Structure of aromatic diazocyanides; synthesis of diazoisocyanides

摘要

2122 J.C.S. Perkin IStructure of Aromatic Diazocyanides ; Synthesis of DiazoisocyanidesBy Teresa Ignasiak, Hydrocarbon Research Centre, Department of Chemistry, University of Alberta, Edmonton,(the late) Jerzy Suszko and Boleslow Ignasiak, Fuel Sciences Division, Alberta Research Council,1.r. spectra of a number of aromatic diazocyanides, prepared under conditions that facilitate isolation and spectralanalysis of pure isomeric forms, suggest that extant published data refer to stable isomers only. Substantial dif-ferences between the spectra of unstable and stable isomers permitted the quantitative determination of theseforms in mixtures and allowed the kinetics of isomerization of unstable to stable forms to be studied.Although the results from i.r. analysis of aromatic diazocyanides are consistent with the concept of geometricisomerism of these molecules, the long-standing uncertainties concerning the type of isomerism could only besettled finally by synthesis of diazoisocyanides. p-Chloro- and p-bromo-benzenediazoisocyanides exhibitproperties, including vibrational and electronic spectra, entirely different from those of the corresponding diazo-cyanides.Detailed analysis of the i.r. spectra of a number of p-halogeno-benzenediazoisocyanides leads to theconclusion that the diazoisocyanide molecule may be represented as a hybrid of two canonical forms.Alberta, CanadaEdmonton, Alberta, CanadaSINCE their discovery,l diazocyanides have attractedmuch interest, and particular attention has been focusedon their isomerism.Generally, two possibilities havebeen discussed, viz. (a) geometric isomerism, cis (1)-trans (11) (syrt-autti) and (b) structural isomerism,cyanide (11)-isocyanide (111). The concept of geo-metrical isomerism received strong support frommeasurements of the dipole moments of selected pairsof diazocyanides2 However, Hodgson and Marsden 3 9 4considered that these measurements do not exclude thepossibility of structural isomerism, previously advancedby O r t ~ n . ~ Over the range 4000-900 cm-l the vibra-tional (i,r.) spectra of several pairs of diazocyanidesmeasured 637 show close similarity in terms of frequencyand absorption intensity, and this was taken as con-vincing evidence of cis-trans isomerism. However,Hodgson * doubted whether the spectra represent thevibrational characteristics of stable and unstableisomers, and suggested that owing to fast isomerizationof the unstable form, the spectra correspond to the stableisomer only.In view of this remaining uncertainty, weattempted to measure the true vibrational characteristicsof both the stable and the unstable isomers.** Since our results confirm the concept of geometrical isomerismof diazocyanides, the results will be discussed in terms of cis-transisomers rather than of cyanide-isocyanide or stable-unstableforms.3 H. H. Hodgson and E. Marsden, J . chew. sOC., 1944, 395.4 H. H. Hodgson, J . chenz.5 K. J. p. orton, J . them. sOC., 1903, 805.6 D. Anderson, R. J. W. Le Fbvre, and J. Savage, J .Ckem.N. Sheppard and G. B. B. M. Sutherland, J . Chem. SOC., 1947.1948, 1097.A. Hantzsch and 0. Schultze, Ber., 1895, 28, 666.(a) R. J. W. Le Fkvre and J. Northcott, J . Chem. Soc., 1949,333; (b) R. J. W. Le FGvre and H. Vine, ibid., 1938, 431, 1878.SOC., 1947, 445.4531975RESULTS AND DISCUSSIONThe spectra of a number of pairs of diazocyanides(Table 1) were measured with a Zeiss UR-10 spectro-photometer. Particular care was taken to ensure thatthe time between the preparation, including purification,and photometric analysis of the samples did not exceed90 min. The time between dissolution of the purifiedisomer and commencement of measurement was 7-432123higher for cis-compounds in carbon tetrachloride than inchloroform ; for trans-compounds this situation isreversed.In trans-diazocyanides the type of sub-stituent also leaves the position of the band unaffected,but its intensity increases with the electron-donatingproperties of the substituent.Depending onthe substituent, cis-isomers show absorption attributable(b) Diazo-grouj5 stretchiqg vibrations.TABLE 1Characteristics of aromatic diazocyanides used for i.r. studiesAnalysis ()Diazoc yanideo-NO2.C6H4N2CNp-CIC6HaN2CNp-BrC6H4N,CNo-CH,OC,H4Namp;Nm-MeOC,H4N,CNfi-Men*C6H4N2CN2,4,6-Cl3C6H2N2CN2,4,6-Br,C6H,N,CNIsomercis *trans *cis *trans *cistranscistranscis *strans *cistranscistranscis *trans *cis *trans *transcis *trans * 7cistrans $ $trans *sect;sect;cis t tLit.m.p.M.p. ("C) ("C) Solvent60 CCl,6850-52C6H14CC1499-100 C6H1446-47? 