Characteristic absorption bands and frequency shifts in the infrared spectra of naturally-occurring long-chain ethers, esters and ether esters of glycerol and various diols

Chem. Phys. Lipids 2 (1968) 114-128 © North-Holland Publ. Co., Amsterdam

C H A R A C T E R I S T I C A B S O R P T I O N BANDS AND F R E Q U E N C Y SHIFTS IN T H E INFRARED SPECTRA OF N A T U R A L L Y - O C C U R R I N G L O N G - C H A I N

ETHERS, ESTERS AND ETHER ESTERS OF G L Y C E R O L AND VARIOUS DIOLS* W. J. BAUMANN and H. W. ULSHOFER University of Minnesota, The Hormel Institute, Austin, Minnesota, 55912, U.S.A.

Received 30 June 1967 The infrared absorption spectra of long-chain ethers, esters and ether esters of glycerol, 1,2-ethanediol and propanediols are recorded and interpreted. The influence of the environment of alkoxy, acyloxy and hydroxy groups on the absorption frequencies of these functional groups is discussed. The infrared spectra of ethers 1) have as yet received comparatively little attention. We have made available several series of alkoxylipids and other long-chain compounds and have initiated a systematic study of the infrared spectra of these synthetic lipids. A comparison of a large number of these closely related long-chain compounds may reveal the factors influencing frequencies and frequency shifts of specific infrared absorption bands, such as inductive effects, resonance phenomena, hydrogen bonding and the presence of different conformational structures. In addition, solution spectra which are relatively unspecific for compounds having different chain lengths 2,3) also constitute an important and convenient means for identifying and characterizing classes of naturally-occurring lipids 4, 5) which usually consist of a variety of homologous compounds. These spectra exhibit bands which usually can be assigned more reliably to particular vibrations of carbon chains and functional groups than bands in spectra of compounds in the crystalline state. The present communication reports interpretations of infrared spectra of solutions of a wide range of long-chain ethers, esters and ether esters of * Naturally-occurring diol lipids, H. For the previous paper in this series, see ref. 9. This investigation was supported in part by PHS research grant No. AM 11255 from the National Institutes of Health, Public Health Service.

I N F R A R E D SPECTRA OF ETHERS A N D ESTERS

115

mono-, di- and trihydric alcohols. The band positions, characteristic frequency shifts, and the O - - H frequencies as related to concentration are described and correlations between structure and band position are discussed.

Experimental The compounds investigated were pure alkyl glycerol-(1) ethers 6), dialkyl glycerol ethers v), trialkyl glycerol ethers v), alkyl glycerides 8) and various ethers, esters and ether esters of 1,2-ethanediol and propanediolsg). Most substances were saturated and had carbon chains ranging from Ci0 to Czo. The spectra of some compounds having up to two isolated double bonds per carbon chain were also measured. The infrared spectra were recorded with a Perkin-Elmer Spectrophotometer, Model 21, equipped with sodium chloride optics. The spectra were taken at 25°C in 10% carbon disulfide solutions; in the ranges of 24002000 cm 1 and 1650-1 400 cm -1 tetrachloroethylene was used. The path length of the sodium chloride cell was 0.1 mm. The O - - H absorption frequencies of some representative derivatives of di- and trihydric alcohols were measured at concentrations varying from 0.002 M to 0.5 M. The accuracy of measurement of the absorption frequencies given is _ 1 cm -~, and above 2000 cm -~ + 2 cm -~. The frequencies were corrected by recording some of the most characteristic bands of polystyrene film (3026, 1 603, 906 cm -~) on the same charts as the spectra to be considered. The reported positions of the bands of the spectra are mean values from up to twenty recordings.

