The stabilities of Meisenheimer complexes. Part 21. Sulphite additions to 2,4,6-trinitrotoluene and 2,4,6-trinitrobenzyl chloride

摘要

1850 J.C.S. Perkin I1 The Stabilities of Meisenheimer Complexes. Part 21.l Sulphite Addi- tions to 2,4,6-Trinitrotoluene and 2,4,6-Trinitrobenzyl Chloride By David N. Brooke and Michael R. Crampton," Chemistry Department, Durham University, Durham DH1 3LE In aqueous sodium sulphite solutions, 2,4,6-trinitrotoluene and 2,4,6-trinitrobenzyl chloride give 1 : 1 and 1 : 2 adducts by addition at unsubstituted ring positions. Rate and equilibrium data for these reactions have been obtained by the stopped-flow method and are compared with similar data for other nitro-compounds. THEreaction of 2,4,6-trinitrotoluene (TNT) with alka- RESULTS AND DISCUSSION line sodium hypochlorite is of current interest 2r3 as Spectroscopic Studies.-The visible spectra obtained in it provides a route to the commercially important 2,2',4,- water indicate the presence of two equilibria between 4',6,6'-hexanitrostilbene.2,4,6-Trinitrobenzyl chloride each nitro-compound and sodium sulphite. In dilute (TNBCl) is likely to be an intermediate in this rea~tion.~.~ (0.01~)sulphite solutions, TNT gives a pink species Likely modes of interaction of TNT, or TNBC1, with alkali are formation of the conjugate base by transfer of a side-chain proton or formation of o-adducts by base additi~n.~In the case of TNT there is good evidence that in the presence of alkoxide ions rapid formation of the adduct (1)is followed by production of the thermo- dynamically more stable anion (2). In liquid ammonia Me NH, (3) TNT gives a 1 : 1 adduct by amide ion addition at an un- substituted ring-position and a 1 : 2 adduct (3), in which geometrical isomerism is possible, by addition at the 1-and 3-po~itions.~ Here we report structural, kinetic, and equilibrium data relating to reaction of TNT and TNBCl with sulphite ions.We chose sulphite for the initial studies since it is known readily to form o-adducts with poly- nitrobenzenes 497-10 but has a relatively low affinity for protons. Hence we expected formation of the con-jugate bases, (2) in the case of TNT, to be of minor importance, thus simplifying the analysis. It has been known for many years that aqueous sodium sulphite solutions will dissolve TNT to give highly coloured solutions; l1 but the only recent work l2 is the observ- ation that in dilute solutions a species of unspecified structure with A,,,,.465 nm and equilibrium constant 5.6 1 mol-l is formed. with A,,,,, 460 and 550sh nm, attributed to 1 : 1 inter-action. In more concentrated ( O.~M) solutions an orange species showing a broad absorption maximum at 420 nm is the predominant species. This is likely to have 1 : 2 stoicheiometry. Previous work with other sub- strates indicates that diniethyl sulphoxide (DMSO) stabilises 1 : 1 adducts relative to 1 : 2 adducts, and we find that in water-DMSO mixtures containing sulphite the visible spectra obtained are similar to those of the lower adduct in water but showing small bathochromic shifts. The behaviour of TNBCl is similar to that of TNT in that at low sulphite concentrations a species with A,,, 460 and 550sh nm is formed while in more concentrated solutions a species with broad maximum at 440 nm predominates.lH N.m.r. data for the parent molecules and sulphite adducts are in Table 1. The spectra of the 1 : 1 adducts TABLE1 1H N.m.r. data for TNT and TNBCl and their adducts with sodium sulphite Solvent a TNT DMSO (4; X = H) DMSO-water70: ~OV/V (5; X = H)TNBCl Water DMSO (4; X = CI) 70 30 V/V DMSO-water (5; X = C1) Water 6 (ring) (side chain) 9.04 2.56 8.50 2.41 (4J24H4 (d J 2 H4 6.0 2.48 9.08 5.00 5.05{ ;:;; { (d, i.;;Hz) (d, J 11 Hz)4.8 (d, J 11 Hz)6'0 5.1(d, J 11 Hz) a Deuteriated solvents were used.were obtained in a 70 : 30 DMSO-water mixture and the observation of spin-coupled bands at 6 8.50 and 6.14 due to ring protons is clear evidence for structure (4; X = H or Cl). The spectra of the 1 : 2 adducts were obtained by dissolving the parent compounds in h-sodium sulphite in D20. In each case a single band due to ring protons was observed at 6 6.0 indicating structure (5; X = H or Cl). There was no evidence for cis-tram-isomerism in the 1 : 2 adducts as has been observed in the di-adduct from 1,3,5-trixiitroben~eiie.