45-47 C5H1285-86 86 C6H1448 49 C5H1277 78 C6H1438 C5H12242910542133(Oil)113(Oil)3150124(Oil)138-13958-5914981LPlT29 C5H12105 C6H144 2 4 3 C6H14132 C6H14C7H16C5H1660-61 C5H1,121-122 C7H16C5H12C6H14Camp;l,LP 759-60 C6H14-Et20149 C6H14'fiH14FormYellowOrange needlesAmorphousOrange needlesYellow needlesRed needlesYellow flakesRed flakes $Brown- yellowneedlesRed needlesOrange flakesRed flakesRed flakesRed flakesRed oilBrown needles oil **Orange needlesOrange needlesBrown flakesOilRed flakesYellow flakesRed flakesRed flakesRequired Found' C H N '47.7 2.25 31.8' C H47.7 2.247.85 2.347.8 2.347.7 2.25 31.847.8 2.2551.15 2.45 25.3559.65 4.4 26.159.65 4.4 26.150.95 2.4551.05 2.4559.5 4.469.4 4.438.85 0.85 17.938.7 0.945.2 3.8 17.6 45.25 3.97 N32.131.6532.031.725.325.325.926.117.7617.4* Not reported previously.t M.p. 46-47 :C was reached after passing a solution of the cis-form in pentane through a silica gel5 cis-Form readily decomposes with oil formation; the isomerization into the ** Freezing temperature -10 "C. tt Addition of sodium acetate (1$ # trans-Form additionally purified by vacuumcolumn.trans-form proceeds with difficulty.moll-l) to diazotised p-anisidine at 0 "C increased the yield of cis-form t o 89.sublimation.$ Obtained by freezing out from C6H14.7 Light petroleum.5s cis-Form immediately isomerizes t o trans-form and could not be isolated.min. All operations were carried out at the lowest suit-able temperature.The spectra of two chosen pairs of isomeric diazo-cyanides are reproduced in Figure 1; some of the fre-quencies characteristic of the remaining investigatedpairs of isomeric diazocyanides are presented in Table 2.Analysis of these data leads to the conclusion that thereare substantial differences in frequency and intensity ofthe absorption bands of isomeric diazocyanides.Thisindicates that the vibrational spectra of aromatic diazo-cyanides in the extant literature are those of the trans-isomers only.(1) Spectral Characteristics of Aromatic Diaxocyanides.-(a) Nitrile group stretching vibrations. In cis-isomersof the diazocyanides, a very weak band in the range2 175-2 150 cm-l shifts, on trasformation into theto diazo-group stretching in the region 1 480-1 420 cm-l,and trarts-isomers between 1460 and 1390 crn-l.Sub-stituents with electron-accepting properties give rise toabsorption at higher frequencies, and those with electron-donating properties to absorption at lower frequencies.Band intensities for the transforms are always sub-stantially greater than for the cis-isomers.Apart from absorption assigned to the diazo-groupstretching vibration, a band also occurs at 1 430-1 400cm-l. This may be assigned to a ring vibration ofdisubstituted benzene derivatives.*(c) C-N Single bond stretching vibrations.Bandsassociated with these vibrations occur in the region1 300-900 cm-1,g310 and in diazocyanides separateabsorptions would be expected for CA,-N and C~N-N,respectively. A weak absorption at 1190 cm-l wastrans-isomer, into the range 2 195-2 190 cm-l and alsochanges its intensity. The type of solvent does notaffect the position of this band, but its intensity isA. R. Katritzky, Quart. Rev., 1959, 13, 353.H. w. Thompson, J . sot., 1948, 328.10 N. v. Colthup. J . opt. SOC. Amer., 1960. 