642s
H. K. HALL,JR.,
(CONTRIBUTION FROM THE ~ I O N E E K I S ( ;I 5-ring > 6-ring = acyclic. It has been suggested that this indicates strain in the 5-membered rings.' The purpose of this article is to examine this suggestion more closely, in connection with a study of the polymerizability of cyclic m ~ n o m e r s . ~ - ~ The infrared carbonyl absorption frequency was measured for 46 cyclic compounds and reference acyclic compounds. The results are recorded in Table I. For comparison, the dilute solution data have been collected in Table 11. It can be seen t h a t the 6-membered ring compounds exhibit absorption maxima a t about the same frequency as the acyclic derivative, whereas the 5-membered rings absorb approximately 40 wave numbers higher, although there is considerable scatter. This agrees with previous work. I n compounds with two equivalent carbonyl groups one usually observed a splitting of the C=O band due to vibrational interaction. I n Appendix I we have shown that this splitting is symmetrical which means that the average of the two absorption frequencies would be a t the position of a single non-interacting carbonyl group in the same ring configuration. I n the case of molecules with two non-equivalent C=O groups each band can be identified with a particular group. In N-acetyloxazolidone, e.g., the 1795 cm.-l band is due to the C=O group in the ring while the 1711 cm.-l band is the absorption of the group in the side chain. As indicated by Bartlett and Stilesj5these data are explicable in terms of the hybridization of the carbon atom in the carbonyl group. As the ring is contracted, the ring bonds to this carbon atom become more p - in character, which confers more s (triple bond) character to the C-0 bond. This strengthening of the carbonyl bond will be reflected in a higher force constant and hence in an increased absorption frequency. Further, since s-bonds are more electronegative than p-bonds,6 the oxygen becomes less ready to donate electrons. This agrees with the results of the hydrogen bonding studies of Tamres and S e a r l e ~ . ~ (1) See I,. J. Rellamy, "The Infrared Spectra uf Complex hlolecules," Methuen Ltd., London, 1954, pp. 111, 115, 127, 128, 150; and other authoritative works on infrared spectra. (2) H. K. Hall, Ji.,THISJ O C R N A I . , 8 0 , 6404 (1958). (3) H. K. Hall, Jr., and A . K. Schneider, i b i d . , 80, 6509 (1958). (4) H. K. Hall, Jr., ibid., 8 0 , 6412 (1958). ( 5 ) P. D. Bartlett and M. Stiles, i b i d , 77, 2810 (1955). ( 6 ) A. D. Walsh, Disc. F O Y Q ~Soc., U ; ~ 2, 18 (1047). (7) M. Tamres a n d S. Searles. Abstracts of 1 3 1 s t A.C.S. Meeting, Miami, Fla., April, 1957, p. 10-R, and earlier papers.
The decrease in frequency for the i-meinbered rings is also explicable in these terms5 I n the earlier papers of this a cyclic carbonyl compound has been defined as unstable if i t can be isomerized to a polymer.s I t is apparent that no correlation exists between this measure of stability and the carbonyl frequencies. y-Butyrolactone and ethylene carbonate (5-membered rings) have never been polymerized, in contrast to the ready polymerization of the corresponding 6-membered rings. Both succinic and glutaric anhydrides fail to polymerize. Therefore the carbonyl frequency cannot be taken as a measure of strain. X further supporting piece of evidence is found in the spectrum of the internally hydrogen-bonded amino acid, N,N-diethylgly~ine~ 0 I
F\
CHn 0 \
(C&)?
