Pentachlorides

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. 

Phosphorus and chlorine combine directly to form either the trichloride or the pentachloride depending on the relative amounts of phosphorus and chlorine used. 

Phosphorus trichloride reacts with chlorine in excess to give phosphorus pentachloride, an equilibrium being set up ... 

However, phosphorus pentachloride in the solid state has an ionic lattice built up of (PC ) and (PClg)" ions and these ions are believed to exist in certain solvents. Thus under these conditions the maximum covalency is reached with chlorine. In phosphorus pentabromide, PBrj, the solid has the structure [PBr4] [Br] . 

Phosphorus pentachloride is prepared by the action of chlorine on phosphorus trichloride. To push the equilibrium over to the right, the temperature must be kept low and excess chlorine must be present. Hence the liquid phosphorus trichloride is run dropwise into a flask cooled in ice through which a steady stream of dry chlorine is passed the solid pentachloride deposits at the bottom of the flask. 

Phosphorus pentachloride sublimes and then dissociates on heating, dissociation being complete at 600 K. It is attacked by water, yielding first phosphorus trichloride oxide, thus ... 

The replacement of the —OH group by a chlorine atom (reaction 9.4) is a very general reaction of phosphorus pentachloride. For example, if concentrated sulphuric acid is written as (H0)2S02 then its reaction with phosphorus pentachloride may be written ... 

The reaction of ethanoic acid and phosphorus pentachloride may be written ... 

The trichloride oxide is also obtained by distillation of a mixture of the pentachloride and anhydrous ethanedioic acid ... 

It is prepared by heating together phosphorus pentachloride and a sulphite, for example calcium sulphite ... 

Chlorine reacts with most elements, both metals and non-metals except carbon, oxygen and nitrogen, forming chlorides. Sometimes the reaction is catalysed by a trace of water (such as in the case of copper and zinc). If the element attacked exhibits several oxidation states, chlorine, like fluorine, forms compounds of high oxidation state, for example iron forms iron(III) chloride and tin forms tin(IV) chloride. Phosphorus, however, forms first the trichloride, PCI3, and (if excess chlorine is present) the pentachloride PCI5. 

One of the general methods for the preparation of acid chlorides is the action of phosphorus pentachloride on the corresponding carboxylic acid ... 

This preparation illustrates the use of phosphorus pentachloride for the preparation of acyl chlorides in this case no difficulty is experienced in separating the 3,5-dinitrobenzoyl chloride from the phosphorus oxychloride formed simultaneously (c/. p. 240), because the former is readily isolated as a crystalline... 

Assemble in a fume-cupboard the apparatus shown in Fig. 67(A). Place 15 g. of 3,5-dinitrobenzoic acid and 17 g. of phosphorus pentachloride in the flask C, and heat the mixture in an oil-bath for hours. Then reverse the condenser as shown in Fig. 67(B), but replace the calcium chloride tube by a tube leading to a water-pump, the neck of the reaction-flask C being closed with a rubber stopper. Now distil off the phosphorus oxychloride under reduced pressure by heating the flask C in an oil-bath initially at 25-30, increasing this temperature ultimately to 110°. Then cool the flask, when the crude 3,5-dinitro-benzoyl chloride will solidify to a brown crystalline mass. Yield, 16 g., i.e,y almost theoretical. Recrystallise from caibon tetrachloride. The chloride is obtained as colourless crystals, m.p. 66-68°, Yield, 13 g Further recrystallisation of small quantities can be performed using petrol (b.p. 40-60°). The chloride is stable almost indefinitely if kept in a calcium chloride desiccator. 

B) Sulphonamides. Mix 0 5 g. of the sulphonic acid or its alkali salt with 15 g. of phosphorus pentachloride, and heat under reflux in a silicone- or oil-bath at 150° for 30 minutes then allow it to cool. 

This method of sulphonamide Tormation may obviously be invalidated by the presence of nuclear —NHj or —OH groups, which could react with ihe phosphorus pentachloride or with the subsequent —SO Cl group. 

Method 2. Mix 1 0 g. of 3 5-dinitrobenzoic acid with 1 5 g. of phosphorus pentachloride in a small, dry test-tube. Warm the mixture gently over a small smoky fiame to start the reaction when the reaction has subsided (but not before), boil for 1-2 minutes or until the solid matter has dissolved. Pour the mixture while still liquid on a dry watch glass (CAUTION the fumes are irritating to the eyes). When the product has solidified, remove the liquid by-product (phosphorus oxychloride) by transferring the pasty mixture to a pad of several thicknesses of filter paper or to a small piece of porous tile. Spread the material until the liquid has been absorbed and the residual solid is dry. Transfer the 3 5 dinitrobenzoyl chloride to a test-tube, add 0-5-1 ml. of the alcohol, and continue as in Method 1. 

The melting points of these esters are usually much lower than those of the corresponding 3 5 dinitrobenzoates their preparation, therefore, offers no advantages over the latter except for alcohols of high molecular weight and for polyhydroxy compounds. The reagent is, however, cheaper than 3 5 dinitrobenzoyl chloride it hydrolyses in the air so that it should either be stored under light petroleum or be prepared from the acid, when required, by the thionyl chloride or phosphorus pentachloride method. 

Malononitrile, obtained by the dehydration of cyanoacetamide with phosphorus pentachloride ... 

Other catalysts which may be used in the Friedel - Crafts alkylation reaction include ferric chloride, antimony pentachloride, zirconium tetrachloride, boron trifluoride, zinc chloride and hydrogen fluoride but these are generally not so effective in academic laboratories. The alkylating agents include alkyl halides, alcohols and olefines. 