40, 3972124 J.C.S. Perkin Iascribed to C,-N in cis-compounds; but in the spectra oftram-isomers this appears at 1330-1 300 cm-l. Theintensity of CAr-N absorption is much higher in trans-than in cis-compounds.0.0 I I I I I I I IIa t 1 I I I I l l I I0.01 I I I I ' --C0 Qi ' I I1 I I I I II I I I 1 Ia D I I I I I I I 11 I I 1 , I 1a ' I 1 I I 1 I2200 2ooo 1800 1600 1400 1200 lo00 800Jlcm-1FIGURE 1 1.r.spectra of cis- and trans-fi-chlorobenzenediazo-cyanide (A and B) and cis- and tvans-fi-bromobenzenediazo-cyanide (C and D) in carbon tetrachloride (2 300-1 300 cm-1)and in carbon disulphide (1 300-700 cm-l)In summarizing the spectral data, it should be empha-sized that the transformation of cis- into trans-isomer isaccompanied by intensification of a number of bandsassociated with ring skeletal vibrations at ca. 1 400 cm-l,C-H in-plane vibrations at 1 250-1 000 cm-l, and C-Hout-of-plane vibrations at 900-700 cm-l.10-12 Theshift towards lower wavenumbers of the diazo-band andthe greater intensity of absorption by trans-forms are dueto stronger conjugation when a planar arrangement ofatoms in the molecule exists, and supports the concept ofcis-trans geometrical isomerism.l1 L.J. Bellamy, ' The Infrared Spectra of Complex Molecules,'Methuen, London. 1960.The surprising shift of the nitrile group stretchingvibrations in cis-forms of diazocyanides towards lowerwavenumbers may be connected with hindered rotationabout CAr-N, an effect caused by interference of N in thenitrile group and H in an orth-position (IV). Since thedistance between H and N is then only 1.5 A (Dreidingmodel), the induction effect brings about a linear re-arrangement of the form (IV). The cis-isomer seems tohave the character of a resonance hybrid of cis-diazo-cyanide (IV) and Linear cumulene (V) ; and since delocal-ization of electrons in the linear cumulene (V) should befacilitated, participation of (V) in hybrid would explainthe shift of nitrile group stretching vibrations in cis-cyanides towards lower frequencies.The properties of the mesomeric structure would beexpected to be affected by the properties of the sub-stituents, with electron-donating groups increasing theparticipation of (V) in the hybrid; this is reflected inspectrum of cis-@-methoxybenzenediazocyanide by thetram-band (due to very fast isomerization) and the verystrong bathochromic shift of the cis-band (Table 2).Transformation of the linear cumulene into the stabletrans-isomer (11) seems to cause no difficulties.(2) Quantitative Determination of cis- and trans-Isomersilz Diazocyanides by 1.r.Spectrophotometry.-Erroneousconclusions from earlier work appear to have been due tolack of a satisfactory method for quantitative deter-mination of both isomeric diazocyanides in their mix-tures, but differences in the vibrational spectra ofdiazocyanides suggested that such a method could bebased on i.r. spectrophotometry.Quantitative measurements were made by using thenitrile stretching vibration band in the range 2 195-2 190 cm-l. This band is absent in the spectra of purecis-forms of f+chloro- and @-bromo-benzenediazocyanide,but appears, and becomes progressively more intense, asisomerization proceeds.The determinations were car-ried out with carbon tetrachloride or chloroform assolvent.In order to determine whether solutions of +hloro-and +-bromo-benzenediazocyanide obey the Lambert-Beer law, transmission measurements were made onstandard solutions of pure trans-cyanide, with varioussolute concentrations. A plot of absorbance vs. con-centration showed that the compounds did obey theLambert-Beer law over the concentration range testedand the method can therefore be used for determiningconcentrations of tram-compounds formed by isomer-ization.With concentrations of tram-isomers of 9-chloro- and@bromo-benzenediazocyanide in chlorofonn and carbon19 N. B. Colthup, L. H. Dely, and S. Wiberley, ' Introduction toInfrared and Raman Spectroscopy,' New York, 19641975 2125DiazocyanidesO-NO,.C,H,N,CNWZ-NO,.C,H,N,CNp-NOz.C,H,N,CNo-CIC,H,N ,CNm-ClC,H,N,CNp-ClC,H,N,CNp-BrC,H,N,CNo-MeOC,H,N,CNtn-MeOC,H4N,CNp-MeOC,H,N,CN2,4, 6-C13C6H,N,CN2,4,6-Br,C,H2N,CNp,-EtOSO,.C,H4N,CNTABLE 2Spectral characteristics of investigated aromatic diazocyanides *GiSAIv ( E N ) v (N=N) v(Camp;-N) and v (CCN-N)A -8 :- - cm-l (E) cm-1 (E) cm-1 (E) cm-l (E)2170 12.5 1460 17.9 1320 (49) 1185 (21) 1 160 (23)2 165 5.0 1480 59.4 1325 (33) 1295 (8.5) 1 180 (33)1 160 (20) 2 165 5.0 1475 44.0 1330 (69.5) 1300 (39) 1190 (18)2170 3.2 1445 48.9 1300 (8.5) 1280 (19.1) I 190 (27) 1 145 (40)2 160 4.6 1475 134.0 1315 (11.5) 1285 (11.5) 1180 (65) 1 165 (109)1460 176.6 1 320 (23.3) 1290 (26.5) 1 190 (15) 1 160 (96)1450 170.8 1 315 (22.5) 1290 (25.7) 1 190 (12) 1 160 (73.5)1450 139.4 1300 (211) 1 190 (47) 1 170 (155)1450 117.0 1300 (97) 1 200 (22.5) 1 140 (94)2 150 17.9 1425 361.8 1 190 (37) 1165 (max.)2 190 14.62 175 20.4 1435 95.52 170 34.4 1420 96.2trans1205 (32.6) 1 160 (51)1 110 (32)1210 (29) 1 130 (57)1 110 (26)V ( C 3 ) v(N=N) v ( CAr-N) and v ( C c r N )A 1 cm-l (E)o-NO2C6H,N$N 2 195 7.5 t 1440 101.4 1 320 (25) 1280 (57.6) 1 215 (72) 1 165 (24)tn-NOzC6H4N2CN 2 195 12.6 t Z 436 109.7 1 330 (63) 1290 (20) 1205 (139) 1 170 (62)fi-N02.C6H4N2CN 2 195; 11.5 1450 68.2 1 330 (205) 1 305 (27.4) 1210 (134) 1165 (76)o-C~C,H~N,CN 2 190 32.8 1420 217.7 1310 (39) 1270 (109) 1 220 (139) 1 135(50)1 145 (43)m-ClC6H4N,CN 2 190 29.9 1455 99.8 1316(39) 1286 (27) 1210 (162) 1 165 (105)p-C1C,H,N,CN 2 195 64.8 1440 204.8 1325 (73.5) 1 225 (199) 1 165 (240)p-BrC,H,N,CN 2 190 67.7 1440 246.7 1320 (85.5) 1220 (162) 1 166 (211)o-Me0 C,H,N,CN 2 190 101.4 1410 361.8 1300(max.) 1210 (218) 1 190 (117) 1 170 (max.)m-MeO*C,H,N,CN 2 190 67.2 1426 167.0 1 300 (100) 1200 (51) 1 160 (177)p-MeOC,H,N,CN 2 190 141.4 t 1400 362.0 1 310 (106) 1245 (155) 1 190 (60) 1 165 (max.)2,4,6-Q,C,H,N,CN 2 195 1390 96.0 1290 (22) 1215 (77.6) 1 160 (111)1 090 (60)2,4,6-Br,C,H2N,CN 2 196 6.5 1 390 104.0 1280 (37) 1215 (69.4) 1115( 70)fi-EtOS02*C,H4NzCK 2 190 25.9 1450 137.9 1 320 (117.5) 1300 (26) 1 210 (185) 1 170 (96.8)- - I cm-l ECClr cm-l ECC4 cm-l (4 cm-1 (tz) cm-1 (E) cm-l (E)1200 (233)* wclr = molar extinction coefficient in CC1,; E = molar extinction coefficient in CS,.t Saturated solution.tetrachloride solutions easily measurable, kinetic studiesof isomerization of cis- to tram-forms could also be under-taken. Transmission measurements were made onsolutions of individual cis-$-chloro- and -p-bromo-benzenediazocyanides at intervals and from the results(assumed) first-order reaction rates were calculated.Rate constants were determined at 25 and 45 *C, and theactivation energy E of isomerization was calculated bymeans of the Arrhenius equation (Table 3).It was found that rate constants depend on the solventused: in a polar solvent (chloroform), k is substantiallylarger (at 25 "C 0.043 and at 45 "C 0.39 x s-l) thanin a non-polar solvent (carbon tetrachloride) (k 0.013;k45 0.13 x 10-3 s-l).Le F6vre and Northcott,13 whocalculated k values from dielectric constants, report thatisomerization of 9-chlorobenzenediazocyanide proceedswith the same rate in chloroform (A? 0.020; k45 0.203 xlod3 s-l) as in carbon tetrachloride (k25 0.020; P5The values for E ( 6 6 ' 7 0 k J mol-l) (Table 3) agree withpublished data.l3 l4TABLE 3Results of kinetic studiesTemp. k x k , x E/kJSolvent ("C) s-l 5-1 mol-l25 0.013 0.01345 0.13 0.1325 0.044 0.04345 0.40 0.3926 0.012 0.01245 0.14 0.1325 0.045 0.04545 0.45 0.40CCl, 0.013 910.14CHCI, 0.043 860.38CCl, 0.012 930.13CHCI, 0.045 860.35i P-ClC,H,NzCNp-BrC,H,N,CN0.214 X' s-l). It is likely that these discrepanciesarise from experimental errors, especially since Ledisturbances at the beginning of each of their kineticl3 R.J. W. Le Fc5vre and J. Northcott, J . Chem. Sot., 1949,944.(3) Synthesis, ProPertiesj and SPectyal Afialysis of P-the results from i.r. analysis of diazocyanides investigatedto clarify the type Of isomerism Of theseFamp;q-e et aJ.13 emphasize the Occurrence of certain ' initial C~ZOYU- and p-Brorno-bexenediazoisocyaazide.-Althoughmeasurements. are consistent with geometrical isomerism, nevertheless,l4 M. A. Saboor, Indian J . Phys., 1943, 17, 223. compounds, aromatic diazoisocyanides had to b2126 J.C.S. Perkin Isynthesized and their properties compared with those ofthe corresponding diazocyanides.During initial attempts to synthesize the diazoiso-cyanides, we used a reaction sometimes employed forpreparation of aliphatic isocyanides,lj i.e.treatment ofdiazonium salts with silver cyanide in dry ethanolreactions (i) and (ii). The i.r. spectra of the resultantcomplexes v,,, 2 095 and 2 140 (isocyanide stretching),2 170 (AgCN cyanide), and 2 290 cm-' (diazoniumstretching) showed that the silver cyanide acts as aR X + 2 AgCN-AgX -t- RNGAgCN ( i )RNSAgCN + KCN +RNC + KAgICN 1 2 ( i i Icomplexing agent for the diazoisocyanide produced aswell as for the diazonium chloride. However, regardlessof precautions to prevent decomposition of the diazo-isocyanides during treatment of the RNC,AgCN com-plex with potassium cyanide, the yield of diazoiso-cyanides obtained in this manner is small (7-9).Themain products were in all cases symmetrical diazo-amino-compounds.obtained from decomposition of complex silver salts aswell as by dehydration of 3-formyl-l-~-halogenophenyl-triazenes proved the identity of products from bothsources.ArN=N.NH*CHO + SOC12 + 2CgH5N1Aromatic diazoisocyanides clearly differ, even inappearance, from the corresponding diazocyanides ; thecharacteristics of diazoisocyanides synthesized in thisstudy are presented in Table 4.$-Halogenobenzenediazoisocyanides proved relativelyunreactive. Attempts to reduce the isocyanide groupwith metallic sodium in n-pentyl alcohol or to hydrate itto give a 3-formyltriazene with aqueous oxalic or aceticacid failed, even though common isonitriles will, undersimilar conditions, readily undergo reduction or hydr-ation.The diazoisocyanides are, however, light-sensi-tive, and decompose almost instantly under a quartzlamp to yield unidentified solids.TABLE 4Characteristics of aromatic diazoisocyanidesAnalysis ()P .Absol; Required FoundM.p. r L bsol; h ;6-C1C,H,N2NC 21 Colourless flakes f 1.26 169 50.75 2.4 25.35 60.5 2.45 25.15p-BrC,H,N,NC 28 Colourless flakes T 1.47 205 40.0 1.9 20.0 39.8 2.0 20.1above 28 "C light yellow oil, insoluble in water.Diazoisocyanide ("C) Form d20 M * C H N C H N* By ebullioscopy in methanol. t Intense smell; abox-e 21 "C light yellow oil, insoluble in water. 1 Intense but pleasant smell;Since aliphatic and aromatic isocyanides can besynthesized by dehydration of N-formyl derivativeswith phosphoryl or toluene-p-sulphonyl chloride,l69l7 wethen concentrated attention on 1-aryl-3-formyltriazenesas possible intermediates in the synthesis of diazoiso-cyanides.1-Aryl-3-formyltriazenes were synthesizedin good yield by treating an ethereal suspension of thesolid diazonium salt and formamide with sodiumhydrogen carbonate reaction (iii) . The products wereremarkably stable, showing no changes in m.p. or i.r.ArNz'CL- + HCO.NH2 + NaHC03e t h e rArN=N.NH.CHO + NaCL + H 2 0 + C 0 2 I i'ii 1 ispectra even after several months in z'aczm. Dehydr-ation of 3-formyl-l-~-halogenophenyltriazenes wasachieved only with thionyl chloride in pyridine reaction(iv).The pure diazoisocyanides were obtained insatisfactory yield by distilling the crude products undervacuum or passing them down a silica gel column(pentane as eluant).Physical constants and i.r. spectra of diazoisocyanidesAs already noted, the individual purified compoundsalways exhibit properties independent of the mode ofsynthesis or of the origin of the compound (p-chloro-or 9-bromo-benzenediazoisocyanide) ; this suggests thatthey do not occur in isomeric forms. However the i.r.spectra of the crude products show bands characteristicof primary amino-groups (N-H stretching vibrations a t3 500 and 3 400 cm-l and N-H deformation vibrations a t1 630 cm-l). This, and the fact that dehydration of 3-formyltriazenes proceeds almost instantaneously (withevolution of gas), indicates that anilines are secondaryproducts, possibly formed by decomposition of the cis-diazoisocyanides.Inspection of a Dreiding modelleads to the inference that the integrity of the cis-diazoisocyanide molecule cannot be maintained, but that/A'N cN II + 2 H 2 0 c A r N H z 4- HC02H 4- N 2 ( v )Nit decomposes yielding formic acid and nitrogen reaction(v). The supposed presence of water in a system con-taining efficient drying agents raises some objections,l6 Houben-Weyl, Methoden der Organische Chemie, VIII,' 16 I. Ugi and R. Meyr, Chem. Ber., 1960, 98, 239.17 W. R. Hertler and E. J. Corey, J . Org. Chem., 1958, 25, 122. p. 3521975 2127and the suggested reaction path (v) ought to be viewedwith caution.However, this path would explain theformation of symmetrical diazonium compounds duringpreparation of diazoisocyanides via the silver