Results and discussion The ester bands

The acyloxy groups of the ester lipids investigated (table 1) give rise to two very strong and characteristic absorption frequencies. The band in the range between 1743-1734 c m - i is due to the C = O stretching vibration 10-17), the band between 1 176-1 162 cm -~ is usually ascribed to a C - - O vibration of the ester group 18). The highest carbonyl frequency measured at 1 743 cm -~ is that of triglycerides (I), the lowest is found for simple long-chain esters (wax esters, X). The C = O frequency is decreased as acyl groups of the triglyceride moiety are replaced by alkoxy groups. The carbonyl band in the spectra of alkyldiglycerides (III) is found at 1739 cm-1, and is shifted to 1736 cm-1 in the case of dialkyl glycerides (VI). Similar frequency shifts are observed for diol

116

W. J. BAUMANNAND H. W. ULSHOFER TABLE 1 Infrared spectra of some ester lipids'~.

No.

Ester

Ester bands (cm 1) C O C O max

Additional characteristic bands (cm 1)t,

CH2--O--CO--R 1 ~'

I

CH--O

[

CO--R

1743(s)

1461(sh), 1420(w), 1358(sh), 1339(sh), l160(s) 1305(w), 1234(m), ll17(m), ll04(sh), 1033(w), 1003(w), 953(w), 823(w)

1743(s)

1419(w), 1347(w), 1272(w), 1255(w), l162(m) 1243(w), ll17(m), ll00(sh), 960(w), 865(w), 802(w)

1739(s)

1420(w), 1301(w), 1258(w), 1245(m), 1162(s) 1117(s), 1096(sh), 1024(w), 962(w), 907(w), 862(w), 803(w)

1738(s)

1423(w), 1358(w), 1300(w), 1243(w), 1168(s) ll15(w), 1096(w), 1048(w), 948(w), 860(w)

1737(s)

3610(w), 1425(w), 1176(s) 1292(w), 1075(m),

1736(s)

1420(w), 1350(w), 1325(w), 1300(w), 1174(m) 1245(w), 1119(s), 1020(w), 860(w), 804(w)

CH2--O--CO--R CH2--O--CO--R lid

I CH~--O

CO--R

CHz--O--CO--R II[ ~'

I

CH--O--CO--R

I

CH2--O--R CH2--O--CO--R IV d

I

CH2

I

CH2 O CO--R CHz--O--CO--R Vd

I

CH2

L

CH2--OH

3525(m), 1393(w), 1247(w), 1056(m),

3470(sh), 1362(w), ll19(w), 979(w),

1723(sh), 1347(w), 1098(w), 860(w)

CH2--O--CO--R VI e

I

CH--O--R

I

CH2--O--R VII d

CHz--O--CO--R [ CH2--OH

1736(s)

3605(w), 3500(s), 1727(sh), 1424(w), 1174(s) 1348(w), 1245(w), ll16(m), 1080(s), 1038(w), 945(w), 882(w)

VIII d

CH2--O--CO--R { CH2--O--R

1736(s)

1420(w), 1352(w), 1299(w), 1259(w), 1174(s) 1248(w),l132-1120(s),1046(w),865(w), 802(w)

1734(s)

1175(s) 1421(s), 1356(w), 1298(w), 1250(w), 1120(s), 860(w)

1734(s)

1422(w), 1354(w), 1301(w), 1247(m), 1173(s) 1231(m), ll13(w), 1070(w), 880(w)

