Murty, C R K

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DIPOLE MOMENTS OF SOME SUBSTITUTED BENZENES AND PYRIDINESC. R. K, MURTYP hysios D kpahtment, Anuhua U nivehstty, Wai t^aih (Received for publication September 16, 1968)A B S T R A C T .  T h e  d i p o l e  m o m e n t a  o f  s e v e n  s u b s t i t u t e d  p y r i d i n e s  h a v e  b e e n  d e t e r ­m i n e d  i n  d i l u t e  s o l u t i o n  i n  b o n z e i i e ,  T h o  m o m e n t  v a l u e s  a r e  a l s o  o a l o u l a t o d  u s i n g  g r o u p  m o m e n t s  a n d  v e c t o r i a l  m e t h o d .  T h o  d i f f e r e n c e s  i n  m o m e n t s  b e t w e e n  t h e  c a l c u l a t e d  a n d  o b s e r v e d  v a l u e s  h a v e  b e e n  d i s c u s s e d ,I N T R O D U C T I O NSince the publication of the electron diffraction results on pyridine by Schomaker and Pauling (1939), considerable interest was shown on the investi­gation of the dipole moments of these compounds. The electron diffraction studies have revealed th a t pyridine has a structure similar to th i t  of benzene. The determination of the dipole moments ol y-substitutod pyridines by Leiss and Curran (1945) has confirmed the similarity between the benzene and pyridine rings. To determine whether or not tho carbon nitrogen linkage can bo adequately represented by a single constant in heterocyclic ring systems, Schneider (1948) has determined the dipole moments of diazines and drawn the following conclu­sions : (1) the carbon nitrogen electric moment apparently has a constant value in the heterocyclic compoujids investigated, (2) a heterocyclic ring of the pyridine type can be represented by a plane hexagon and (3) in the compounds investigated, all bonds between nitrogen and carbon seem to be electrically equivalent so far as dipolar characteristics arc concerned.The present investigation on substituted pyridines has been undertaken pri­marily to obtain dipole moment data on as many pyridine compounds as possible in order to understand their structure.The experimental arrangement used for measurement of dielectric constant and refractive index ol solutions and the method of calculation ot dipole moments from the experimental observations had been described previously by the author (1957). All the substituted pyridines were supplied by L. Lights and Co., England and were reported to be of a high standard of purity. The solvent benzene was also of A.R. grade of purity.C A L C U L A T I O N  O F  n F R O M  G R O U P  M O M E N T SFor the calculation of dipole moment in the case of 2-and 3-acetyl pyri- dines, group moments of 2.22 and 2.89 have been assumed for N - and acetyl63516Dipole Momenta of some Suhstitued Benzenes and Pyridines 517groups respectively. I t  has also been assumed that the acetyl group moment makes an angle of 55® with the C-C bond to the ling (Smyth, 1955). This value of the angle has been obtained from the dipole moment of p-diacetyl benzene.In  the case of amino methyl pyridines, the group moments 0.40 and 1.53 have been assumed for CH^ and NHg. I t  is also assumed that the moment of NHg acts along the C-C bond to the ring following the example of other substi­tuted pyridines investigated by Curran (1945). I f  it is assumed that NKg group moment makes an angle of 38® with