cyanide-diazonium salt complexes.In the light of these observations it seems that therelatively stable diazoisocyanides studied in this investi-gation are best represented by a trans-model (111).The i.r. spectra of p-halogenobenzenediazoisocyanides(Figure 2) exhibit remarkable differences from those of0.0 I I I I ,. I I v r0.2 -A0 0 - , I , I I0.2 -0.4 - BII I I I I I I I I I t 1 t j 0.8 I- L O O ' 3000 2600 2200 1800 1400 1000Jrcrn"FIGURE 2 1.r. spectra of p-chloro- (A) and p-bromo-benzene-diazoisocyanide (B) in carbon tetrachloride (3 500-1 300 crn-')and in carbon disulphide (1 300-700 cm-l)corresponding cis-trarts-diazocyanides (Figure 1) or p -halogenophenyl isocyanides.The most important differ-ences occur in the region assigned to stretching vibrationsof cyanide and isocyanide groups. In spectra of p -halogenophenyl isocyanides, the isocyanide stretchingvibrations appear at 2 135 (for p-chloro-) and 2 130 cm-l(for 9-bromo-), whereas in the spectra of diazoisocy-anides, characteristic doublets are observed a t 2 135-2 130 and 2 100-2 095 cm-l respectively. A similardoublet occurs in the spectra of compounds (e.g. carbodi-imides or azides) which contain a bond system typical ofcumulenes,12 but the reason for it remains unknown.We emphasize that the doublet in the spectra of 9-halogenobenzenediazoisocyanides is the strongest absorp-tion and that its location does not depend on the methodby which the spectra are recorded (i.e.potassiumbromide pellets or polar or non-polar solvents). This isremarkable in view of the findings1* that in aromaticisocyanides polar solvents will stabilize the chargeseparation of the triple bond ( k c ) and so increase thebond force constant with a resultant shift towards higherwavenumbers.More detailed measurements also showed that inaromatic isocyanides the molar absorption coefficient ofthe isocyano-group is ca. ten times larger than thatassigned to cyano-group stretching vibrations in trans-diazocyanides. Gillis and Occolowitz l9 explain this byassuming greater separation of charges in the isocyano-( k c ) than in the cyano-group (CEN).In turn, the+ -isocyano-group absorption in p-halogenobenzenediazo-isocyanide is twice as strong as in the correspondingisocyanides.In accord with our previous observations, absorptionnear 1440 cm-l was ascribed to the diazo-group stretch-ing vibration.In the spectra of p-halogenobenzenediazoisocyanides,there is also a doublet at 1 305-1 280 cm-l. Its weakerwing at 1 280 cm-l seems to result from vibrations of thephenyl-nitrogen bond; the strong peak at 1 305 cm-l isascribed to N-Nxc stretching vibrations.To summarize, the i.r. spectra of benzenediazoiso-cyanides are characterized by (1) the presence of aremarkably strong doublet at 2 135-2 095 cm-l, withabsorptions at 2 135 cin-l assigned to v (kc) and at 2 095cm-l to a bond system typical of a cumulene (-h=fi-C) ;(2) the weak diazo-group (-N=N-) absorption at 1 440cm-fprobably resulting from participation of the cumulenesystem in a mesomeric structure (-N=N-N-C=N-N=N=C) ; and (3) the high wavenumber absorptionassigned to N-NxJ~ stretching vibration and associatedwith the increase of bond strength due to participation ofthe cumulene system in the above inesomeric structure.These correlations allow the conclusion that the mole-cule of *-halogenobenzenediazoisocyanide is a hybrid oftwo canonical forms (VI) f-t (VII).Although theexistence of (VII), characterized by a rarely encountered+ -+ 2-separation of charges, may seem questionable, theelectronic spectra of 9-halogenobenzenediazoisocyanidesdo not rule out the participation of this structure in aresonance hybrid.TABLE 51j.v.absorption bandsA,,,,/nni (euro;/I mol-l cm-1)P-C1C,H,N,NCp-BrC,H,N,NC?