636
ortho-Disubstituted Compounds with One Cylindrically Symmetrical and One Asymmetrical Substituent. A treatment similar to that outlined above for the metadisubstituted compounds leads t o eq 17 for this type
(aR"2)' - ~ R " l ( y
&z)}
- 80 (17)
of compound, where the alternative values apply to the two possible planar configurations. Using the previous values of x and y , alternative values of Aorfho(as/s) are calculated, and compared in Table XI with the Aobsd values. The results are shown graphically in Figure 4. It is clear that some of the kiobsd values lie quite outside
the possible ACIS/AtTan9 ranges: we believe that this indicates the effects of steric hindrance and rotation of substituents out of the ring plane. Compounds of fixed orientations (XXXIX; Z = CO, 0, and COO; n = 1, 2, and 3) will be the subject of future investigations. When these are complete it will be easier t o assess the significance of the cisltrans proportions shown in Table X I ; the pattern appears to be quite different from that appertaining to the meta compounds. Acknowledgments. Part of this work was carried out during the tenure of a Nufield Traveling Fellowship (to R. D. T.). We thank the U. S. National Institutes of Health for a Postdoctoral Fellowship (to T. T. T.) and the Science Research Council for a Research Studentship (to M. v. s.).
Infrared Intensities as a Quantitative Measure of Intramolecular Interactions. VI.' Pyridine, Pyridine 1-Oxide, and Monosubstituted Derivatives, The Band near 1600 cm-1 A. R. Katritzky,2a C.K. Palmer? F. J. Swinbourne,2a T. T. Tidwell,'" and R. D. TopsomZb Contribution from the School of Chemical Sciences, Universitr of East Anglia, Norwich, England, and the Sciiool of Physical Sciences, La Trobe Uitiversity, Melbourne, Australia. Received June 176, 1968 Abstract: Integrated intensities for pyridine, pyridine 1-oxide, and I-substituted pyridinium compounds are used to derive UR' values for :N . and :N+(X). "substituents." Intensities for series of 4-substitutedpyridines and pyridine 1-oxides provide evidence for direct interaction between the substituent and the hetero group for donor pyridines and for both acceptor and donor pyridine oxides. Intensities for 3-substituted pyridines show fair agreement with those calculated using the treatment discussed for meta-disubstituted benzenes. 2-Substituted pyridines are compared and discussed with reference to ortho-disubstitutedbenzenes.
We
have previously shown that the total integrated intensity area of the infrared ring stretching bands near 1600 cm-I for monosubstituted3 and for para-,4 meta-, and ortho-disubstituted benzenes are related t o the uR0 values of the substituent(s) by relations 1-4. These equations refer to substituents of at least Czv symmetry-for less symmetrical substituents, appropriate correction factors have been derived. For para-disubstituted benzenes a further correction must be applied where direct interaction O C C U T S . ~ For durenes an equation similar to (1) applies in the absence of steric hindrance. l l 4
+ = 11,80O(~R"l- UR'~)' + 170 A,,,,
A,,,,
=
17,60O(~R")* 100
(1) (2)
(1) Part V: A. R. Katritzky, M. V. Sinnott, T. T. Tidwell, and R. D. Topsom, J . A m . Chem. SOC.,91, 628 (1969). (2) (a) School of Chemical Sciences, University of East Anglia, Norwich, England; (b) School of Physical Sciences, La Trobe University, Melbourne, Australia. (3) R. T. C. Brownlee, A. R. Katritzky, and R. D. Topsom, J . A m . Chem. Soc., 88, 1413 (1966); R. T. C. Brownlee, A. R. Katritzky, T. T. Tidwell, and R. D. Topsom, ibid., 90, 1757 (1968). (4)P. J. Q . English, A. R. Katritzky, T. T. Tidwell, aiid R. D. Topsom, ibid., 90, 1767 (1968).
Aoim = 15,90O((~R'1)'
+
(UR'2)' - uR01uR021 - 80 (4) We have now studied pyridine, pyridine I-oxide, and several series of their monosubstituted derivatives to examine the generality of these equations and as part of a general investigation of substituent-ring interactions in heteroaromatic compounds. Earlier semiquantitative work on the infrared intensities of 2-,j 3-,6 and 4-monosubstituted7 pyridines and 2-,8 3-,y and 4-monosubstituted lo pyridine 1-oxides had indicated that correlations did indeed exist with the nature of the substituent, and especially for compounds of CBvsym(5) A. R. Katritzky and A. R. Hands, J . Chem. SOC.,2202 (1958). (6) A . R. Katritzky, A. R.Hands, and R. A. Jones, ibid., 3165 (1958). (7) A. R. Katritzky and J. N. Gardner, ibid., 2198 (1958). (8) A . R. Katritzky and A. R. Hands, ibid., 2195 (1958). (9) A . R. Katritzky. J. A. T. Beard, and N. A . Coats, ibid., 3680 (1957). (10) A . R. Katritzky and J. N. Gardner, ibid., 2192 (1958).
