1,1.1-Trihalides

All of the trihalides of the Group VA elements are known. In principle, the direct action of the appropriate halogen on the elements leads to the formation of the trihalides. However, such reactions may not always be the best preparative methods. The fluorides are prepared by the following reactions  

Bismuth trifluoride is only slightly soluble, and it precipitates from solutions containing Bi3+ when an excess of F is added. 

The chlorides of the Group VA elements are prepared in the following ways. Phosphorus trichloride is obtained by the reaction of white phosphorus with chlorine using an excess of phosphorus  

The trichlorides of As, Sb, and Bi are produced when As406, Sb2S3, and Bi203 react with concentrated HC1  

A second trend that is interesting is shown by the bond angles of the molecules. For example, the bond angles for the PX3 molecules are all in the 101° to 104° range that is 

Most halides are LnXs, but a number of LnX2 are known, as are a handful of tetrafluorides. 

Although the halides can be obtained as hydrates from reaction of the metal oxides or carbonates with aqueous acids, these hydrates are hydrolysed on heating to the oxyhalide and thus the anhydrous halides (which are themselves deliquescent) cannot be made that way. 

The fluorides are obtained as insoluble hydrates LnF3.0.5H2O by precipitation and the hydrates dehydrated by heating in a current of anhydrous HE gas (or in vacuo). 

Otherwise the anhydrous halides can generally be made by heating the metal with the halogen (except for Euls) or gaseous HCl. 

Another method for the chlorides involves refluxing the hydrated chlorides with thionyl dichloride (SOCI2) for a few hours an advantage of this method is that the other reaction products are gaseous SO2 and HCl. 

Attempts at investigation of the kinetics of exchange in triiodide have been made using nmr line-width methods . It is concluded in the most recent publication that the relaxation times observed to date for CI, C1, Br and l are all probably due to relaxation, viz. 

Apart from reactions of halides with inorganic (Chapter 3) and organic species (Section 4) and their oxidations by sundry anionic species, there remain oxidations by nonionic species and some substitution reactions not previously discussed. This section deals with oxidation by hydrogen peroxide and by oxygen, then with the oxidation by chlorine dioxide, and finally picks up some observations on phosphorus species. 

Extensive studies have been made of the oxidations of all the halides by hydrogen peroxide. Mellor in 1904 was already able to cite fourteen investigations of kinetics of the hydrogen peroxide-hydrogen iodide reaction including studies of the temperature dependence of the rates and the kinetic form of catalysis by salts of molybdenum and iron. Since the processes (1) and (2) 

Studies of the chloride ion reaction required careful attention to impurities which at first gave rise to rates of hydrogen peroxide decomposition comparable to those under study. The reaction obeys the rate law 

Salt effects and catalysis by copper have been examined. 

IrFs (also Pt(0, Fla) Intermediate packings RuFa.MoFal ) 

CoFa MnFaW 

W Unique distorted (monoclinic) VFa tyP due to Jahn-Teller effect. ( ) Close to cubic ReOa structure. 

Other trihalides. Most of the other halides of metals of this group adopt one or more of the following structures (Table 9.17)  

L = Bila or YCla layer structure C = Zrla chain structure D  

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White and red phosphorus combine directly with chlorine, bromine and iodine, the red allotrope reacting in each case at a slightly higher temperature. The reactions are very vigorous and white phosphorus is spontaneously inflammable in chlorine at room temperature. Both chlorine and bromine first form a trihalide ... 

A complete set of trihalides for arsenic, antimony and bismuth can be prepared by the direct combination of the elements although other methods of preparation can sometimes be used. The vigour of the direct combination reaction for a given metal decreases from fluorine to iodine (except in the case of bismuth which does not react readily with fluorine) and for a given halogen, from arsenic to bismuth. 

In addition to the trihalides, arsenic and antimony form penta-fluorides and antimony a pentachloride it is rather odd that arsenic pentachloride has not yet been prepared. 

