Indian Journal of Chemistry Vol. 15A,January 1977,pp. 23-26

Chlorosulphuric Acid as a Non-aqueous Solvent: Part VI• Conductometric & Spectral Studies of Various Inorganic & Organic Acid Annydrides in Chlorosulphuric Acid

R. C. PAUL*, D. S. DHILLON, D. KONWER & J. K. PURl Department of Chemistry, Panjab University, Chandigarh 160014

Received 25 February 1976; accepted 2 June 1976

Conductometric and spectral studies of the solutions of acid anhydrides in chlorosulphuric acid indicate that nitric and nitrous oxides behave as non-electrolytes, whereas dinitro~en trioxide, dinitro~en tetroxide and dinitro~en pentoxide form nitrosonium and nitronium ions. Phosphorous pentoxide ~ives a mixture of various sulphur oxychlorides which behave as non• electrolytes. Arsenic trioxide, antimony trioxide and bismuth trioxide ~et solvolysed whereas boric anhydride forms tetrachlorosulphato boric acid when dissolved in chlorosulphuric acid. Or~anic acid anhydrides behave as bases. From the extent of protonation of these solutes, it has been inferred that chlorosulphuric acid is a stron~er acid than sulphuric acid.

as such. Antimony pentachloride (BDH) was used without further purification. SYSTEMATICthe solutions physico-chemicalof various anhydridesinvestigationsof inorganicof and organic acids in fluorosulphuric acid1 have Results and Discussion shown that the acid gets dehydrated and hydronium (H;O) ions are formed. Such an investigation is It has already been shown that (SOaCI-) ions are lacking in chlorosulphuric acid as a non-aqueous responsible for the conductance of these solutions solvent. As chlorosulphuric acid is weaker than and (50aCl-) is the strongest monoacid base'. Specific disulphuric and fluorosulphuric acids, investigations conductance versus molality curves of various -on the solution behaviour of tlie inorganic and solutes in chlorosulphuric acid are given in Fig. 1. -organic acid anhydrides shall be of greater help in The conductometric factor, y (the number of SOaCl• differentiating the basicities of these anhydrides. bns produced per mole of solute) has been obtained Materials and Methods by comparing the observed conductance curves with those of potassium chlorosulphate (a fully The design of the conductivity cells used has ionized base). Acid-base titration curves in chIoro• been described earlier2. Conductance measure• sulphuric acid medium are given in Fig. 2. ments were made as described previouslys. Chloro• When sulphur trioxide is added to chlorosulphuric sulphuric acid (BDH technical) was first fractionally acid the conductance of the solution decreases. nitrogen the formation of higher chloropolysulphuric acids atmosphere before use. of the type HS208CI, HSsOgCIand probably HS,01ICI Nitrous oxide, nitric oxide, dinitrogen trioxide, and even higher polyacids which further behave as Arsenic trioxide, antimony trioxide, have already been reported by Gillespie and antimony pentoxide, bismuth trioxide, phosphorous Robinson8• The non-electrolytic behaviour of these pentoxide and boric anhydride were commercially higher acids in chlorosulphuric acid has been con• available samples and were used without further firmed by the fact that on addition of potassium or purification. Succinic anhydride was washed with sodium chloride there is always an increase in dry ether and then dried over phosphorous pent• conductance; at no stage there is any decrease. oxide for 24 hI. Commercially available samples of The reaction occurring could be represented by acetic anhydride (b.p. 140°/740 mm), isobutyric Eq. (1) or (2). anhydride (b.p. 182°/745 mm) and dichloroacetic KCI + SOa---+KSOaCI--+K'" + SOaCI- '" (1) anhydride (b.p. 101°/16 mm) were distilled in an inert atmosphere before use. Maleic anhydride was KCI+HSOaCI-+HCI+KSOaCI-+K'" +SOaCI-+HCI recrystallized from chloroform and dried over '" (2) phosphorous pentoxide. Potassium chIorosulphate was prepared by the literature method' and used The sulphur trioxide dissolved in HSOaCI may convert KCI to KSOsCI or KCI may be solvolysed *Present address: Vice-Chancellor, Panjab University, by chlorosulphuric acid to KSOaCl. In both the Chandigarh 160014. cases SOsCI-ions are produced which are responsible INDIAN J. CHEM., VOL. I5A, JANUARY 1977

