Qadeer, (Mrs) S.

Zaidi, S. A. A.

English

NOPR

lndian Jourl\al of CbemistryVol. 14A. March 1976. pp. 15.4-155A Stable Dipositive Chlorocarbonium IonS. A. A. ZAIDI & (Mrs) S. QADEERChemistry Department, Aligarh Muslim University. Aligarh 202001Received 20 February 1975, accepted 19 April 1975a.a.a.a'.a'.a'-Hexachloro-p-xylene dissolves slowly in 100% H2S04 and rapidly in HS03Cl,formin~ li~ht orange and dark red solutions respectively. Conductance, NMR and UV studiesindicate that it; forms the stable dipositive carbonium ion C12CC6H/;C12, in the former medium+and the monopositive carbonium ion Cl3CC6H.CCI2, in the latter.MULTICHARGED aryl carbonium ions. wherethe positive centres are stabilized by the. delocalization of the positive charge over morethan one benzene nucleus, are well known both insolution! and in the form of their crystalline salts".Salts of di- and tri-positive carbonium ions derivedfrom a single benzene nucleus and stabilisedby tropylium'' or oxolenium rings- directly attachedto the ionizing centre, have also been reported.However, attempts at preparing dipositive chloro-or fluoro-carbonium ions stabilized by only one ben-zene ring have, so far, involved obtaining 'extra-ordin ary' dipositive carbonium ions requiring a doubleionization at the same centre for their formation=".It was thought that a carbonium ion of this typemight be obtainable if ionizati on occurred atdifferent centres attached to the same benzenenucleus. With this end in view, the behaviourof a,a,a,a' ,a' ,a' -hexachloro-p-xylene, CClaC6H4CCla(I) was studied in sulphuric and chlorosulphonicacids.Materials and Methodsa,a,a,a' ,a' ,a' -Hexachloro-p-xylene (Eastman-Kodakpractical grade) was used after recrystallization fromacetone. Chlorosulphonic acid (BDH, reagent grade)was used as such. Sulphuric acid of minimum con-ductance (corresponding to 100% sulphuric acid)was prepared by mixing the requisite amounts of20% oleum (Swastik Chemicals) and 98% sulphuricacid (BDH). The conductivity cell was similar tothat used by Ciruna et ai', Conductance measure-ments were carried out at 35 ± 0.10 using a Systronicsconductivity bridge, model 321.The NMR spectra were recorded on a Varian A-60spectrometer, at room temperature using hexamethyldisilazane as an external standard.Results and Discussiona,a,rt ,a' ,a' ,a' -Hexachloro-p-xylene (I) dissolvesreadily in chlorosulphonic acid with evolution ofHCI to form a red coloured solution. Conductancemeasurements made in the concentration range up toO·lm showed it to be fully ionized and behaving asa 1:1 electrolyte, giving a conductance curve which154closely followed that of KCF. The following re-actions may be taking place in this medium:ClaCC6H4CCla+HC1SOa->-ClaCC6Hic12 + HCI+SOaCl- ... (1)+ClaCC6H4CCl2+S03Cl- ->-ClaCC6H4CC12(OS02Cl) ... (2)The conductance decreases with time, that of O'lmso!u~ion decreasing to roughly one-third of itsoriginal value after a week, indicating that perhapsthe carbonium ion was slowly converting into itschlorosulphate (Eq. 2).In 100% sulphuric acid (I) dissolves slowly, overa period of 7-8 hr to give a light orange solution anda conductivity Y value (in the concentration rangeup to O·lm) between 2 and 3 (ref. 8). Acid-basetitrations with O'lm oleum? gave a conductanceminimum at a nb/na value between 0·24 and 0·19.Keeping in view the possibility of some loss of HClin these experiments, these results were interpretedto indicate the formation of a dipositive carboniumion according to the reaction represented by Eq. 3.ClaCC6H4CCla+ 6H2SO4->-cli;c6Hl C12+2HC1SOa+2HaO+4HSO;l ... (3}The formation of 4 moles of bisulphate per moleof carbonium ion gives rise to theoretical Y and nblnavalues of 4 and 0·18 respectively. The conductanceof a solution of (I) in 100% sulphuric acid was foundto remain unchanged up to one week, indicating thedication to be quite stable in this medium.Hydrolysis of the solution of (I), in both HC1S03and 100% H2S04, resulted in the formation ofterephthalic acid.No interaction between SbCls and (I) was notedin CC14solution although a large number of carbo-nium ion hexachloroantimonates have been preparedin this way2a-c,1O.The NMR spectrum of (I) in 100% sulphuric acidgave a singlet at 8·683. A solution of (I) in 20%oleum aged for 44 days gave a similar spectrum.