the C-C bond following the example of substi­tuted anilines, the calculated values are very much different from the observed values.K K S U L T S  A N D  D I S C U S S I O NDipole moments of seven substituted pyridines have been determined and the results are given in Tables I  to VII. The observed and calculated values of dipole moments are given in Table VIII,'fABLE I 2-Acetyl pyridinew Ae A eIW U j, n ’‘ 1 n A«2 A»i'“/W0 01204 2. .30.39 0.0999 8 290 1.49416 2 23262 0.00187 0.15R0 02223 2.4398 0 1768 7 9J1 ] 49432 2 23289 0 00224 0.1010 03479 2 6336 0.2695 1.49464 2.23304 0.00299 0.0860 04Hf)] 2 6360 0 3720 7.670 1.49454 2 23364 0.00299 0.0620.06299 2.7448 0 4808 7.632 1.49469 2 2338) 0.00316 0.0500.07646 2.8489 0 6849 7.600 1.49474 2.23425 0.00360 0.047= 8 .0 8 , W — 124. Prf== 163.4 o.c. fi ^ 2.86DTABLE II3-Acetyl pyridineW E,iS Ae A e lW «i2 71213 ^n2 An2/TT0.0227C 2.4109 0.1469 6.465 ].49622 2.23669 0 00504 0.2210.03499 2.4829 0.2189 6.263 1.49577 2.23733 0.00668 0.1910.04908 2.6722 0.3082 6.280 1.49598 2.23796 0.00730 0.1490.06272 2.6560 0.3910 6.234 1.49633 2 23901 0.00836 0.4330.07362 2.7247 0.4607 6.266 1.49668 2.24007 0.00942 0.128(Ab/TT)^_^0==6.48, {An^lW)j^ ^ q=0.2Q2, Pd=xl28.6 c.o. t* ^2.59D518 G, E, K , M urtyTABLE III2-Amino-3-methyl pyridinew ®ll! AE Ae/TT Ml. M®1 2 AW20.00263 2.2776 0.0136 5.190 1 49383 2.23164 0 00089 0.3830.00448 2.2880 0.0240 5 348 1 49399 2.23202 0.00137 0.3060,008.56 2.3126 0 048.5 .5.666 1.49421 2 2.3267 0 . 0 0 2 0 2 0.2300.01417 2.3477 0.0837 5.912 1.49467 2.23404 0.00339 0.2390.01820 2 3726 0 1086 5.971 1.49473 2.23423 0.00358 0.197{AefW) 0 " —> 0 ^  ^ .08, 0..342. P^/ =  94 .9  0.0 n 2 17 1)1TABLE IV2-Amino-4-methyl pyridine \W El? AP A eIW 912j 2 A»i“ aW2/ if0.00207 2.2843 0.0203 9 766 1.49386 2.23163 0 00098 0.4730.00436 2.2987 0 0347 7 943 1.49414 2 23246 0.00181 0.4150.00721 2.3179 0.0539 7 482 1.49419 2.23261 0.00196 0.2720 . 0 1 0 2 1 2.3363 0.0713 6 984 1 49444 2.23335 0.00270 Oj. 2040 01267 2 3438 0 0798 6.306 1 49464 2.23396 0 00331 0.261( A " W ) , r - > o = » - 550. /*,^==r]41 .0  c.c. iU -  2 .65DTABLE V2-Amino-6-methyl pyridinew E)2 AE Ae/TT An'-i/W0 . 0 0 2 0 2 2 2782 0.0142 7.021 1.49411 2.2.3237 0.00172 0.8510.00294 2.2819 0.0179 6.103 1.49427 _ 2 .23284 0.00219 0.7470.00678 2.3026 0.0386 5.689 1.49462 2.23300 0.00325 0.4790.0099.3 2 3181 0.0641 5.450 1.49462 2.23390 Q.00326 0.8270.01371 2.3363 0.0713 6.198 1.49464 2.23306 0.00331 0.241=  Prt=111.4o.c. /*=;=8.35DDipole Moments of some Substituted Benzenes and Pyridines 519TABLE VI2 - Amino - b -methyl pyridinew cia ^ejW0.01405 2 3157 0.0517 3.678 1.49561 2.23686 0.00621 0.4420.01899 2.3348 0.0708 3.729 1.49633 2.23900 0.00836 0.4400.028JO 2.3699 0 1059 3.761 1 49692 2 24078 0 01013 0.3600.0300J 2 4023 0 1383 3..S40 1.49756 2.24269 0.01204 0.3340.0456J 2.4400 0 1760 3.859 1.49783 2,24350 0.01285 0.282— 3 .,60.5, = 0 510. J\( == 63 2 (i.e. M i1.77 DT A BL E  V II2-Am iiio-4-6-dim othyl pyridineW f i i Ae ^ejW AW 2 An'ijW0 01112 2 3138 0 0498 4.483 1 49528 2.23586 0.00621 0 4(t90.01738 2.3389 0.0749 4 308 1 49587 i  23763 0 00698 0 4020 02518 2 3064 0 1024 4.069 I 49629 2 23890 0 00825 0.3280.03487 2.3999 0 1359 3.896 1.49668 2.24007 0.00942 0.2700.04056 2 4133 0.1493 3.696 1.49747 2 24242 0.01177 0 290, 520 P - 9 7  7 e.e, M ^ 2.20DT A BL E  