CH2 O--CO--R

I

IX d

CH2 CH2 O--R

Xr

R--O--CO--R

INFRARED SPECTRA OF ETHERS AND ESTERS

117

derivatives and when acyloxy g r o u p s are replaced by h y d r o x y groups. The C = O b a n d o f the diesters o f 1,3-propanediol (IV) at 1738 c m - 1 is f o u n d at 1737 c m - 1 in the spectra o f m o n o e s t e r s V, a n d is shifted to 1734 c m - 1 for the ether ester IX. The a b s o r p t i o n s o f the c o r r e s p o n d i n g derivatives o f 1,2-ethanediol are observed at 1743 c m - 1 and at 1736 c m - ~ (VII, VIII). The C = O frequencies o f glycol diesters (ll) or alkyl diglycerides ( I l l ) are as high, or a l m o s t as high as those o f triglycerides. Structures in which two or m o r e vicinal ester g r o u p s are located at the short-chain moiety, so they have strong influence on each other, show a c a r b o n y l frequency which can be a d e q u a t e l y explained by the presence o f the " c a r b o n y l o x y " f o r m o f the ester group, whereas t h a t o f wax esters (X) is r a t h e r explained by p a r t i c i p a tion o f an " e n o l i c " canonical form. The effect o f a h y d r o x y or an a l k o x y g r o u p on the c a r b o n y l frequency o f the a d j a c e n t ester g r o u p is similar to t h a t o f an acyloxy group, but it is significantly weaker. The influence o f a functional g r o u p on the ester g r o u p and, thus, on the ester bands is greatly diminished as the distance between the g r o u p s is increased. Esters o f 1,3-propanediol (IV, IX) exhibit c a r b o n y l a b s o r p t i o n s at a lower frequency t h a n do the c o r r e s p o n d i n g 1,2-ethanediol derivatives (II, VIII). However, the m o n o e s t e r s o f 1,3-propanediol (V, 1 737 c m - 1 ) a n d o f 1,2-ethanediol (VII, 1736 c m - 1) do n o t a b s o r b a c c o r d i n g to this rule. The deviation can be explained by differing stabilities o f the i n t r a m o l e c u l a r h y d r o g e n b o n d s f o r m e d between ester and h y d r o x y groups. The shoulders observed near 1 723 c m - i a n d 1727 c m - ~ in the spectra o f V a n d VII, only, can be assigned to a C = O a b s o r p t i o n lowered by h y d r o g e n bonding. The m o s t intensive C - - O b a n d o f the ester lipids I-X (table 1) is observed in the range o f 1 176-1 160 cm - I . Its frequency is altered roughly in inverse p r o p o r t i o n to the c o r r e s p o n d i n g c a r b o n y l vibration. T w o a d d i t i o n a l peaks in the regions between 1250-1234 cm -1 and 1 119-1 l l 3 cm -~ were considered to be characteristic for C - - O vibrations o f long-chain esters Z,19, z°).

a Relative absorption intensities are given as follows : (s) -- strong, (m) medium, (w) weak, (sh)--shoulder. b The following bands associated with vibrations of the aliphatic chains occur in all the spectra quoted: 2930-2925 cm 1 (s), and 2855-2850 cm -1 (s) vC--H of --CH2--; near 1470 cm a t~C--H of --CH2--; 1448-1445 cm i (sh) asymmetrical and 1380-1375 cm 1 (w) symmetrical 6C--H of C--CHa; 719-717 cm -1 (m) C - - H rocking vibration of --(CHz)n--. The bands occurring due to unsaturation are not listed in the tables: the c i s - - C - - H stretching vibration near 3010 cm -1, the trans-C--H out-of-plane deformation near 965 cm i, and the cis-C--H out-of-plane deformation near 695 cm -1 (sh). e Pure triglycerides were obtained from The Hormel Institute. See ref. 9. e See ref. 8. f Pure wax esters were a gift from Dr. H. H. O. Schmid, The Hormel Institute.

| 18

W. J. BAUMANN AND H. W. ULSH()FER

Recently, however, Jones 21) has found evidence that, at least in methyl palmitate, the bands in the range of 1 176-1 160 cm-1 and 1250-1 234 c m are due to C - - O skeletal modes coupled with the deformation of the ~-CH 2 groups of the long-chain moiety, whereas the band near 1 119-1 113 cm -1 appears to be associated with a C - - C skeletal vibration. Three such bands occur at 1234, 1 160 and 1 117 c m - 1 in the spectra of triglycerides. They are also present in the spectra of all diol monoesters, diesters and ether esters listed in table 1, but in alkoxylipids IIl, VI, VIII and IX the band near 1 119-1 113 c m - 1 is submerged beneath the stronger C - - O absorption of the ether group. The ether band