-ClC,H,N2CN196 (30 276), 253 (19 796), 286 (2 906),199 (46 964), 2 (24 084), 287 (4 236),191 (8 222), 193.5 (8 996), 233.5 (6 108),296 (3 000)297 (3 064)241 (4 990), 355.5 (10 995)342 (5 548), 337 (12.766)p-BrC,H,N,CN 194 (10 314), 195 (10 174), 236.5 (5 886),p-BrC,H,NC 199.5 (5 144), 236 (2 845), 244.5 (2 838)The essential difference amongst the U.V. spectra ofdiazoisocyanides, corresponding trans-diazoc yanides, and9-bromophenyl isocyanide (Table 5) is an absence ofstrong absorption at 335 nm in the spectra of diazoiso-cyanides. This absorption is due to n-n* type transitionsattributable to a delocalized rc-electron system in themolecule and, in trans-diazocyanides, characteristic of achromophoric cyanide group which participates in theresonance of the entire molecule through the diazo-group.lS R.G. Gillis and I. L. Occolowitz, Sfiectrochim. Acta, 1963, 19, l8 W. D. Horrocks and R. H. Mann, Spectrochim. Acta, 1963,9, 1375. 8732128 J.C.S. Perkin IThis absorption is strong, but in diazoisocyanides thecorresponding absorption at 297 nm is weak which indi-cates that the x-x* transition probability is significantlyreduced and that the rigid, semi-polar bonded isocyanidegroup (--kc) is non-chromophoric and hindering form-ation of polar resonance forms.On the other hand, absorption at 196-199 and 253-255 nm in the spectra of diazoisocyanides is batho-chromically shifted relative to corresponding absorptionin diazocyanides (191-194, 193-195, and 233.5-236.5nm).This indicates that the remainder of the diazo-isocyanide molecule, comprising the substituent, aro-matic ring, and diazo-group, is much more conjugatedthan in diazocyanides. This is likely in view of the factthat the positively charged nitrogen atom in the iso-cyanide molecule is an electron-attracting centre and ahalogen in the $ara-position will facilitate electrondelocalization in the same direction.EXPERIMENTALPreparation of Aromatic Diazocyanides.-Aromatic aminesused were purified either by crystallization or, in the case ofliquid amines, by distillation.Diazotization in aqueous solutions was conducted withsodium nitrite ; in non-aqueous solution n-pentyl nitritewas employed.With the exception of 2,4,6-trichlorobenzene- and 9-ethoxysulphonylbenzene-diazocyanides, diazocyanides weresynthesized by Le Famp;re and Vine's procedureJ2* modifiedas follows.To a stirred solution of amine (0.1 mol) inconcentrated hydrochloric acid (d 1.19; 45 mol) and water(45 ml) sodium nitrite (0.1 mol) in water (20 ml) was slowlyadded a t 0 "C. Following filtration, the solution wascooled with solid carbon dioxide in acetone to -10 "C.In order to decrease the freezing point, ethanol (50 ml) wasadded, and potassium cyanide (0.2 mol) in water (26 ml)was then slowly added with vigorous stirring. After thereaction was complete (a drop of potassium cyanidesolution and the reaction mixture placed on filter paper didnot yield a coloured product), ice-water (100 ml) wasadded.This speeded up filtration and prevented deconi-position of the cis-isomer on the filter paper. The wetprecipitate of cis-diazocyanide was then dissolved in theminimum volume of a non-polar solvent, usually n-pentane,carbon tetrachloride, or n-hexane, and cooled to 10 "C.The solution was dried with sodium or magnesium sulphate,filtered, and cooled with solid carbon dioxide. Theresultant cis-diazocyanides were either purified by freezingout from the solvents (Table 1) or directly isomerized intothe respective trczns-forms. The latter was accomplishedby warming the hexane, carbon tetrachloride, or chloroformsolution of the unstable cis-form at 60-60 "C for ca.6 h.Evaporation on a water-bath left the crude trans-compound,which was crystallized (Table 1). In order to obtain purecompounds, the freezing out (of cis-forms) or crystallization(of trans-forms) from boiling solution was repeated severaltimes. The cis-diazocyanides were dried in the dark overpotassium hydroxide.The preparation of 2,4,6-trichlorobenzenediazocyanidewas based on Hantzsch's procedure.aop-Ethoxysulphonylbenzenediazocyanide.--Because of thedifficulties encountered during direct esterification ofsulphanilic acid, synthesis of this compound included :(a) preparation of p-nitrobenzenesulphonic acid ; (b) pre-paration of p-nitrobenzenesulphonyl chloride ; (c) reactionof p-nitrobenzenesulphonyl chloride with ethanol; ( d ) reduc-tion of ethyl p-nitrobenzenesulphonate.The details aredescribed elsewhere.21Preparation of Triazenes and Diazoisocyanides.