This molecule exhibits the characteristic frequency of a 5-membered lactone (1786 cm.-l in C C t ) even though any strain could be relieved with negligible activation energy by breaking the H-bond or by dimerization. We conclude that the carbonyl frequency is determined by the hybridization or bond angle a t the carbonyl group. This angle can be varied considerably from 120" without straining the molecule appreciably. For example, the 0-C-0 angle in ethylene carbonateloa is 11l0,yet this compound does not polymerize. Nevertheless the variation in bond angle is sensitively reflected by the carbonyl frequency as indicated above. I n Appendix I1 we have roughly estimated the energy required for a bond angle deformation of 9" (ethylene carbonate) loa and 36" (propiolactone) .lob,c The values are 1.6 and 25.6 kcal.imole, respectively, the latter value being supported by the ease of polymerization of propiolactone. The strain in the molecule increases with the square while the frequency shift seems to increase roughly linearly with the bond angle deformation. l1 The knowledge of bond an(8) (a) G. E. Wheland, "Advanced Organic Chemistry," J n h n Wiley and Sons, Inc., h-ew York, S . Y., second edition. 192!1, p 3(j7. (b) F. S. Dainton and K. J . I v i n , O u a r t . Reus., 12, 82 (1058). (9) G. M. Barrow, THIS]OURNAL, 80, 86 (1958). (10) (a) C. J. Brown, Acta Crrst., 7, 92 (1954); (b) S . K(wak, J . If Goldstein and J. W. Simmons, J . Chem. Phys., 26, 1203 (1956); ( r l G. W. %'heland, "Resonance in Organic Chemistry," John Wiley a n d Sons, I n c . , A'ew York, S. Y . , 1!155, p . 720. (11) C,f J , 0. Haliurd, I. C A t i u P h > b . , 24, 830 (1956).
Dec. 5, 1958
6439
I N F R A R E D S P E C T R A .4ND S T R A I N I N C Y C L I C CARBOXYLS
TABLE I ,---Infrared
Compoundi
R i n g size Acy- Mono- Biclic cyclic cyclic Liquid
C=O stretching Solid Solid Nul01 in K B r mull
King s i z e Acy- M o n o - Biclic cyclic cyclic
1,lijtlid
li, 8
34.57
1655im)
S-Acetyl urea5 1749(s) I(Sl(m)
1749(s) lG49(ni)
1754(s) 1739(w) 1718(m) 1695(m)
3469
,
Imides 1654(s) 11; I 1 ( K ) Siiccininiide
I
177 1( w )
1771(m) 1770b
1 098(s)
1090(s)
3421
1714(s) 1F90(m) 1725'r' 1753(s) 1727(s)
1894b ii
C ) cloliexane-l.3dicarboximidr I
6.8
1697(s)
1701(s)
1tiC5(\Y)
lh05(w)
1704(s) 1682(s)
1705(s) 1671(m)
1742(m) 1730(s) 1718(s) 1730(m) 1714(s)
1756(w)
1758(w) 1702(s)
l733(s) 1715(m)
XX)
J3ic~-cl,,!2:2:l]heptane-2.R-dicarboximlde
>
1702iS)
320 L 3072
1-Methylimides S-Xlethyldi:rcetamide K-&lethylsuccinimide S-hIethylglutarimid? S- hlethylcyclohexane-l,3-dic:irbnrimide (SXIj
1710 (s) lf95(s) I
1708 17136(w) 16%(s)
' 1703(m)
1722 (w) 1072(s)
1729 (w) 108C(s) 17'8(n j 1679(s)
1721(s)
1722(w) lij70(s)
li 6, 8