The characteristic ether band of the alkoxylipids studied (table 2) occurs in the range between 1 132-1 110 c m - 1, and is assigned to an asymmetrical C - - O stretching vibration z2, 23). The frequency depends on the environment of the ether gro up, i.e., the degree of polarization of the C - - O bond. Stronger polarization results in a lower absorption frequency. The C - - O frequency found at 1116 cm -1 in the spectra of long-chain dialkyl ethers (XVIII) occurs at 1 120 c m in dialkyl ethers of 1,2-diols (XIII, XIV) and at 1 121 cm -1 in trialkyl ethers of glycerol (Xll). The accumulation of alkoxy groups along the short-chain moiety shifts the ether band to higher frequencies. This shift is characteristic for vicinal ether groups. It is not observed in the spectra of dialkyl ethers of 1,3-propanediol (XVII). In these compounds the two functional groups are methylene-interrupted, and thus diether XVII absorbs at 1 116 c m - 1 as do dialkyl ethers (XV|II). Ester groups influence adjacent ether groups similarly as do ether groups, but the effect is somewhat weaker. The C - - O absorption band, which occurs at 1 121 cm-~ in the spectra of trialkyl glycerol ethers (Xll), is shifted to 1 119 cm-1 for dialkylglycerides (V I), and to 1 l l 7 cm-1 for alkyldiglycerides (ll|) as alkyl residues in XII are replaced with acyl functions. In contrast the C - - O ether bands of the d iol ether esters V lll (l 132-1 120 cm-~) and IX (1 120 cm -1) have higher frequencies than the bands of the corresponding diol diethers X I I I (1120 cm -1) and XVII (1 116 cm 1). In addition, the band of VIII is broad indicating that the formation of a definitely oriented structure may be hindered. In the spectra of alkoxylipids having free hydroxy groups the ether band occurs at a position higher or lower than that of dialkyl ethers (XVIII). Basically, hydroxy groups seem to strengthen an adjacent ether bond, but less than do ester groups. Thus the broad band of glycol ether ester (VIII) at 1 132-1 120 cm -1 occurs at I 122 cm -1 in the case of glycol monoethers (XI)24,25). Similarly, the bands of the corresponding derivatives of 1,3-

INFRARED SPECTRA OF ETHERS AND ESTERS

119

TABLE2 Infrared spectra of some ether lipids ~. No.

Ether band C--O (cm 1)

Ether CH2--O--R

VIII e

X[ c

broad

1736(s),1420(w),1352(w),1299(w),1259(w), 1248(w),l174(s),1046(w),865(w)

1122(s)

3590(m),3470(w),1350(w),1296(w),1238(w), 1205(w),1056(m),885(w),852(w)

1121(s)

1355(w), 1328(w), 1304(w), 1262(w), 863(w), 803(w)

1120(s)

1353(w), 1322(sh), 1300(w), 1242(w), 974(w), 866(w)

1120(s)

1345(w), 1307(w), 1262(w), 1024(w), 920(w), 802(w)

1120(s)

1734(s),1421(w),1356(w), 1298(w),1250(w), l175(s),860(w)

1120(s)

3560(sh), 3400(s) broad, 1321(w), 1301(w), 1250(w), 1056(s), 1045(s), 980(w), 930(w), 858(w), 802(w)

1119(s)

1736(~, 1420(w),1350(w),1325(w),1300(w), 1245(w), 1174(m), 1020(w), 860(w), 804(w)

1118(s)

3575(w), 3440(sh), 1322(w), 1303(w), 1260(w), 1220(w), ~ ll00(sh), 966(w), 800(w)

1117(s)

1739(s), 1420(w), 1301(w), 1258(w), 1245(m), 1162(s), 1096(sh), 1024(w),962(w), 907(w), 862(w), 803(w)

