. HBrOs HHrOl HRrO Hms HIO.
HIO,
HI0 HMDOI
H2Mn04 HkO. HITcO. HReO.
arid pemrodbulfurie add ihiorulfuricadd dithionicecid ~uilumusacid dlaulfurour acid thtorrullvrouracid d,th;onour acid sulfory1,cacid polyfhionicacids aelenicaeid seleniour acid orthotelluric acid chmmic acid dichmmicacid pgrehlorie acid ehloricaeid chiorous acid hypochiomusacid perbmmic seid bmmic acid hmmous acid hypobmmousacid orthoperiodic acid periodic acid iodicacid hypoioduus acid permanganic acid manesnccacid pertechneticacid fechneiir acid pmhenicaeid rhenicaeid
Werner names by substituting Roman numerals for the cumbersome infixes. Apparently, the Roman numeral had a good deal of appeal for these names are in common use for the haloacids (Table 2 ) . However, as yet they have not been a c c e ~ t e dfor the oxoacids. I U P A C - A ~ ~ ~Names O U ~ ~(1957). The IUPAC Commission on the Nomenclature of Inoraanic Chemistrv made this statement in the 1947 edition of the Rules The nomenclature of acids has a lone historv of established
well-known substances. The present rules are aimed at preserving the more useful of the older names while attempting to guide further development along directions which should allow new compounds to be named in a more rational manner. IUPAC-Approued Names (1971). Yet the list of names (Table 3)' in the new revision remains practically unchanged. T h e student still has a major feat of memory to distinguish the various 0x0 acids. T h e few changes which have been made deserve comment. HzS~0-idisulfuric acid-the alternative name pyrmulfuric acid has been dro~oed. .. HzPH03 phcsphonic aeid these names are adopted in place H2P2H203 diphosphonic of phosphorous and diphosphorous acids. Because only two hydrogen atoms in H3P03 are replaceable in neutralization, the acid behaves like the organic compounds which are known as phosphonic acids, RPO(0Hh. Then the condensed acid derived from phosphonic aeid should be diphosphonieacid. (H0)20P-PO(OH)2hypophosphoric acid or diphosphoric(1V) acid. The alternative name is introduced in analogy to the name diphcsphoric (LU, V) acid for (HO)2P-OPO(OH).. The Task Remaining. It is obvious that the changes only skirt around the real problem. What can he done to disengage ourselves from the t a n d e oresented hv the oresently-a&epted names for the oioacids? ~ r o f e k o rK. A. Jensenz has suggested for the acids of chlorine the following names anions
HClO HCIOl HCIOI HCIOl
chloric(1)aeid chlorie(1II)aeid chloric(V)acid, ehlaric(VI1)ae~d
C10C10~C10c C104
One could object immediately that these names do not specify composition hut then neither do the currently accepted ones. However, remembering the formulas, the names follow directly so half of the burden of memory is removed. Further, the names in two instances are shorter than the ones presently in use. Even if these names were adopted, it is questionable if they would serve as the basis for a completely general pattern. Quite aside from the acids containing a single atom of the acid element there are those with two such atoms directly attached, [(H0)20P-k, [(H0)02S-1, etc.; those with two such atoms linked through oxygen, [ ( H 0 ) 2 0 P ] 2 0 , [(HO)OzS]zO, etc.; and those with bridging groups other than oxygen, [(H0)02S]2SX. Simple ways for indicating these differences would he helpful. '"Nomenclature of Inorganic Chemistry" (2nd ed.), Pure and Applied Chem. 28 (I),32-3 (1971). Jensen, K. A., Private communication.
124 /Journal of Chemical Education
chlarate(1) ion ehlorate(II1)ion chlorate(V)ion ehlorate(VI1) ion