S-Acetyl Iactaiiis S Acetyl-2-pyrrn-
lirliiione S -Acetyl-2-piperiilr~ne S -Acetyl-2-orc,heaameths leneimine S Acetyl-G-azahicyclo : 3 :2 : 1 ;octan-7-one X-Actryl-2-azabicyclo [ 2 : 2 : 21,ictan-3-une
1
c,
Ti, 7
1739(s) 1893(s) 1695
1715(s) 1702(>) 1701
1095
1701
I741(6)
17dG(s) 1F97( s )
1092(s)
1719(s) 1700(s)
1717(s) lGSG(s)
0 . fi
Urethans 3409
1731 1710
1675(m)
1783 1760a in CHCla 1743 1725
l642(s)
7,
i
1090
lCSl
1717
3250(s) 8478 8163(m) 3246(s) 3140(s) 3237(s) 3127(s)
3454 3457
3454
2932f.s)
285G(m)
)Ih C K
I , ,I 11 (1 Cll
7 -
C=O stretching Solid Solid Sujol in R B r mull
_I
si rC.1 (-11 lilX Y*),,,
7D
ire~1011
INFRARED SPECTRA AND STRAIN IN CYCLIC CARBOXYLS
Dec. 5, 1953
TABLE I (Continued)
Compoundi
Ring size Acy- hlono- Biclic cyclc cyclic Liquid
-Infrared
absorption frequencies in cm. -1.7 N-H stretching in CCl various concu. CHr stretching -lurethans S-.4cetyloxazolidone X-Acetyltetrab ydrooxazinone
x
5
x
6
1779 1698
1795 1711 1748
1738 1698
1707
Anhydrides Acetic
1832(s) 1824b 1762(s)
1822(s)
X
1758(m)
174Sb 5
Succinic
1858(w)
1872(m) 1865b li96(s) 1782b 1858(w) 1795(s) 1811(m) 1744(s) 1802b 1761b
1783(s) cis-1 ,2-Cyclohexanedicarborylic cir-1,3-Cyclohexanedicarboxylic Glutaric
5
1852(w) 178G(s) 1802(m) 1771(s)
6,s 6
S. Pinchas and D. Ben-Ishai, THISJOURNAL, 79,4099 (1957). R. 4. Jones and C . Sandorfy in “Technique of Organic R. Mecke Chemistry,” T’ol. IX, A. U’eicsbergcr, Ed., Iriterscience Publishers, Inc., Kew York, 5 .Y., 5956, pp. 444-459. C. L. arid R. Mecke, Jr., Che7n. Ber., 89, 343 (1956); R. Mecke, R. Mecke and A. Luttringhaus, ibid., 90, 975 (1957). Angell, Tmns. Faraday SOC.,5 2 , 1178 (1956); cf. C. J. Brown, .4cfn Ciyst., 7, 92 (1954). e R. Grave, A. Heinke and C. 78, 2173 (1056:. 0 P, Wilder, Jr., Sommer, Chctn. Bcr., 89, 1978 (1956). f P. D. Bartlett and B. E. Tate, THISJOURNAL, arid A. Winston, ibid., 78, 868 (1956). h C. F. H. Allen, T. Davis, D. 11:. Stewart and J. A . T‘anAllan, .I. Org. Chenz., 20, 306 (1955), and following papers. i Raman band. i The Roman numerals refer to paper I11 of this series.
TABLE I1 DIFFERESCE BETWEEN.4BSORPTION FREQUENCIES O F nMEMBERED RINGA?in O F ~ c v c L r c .4?iAI,oG I N c M . - ~ Clans of compouiid
n =
5
4
Rloriocyclics Lactams N-Methyl lactams 1.actones Carbonates Ketones Ureas Imides N-Methylimides X-Acetyl lactams Urethans Anhydrides Average
..
+31 +4F 40 72 f30 23 +36 5. +37 53 37
..
+83
+
+ + + +
+fig
..
.. ,.
.. ..
+ ..
+76*7
+37 rt 11
0
7
- 13
- 15
..
0
-8
+ 5 +31 0 23
,.
- 10
+
- 6
..
f 3 2
..
- 1
- 7 12 14
- 7
+-
+7 & 1 4
.. ..
-
~ 8 * 3
Bicyclics‘ Lactams Lactones Carbonates Ketones Ureas Imides N-Methylimides AT-Acetyl lactams Urethans Anhydrides
.. .. ..
..
..
+37 +29 , .
+35 ..
- 1 + 4 +16
..