1132-1120(s)

[

CHz--O--CO

R

CH2--O--R I CH2--OH

Additional characteristic bands (cm-1) b

CH~--O--R

I

XII a

CH--O--R CH2--O--R

XllI~

CHz--O--R I CH2--O--R CH2--O--R

I

XIV e CH--O--R

I

CH3 CH~--O--R

I

IX c

CH2

I

CHz--O--CO--R CHz--O--R XV f

CH--OH

I

CH2--OH

cHi~o--R I

VI g

CH--O--R

I

CH2--O--CO

R

CH~--O~R I

XVI a CH--OH

I

CHe--O--R CH2--O--R III g

I I

CH--O--CO--R CH2--O--CO--R

W. J. BAUMANNAND H. W. ULSHOFER

120

TABLE2 No,

Ether

Ether band C--O (cm ~)

(continued) Additional characteristic bands (cm ~)~'

CH2--O--R

I

XVil ~ CH2

I

I 116(s)

1355(w), 1302(w), 1262(w), 860(w), 800(w)

l 116(s)

1300(w), 1258(w), 860(w), 803(w)

I l14(s)

3575(sh), 3425(s), 1398(w), 1349(w), 1302(w), 1203(w), 1056(s), 1007(sh),970(w), 897(w), 817(w)

I l14(s)

3575(m), 3530(sh), 3475(sh), 1398(w), 1348(w), 1300(w), 1203(w), 1041(m), 978(w), 855(w)

1110(s)

3610(w), 3530(m), 3445(sh), 1486(w), 1351(sh), 1302(w), 1266(w), 1078(s), 1033(sh), 965(w), 860(w), 809(w)

CH2--O--R XVIII" R - - O - - R

C.2--OH I

XIX i C H - - O - - R

I

CH2--OH

XX"

CH2--O R I CH--O R [ CH2--OH

I XXI ~' CH~

I

CH2--OH a See table I. b See table 1. c See ref. 9. d See ref. 7. e 1,2-Dioctadecyloxypropane (XIV) was prepared from the corresponding dialkyl glycerol-(l,2) ether XX by reaction with methanesulfonyl chloride in pyridine, and subsequent lithium aluminum hydride reduction of the methanesulfonate formed. Calcd. for C39H8002: C, 80.62; H, 13.88. Found: C, 80.20; H, 13.82. M.P. 45-46'. See ref. 6. g See ref. 8. " Samples of long-chain dialkyl ethers were gifts from Dr. H. H. O. Schmid, The Hormel Institute. i Pure alkyl glycerol-(2) ethers were gifts from Dr. F. Snyder, Oak Ridge Institute of Nuclear Studies.

p r o p a n e d i o l IX a n d X X ! a p p e a r at 1 120 c m - ' a n d I l l0 c m -~, respectively. This large decrease in the a b s o r p t i o n f r e q u e n c y is due to the f o r m a t i o n o f s t r o n g h y d r o g e n b o n d s . W i t h the ether o x y g e n serving as electron d o n o r , the C - - O b o n d is l o o s e n e d to a degree d e p e n d i n g m a i n l y o n the stability o f the associate f o r m e d . T h u s , the ether f r e q u e n c y is largely a f u n c t i o n o f the g e o m etry o f the ether a n d h y d r o x y g r o u p s i n v o l v e d . G l y c o l m o n o e t h e r (XI) shows a very sharp a b s o r p t i o n b a n d at 1 122 c m -1. T h e C - - O b a n d o f the