Journal of the American Chemical Society 1 91:3 1 January 29, 1969
637 Table I. 1-Substituted Pyridinium Derivatives Compound
Solvent
1600-cm-l band Postn 6s
1585-cm-l band Postn €8
Pyridine 1-oxide I-Methoxypyridinium iodide 1-t-Butoxypyridinium perchlorate Pyridine Pyridine borane 1-Methylpyridinium iodide Pyridine 1-oxide borofluoride
CHCItn CHCN CHICN CCla CHC13 CHaCN CHaCN
1618 1617 1639 1598 1623 1635 1682b
1608
-~ Q
17 43 3 30 10 15 4
87
...
...
1613 1582 1578 1588 1621
46 100 1 2 23
Substituent, Aobsd
UR
-0.212 -0.171 01268 0.209 0.281
...
894 615 437 1360 870 1490 369
~
Values for other solvents are shown in Table VI.
Shoulder.
Table 11. 4-Substituted Pyridines" Substituent N(CH3)z OCH, SCHa
c1
i-CaHl C?Hj CH, CH=CHz CHIOHe CN NO2 COOC2Hs CHZCO 4'-CjHaN
1600-cm-1 band Postn fs 1598 1593 1577 1590 1602 1602 1606 1597 1606 1593 1606 1598 1594 1604
843 623 382 135 247 20 1 270 327 97 159 26 52 19 98
a Solutions in carbon tetrachloride. tion in chloroform.
b
1585-cm-' band Postn €8
Substituent, UR'
...
1573 1538d 1580d 1585 1560 1565
-0,533 -0,428 -0,250 -0.217 -0.115 -0.103 -0,099 -0.050 0 0.085 0.174 0.180 0.219
67 27 43 22 36 22
...
*..
1563 1552 1573 1564 1555 1590
35 75 109 83 41 335
Calculated using eq 2 or 5.
ApLd
...
7740 5860 3340 2950 1900 1790 1760 1360 1018 565 274 447 225
...
Apamc
Aobsd
13,600 9,890 5,040 4,320 2,450 2,270 2,210 1,550 1,020
13,600 9,100 4,670 4,870 2,800 2,730 2,600 2,400 2,280 1,550 1,080 1,290 700 2,730
... ...
...
... ...
Calculated using eq 6 for donor substituents.
Shoulder.
e
Solu-
Table 111. 4-Substituted Pyridine I-OxidesSubstituent
1600-cm-1 band Postn fa
N(CH3)z OCHa CI GCIHI CZHS CH3 H CN NOz CO?CzHj CH3CO a
Solutions in chloroform.
1632 1634
194 69
1620 1623 1624 1618 1614 1603 1615 1618d
6 3 6 17 278 385 223 33
...
...
1585-cm-1 band Postn €8 .
.
...
I
...
... ...
... ...
... 1609
2
...
...
1608
87
...
...
1582
149
1609
379
...
...
Calculated from eq 2 or 5.
Substituent, bR
(11) A . R . Katritzky, J . Chem. SOC.,4162 (1958). ( 1 2 ) E. D. Schmid and R. Joeckle, Spectrochim. Acta, 22, 1645 (1966). (13) R. Joeckle and R. Mecke, Ber. Bunsenges. Phys. Chem, 71, 165 (1967). (14) E. Wachsmann and E. W. Schmid, Z . Phys. Chem. (Frankfurt), 27, 145 (1961).
A,,,,c
Aobad
1390 743 170 28 1
5070 2160 199
310
203 213 894 1960 4650 3570 4660
5370 1830 0 115 100 48 894 2180 5290 3390 4580
-0.533 -0.428 -0.217 -0.115 -0.103 -0.099 0 +O. 085 10.174 +O. 180 +0.219
Calculated from eq 7.
metry the general pattern of the variations was qualitatively well understood. l 1 The Freiburg school have demonstrated that their quantitative relation between the intensity of CH stretching bands and the inductive constant, uI,holds for substituted pyridines, l 2 and they have also investigated solvent variation. l 3 The limited published work on the normal coordinate analysis of pyridines14 indicates that the forms of the 1 6 0 0 - ~ m - ~ vibrations are not very different from those in substituted benzenes. Some quantitative intensity work has been reported on pyridine substituent vibrations; thus the v(C=N) intensity in coordinated 3-cyano-
Aporab
321 894 1210 1930 1890 2290
...