~~~~The shifts of CHzX CHZX O~Namp;NO~ -03s"U:Oi NO * NO1 tlie side-chain protons vary little on complex formation but in tlie adducts formed from TNBCl non-equivalence of the CH,C1 protons is observed presumably due to restricted rotation of this group.Kinetic and Equilibriuztm Data.-Examination by stopped-flow spectrophotometry of the reactions of TNT, or TNBC1, with sodium sulpliite in water indicated the presence of two processes whose rates were well separated. We attribute these to the formation of 1 : 1 and 1 : 2 adducts. Since all measurements were made with kiParent + -4(4)k-, kz(4)+ S032-~(5) k-2 sulphite in large excess over the parent, equations (1) antl (2),respectively, will apply to these processes.* k,K1S032-2kslow = h-+ 1 + K1S032- 2,4,6-Trinitrobenzyl chloride. Rate and equilibrium measurements were made at 460 nm, the absorption maximum of (4;X = Cl), and are in Table 2. A plot according to equation (1)of the rate data for the faster reaction was linear and yielded values of k,, 4000 1 mol-l s-l, and h-l, 77 s-l.Combination of these values gives K, (=k,/k-,) 52 1 mol-l, in good agreement with the value obtained from the optical densities at completion of the rapid reaction. Extrapolation to zero sulphite concentration of the rates of the slower reaction gave a value for k-, of 1.7 s-l. Using this value and the known value of K,, values of k, were calculated using equation (2). Good agreement between the observed and calculated rates is observed in Table 2. 2,4,6-TrinitrotoZuene. In water a very fast process is associated with the formation of (4; X = H). Rate coefficients were in excess of 250 s-l in the most dilute sulphite solutions which gave measurable absorption, and were too rapid for accurate measurement by tl~e stopped-flow method.We made use of the stabilisation of the 1 : 1 adduct by dimethyl sulphoxide4V7 to obtain data in mixed solvent systems. Rate measurements ob-1851 TABLE2 Kinetic and equilibrium data for reaction of 2,4,6-trinitro-benzyl chloride (5 :.: 10 5~ with sodium sulphite in) water at 25'' lfnlow CNa,SO,I "/ Lt/ 0.11. I,/ :y/ (talc)/s--1 hl 5-l (460 nm) b 1 mol 1 0.002 86 4-5 0.0131 GI 0.004 9s b.0214 A4 O.OO(i 110 0.0302 56 0.OOK 110 0.0371 5(i 0.010 118 0.0427 55 1.94 )r 0.1 1.90 0.015 1"ti 0.0537 54 0.W) 2.27 2.27 0.040 3.10 3.20 0.060 4.05 4.20 0.080 5.32 5.25 0.100 0.1OG 6.33 G.35 n.ith sodium sulphite.After coin-loiiic strcngth 0.3~ plction of rapid colour-forining process, but before second process. Measurcnicnts relate to a 2 mni pathlength cell. A Renesi-Hildebraid 1)lot gives a value of 0.120 for complete conversion. Calculatctl froin equation (2) with k-, 1.7 s-I, k, 55 1 mol-* s-l, antl K, 58 1 nio1-I tained at 460 nm in media containing 10, 20, and 307{, diinethyl sulphoxide by volume are in the Figure and derived parameters are in Table 3. Measurements of optical density at the completion of the rapid colour- forming reaction were also obtained using tlie stopped- flow spectrophotometer and were used to calculate values of K,, which were in each case in excellent agreement with those derived from rate measurements.In agreement with previous work we find that log plots of K, and k-, 7wrsus inol ' dimethyl sulphoxide were linear and allowed values for these parameters in water to be fountl. 0.050 0.10 Na,SO31 / M Rate data for formation of (4; X = H) in water containing the following percentage of DMSO by volume: A, 10; B, 20; C 30 1852 The increase in stability of the 0-adduct with increasing proportion of DMSO in the solvent probably reflects the poor solvation by DMSO of the sulphite ion and the good solvation of the polarisable adduct by this component. The kinetic data show that tlie changes in value of I, with solvent composition derive largely from changes in k1.This indicates that, as found in previous work,8 the transition state for formation of tlic 1 : 1 atlduct is ' reactant-like '.Rate measurements of tlitt slower process relating to the formation of (5; X = H) were conveniently madc in water at 25". Data are in Table 4. Tlie values for this process depend markedly on tlle ionic strength of the medium as expected for formation of a multi-charged adduct.