INFRARED SPECTRA OF ETHERS AND ESTERS

121

corresponding derivative of 1,3-propanediol (XXI), however, appears at ! 110 c m - ', as this compound only forms a reasonably strong intramolecular hydrogen bond without ring strain. The spectra of monoalkyl6. 26) and dialkyl ethersT, 27) of glycerol (XV, XIX, XVI, XX) show bands in the same region as do ethers of 1,2-ethanediol and 1,3-propanediol. ~-Alkyl glycerol ethers (XV, 1 120 c m - ' ) exhibit the C - - O band at a lower frequency than do glycol monoethers (X[, 1 122 c m - ' ) probably due to hydrogen bonding between the ether oxygen and the ~'hydroxy group. In the spectra of 1,2-dialkyl glycerol ethers (XX) the C - - O band occurs at 1 114 c m - ' , i.e., the frequency for dialkyl glycol ethers (XII1) found at 1 120 c m - ' is lowered by intramolecular hydrogen bondings as it is in 1,3-propanediol monoethers (XXI). 1,3-Dialkyl glycerol ethers (XVI, 1 118 cm- ') show a C - - O frequency higher than that of dialkyl ethers (XVIII, 1 116 c m - ' ) , as they are unable to form such strong intramolecular bonds.

The O - - H stretching absorptions ! n the range of 3 650-3 400 cm- ' usually several O - - H bands are encountered 2s, 29) due to various intramolecular or intermolecular associations. The O - H frequency of a hydroxy group serving as proton donor is significantly shifted to a lower value, and the magnitude of the spectral shift (A VoH) of the fundamental O - - H stretching band is considered to be a measure of the strength of the hydrogen bond 30, 3l). The effect of hydrogen bonding on the acceptor is usually smaller. The O - - H bands of compounds with functional groups capable of taking part in hydrogen bonding show higher molar absorptivities, higher integrated intensities and greater half band widths. The effect is more pronounced for intermolecular associations than it is for intramolecular ones 32). The O - - H frequencies which were measured in 10~ solutions are listed in tables 1 and 2 (bands above 3000 c m - ' ) . The O - - H absorption frequencies of some compounds were also measured at various molar concentrations in order to study the influence of concentration on the frequencies and intensities of the O - - H bands. The O - - H absorption curves of O-octadecanoyl-l,2-ethanediol (VII) and O-octadecanoyl-l,3-propanediol (V), as depicted in fig. l, exhibit two welldefined peaks. The stretching vibrations of the free O - - H groups at 3602 c m - ' (VII) and 3 606 cm-~ (V) occur at relatively low values which may be partially due to association with the solvent33). The second band at 3505 c m - i (Vll), and 3 529 c m - ' (V), respectively, at concentrations lower than 0.125 M can be assigned to intramolecularly bonded O - - H groups, as extrapolations of the absorptivity of both bands to zero concentration do not pass through the origin. At concentrations higher than 0.125 M the bands

122

W. J. BAUMANN

AND H. W. ULSH()FER

for intramolecular hydrogen bonding become obscured by the strong intermolecular ones. The absorption frequencies associated with bonded O - - H of glycol monoester VII are shifted to a much greater extent (A VoH=97)* than are the bands of the 1,3-propanediol derivative V (AvoH=77, see table 3). This may be explained by the greater stability of the smaller sevenmembered ring formed by the glycol monoester VII. The probability of ring formation, however, is greater in the case of 1,3-propanediol monoester V as 00 ~---

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3400

3200

t

3600

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3400

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3200

cm"

Infrared absorption curves of 1-O-octadecanoyl-l,2-ethanediol (A) and l-O-octadecanoyl-l,3-propanediol (B) at various concentrations.

* Values reported for short-chain glycol monoesters are Avo,a=81--137cm 1 (see ref. 34. Corresponding data for 1,3-propanediol derivatives have not been published.