Shoulder.
pyridines is proportional to the coordination constant. l5
Experimental Section Compounds were obtained commercially or prepared by published methods and purified by preparative vapor phase chromatography or recrystallization. Purities were checked by vpc or melting point. Spectra were obtained on a Perkin-Elmer 125 spectrophotometer under the conditions previously specified.3 Solvents were purified as b e f ~ r e . ~Intensity area, A , values are quoted as averages of eight readings. The effect of concentration on the measured A values was checked for pyridine in carbon tetrachloride and no significant variation was found.'B
Results and Discussion Compounds were measured where possible in carbon tetrachloride ; the integrated intensities and frequencies (15) D. G. Brewer and P. T. T. Wong, Can. J . Chem., 44,1407 (1966) (16) For full details see C. R. Palmer, Ph.D. Thesis, University of East Anglia, 1967.
Katritzky, et al. / Substituted Pyridines
638 Table IV.
a
3-Substituted Pyridines0
Substituent
1 6 0 0 - ~ m -band ~ Postn ea
1585-cm-l band Postn ea
N(CHdz NDz
1586 1600
295 103
1562 1579
35 235
-0.533 -0.467
OCHs
1589
78
1578
126
-0,428
SCH3
1570 1572 1572 1588 1596 1596 1594
20 51 53 12 14 31 14
1560 1559 1567 1575 1580 1580 1588
55 22 39 57 48 60 81
-0.250 -0.231 -0.217 -0.103 -0,099 0 0.085
Ameta
4370 3450 2990 1610 1530 1480 1370 1380 1690 2260
183
1576
48
0.180
3210
1587
226
1570~
52
0.219
3700
* Solution in chloroform.
Ameta(as1s)
Aobsd
...
4210 2940
...
3480 ;2550 . . I
... ... ... ... ...
230 630 790 570 702 830 1110 650 ~~
3850 2410 4330 2960
~
1760 2490
Shoulder.
2-Substituted Pyridines"
Substituent
1600-cm-1 band Postn ea
1580-cm-1 band Postn Es
UR
Aorihob
1,600
1,561
133
-0.533
202
209 192 138
1,564 1,569 1,571
264 134 69
-0.428 -0,250 -0.231 -0.217 -0.103
9,547 112,500 '[ 6,030 3,620 3,330 3,140 1,810
10,200
1,573
7,840 S ,530 14,940 3,110 2,880 2,730 1,660
103
1,572 1,574
57 66
-0.099 0
1,630 1,120
1,780 1,060
2,130 1,860
1,571
65
...
1,540
... ...
1,640
{;:% 1,572 1,580 1,592 1,596d {1,590 1,597 1,595d 1,583 1, 596d 1,586$
i c
URo
1592
Solutions in carbon tetrachloride.
Table V.
Substituent,
{I;;
{:; {::::;
1,584
86
a Solutions in carbon tetrachloride. Calculated using eq 10. Shoulder.
1,569
Substituent,
0.085
2;{
Aortho'
809
0.180
81
* Calculated using eq 4 and 9;
0.219
Aabsd
5,790 4,200 3,130 3,110 2,110
9 50
for asymmetric substituents the two alternative values are shown.
Table VI. Intensity Variations with Solvents Compound 4-Acetylpyridine 3-Acetylpyridine 2-acetyl pyridine
4-Dimethylaminopyridine 3-Chloropyridine 2-Met hoxypyridine Pyridine Pyridine 1-oxide 4-Nitroovridine 1-oxide a
In A units.
* Immiscible.
CCI4
CHCls
--Intensity" CHICN
1,170 2,490 949 13,600 571 5,790 1,360 c
1,090 2,510 1,040 13,800 66 1 5,360 1,230 841 5.290
1,070 2,260 960 13,000 562 5,430 1,080 650 4.200
C c
1
I-C~HTOH 1,040 2,470 1,100 14,600 686 5,670 1,250 418 c
CeHiz b b 992 C
578 5,660 1,230 C C
Insufficient solubility.
