a
ANALOGUES OF THE AMMONIUM COMPOUNDS IN PERIODIC GROUPS FIVE, SIX, AND SEVEN H. 6. HEAL University of British Columbia, Vancouver, Canada
T H E ammonium salts are the best known representatives of a large genus of compounds with the following characteristics. The positive ion consists of a central atom, which may be an element of the nitrogen or oxygen group, or a halogen, surrounded by ligands which are hydrogen atoms, organic radicals, or some combination of these. The elements of the nitrogen group take on four ligands, those of the oxygen group three, and the halogens two. All these "onium" ions, as they are often called, have a single positive charge, and in all of them the central atom has built up its electron number, by combination with the ligands, to an inert gas number, there being always an octet of valence electrons. Other examples are the phosphonium salts (containing pH4+), the hydronium salts (H30+), the trimethyl sulfonium salts (Mess+) and the diphenyl iodonium salts (Phd+). A great part of our knowledge of these compounds dates back many years. There have, however, been a number of recent developments in the subject which, in the writer's opinion, justify the presentation of this short review. Among these are the discovery of the aryl bismuthonium salts and of the triaryl oxonium salts, structural studies of the hydronium compounds, and theoretical work on the hydronium ion in solution. A further justification is the strong current interest in the applications of the onium salts, which is giving rise to hundreds of journal papers and patent applications annually. Most of these cover organo-substituted ammonium salts, which are under investigation as drugs, anesthetics, disinfectants, fungicides, plant growth regulators, detergents, flocculating agents, rubber fillers, gasoline and fuel oil additives, etc., and for polymer productiou. Phosphonium and arsonium salts are being studied to a lesser degree as drugs, and stibonium compounds as analytical reagents. A complete account of this subject would require a t least a large book. No more will be done here than to survey some aspects of the preparative and physical chemistry of the onium salts, with emphasis on recent discoveries, and on some regularities in properties which have not received sufficient notice in textbooks. I n order to limit the material to be covered, the chemistry of the pyridinium, pyroxonium and similar salts containing doublebonded nitrogen or oxygen will be omitted.
CONSTITUTION AND CHEMICAL PROPERTIES
can be hydrocarbon radicals. For example, the ions PH4+, PH3R+, PHaR2+,PHRs+, and PEL+ have been reported. It will be noticed that only the fully substituted salts of antimony, bismuth, sulfur, selenium, tellurium, chlorine, bromine, and iodine have been made, and that no unsubstituted arsonium salts exist. No bismuthonium compounds were known until 1952, when the extremely unstable tetraphenylbismuthonium chloride and bromide (decomposing a t room temperature) were prepared by the skillful experimental work of Wittig and his group (41). Chloronium and bromonium salts containing two separate ligands were not obtained until 1955 (26),though cyclic salts of this type were described in 1952 (SO). Polonium and astatine are not known to form "onium" salts, but may well be capable of doing so. A great variety of hydrocarbon radicals, and also other types of organic substituents, can be introduced, and the substituents need not all be alike. A chelating group may fill two ligand positions. The substituents can all be phenyl groups, except in the ammonium ion. No quaternary ammonium compounds containing more than two directly linked phenyl groups have been prepared. (There is, however, a compound between triphenylamine and perchloric acid, which may be [(CsHa)3HN]+C104-). It is conceivable that the nitrogen atom may be too small to accommodate four phenyl groups around it, but it seems likely that a new preparative approach might yield the tetraphenylammonium compounds, especially when one remembers that the isostere of tetraphenylammonium, tetraphenylmethane, has been known for many years and is exceptionally stable. When they can be prepared at all, the fully arylated "onium" compounds are usually more stable than the corresponding fully alkylated compounds. Mention must be made here of a large group of compounds obtained by the addition of halogens t o tertiary phosphines, arsines, and stibines, and to thio-, seleno-, and telluro-ethers, such as the substance (CH3)2SBr2. Although these compounds are hydrolyzed by water and are not always regarded as salts, recent work (5) on (CH3)?SBr2has shown that it is a good conductor in liquid sulfur dioxide solution, and that it will undergo - double decom~osition in Number of Organic Ligands i n Onium Salt Cations (All additional lieand nositions are oceunied bv hvdroeen.)