The trihalides closely resemble those of antimony. Bismuth(V) fluoride is known. It is a white solid, and a powerful oxidising agent. 

The melting and boiling points of a series of similar covalent halides of a given element are found to increase from the fluoride to the iodide, i.e. as the molecular weight of the halide increases. Thus, the trihalides of phosphorus have melting points PF3 = 121.5 K. PCI3 = 161.2 K, PBrj = 233 K, PI3 = 334 K. 

Reaction with phosphorus trihalides (Section 4 13) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides... 

Alcohol Phosphorus trihalide Alkyl halide Phosphorous acid... 

Section 19 16 Halogenation at the a carbon atom of carboxylic acids can be accom plished by the Hell-Volhard-Zehnsky reaction An acid is treated with chlorine or bromine m the presence of a catalytic quantity of phospho rus or a phosphorus trihalide... 

With tetiaaiyltin compounds, the reaction can proceed further to the aryltin trihalides ... 

Table 8. Physical Properties of Typical Organotin Trihalides...

Some of the general reactions for the boron trihalides where X represents Cl, Br, or 1 and X a different halogen, ate... 

Addition to the Double Bond. Chlorine, bromine, and iodine react with aHyl chloride at temperatures below the inception of the substitution reaction to produce the 1,2,3-trihaLides. High temperature halogenation by a free-radical mechanism leads to unsaturated dihalides CH2=CHCHC1X. Hypochlorous and hypobromous acids add to form glycerol dihalohydrins, principally the 2,3-dihalo isomer. Dehydrohalogenation with alkah to epicbl orobydrin [106-89-8] is ofgreat industrial importance. 

Pyridazines form complexes with iodine, iodine monochloride, bromine, nickel(II) ethyl xanthate, iron carbonyls, iron carbonyl and triphenylphosphine, boron trihalides, silver salts, mercury(I) salts, iridium and ruthenium salts, chromium carbonyl and transition metals, and pentammine complexes of osmium(II) and osmium(III) (79ACS(A)125). Pyridazine N- oxide and its methyl and phenyl substituted derivatives form copper complexes (78TL1979). 

By incorporation of some trihalide to give a branched polymer such as Thiokol ST (about 2% of 1,2,3-trichloropropane is used in this instance). The resultant vulcanisates have lower cold flow and compression set than obtained with Thiokol A. 

Hell-Volhard-Zelinsky reaction (Section 19.16) The phosphorus trihalide-catalyzed a halogenation of a carboxylic acid ... 

The boron trihalides are volatile, highly reactive, monomeric molecular compounds which show no detectable tendency to dimerize (except perhaps in Kr matrix-isolation experiments at 20K). In... 

The boron trihalides form a great many molecular addition compounds with molecules... 

The importance of the trihalides as industrial chemicals stems partly from their use in preparing crystalline boron (p. 141) but mainly from their ability to catalyse a wide variety of organic reactions.BF3 is the most widely used but BCI3 is employed in special cases. Thus, BF3 is manufactured on the multikilotonne scale whereas the production of BCI3 (USA, 1990) was 250 tonnes and BBr3 was about 23 tonnes. BF3 is shipped in steel cylinders containing 2.7 or 28 kg at a pressure of 10-12 atm, or in tube trailers... 

Many other routes are now available but the most used involve the reaction of Grignard reagents or lithium alkyls on orthoborates or boron trihalides ... 

AIF3 is made by treating AI2O3 with HF gas at 700° and the other trihalides are made by the direct exothermic combination of the elements. AIF3 is important in the industrial production of A1 metal (p. 219) and is made on a scale approaching 700000 tonnes per annum world wide. AICI3 finds extensive use as a Friedel-Crafts catalyst (p. 236) its annual production approaches lOOOOOtpa and is dominated by Western Europe, USA and Japan. The price for bulk AICI3 is about 0.35/kg. 

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