~ '" 4.4 ~- -Sb20s -1Il-J8(--:-+-8"203--"-"-IS08UTYRICANHYOI?IOE-0-0--,-PaOsPI?OPlON/C(CIC~COz)20O.z ANHYDRIQE JCE .e 2:4'.6 -~-4-T~ '"a~~~, 2.03.6t3.2t -fl__fIl-8ENZOIC-t(-t{-"'N.EIC-4-4-HaOs-4---4r--8_._/(5°3'/-~-¢-SUCCINIC______-9--¢_ACETIC-PHTHALLIC-)(--+-l-B20]-Cl-()-Sb20) A¥'JANHYDRIDEANHYDRIDEANHYDRIDEANHYDRID£SeOzANHYDRID£ \ 2.8 ~ 0.84.ol-o-O-NaOJ'·2 _~-Nl.04

Fig. I-Conductance of various inorganic and organic acid anhydrides in chioro• sulphuric acid at 25°

0.4,! I

t ,--4.-..--.--'-_ o 2 4 8 '0 -z ,i 14 16 iB MOLALITY XIO

3.6 for an increase in the conductance (HCI produced __ IV,~ VS SbCls in the latter case behaves as a very weak electro• ~-A1z0+ VS SbCls lyte4). Even the addition of potassium chloro• _____ IVZ Os vs SbCls e-e-- 8z~ VSICSOJCI sulphate directly does not bring about a fall in the ~ 8z03 V5 CHlCOOH conductance of the solution which confirms the ~ 8a 03 VS PVRIDINE observation recorded above. In the case of selenium trioxide, however, there is no significant change in the conductance of the solution indicating thereby that it behaves as a non-electrolyte, probably due to the formation of a compound of the type HSeSOoCI, analogous to the species HSeSOoF in fluorosulphuric7 acid which behaves as a non-electrolyte. Both selenium and tellurium dioxides are highly soluble in chloro• sulphuric acid and give highly conducting solutions. From the conductometric factor (Y), it is seen that in the case of selenium dioxide monoprotonation takes place whereas in the case of tellurium dioxide incomplete diprotonation takes place. It seems to 0·8 be quite reasonable to suggest the following mode of protonation (Eq. 3) in the case of selenium dioxide. + 0.4 Se02+HS03CI.-+HO=Se=O+S03CI- ... (3) In the case of tellurium dioxide the second in• complete proto nation may take place at the other o ! 0.4 ()·B '·2 ,·6 2.0 2.4 -2·8 3.2 3.6 oxygen atom. MOLAR RATIO Both nitrous and nitric oxides are quite soluble Fig. 2 +Conductometric acid-base titrations of different in chlorosulphuric acid, but behave as non-electro• anhydrides in chiorosulphuric acid at 25° lytes in it. 24 PAUL et al.: CHLOROSULPHURIC ACID AS A NON-AQUEOUS SOLVENT