The table summarines the types of compounds known in which the ligands are hydrogen atoms, hydrocarbon radicals, or both. The numbers 0, 1, 2, etc., indicate how many ligands in the "onium" ion JOURNAL OF CHEMICAL EDUCATION
methanol to give unstable compounds such as (CH& SBrCI04 and (CHa)&BrBF4. Thus (CH&SBrz is a sulfonium salt with hromine ligand, [(CH&SBr]+Br-, and the selenium and tellurium analogues should probably be similarly formulated. The compound (CH& NBrZ, formed by addition of bromine to trimethylamine, probably contains an ammonium ion with one ligand position occupied by bromine [(CH&NBr]+. There is evidence that the so-called salts of trimethylamine oxide and the Group V analogues, such as (CH&NO.HCl, should he formulated as onium salts with OH ligands, e.g., [(CH&NOH]+Cl- (16, 21). More work on the structure of these compounds is needed. There is little doubt that all the substances covered by the table are really ionic compounds, except for a few doubtfully constituted "oxonium" compounds which will be discussed later. The evidence is as follows: The nitrogen compounds cannot even he formulated as fully covalent without exceeding the octet of valence electrons for nitrogen, which is not known to happen in any other compound. This restriction does not apply to the heavier elements of the nitrogen and oxygen groups, or to chlorine, bromine, and iodine, and fully covalent structures are formally permissible here, yet t,he physical evidence indicates that these compounds, as well as the nitrogen ones, are really ionic. Firstly, the solubilities show ionic character; water, liquid ammonia, and lower alcohols are the best solvents, acetone is worse, and the substances are usually only slightly soluble in ether and hensene. (Chloroform, however, dissolves the tetraphenylphosphoninm, arsonium and stibonium compounds readily, also other pheriylated "onium" salts.) Secondly, the solutions are good electrical conductors, and undergo instantaneous metathetical reactions with, e.g., silver nitrate, just like solutions of sodium salts. Diphenyliodonium iodide, (C6H&12exchanges half its iodine instantaneously with radioactive iodine ion in aqueous solut,ion (17), whereas the other half will not exchange even under drastic conditions: this is just what one would expect for an "onium" salt, [C6H6-I-C,H,] +I-. Thirdly, the compounds of colorless anions are themselves nearly colorless, which seems to rule out any pronounced degree of covalent binding of the anion; for instance, triphenyltellurium bromide is colorless though tetraphenyltellurium is yellow and tellurium tetrabromide is red. Even the iodides are practically colorless. The bases from which the partially substituted and unsubstituted salts are derived are all weak, so that the salts are hydrolyzed in aqueous solution to various degrees; the hydrolysis is, for example, slight with ammonium salts, hut more marked in the phosphonium series, indeed almost complete with the unsuhstituted phosphonium salts. I n contrast, the bases from which the fully substituted salts are derived are all very strong, being fully dissociated like the alkali hydroxides. They are made in solution by treating the iodides with silver oxide. Sidgwiek (82) explained the great jump in base strength accompanying the substitution of the last hydrogen atom by the fact that hydrogen honding of the cation to the hydroxyl ion then ceases to be possible. VOLUME 35. NO. 4, APRIL, 1958
Many of the "onium" ions share the unusual characteristic of giving sparingly soluble salts with a variety of large oxyanions, such as Clod-, Mn04-, and also with large chlorocomplex ions such as SnCla--, ShCls-, etc. There have been a number of applications of this property to analysis (38). The bond con$guration in the nitrogen group ions is tetrahedral: this has been proved by X-ray diffraction studies on the crystals of tetramethyl ammonium salts (S),by the Raman spectra of ions of this group in aqueous solution (88) and by the optical resolution of the compound
whirh would not he possible if the configuration were planar or square pyramidal (Z5). The bonding orbit,als of nitrogen (+) are sp3 hybrids, as in the isosteric neutral carbon atom. The oxygen group "onium" ions are presumably pyramidal in shape, like the isosteric ammonia molecule. The only direct evidence for this appears to he the fact that sulfonium ions containing three different organic radicals are optically resolvable, which would not be the case if the structure wei.e planar (27). Since the orbitals available for honding in divalent iodine (+) are like those in divalent oxygen and sulfur, the two carbon-halogen honds in the halogenonium salts are probably nearly a t right-angles. The existence of unusually stable chelate halogenonium ions (Z%),surh as r
1/ 7 7+
which could not he formed aithout strain if the bonds from the halogen were collinear, supports this belief. PREPARATION
The following is a summary of the most important methods of preparation of the "onium" salts. (1) For all compounds containing hydrogen as a ligand, the acid may be directly combined with the base; this can he done in the vapor state, or, if the salts are not too extensively hydrolyzed, in aqueous solution. Examples: NH3
+ HCI
PH8
+ HI
vapor [NHd +CI or solution vapor d
-
+ HC1
vapor or
(CHMsH
+ HBr
vapor at
(CH&N
+ HCI
CHINHe
[PHd '1
only
ICHJNHal+CI-
solution
-A
low temps. vapor or
-solution -
[(CH&AsH?I+BI
[(CH,),NH]+C1
Reaction is usually instantaneous if the conditions of temperature and pressure are such that the product compound can exist; however, it is well established that the combination of ammonia and hydrogen chlo-
ride is very slow in the complete absence of moisture (15). (2) For compounds in which the ligands are all alkyl radicals, or are a mixture of alkyl and aryl radicals, the combination of alkyl iodide or sulfate with alkyl or aryl nitrogen, sulfur, etc., is a general method, e.g.,
--
+
(CH&N CHJ (CHa)%S CHJ
+
[(CHdrNl +I[(CHx)aSl +I-
Reaction often takes place slowly on mixing the reactants a t room temperature, but sometimes heating is required. The iodides form most easily, and can be used as a starting point for preparation of other salts by metatheses in aqueous solution. (3) Method (2) does not work with aryl iodides; for fully arylated compounds, syntheses by the Grignard or Friedel-Crafts methods are often employed, e.g., AICI, (C.Ha)BeCIa C8H6----t[(CaHs)sSel +ClHCI C,H,MgBr L [(C,H,),Asl+ (C,H,),AsO
+
+
+
The starting materials for these methods are readily accessible. The action of phenyl magnesium bromide on phenyl iodide chloride (PhICI2) does not, however, give a good yield of aryl iodonium salts; the best route to these compounds is the following: (4) Masson and Race's (23) synthesis of iodonium salts. HIOs iodic acid
---
+ 2ArH + H2SOI
conr. HBO,
[ A d +HS04-
solution
and other products
(5) Tetraphenyl bismuth chloride, bromide, nitrate, and perchlorate have only been obtained (Wittig, 1952) by the following sequence based on the use of phenyl lithium, and giving as intermediate the remarkable compound pentaphenyl bismuth (41):
+ 2CaHJ.i
(CrH&Bi
+ Br2
(CsH&Bi
ether
(CsHJrBiCI2
(CaHJsBi
soh. at -75"
ether ---t
soh. at -70'
+ HCI
[(C.H.)
