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Electric dipole moments and molecular conformations of acetophenone oximes and benzaldoximes |
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Journal of the Chemical Society, Perkin Transactions 2,
Volume 1,
Issue 4,
1974,
Page 325-328
Saumyendunarayau Basu,
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摘要:
1974 325Electric Dipole Moments and Molecular Conformations of AcetophenoneOximes and BenzaldoximesBy Saumyendunarayau Basu and Charles W. N. Cumper," School of Chemistry, Thames Polytechnic, LondonSE18 6PFThe electric dipole moments of 30 oximes have been measured in benzene or 1,4-dioxan solution a t 25.0 "C andthe results interpreted in terms of molecular conformations. The hydroxyimino-group has either conformation(I) or (1') in every molecule studied.ANALYSIS of the electric dipole moment of a moleculeprovides one method for obtaining information aboutthe conformation in solution. Results are available foracetophenone oxiines and for benzaldoximes but it seemsdesirable to extend these to a wider range of compoundsincluding some with substituents in the phenyl ring.The electric dipole moments were determined as describedprevious1y.l The total polarisation (PT) molar refractivity(I?,,), and dipole monients (p) are recorded in Table 1 wherethe other pararncters are the limiting slopes of the electricpermittivities ( K ) , specific volumes (p), and refractive indicessquared (7) against weight fraction of the solute. Themeasurements were made in benzene solution except forfive compounds whose solubility in benzene was too lowfor the results to be reliable and lJ4-dioxan was employedinstead.Prepavatioiz n.nd Purificatiotz of Compounds.-The purific-ation of the solvents has been described2 Each oxime wasprepared by a standard method and recrystallised fromone or two solvents t o a constant m.p.which was in agree-ment with literature values ; syn-benzaldoxime waspurified by fractionation, b.p. 92.5 "C a t 1.4 mmHg. Thefinal purification of each compound was performed immedi-ately before its solutions were made and their electricpermittivities measured.DISCUSSIONIn order to analyse electric dipole moments in termsof the structure and conformations of molecules aknowledge of relevant angles and bond or group momentsis required. The latter are frequently available4 butthose for the C=N and N-0 bond moments in solutionare not easily obtained and their literature values arenot particularly reliable. In oximes the situation iscomplicated by an interaction between these two bondmoments; conjugation between the three atoms resultsin some delocalisation of lone pair electrons on theoxygen atom into the C=N bond.The results for theparent and para-substituted compounds studied arebest satisfied by taking the C=N bond moment as 1.80 DC. W. N. Cumper, ,4. A. Foxton, J. F. Read, and A. I.I-ogel, J . Chesiz. SOL., 1964, 430. * C . W. h'. Cumper and A. Singleton, J . Ckem. SOC. (B), 1967,1096.3 A. I. Vogel, ' Practical Organic Chemistry,' Longmans,Green, London, 1966, 3rd ecln.; 0. L. Brady, A. N. Cosson, andA. J. Roper, J . Chem. SOC., 1925, 127, 2427; C . R. Hauser andU. S. Hoffenberg, J . Org. Chem., 1955, 20, 1481.and that of the N-0 bond in conjunction with it as-0-20 D, relative to a zero value for the C-H bond.The group moment from the phenyl ring to the C(=N)bond has been taken as 0.50 D and the other values froma compilation of electric bond and group moments.4The bond angles employed in