-l-p-Chlorophenyl-3-formyZtriazene. To a suspension of p-chlorobenzenediazonium chloride (4.02 g) in diethyl ether(50 ml), freshly distilled formamide (2.5 g; b.p. 111 "C a t20 mmHg) was added with vigorous stirring. The mixturewas then brought to boiling and aqueous 7 sodiumhydrogen carbonate (35 ml) was slowly added (5 ml every15 min). The product passed into the ether layer which,over ca. 85 min, was changed three times. After the re-action was complete, the extracts were combined, dried(MgSO,), and evaporated to leave a crystalline product(3 g).This was shaken for several minutes in previouslycooled 1 : 1 ether-hexane (20 ml). Oily compounds passedinto solution, and the residue was transferred to the ether,to which a small quantity of activated charcoal had beenadded, and shaken for a few minutes. After filtration andevaporation the triazene obtained was recrystallized fromboiling hexane or 70 ethanol; m.p. 130-131 "C(decomp.); yield 74 (Found: C, 45.55; H, 3.3; N, 22.7.C,H,ClN,O requires C, 45.75; H, 3.25; N, 22.85).1-p-Bromophenyl-3-formyltviazene. This triazene wasobtained in a manner similar to that for the chloro-deriv-ative; yield 75 (Found: C, 37.0; H, 2.6; N, 18.25.C,H,NrN,O requires C, 36.85; H, 2.65; N, 18.4).p-Chlorobenzenediarzoisocyanide from the triazene.-Tol-~-chlorophenyl-3-formyltriazene (340 mg, 1.8 mmol)dissolved in anhydrous pyridine (4 ml), freshly distilledthionyl chloride (0.22 ml, 3 mmol) was added dropwise over10 min at -6 O C .The mixture was protected againstmoisture with a Drierite-filled absorption tube. After 1 hstirring in an ice-bath, the excess of pyridine was removedunder reduced pressure, and the residue was shaken withpentane. The pentane extract, containing diazoisocymzide,was further purified by short-path distillation at 60 "C and0.8 mmHg or by separation on silica gel; yield 50; forcharacteristics see Table 4.To acooled sohtion (0 "C) of l-~-bromophenyl-3-formyl triazene(0.114 g 0.5 mmol) in anhydrous pyridine (2 ml), thionylchloride (0.07 ml, 0.9 mmol) was added dropwise. There-after, the procedure for the bromo-compound was as fordehydration of the chloro-analogue ; yield 40 ; forcharacteristics see Table 4.S$ectra.-I.r. spectra of diazo- and diazoiso-cyanideswere obtained with a Zeiss UR-10 spectrophotometer.Solutions (0.09-0.02 mol) were examined in sodiumchloride (700-1 800 cm-l) and lithium fluoride (1 800-3 600 cm-l) cells of 0.6 mm thickness; compensating cellscontaining pure solvents were placed in the reference beam.U.V.spectra of compounds listed in Table 5 were obtained,over the range 187-360 nm, with a Unicam SP 700 instru-ment (n-hexane as solvent). Standard quartz cells (1 cmthickness) were used. Below 200 nm the spectra wererecorded in a nitrogen atmosphere.Quantitative Determination of Benzenediazocyanides.-The cis-forms of p-chloro- and 9-bromo-diazocyanides were2o A. Hantzsch, Ber., 1838,81, 340.21 T. Ignasiak, Ph.D. Thesis, A. Mickiewicz University, Poland,p-BromobenzenediazoisQcyanide from the triazene.19671975 2129purified by freezing out from n-pentane, with lack ofabsorption at 2 190 cm-1 used as a criterion of purity(m.p. P-Cl, 29 "C; p-Br, 42 "C). The trans-forms,obtained by isomerization of the respective cis-forms,were purified by crystallization from n-hexane (m.p. p -C1, 105 "C;For kinetic measurements, 0 . 1 ~ solutions of the cis-formsin chloroform and in carbon tetrachloride were placed inthermostatted baths a t 25 and 45 "C ( f O . 1 "C). At definiteintervals, the spectra of the samples were scanned over therange 2 150-2 200 cm-1 (Zeiss UR-10) spectrophotometer,slit width 4; scanning rate 32 cm-l min-l; recording timeP-Br, 133 "C).50 s; recording scale 32 mm per 100 cm-l; gain 6,5; bandwidth 2; time constant 2; diaphragm correction 3, timedelay on '). Solutions were examined in sodium chloridecells of 0.6 mm thickness, a compensating cell containingthe pure solvent being placed in the reference beam.We thank Dr. N. Berkowitz for assistance in preparationof the manuscript, and Drs. R. M. Elofson and 0. Strauszfor critical reading of the paper. Suggestions made duringexperimental work by Professor M. Wiewiorowski aregreatly appreciated.6/027 Received, 7th January, 1976