VIIIiSubsianco fi ohnerved fi ealeiiluted2-AeelyJ pjridino 2 85 2 81 1>3-Aoetyl pyi-idino 2.53 0.712-Aiiiirio-3-methyl pyridine 2 17 2.352-Amino-4-methyl pyridme 2.65 2,282-Araino-6-mothyl pyridine 2.36 1.92'2|.Amino-6-inothyl pyridine 1.77 1.592-Amino-4 : 6-dimethyl pyi'idine 2 20 1.92la  the case of acetyl pyridines, the agreement between calculated and observed values is satisfactory when the acetyl group is in the a-position. Eor //-acetyl52 0 0. n. K.pyridine the calculated value is very low compared to the observed value of 2.63. This difference is so much that it cannot be accounted for by experimental errors. The discrepancy may be understood when the y-acetyl pyridine is also investi­gated.The group moments of CHg and NHa have the same sign hut opposite to that of pyridine moment. As the group NHg is fixed in the a-positioii in all the compounds investigated, the moment to be expected for 2-amino-3-methyl pyridine is th a t of 2-amino pyridine plus the small contribution of CH3. The observed value appears to be somewhat less than the expected value. When CH3 is in the y-position the moment observed is maximum indicating negli­gible effect of NHg on CH3 and an approximate moment of 2.17-f-0.40 =  2.57 may be expected for this compound. The observed value of 2.65 shows fan agreement with the expected value though considerably greater than the cal­culated value obtained by simple vectorial method. The value of 2.17‘foir 2-amino pyridine includes the inductive and resonance effects between pyridine and group. \In  the case of 2-amino-5-methyl pyridine the CH3 group is in the m eta­position relative to N - and away from NHg unlike in 2-amino-3-mothyl pyridine. Assuming negligible effect of NHg, a moment of 2.39 is to be expected which is close to 2.35 observed for this compound. 2-amino-6-methyl pyridine should have the minimum moment aiiico both NHa and CHg are ortho-to N. The observed value of 1.77 is minimum for all those compounds and is slightly greater than the value of 1.65 observed for 2 : 6-dimethyl pyridine as is to be expected.The moment value for 2-amino-4-6-dimotliyl pyridine should be obtained by adding CH3 group moment to that of 2-amino-6-methyl pyridine since GHg which is added is para-to N. The observed value of 2.20 is in reasonable agree­ment with 2.17 which is the expected value.In  conclusion, the author considers th a t a much larger amount of data on pyridine compounds with various substitutions in different positions have to be collected before the structure and resonance effects in these compounds can be completely discussed.A C K N 0 W E D G M E N 1’The author is very much indebted to Prof. K. R . Rao for his kind and cons­tan t guidance throughout the progress of the work.R E F E B E N C E S  _Curran, B. 0., and Leiss, D. G., 1945, J. A7n, Chettv. Soc,, 67, 79.Murty, C. R. K., 1967, Ind, J. Phij8., 31, 266.Schneider, W. C., 1948, J, Am. Chem. Soc., 70, 027.Schomaker, V., and Pauling, L., 1939, J . Am. Chmi. Soc., 61, 1760,Smyth, C. P., 1955, Dielectric Behavior and Structure, McGraw-Hill Book Co„ pp. 343.

Dipole Moments of Some Substituted Benzenes and Pyridines

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Dipole Moments of Some Substituted Benzenes and Pyridines

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