Dinitrogen trioxide, dinitrogen tetroxide and sulphuric acid. Paul and coworkers1o-a have re• dinitrogen pent oxide are readily soluble in chloro• ported that fluorosulphuric. acid is dehydrated by sulphuric acid forming highly conducting solutions. phosphorous pentoxide to ,form di$ulphuryl difluo• At lower concentrations these solutions are colour• ride and trisulphuryl fluoride whereasdisulphuric less but when the concentrations are increased acid is dehydrated to form free sulphur trioxide yellow colour develops. From the conductance which further gives trisulphuricacid. Phosphorous data and the Y values obtained (Fig. 1), the follow• pentoxide gives solutions of low conductance in ing modes of reactions (4-6) may be represented: chlorosulphuric acid, accompanied probably with NzOa+ 3HSOaCI-+2NO++ 2S0aCl-+ HzSO4+HCl the formation of sulphur oxychlorides which are known to behave as non-electrolytes'. The pos• ••. (4) sible mode of solvolytic reaction can be represented N20,+ 3HSOaCl-+NO~+ NO++ 2S0aCl• as shown in Eq. (to). +HZS04+HCl ... (5) PZ05+ 9HSOaCI-+2HaP04+4Sz05Clz+~SO,+HCI NzO5+ 3HSOaCl-+2NO~+ 2S0aCl-+ HZS04 + HCl ... (10} •.. (6) Arsenic pentoxide is extensively soluble in chlqro• The possibility of the reaction (7) sulphuric acid on warming to give conducting NzOa+3HSOaCl-+2NO·+H;0+3S0aCl- ... (7) solutions. It may be assumed that when this solute is added to chlorosulphuric acid solvolysis has been ruled out as the Y value obtained is 2. takes place and the solvolysed product is slightly Chlorosulphuric acid is molecularly self-dissociated ionized to produce a significant change in con• according to Eq. (8) ductance. Arsenic trioxide has a very limited HSOaCl-+HCl+SOa ... (8) solubility in chlorosulphuric acid yielding solutions of low conductance (Fig. 1). The possible modes and the water formed during the above reaction of reactions occurring. in these ~olutions may be combines with SOa to produce sulphuric acid rather represented as in Eqs. (11) and (12). than getting protonated as hydronium ions. When the concentration of these anhydrides is increased ASzOa+9HSOaCl-+2As(SOaCl)a+3H.SO, + 3HCl light yellow solids separate out. IR spectrum of ...(ll} the solid obtained from NZ05 in silver chloride or plates show a strong band at 2310 cm-1 indicating AszOa+3HSOaCl-+2AsOSOaCl+HzSO,,+HCl ••.(12} the presence of NO; ions. The other frequencies One can expect the possibility of the formation of of NO~ ion and those for the anions could not be resolved as the solvent itself covers the whole the 3pecies As(SOaCI)a, analogous to As(SOaF)a in fluorosulphuric acid. If As(SOaCl)a is formed it region. In the case of solid obtained from NzO" the bands at 2280 and 2145 cm-1 may be assigned may further show amphoteric behaviour in chloro• to the species NO+zand NO'"respectively. Similarly sulphuric acid according to the reaction (13) in the case of the solid obtained from NzOaa sharp As(SOaCl)a-+As(SOaCl)~+SOaCl- ... (13} peak at 2165 cm-1 indicates the presence of NO'" and should give higher value of Y. As(SOaCI)s ion and supports the modes of ionization as shown may further react with another SOaCl- ion from in Eqs. (4-7). chlorosulphuric acid to form the species of the Further, to check the validity of the above modes type As(SOaCl), (Eq. 14) of ionization some acid-base titrations between As(SOaCl)a+SOaCl-~As(SOaCl)4 ... (14} antimony pentachloride (a strong acid of the system) and the solutions of these anhydrides in chloro• which will further lower the Y value to a greater sulphuric acid have been carried out. Robinson8 extent. But as there was no change of conduc• has reported that antimony pentachloride behaves tance during these investigations, the possibility as strong Lewis acid in chlorosulphuric acid. of the Eq. (11) representing the mode of reac• When antimony pentachloride is added to the tion must be ruled out. The low conductance solutions of these anhydrides in chlorosulphuric values of the solutions are either due to the weak acid there is a decrease in conductance (Fig. 2) and electrolytic behaviour of sulphuric and hydrochloric this decrease in conductance is due to the reaction (9) acids or due to the slight ionization of AsOSOaCl which further supports the mode of reaction repre• H~SOaCl+SOaCI-~2HSOaCI ... (9) sented by Eq. (12). After complete removal of the (SOaCI-) ions Bismuth trioxide and antimony trioxide dissolve from the solution, further addition of antimony in chlorosulphuric acid to give milky, stable solt.· pentachloride causes an increase in the conduc• tiom. The possible mode of reactiom is shown in tance which is probably due to the formation of Eq. (15). an acid of the type H,[SbClz(SOaCI)aSOaCI]similar SbzOa+ 3HSOaCl-+2SbO.SOaCI+ HzSO,+HCl to H[SbFz(SOaF)aSOaF] already known in the case of fhlOfOsulphuric acid system. From the 2SbO.SOaCl~2SbO"'+2S0aCl- ... (15) positions of minima in the conductance composi• Eq. (15) requires a value of two for the conducti• tion curves it is evident that all these solutes vity factor (Y) but the low conductance and milky produce two moles of SOaCl- and behave in a appearance show that SbO.SOaCl species formed similar manner. are insoluble in chlorosulphuric acid possibly Engelbrecht and Stolle have reported that P205 because of extensive polymerization. Similar forms phosphoryl fluoride when dissolved in fluoro- observations have already been made in the case 25 I INDIAN J. CHEM., VOL. 15A, JANUARY 1977 I