are recorded in Tables I-V. Some compounds were insufficiently soluble in carbon tetrachloride; the solvent in these cases is noted in the tables. Solvent variation studies are recorded in Table VI. Pyridine and N-Substituted Derivatives. Pyridine and N-substituted pyridinium derivatives (I) are considered as monosubstituted benzenes, and the values of Aobsd (Table I) have been used with eq 1 to derive uR O values for the "substituents" : N . and :N+(Z). (Z = CH,, BH3-, 0- etc.) where the substituent replaces a ring CH group. In Table VII, the values of b R 0 are Journal of the American Chemical Society
91:3
compared with the corresponding benzenoid substituents to give the net effect of replacing CH by N, or replacing C by N+. Values for acetonitrile are quoted where possible to facilitate comparison. Whereas the aza substituent is a moderately strong electron acceptor, the N-oxide group is a donor of considerable magnitude particularly in nonpolar solvents where little solute-solvent interaction occurs (the directions of these effects are confirmed by results for substituted derivatives). It is of interest that quaternization of the nitrogen atom does not increase its acceptor
1 January 29, 1969
639 Table VII. Resonance Effects of N and NC in Pyridine and Pyridinium Derivatives ’6000
UR’ of OR’ of Net effect of N heterocyclic benzenoid or N+ replaceSolvent “substituent” substituent ment by CH
“Hetero substituent”
A-0
CHCI,
-0,212
-0.593
$0.381
CHXCN CHaCN
$0,281 $0,236
-0,099
$0.380
CHICN
$0,171
-0.428
1-0.599
N-O-t-Bu
CH3CN
$0.138
...
N-OBFI
CHICN
$0.123
CHCI,
$0.209
... ...
... ...
k-CH3 N + N-OCH, t
A
.
N-BHI
/ / ’
12000-
Abr
...
t -
8COO+
I
...
1
08 0
power very markedly, and coordination with BHJ apparently significantly decreases it. The m a l l increase in electron withdrawal on quaternization is in agreement with the effect of ring substituents on the basicity of pyridine being governed by normal u values rather than u+ values.17 For u values determined by reactivity methods, there is a very large difference between the uncharged and protonated pyridine nitrogen: l8 evidently this is mainly due to inductive effects. We were unable because of solubility difficulties to obtain a value for the protonated aza group NH+, but we should expect it to be ca. +0.35 (cf. Table VII). Previous values of u R Ofor aza nitrogen are not available for comparison, and other u values vary considerably:’’ u, and up are both positive with up ca. +0.3 greater than um. ‘The value now found for u R 0 is not incompatible with these data. Resonance interaction of OR groups with a ring is evidently impeded when they occur as part of a :N+(OR) group, compared to the usual :C(OR). environment (qf. Table VII). 4-Substituted Pyridines. The 4-substituted pyridines may be considered as a special type ofpara-disubstituted benzene. Values for A,,,, (Table 11) were calculated from eq 2, using the uRO of +0.268 for the pyridine nitrogen and the appropriate u R o for the other substituents ; for asymmetric substituents, a correction according to eq 5 has been made, a s p r e v i ~ u s l y . ~
8000
4000
12000
16000
A pp Figure 1. Observed intensities for 4-substituted pyridines with and acceptor (0)substituents plotted against Apara(eq donor (0) 1) or Apain(as!a) (eq 5 ) . Corrected values for the donor substituents by eq 6 are shown ( x). The line shown has unit slope.
Moderate deviations shown by the electron-acceptor substituents are less easy to account for. Apparently electron-acceptor substituents are able to conjugate far less effectively with the 4 position of a pyridine ring than with benzene, as indicated by the tabulation in Table VIII. Table VI11
CN urt0 (monobenzene) Effective U R ’ (4-pyridine) Effective CTR’ (p-nitrobenzene)
Substituent--NO? COyEt
CHIC0
+O. 18 -0.04
$0.22 t0.06
$0.08 -0.07
$0. 18 -0.01
-0.18
...
.
.
I
--7
-0.18
(6)
Other evidence is available for this reduced interaction. Thus Schofield and coworkers1Y found that 3 substituents of varying size did not apparently alter the conjugation of a 4-position nitro group with the pyridine ring, and concluded that the resonance interaction was small even in 4-nitropyridine itself, and a similar conclusion was reached by other workers. 20 The work of Exner21 (cf. discussion in ref 3) indicates that the conjugation of an electron-withdrawing group with the benzene ring is reduced significantly by another withdrawing group in the para position. The u values derived for the 4-pyridyl group from carboxylic acids is lower than for acetic acid ionization,18 again suggesting reduced interaction of the substituent with the ring. 4-Substituted Pyridine 1-Oxides. These compounds were also treated as a class of para-disubstituted benzenes (Table 111). Values of A o b s d are plotted against A,,,, in Figure 2 . The line shown has unit slope. Here marked discrepancies are noticed for both strong
(17) 1% H. Jaffe and H . L. Jones, AdLan. Hererocqclic Chem., 3 , 209 (1964). (18) J. €I. Blanch, J Cltem. Soc., B, 937 (1966).
(19) J. M. Essery and I