~on~rming the formation of a dipositive carboniumion in these media. The peak position is at a higher3 value than that of a solution of (I) in CC14(8·23).However, the extent of deshielding is less than thatstage than at the initial stage, (on account of theinductive effect of the positive charge which is de-localized onto the carbon atom adjacent to theionizing centre) and require a medium of higherdielectric constant than chlorosulphonic acid fortheir ionization. However, the possibility that thedifference in the behaviour of (I) in the two solventsis due to some inherent difference in their properties,cannot be ruled out.AcknowledgementThe authors thank Prof Wasiur Rahman, Head,Department of Chemistry, Aligarh Muslim Univer-sity, for providing facilities for this work, and DrNitya Nand and Dr A. Shoeb of the CDRI Lucknow,for NMR and UV spectra. One of them (S.Q.)thanks the CSIR New Delhi, for a junior researchfe lowship.References1. (a) HART, H., SULZBERG, T. & RAFOS, R. R., J. Am.chem, Soc., 85 (1963), 1800; (b) HART, H., SULZBERG,T., SCHWENDMANN, R. H. & YOUNG, R. H., Tetra-hedron Lett., (1967),1337.2. (a) VOLZ, H. & VOLZ DE LECEA, M. J., TetrahedronLeU., (1966), 4675, 4683; (1964), 1871. (b) VOLZ,H. & SCHNELL, H. W., Angew. Chem., 4 (1965), 873.(e) VOLZ, H., Angeiu.cte«; 7 (1968), 2.34. (d) BAENZIG-ER, N. C., BUCKLES, R. E. & SIMPSON, T. D., J. Am.chem, Soc., 89 (1967), 3405. (e) PAUL, 1. C. &McKECHNIE, J. S., J. chem, Soc. B, (1971), 918.3. MURRAY, R. W. & KAPLAN, M. L., Tetrahedron Lett.,(1965), 2903; (1967), 1307.4. HART, H. & TOMALIA, D. A., Tetrahedron Lett., (1966),3389.5. (a) HART, H. & FISH, R. W., J. Am. chem, Soc., 82(196m, 5419. (b) GILLESPIE, R. J. & ROBINSON,E. A., J. Am. chem. Soc., 87 (1965), 2428. (e) ROBlXSON,E. A. & CIRUNA, J. A., J. Am. chem . Soc., 86 (1964),5677. (d) CrnuNA, J. A., NG, K., ROBINSON, E. A.,ZAIDI, S. A. A., GILLESPIE, R. J. & HARTMAN, J. S.,Tetrahedron Lett., (1965), 1101.6. ZAIDI, S. A. A., The formation of carbonium ions in ~strong acid solvents, Ph.D. thesis, University of Toronto,1967.7. CIRUNA, J. A. & ROBINSON, E. A., Can. J. cu-«, 46(1968), 1719.8. GILLESPIE, R. J. & ROBINSON, E. A., Non-aqueous "-solvent system, edited by T. C. Waddington (Academic \..JPress, New York), 1965.9. FLOWERS, R. H., GILLESPIE, R. J. & ROBINSOX, E. A.,Can. J. cie«, 38 (1960), 1363.10. (a) VOLZ, H. & VOLZ DE LECEA, M. J., TetrahedronLett., (1965), 3414; (b) BRESLOW, R., GROVES, J. T. &RYAN, G., J. Am. chem. Soc., 89 (1967), 5048; (e) TOBEY,S. W., SADO, A. & 'VEST, R., JAm. chcm, Soc., 88(1966), 2488; (d) WEST, R. & KWITOWSKI, P. J., J.Am. chem. Soc., 89 (1967), 5280. (e) BALLESTER, M.,RIERA-FIGUARAS, J. & RODRIGUEZ-SrURANA, J., Tetra-hedron Lett., (1970). 3615. .11. STREITWIESER, A., Molecular orbital theory for organic "/chemists (John Wiley & Sons, New York). 1961.ZAIDI & QADEER: STABLE CHLOROCARBONIUM IOrobserver in the case of monoposive carbonium ionobtained in chlorosulphonic acid. The spectrumin sulphuric acid was unchanged after a weekconfirming the high stability of the dication inthis medium.In chlorosulphonic acid, the NMR spectrum of(I) consisted of doublets centred at 9·15 a and 8·58 a(] = 8-9 Hz) indicating the formation of amonopositive carbonium ion in this medium. Thespectrum was found to change slowly with time,converting almost completely into an unidentifiedspecies (,.....,8·5a), presumably the chlorosulphate,after five days.The electronic spectrum of (I) in 100% sulphuricacid showed bands at 265 and 320 nm while in CHC13solution, three bands at 250, 274 and 281 nm areobtained. In chlorosulphonic acid, a strong bandat 390 nm was observed. The marked bathochromicshift observed in proceeding from chloroform to thestrongly acidic media was a further indication ofcarbonium ion formation.Simple HMO calculations using an IBM 1120computer, were carried out on the monopositive (II),dipositive (III) and tripositive (IV) species anddelocalization energies and charge densities werecomputed for them. Approximate values for theheteroatom parameters, as recommended by Streit-wieser-' were used. II, III and IV were foundto have delocalization energies of 2·94~, 3·77~ and4·79 ~ respectively, showing that inherent instabilitywas not a barrier to the formation of III and IV andindicating them to be stable relative to II. II hasalready been reported as a fairly stable speciesin chlorosulphonic acid" and in the form of itshexachloroantimonate saltlOa•The differences in the behaviour of (I) in 100%H2S04 and HClS03 are probably related to the largedielectric constant of the former medium (e = 100)as compared to that of the latter (e = 60) (ref. 7).If dicarbonium ion formation takes place by atwo-step mechanism as shown below:-Cl- + -Cl- + +CI3CC6H4CCI3-->-CI3CC6H4CCI2--»-CI2CC6H4CCl2the chlorine atoms of the trichloromethyl group willbe bound more firmly to carbon at the intermediateCI C\'"'!.053'+/C~.601~."66~ •. rC t·.05e( + 'elIII.....••ct ,,+/CI +.048e""' •• 5707-'o:a4.-: ,- •. 3494Cl ••••C e •..•.ct• I 1+el ctIVII155

A Stable Dipositive Chlorocarbonium Ion

http://nopr.niscair.res.in/bitstream/123456789/53643/1/IJCA%2014A%283%29%20154-155.pdf

A Stable Dipositive Chlorocarbonium Ion

Abstract

Similar works

Full text

Available Versions

NOPR