Sulphonamides. Mix together 1 0 g. of the dry acid or 1 - 2 g. of the anhydrous salt with 2 5 g. of phosphorus pentachloride f and heat under a reflux condenser in an oil bath at 150° for 30 minutes. Cool the mixture, add 20 ml. of dry benzene, warm on a steam bath and stir the solid mass well to extract the sulphonyl chloride filter. Add the benzene solution slowly and with stirring to 10 ml. of concentrated ammonia solution. If the sulphonamide precipitates, separate it by filtration if no solid is obtained, evaporate the benzene on a steam bath. Wash the sulphonamide with a little cold water, and recrystallise from water, aqueous ethanol or ethanol to constant m.p. 

The procedure is not usually applicable to aminosulphonic acids owing to the interaction between the amino group and the phosphorus pentachloride. If, however, the chlorosulphonic acid is prepared by diazotisation and treatment with a solution of cuprous chloride in hydrochloric acid, the crystalline chlorosulphonamide and chlorosulphonanilide may be obtained in the usual way. With some compounds, the amino group may be protected by acetylation. Sulphonic acids derived from a phenol or naphthol cannot be converted into the sulphonyl chlorides by the phosphorus pentachloride method. 

The Y-phenylbutyric acid is cyclised to a-tetralone by converting it into the acid chloride with thionyl chloride or phosphorus pentachloride and then an intramolecular Friedel and Crafts reaction is carried out ... 

By treatment of this oxime with phosphorus pentachloride or thionyl fhloride in ether solution, smooth conversion into benzanilide, m.p. 163°, results. The change of any oxime into a substituted amide under the conditions mentioned is usually termed the Beckmann rearrangement. The above example may be represented ... 

Beckmann rearrangement of benzophenone oxime to benz-anilide. Dissolve 2 g. of benzophenone oxime in 20 ml. of anhydrous ether in a small conical flask and add 3 g. of powdered phosphorus pentachloride (or 3 ml. of pure tbionyl chloride). Distil off the solvent and other volatile products on a water bath CAUTION ether), add 25 ml. of water, boil for several minutes and break up any lumps which may be formed. Decant the supernatant liquid, and recrystallise, in the same vessel, from boiling alcohol. The product is benzanilide, m.p. 163° confirm this by a mixed m.p. determination with an authentic specimen. 

Method 2. Place 90 g. of sodium benzenesulphonate (Section IV,29) (previously dried at 130-140° for 3 hours) and 50 g. of powdered phosphorus pentachloride (1) in a 500 ml. round-bottomed flask furnished with a reflux condenser heat the mixture in an oil bath at 170-180° for 12-15 hours. Every 3 hours remove the flask from the oil bath, allow to cool for 15-20 minutes, stopper and shake thoroughly until the mass becomes pasty. At the end of the heating period, allow the reaction mixture to cool. Pour on to 1 kilo of crushed ice. Extract the crude benzenesulphonyl chloride with 150 ml. of carbon tetrachloride and the aqueous layer with 75 ml. of the same solvent. Remove the solvent under atmospheric pressure and proceed as in Method 1. The yield is about 170 g., but depends upon the purity of the original sodium benzenesulphonate. 

Alternatively a mixture of 90 g. of sodium benzenesulphonate and 60 g. (36 ml.) of phosphorus oxychloride may be used. The experimental procedure is identical with that for phosphorus pentachloride, but the yield is slightly better. 

Place a mixture of 30 g. of 3 5-dinitrobenzoic acid (Section IV,168 and 33 g. of phosphorus pentachloride in a Claisen flask fit a reflux condenser into the short neck and cork the other neck and side arm (compare Fig. Ill, 31, 1). Heat the mixture in an oil bath at 120-130° for 75 minutes. Allow to cool. Remove the phosphorus oxychloride by distillation under reduced pressure (25°/20 mm.) raise the temperature of the bath to 110°. The residual 3 5-dinitrobenzoyl chloride solidifies on cooling to a brown mass the yield is quantitative. Recrystallise from carbon tetrachloride the yield is 25 g., m.p. 67-68° and this is satisfactory for most purposes. Further recrystallisation from a large volume of light petroleum b.p. 40-60°, gives a perfectly pure product, m.p. 69 -6°. 

When unsubstituted, C-5 reacts with electrophilic reagents. Thus phosphorus pentachloride chlorinates the ring (36, 235). A hydroxy group in the 2-position activates the ring towards this reaction. 4-Methylthiazole does not react with bromine in chloroform (201, 236), whereas under the same conditions the 2-hydroxy analog reacts (55. 237-239. 557). Activation of C-5 works also for sulfonation (201. 236), nitration (201. 236. 237), Friede 1-Crafts reactions (201, 236, 237, 240-242), and acylation (243). However, iodination fails (201. 236). and the Gatterman or Reimer-Tieman reactions yield only small amounts of 4-methyl-5-carboxy-A-4-thiazoline-2-one. Recent kinetic investigations show that 2-thiazolones are nitrated via a free base mechanism. A 2-oxo substituent increases the rate of nitration at the 5-position by a factor of 9 log... 

Treatment of a-thiocyanatoketones at low temperature with dry hydrogen chloride in ether solution gives satisfactory yields of 2-chloro-thiazole derivatives (188). The use of phosphorus pentachloride leads to the same results, but in this case chlorination can also occur at the 5-position (Scheme 97) (18, 68). 

The oximes of ketones undergo the Beckmann rearrangement on treatment with phosphorus pentachloride (118). 

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