possi ility of the reaction (16) TABLE 1 - DISSOCIATIONCONSTANTSOF ACID ANHYD1boron tri• maleic anhydride, phthalic anhydride and mono• chIo osulphate behaves as an acid of this solvent chloroacetic anhydride in chlorosulphuric acid have syst m and combines with SOsCI- ions to form lower conductance values which indica.te that all HB( 0sCI), according to the Eq. (18) these anhydrides behave as weak bases in this solvent. B(S sCI)s+SOaCI-...,..B(SOaCI-), ... (18) The dissClciation constants of these acid an• hydrides have also been determined in chloro• Sosho~n~heI inoverallEq. (19)reaction may be. represented as sulphuric acid and when these values are compared B20~+ 13HSOaCI...,..2B(SOaCI),+ 2H~SOaCl with those obtained in the case of sulphuric, fluoro• i +3HCI+3H2SO, ... (19) sulphuric and disulphuric acids (Table 1), it has been inferred that chlorosulphuric acid is a weaker proton donor as compared to fluorosulphuric and ten ncy to polymerize. A similar type of poly• disulphuric acids, but stronger than sulphuric acid. mer' ation with B-O-B bond has already been Atrepo!igherted by concentrationGillespie and coworkers1a,l'B(SOaCI), ionfromhas thea References B20 in sulphuric acid. 1. PAUL, R. C., SHARMA,S. K., SHARMA,R. D., PAUL, K. K. & MALHOTRA, C., inorg. nucl. Chern., 33 (1971), I order to confirm further the nature of B20a 2905. K J. in c lorosulphuric acid, some acid-base conducto• 2. GILLESPIE, R. J. & MALHOTRA,K. c., J. chern. Soc., Ra~nmet ic titrationsspectroscopyhaveofalsoconcentratedbeen carriedsolutionsout be•of (1967), 1994. B20S 3. PAUL, R. c., DHILLON, D. S. & PURl, J. K, Indian J. twe1n the solutions of in chlorosulphuric acid Chern., 13 (1975), 1058. 4. ROBINSON,E. A. & CIRUNA, J. A., Can. J. Chem., (1968), Fro the shape of the conductance curves which 1719. hav sharp end points, it has been inferred that andfhe strong base KSOaCI or benzoic acid (Fig. 2). 5. GEORGE BRAUER, Hand book of preparative inorganic the, acid HB(SOaCI), thus formed behaves as a chemistry, Vol. 1 (Academic Press, New York, London), 1963, 484. 6. GILLESPIE, R. J. & ROBINSON,E. A., Can. J. Chem., 40 T e behaviour of various organic acid anhydrides (1962), 645. has already been reported in other strong acids 7. PAUL, R. C., PAUL, K K & MALHOTRA,K C., J. inorg. whe e they are shown to behave as weak bases. nucl. Chern., 31 (1969), 2614. Acestrotgic acidanhydridein this solvent.undergoes self-dissociation as 8. ROBINSON, E. A., Paper presented at the International A~ ~Ac++OAc. 1967.Conference on Non-aqueous Solvent, Canada, June A though it contains three basic oxygen atoms, 9. ENGELBRECHT, A. & STOLL, B., Z. anorg. Chern., 292 yet 't is monoprotonated in sulphuric and fluoro• (1957), 20. 10. MALHOTRA,K C. & PURl, J. K, Indian J. Chem., (in sulp uric acids whereas it forms acyl ions in di• press). suI uric acid. But the solution of acetic anhy• 11. PAUL, R. C., MALHOTRA.K. C. & VAIDYA, O. C., Indian dri in chlorosulphuric acid are highly conducting J. Chem., 3 (1965), 1. and quite stable. When its conductance is com• 12. PAUL, R. C., MALHOTRA,K. C. & KHANNA, K. C., Indian J. Chem., 3 (1965), 63. par~dbeh~veswithas a thatstrongofmonoKSOsCI,acid baseit isinfoundchIorosulphu•that it 13. GILLESPIE, R. J., J. chem. Soc. (C), (1950), 2542. 14. BARR, J., GILLESPIE, R. J. & THOMPSON,R. C., Inorg. ric ~cid. Similarly benzoic anhydride, isobutyric Chern., 3 (1964), 1149.

26

II.: I 1i' liP I