these calculations arethe X-ray values reported for syn- and annti-9-chloro-benzaldoximes : synA Ph-C=N 123", GG-0 112" ;anti Ph-e=N 130", C=N-0 118" (the N-0 bond is 19"out of the molecular plane).The parent molecules have low dielectric relaxationtimes measured7 a t 9-82 GHz in benzene solution at25-0 "C; acetophenone oxime, 5.4 ps ; syrc-benzaldoxime,9.0 ps; anti-benzaldoxime, 6.2 ps.These resultsindicate that relaxation occurs not only by rotationaldiffusion but also by an internal mechanism such as thelibration or rotation of the hydroxy or phenyl group.Further, over an 80" temperature range down to-40 O C , the IH n.m.r.spectra of anti-benzaldoxime andof its $-chloro-derivative indicated that there was nodistinction between the two ortho-protons thus implyingrotation about the Ph-C bond. In acetophenone oximeconformation (I) (Figure) is sterically more likely than(11) and might be stabilised by a favourable interactionbetween the oxygen atom and a methyl hydrogen atom;this could also be one reason why the anti-isomers ofacetophenone oxime have not been isolated. Thesituation in syn-benzaldoximes is similar except that thepossible stabilising effect in conformation (I) would notbe present. Conformation (11') for the anti-isomers issterically impossible ; even in (1') an interaction betweenthe oxygen atom and phenyl hydrogen atom forces theformer 19" out of the molecular plane in the crystallinephase.Table 2 lists the dipole moments computed for thevarious molecular conformations.For each series it isevident that the molecules exist primarily, if notentirely, in conformations (I) or (1').Acefophenone and Related 0ximes.-The replacementof the phenyl ring by a saturated hydrocarbon or by ahydrogen atom (acetoxime) gives a marginal increase inthe experimental dipole moment but with the twonaphthyl compounds the values would seem to be in-fluenced slightly by altered mesorneric moments. AAC. W. N. Cumper, Tetvahedrovz, 1969, 25, 3131.C. P. Smyth, J . Amev. Chem. SOC., 1938, 60, 183.B. Jerslev, Nature, 1957, 180, 1410.7 G. A. Balogun and C. W.N. Cumper, unpublished results326 J.C.S. Perkin I1TABLE 1Polarisation data and dipole moments (p) of oximes in benzene or 1,kdioxan solution at 25.0 "CCompoundAcetoximeCyclohexanone oximeCyclohexyl methyl ketoxiineMethyl a-naphthyl ketoximeMethyl 6-naphthyl ketoximeAcetophenone oximep -Me t hyl-p-Chloro-p-Bromo-p-Nitro-p - Hydroxy-p-Methoxy-sy n-Benzaldoximep-Methyl-p-Isopropyl-p-Chloro-$-Nitro-$- Met ho xy-o-Chloro-o-Nitro-o-Hydroxy-m-Nitro-$-Methyl-e-Isopropylp-Chloro-p-Nitro-fi-Methoxy-nz-Nitro-anti-BenzaldoximeO-Chloro-* Measurcd in I ,4-dioxan solution.a0.71,0.75,0-75,0.64,0.67,0.71,0.4161.21,1-02,8-80,3.80,1-75.,0.95,1-04,1-13,2-38,2.06,1.48,0.94,1.10,9.49,10.0,9.4711.20,1.00,2.93412.1,2.50,12.4,2.138P- 0.060-0.177-0.133- 0.322-0.310-0.254-0.170- 0.369- 0.472- 0.363-0.161-0.338-0.251-0.227-0.171-0.357-- 0.409- 0.297- 0.301-- 0.405- 0.349-0.413- 0.241- 0.2290.201-0.211- 0.245- 0.220- 0.368- 0.242Y-- 0.2450.044- 0.0420.3120.40 10.2960.2660.2250-2820.5430.2480.3000.2270.1670.5560.3430.3030.2140.1390.2660.2700.2030.2770.1830.5040.4680.5040,2450.475PTlCrnS33-5942.0253.6971.7568-4483.2662-3774.6584.03535-8139.894.3053.9965.1 379.7069.92476.5106-1320.7303.999.3970.3154.4387-8596-8892-04386.4113.3364.299.43R ~ l c m ~19.8128.3740.6859.6559.4642.8749.8843.9453-70(49.2)46.7546-8633.6342.1951.7452.3944.7346.3044.8386-5038.6489.9036.7543.3950.0044-0244.8142.4642.6944.54ECP0-820.820.800.770.660.710.811.221.224.872.15 *1-620.871.061.170.924.601.711 *633.731.253.600.911.091.521.53 *4.09 *2.86 *1.673.95 *Lit.p/D0.88 a 1-060.83 b0.87 C 0.99 '0.71 6a R. Raman and S. Soundararajan, Pvoc. Indiapz Acad. Sci. 14, 1958, 47, 357.Steel, Austral. J. Chem., 1966, 19, 1769. C J . Brassin and H. Lunibroso, Bull. SOC. chi&. Frame, 1961, 492.b K. T. W. Le Fhvre, R. K. Perrins, and I<. D.TABLE 2Calculated dipole moments of oximes corresponding to the conformations in the FigureAcetoximeCyclohexan one oxi meCyclohexyl methyl ketoximeMethyl cc-naphthyl ketoximeMcthyl p-naphthyl lretoximeAcetophenone oxime$-Methyl-p-Chloro-p-Bromo-p-Nitro-p-Hydrox y-p-Methoxy-sy n-Benzaldoximc$-Methyl-p-Isopropyl-p-Chloro-p-Nitro-$-Nethos y-o-Chloro-o-Nitro-n-Hydroxy-na-Nitro-p-Methyl-9-Isoprop yl-p-Chloro-p-Nitro-p-Methoxy-o-Chloro-m-Nitro-anti-Bcnzaldoximc* 3Ieasurecl in 1,4-dioxan solution.ilA)1-951.901-611 *764.092-18 a1-10 b3.491.921.893-48Dipole moment @ID)0.800-800.710.710.711-021.171.143.510.701.061-120.953-350.861.161.211.153.441.05 4.251 .oo 4.201.17 3.392.15 2.024-58 2.962.05 3.571.60 1 -2811-01 1.441-12 3.552-29 3-974-02 2.503.113-1 12.942-942-943.062-942.934-282-722-852-892.654.003.483-723-782.401.52- (W1.491.561.514.306.733-873.946.192.404.784.91Experimentalvalue0.820.820.800.770.660.7 10.811.221.224.872-15 *1-520.871-061.170.924-601.711.633.731-253-600.911.091.521.534.09 *1.86 *1-673.95 *With the O-H group directcd away horn the C=N bond.6 With the O-K group directed towards the C=N bond1974 327small interaction moment in 9-chloro- and bromo-acetophenone oximes would explain why their pre-dicted values are slightly low. The P-hydroxy- and,H (1‘1Thc syn-conformations of acetophenone oximes and thc syn- andanti- conformations of benzaldoximes.The letters A and Bdifferentiate the two sides of the phenyl ring in substitutedcompounds-methoxy-groups can undoubtedly rotate about thePh-0 bond and results for these oximes are consistentwith them spending roughly equal times in the two stableplanar conformations (IA) and (IB) with the O-H orO-CH, on either side of the phenyl ring. (The experi-mental result for @-hydroxyacetophenone oxime wouldbe expected to be ca. 0.3-0.4 D high2.9 since themeasurements were made in 1,4-dioxan solution.)One apparently inconsistent result is that for #-nitro-acetophenone oxime; if it also exists in conformation(I) the predicted dipole moment is 1.36 D less thanthe experimental value. Nitro-compounds frequentlypossess considerable interaction moments and a sub-stantial increment might be expected in this case fromthe canonical form (2) which would increase the dipolemoment calculated for conformation (I) but would havea smaller resultant effect on (11).That some electroniceffect is present in this molecule is confirmed by the lHn.m.r. resonance of the methyl group being shifted by0.07 p.p.m. upfield compared with the other aceto-phenone oximes studied.BenmZdoximes.-The syn-isomers are structurally verysimilar to the acetophenone oximes but in the anti-isomer conformation (11’) is sterically impossible andeven in (1’) the molecules cannot be planar.6The calculated and experimental dipole moments(Table 2) for the benzaldoximes and their derivativeswith substituents in the $ara-position are in reasonableagreement bearing in mind that in four cases the experi-mental results are 0-3-06 D high because measure-ments had to be made in 1,4-dioxan solution. The lowsolubility of some of the anti-benzaldoximes in benzenesolution is presumably due to their tending to