* Co?i@arisoizwith Other Compozinds.-Ratc and eyuilib- rium data at I 0.3~are summarised in Table 5 wlierc they are cornpared with similar data for 1,3,Ltrinitro- benzene and 2,4,6-trinitroanisole. As with other com- pounds we were unable to detect addition of sulphite at TABLE3 Variation with solvent composition of rate nntl equilibrium data for formation of (4; S --li) from 'l'S'1' ant1 sodium sulpliite at 25' Vol Mol 30 9.6 1 100 amp; 50 14 f0.5 SO f5 so 8 20 6.9 920 amp; 70 45 * 2 21 * 2 21 f2 10 2.7 900 amp; 100 135 amp; 10 7 j;1 7 1 oc 0 800 f100 300 amp; 30 2.6 .k 0.5 2.6 f0.5 * From kinetic data, K, = /i1//t-*.b From optical dcnsity data. c Extrapolated values. the l-substituted position.* The higher value of K, for TNBCl than for TNT can be attributed to the greater inductive effect of the CH,CI substituent compared with J.C.S. Perkin I1 We were unable to detect either from the n.m.r. or kinetic measurements any evidence euro;or cis-tram-iso- merisrn in the 1 :2 adducts.8*9~13~15As in other cases * it s~enislikely that one isomer, probably tvutcs, is favoured. *r.kLIaI: 4 I;itcdata for tlie forin;ltion of (5; S H) in wLter at 25" Na,SO3/~i /i,lnMn/s-l (calc) 0.010 0.020 l.l!) 0.03 1 80 1.17 I .a0 (1.040 1.27 1 32 0 050 0.060 I40 1.50 0.08(1 1.75 1.74 0.10 2 I0 2.03 0.20 0 :lo 0.40 /f91nwr/s kvlowd(calc) 1.o1 1.01 1.711 1.77 4.00 4.13 11.3 11.7 22.3 21.9 34.3 33.6 " I 0.3~with sodium sulphatc.Z = CciZi2. Calculatcd nit11 /-, 1.16 5 I; 11, 42 1 ~nol-~ and A', 2.0 1 mol-'. Z 1.5~bsol;-I; with sodium sulphatc. Calculatccl with k ,0.85 50; k, 1GO 1 n101-l s-l; antl I, 2.6 1 11101 EXPE RIM ENTAL 2,4,Ci-lrinitrotolucnc and 2,4,amp;trinitrobenzyl chloride were recrystallisecl spccimens supplied by Ministry of Defence, P.E.K.N!.E., Waltham Abbey.Analytical grade sodium sulphite was used without purification. The distilled water used was boiled to remove carbon dioxide and subsequently protected from the atmosphere. IH N.1ii.r. nieasurenients were made with a Bruker HXSOE instrument modified for Fourier transform operation and using a deuterium lock. Measurements in media containing DMSO were made relative to internal tetraniethylsilane. In water where this standard was insoluble shifts were measured relative to internal dioxan assuming a difference of 3.70 p.p.m. between the two references.' Kinetic measurements were made by mixing freshly prepared solutions of reagents in a Canterbury stopped-flow apparatus. All measurements were made at 25" wi tli TABLE5 Summary of equilibrium antl kinetic data for sulphite additions in water at 25" with I 0.3~ k,/l m01-'s 1 k-,/s Kl/l Ill01 1 /z2/ 1 mol-' s-1 k-,/sP K,/1 Inol-1 1,3,6-Trinitrobenzene 3.5 x 104 12 290 { 19;.4 * 21 0.13 9.2 9.3 2,4,G-Trinitroanisole 4.8 x 103 36 140 170 0.12 1.4 x 103 2,4,8-Trinitrotoluene 800 :300 2.6 42 1.16 36 2,4,6-Trinitrobenzyl chloride 4 x 103 77 55 55 1.7 32 0 Data rcfcr to formation of cis-and tvans-isomers.From ref. 9. c From ref. 8. the CH, group. However steric effects are also likely to be important and the presence of bulky groups at the 1-position Will hinder planarity Of nitro-groups, an important factor in charge delocalisation in the adducts. The decreased stability of the 1 : 1 adducts from TNT and TNBCl relative to TNB may be due to this factor.Buncel and his co-workers lo have sutrtrested that tlif- "" ferences in the extent of solvation of adducts plavs an sulpliite in large excess over nitro-compound, so that first- orcler kinetics were ~bsa~~~. bsol;bsol;re tllank S.1I.C. for a maintenance grant to 11.N. B. a~itl for a grant to pLlrcllase tiie stopped-flow spectropliotometer, ant1 RiIinistrv of IIefcmx for ;L gift of clieniicals. 0/551 Receiued, 14th Apvil, 19801 C. F. Bernasconi, J. Org. Chem., 1971, 36, 1671; E. Buncel, A. 12. Norris, I. E. Russell, and I. Tucker, J. Amer. Chem. Soc., 1972, 94, 1646; E. Buncel, A. 12. Norris, TI;. E. Russell, I. Shcri-clan, and 14. Wilson, Canad. J. Chem., 1974, 52, 1750, 2306; C. A. Fyfe, C.11. Malkiewich, S. W. H. Uainji, and A. K. Norris, J. Amer. Chem. SOC.,1976, 98, 6983. I. Foster and J. A. Chudek, .J.C.S. Perkin II, 1979, 62s. M. R. Crampton, J. Chew. 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