+ 17 +- 418
Although the carbonyl frequencies fail to correlate with ring they in the same order as rates of alkaline hydrolysis for a given ring size. This is as expected12 because the position of absorption and rate of hydrolysis of X-C=O vary according to the electronic properties of X.13a We have compared the carbonyl frequencies of the bicyclic compounds listed in Table I with expectations based on conformational analysis. The bicyclo [3:3 : 1Inonane derivatives, namely, the 1,3-carbonate, imide, N-methylimide, urethan, urea and anhydride, are expected to occur in the two-chair form I
- 29 ..
+ 7
..
..
-
Y
\\
w I
~
Y Z‘ \
x& I1
Since this will be identical in conformation with the ., .. .. chair form of the 6-membered rings (II), the fre, , +38 +I1 .. quencies should be very similar. This is seen to be .. .. - 6 - 14 the case for the six compounds examined. .. . . 19 .. ~The bicyclo [3 : 2 : 2lnonane - derivatives should~ Average .. +35 A 3 + 4 * 10 -12 *13 occur as a slightly twisted two-boat form 111. The a Smallest ring is chosen. urethan, carbonate and lactam absorb a t the same gles would be very useful for a detailed study of the frequency as the 6-membered and 7-membered rings. latter correlation. The bicyclo [3 :2 : 1]derivatives should present a Since degree of strain, as evidenced by polymerizability (at equilibrium), does not correlate with the cyclopentane ring fused to a chair cyclohexane ring carbonyl frequency, the strain is not caused by an- (IV). The spectra of the lactone and lactam are gle deformation in 5- and 6-membered rings. quite close to those of the corresponding 5-memTherefore, hydrogen-hydrogen and hydrogen-lone bered compounds. (12) H. A. Staab, W. Otting and A. Ueberle, 2. Hleklrochem., 61, pair repulsions remain as the determining factors (1957). in whether or not a cyclic compound will polymer- 1000 (13) R. N. Jones and C. Sandorfy, Table I , ref. b; (a) pp. 472-473; ize. (h) p . 360. ..
..
..
H. K. HALL,J R . ,
6432
I
X-Y/z I11
IV V Finally, the bicyclo 12 : 2 : 21lactone and lactani
(two-boat form) are found to absorb a t the same frequencies as the corresponding six-membered rings (17). To sum up, the carbonyl frequency in a bridged bicyclic compound is that of its smallest ring, 7norcamphor being the only exception. It appears also that carbonyl groups in boat or chair forms of cyclohexane absorb a t about the same frequency. This agrees with our conclusion that H-H strain in a ring does not determine the carbonyl absorption frequency. Other Frequencies.--?jH stretching vibration frequencies in cyclic amides, ureas, imides and urethans also increase with decreasing ring size. They cannot be compared with the corresponding acyclic compounds which are in the trans form and absorb a t a frequency about 10-50 cm.-' higher than the large ring cis compound as shown in Table I. Similar band shifts have been observed €or the symmetric and antisymmetric CH2 stretching vibration frequencies. This effect has been noted previously in cyclic hydrocarbons,13b halohydrocarbons,13b and cyclic olefins.l 4 The difference between the two coniponents is about 60-80 cm. -I, but the values of the frequencies themselves are about 10-40 cm.-' higher for -7-memberedthan for 6-membered rings. It is interesting to notice that some compounds show a rather complex spectrum in the C-H stretching vibration region. S-Pyrrolidone has four bands (see Table I) which indicates that the skeletal bond angles are not the same for all three CH2 groups. 9 "strained" one causes the bands a t 2982 and 2922 c m - ' while a relatively "unstrained" CHz group would absorb a t 2950 and 2878 cm.-'. This conclusion could be checked if the bond angles were known. Experimental T h e prcp:iration rif t h e ccmpoumii i; ticmibed in preceding articles. -4, l5 The spectra were incasiiretl using a Perkin-Elmer singlebeam, double-pass, spectrophotometer equipped with a CaF? prism. Both K B r pellets and S u j o l mulls o f t h e solid compounds wcre prepurerl . T h e liquid compounds _.______~ ~~.
(14) P. I