poly-merise ; the anti-isomer of P-chlorobenzaldoximes ispolymeric in the solid state whereas the syn-form isdimeric.J. W.Smith, ‘ Electric Dipole Moments,’ Butterworths,London, 1965, p. 209.It would seem that a p-methoxy-substituent in theafinti-isomer has no great preference for either of the twoconformations (IA or B) in which it is coplanar with thephenyl ring but in the syn-isomer there appears to be apreference for (IA).The experimental result for thead-isomer of p-isopropylbenzaldoxime exceeds thecalculated value, as was also the case for the correspond-ing acetophenone oxime.The para-nitro-derivative of syn-benzaldoxime issimilar to the corresponding acetophenone oxime butthe interaction moment in the anti-isomer would appearto be small; this could be associated with the non-planarity of the molecule and is supported by a 4 nmreduction in the wavelength maximum of its P absorptionband. When the nitro-group is in the meta-positionthere should not be a significant interaction moment andthe dipole moment is consistent with it being on eitherside A or side B (Figure), of the phenyl ring. There isn.m.r. evidence that conformation (IB) predominates.The lH chemical shifts, measured relative to tetra-methylsilane in (CD,),SO solution, of the aromaticprotons are shown in Table 3.Comparing the twoTABLE 3IH ChemicaI shifts [a (p.p.m.)] in 1%-nitrobenzaldoximesBen zalcioxinieProton sy” anti Differeiicc2-H 8.40 8.89 - 0.494-H 8.21 8.2 9 - 0.085-H 7.70 7-72 - 0.026-H 8.05 8.2 1 --0.16benzaldoximes the chemical shifts of 5-H are nearly tliesame whereas those for 4-, 6-, and 2-H differ progressivelyin this order, that for 2-H being particularly great.With the (IA) conformation the greatest differencewould be expected for 6-H, since this atom would beaffected by its proximity to the oxygen atom in theanti-isomer and to the lone-pair electrons of the nitrogenatom in the syn-isomer. With (IB), however, 2-Hshould display the greatest difference which is in agree-ment with the experimental results.When a chlorine atom or a nitro-group is substitutedin the ortho-position of the phenyl ring then conformationA would be stabilised by favourable C1.H orNO, H interactions, whereas B would be destabilisedby the interactions of these substituents with thehydroxyimino nitrogen atom. This is supported by thedipole momcnt results. The H-C=N proton resonancein the n.m.r. spectra of the four ortho-substitutedbenzaldoximes was at a lower frequency than in theother benzaldoximes which is also consistent with (IA)being the predominant conformation.The ortlzo-hydroxy-substituent in sy%-benzaldoximemay have many conformations and it is difficult todeduce from dipole moment evidence alone which pre-dominates. Conformation (IB) with the hydroxyC. W. N. Cumper and J. W. M. Woad, J . Chew$. SOC. (B),1971, 1811; C. W. N. Cumper and A. Singleton, ibid., 1967,1100; 196S, 645328 J.C.S. Perkin IIhydrogen atom directed towards the nitrogen atom(pCalc. 1.60 D) would be stabilised by an O-H * * * Ninteraction, which in turn would modify its dipolemoment, and conformation (IA) in which this hydrogenmight be stabilised by a weak H-0 - - *H-C=N inter-action. On the other hand, the remaining two conform-ations with the oxime in structure (I) would be de-stabilised by unfavourable interactions.atom is directed away from the C=N bond (pcdc. 2.16 D) [311692 Received, 9th August, 1973
ISSN:1472-779X
DOI:10.1039/P29740000325
出版商:RSC
年代:1974
数据来源: RSC
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