INFRARED SPECTRA OF ETHERS AND ESTERS

123

can be concluded from its lower absorbance of the fundamental O - - H band (fig. 1). The assumption that both free O - - H groups exhibit very similar specific intensities appears to be reasonable. Steric hindrance or the presence of stable conformational structures may reduce the amount of hydrogen bonding in glycol monoester VII. The infrared absorption curves of glycol monoether XI (fig. 2A) show sharp peaks at 3 589 cm-1 at all concentrations measured. The band can be correlated with the O - - H stretching frequency of the intramolecularly bonded O - - H group. The band position is in good agreement with the frequency reported for 2-methoxy-ethanol (3 593 cm-i)35). The corresponding band of the 1,3-propanediol derivative XXI is very strong and occurs at 3531 cm-1. The position of the bands for intermolecularly bridged O - - H groups, which is somewhat dependent on concentration, occurs in the spectra of glycol monoether XI in the range between 3465 cm -1 (0.50 M) and 3490 cm-1 (0.03 M). In the spectra of propanediol monoether XXI the weak band is shifted from ~3420 cm -1 (0.50 M) to 3465 - I (sh, 0.125 M). Monoether XI shows a strong peak even at 0.03 M concentration due to well-defined intermolecular associates. The fundamental O - - H stretching absorption of XXI is found at 3605 cm - l , but it is missing in the spectra of glycol monoethers XI. The fundamental O - - H absorption of 1,2-ethanediol monoethers is the least intense of all monoethers of short-chain diols reported 32, 34, 36 39). This fact has been correlated with the conformations possible. The ratio of intramolecularly bonded to "free" hydroxy groups decreases in the sequence 2-methoxy-ethanol (2) 3-methoxy-propanol (2/7) 4-methoxybutanol (4/23)*. In a nonpolar solvent such as carbon disulfide long-chain compounds exhibit a particularly high degree of association, i.e., very weak fundamental O - - H bands**. Propanediol monoether XXI exhibits a A yon value of 74 c m - 1 indicating the formation of a relatively stable intramolecular associate. A quantitative evaluation of the A VoH values of the ethers XI and XXI in comparison with those of the esters VII and V (table 3) is not possible as different proton accepting sites are involved. However, the values show that in the case of the esters the 1,2-ethanediol derivative and, in the case of the ethers, the 1,3-propanediol derivative forms the more stable intramolecular associates. The frequencies of the intramolecularly bonded O - - H groups of monoalkyl glycerol ether XV (3570 cm -1) and dialkyl glycerol-(1,2) ether XX * In the same series of monoethers a marked increase in hydrogen bond strength, i.e., in 3VOHvalues (30 to 86 to 180 cm 1, in CC14) has been reported (see ref. 32, 40). ** This result could be partially due to an unsatisfactory resolution of the instrument. For glycol monoether XI a AvoH value of about 30 cm-1 could be expected.

BAUMANN AND H. W. ULSH()FER

W.J.

124

(3573 cm -~) are almost identical, though not necessarily due to identical types of bonding (fig. 3). The comparison of the A VoH values of some shortchain diol monoethers with those of their parent diols41, 42) shows that replacement of one alkoxy group by a hydroxy group 40, 43) does not significantly change the type and strength of the intramolecular bonds involved. By virtue of its two donor groups capable of forming a variety of associations, the monoalkyl ethers of glycerol XV gives rise also to a very intensive band for intermolecularly bonded O--H, which is much broader than that of diether XX. The absorption curves of glycerol ethers XV and XX do not exhibit fundamental O--H bands. O0 -O03M 0 06M

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/'

3600 Fig. 2.

Infrared

i

3400

i

3200

-10

i/

1 i

-06

-15

i

3600

absorption curves of 2-octadecyloxy-ethanol propanol (B) at various concentrations.

i

3400

3200 (A)

and

crn -1

3-octadecyloxy-

125

INFRARED SPECTRA OF ETHERS AND ESTERS O0 0016M

/S /

/

0 03M

0 03M

/

006 M

/

/ /

/ 0 06M

\

0125M

/-

/

z <

025M

g

/

,/

< -02

?

0125M

05M

-0.4

-06

-08

-1.0

/ -15 [

3600

i

3/-,00

l

3200

I

3600

1

3400

i

3200

c rn"1

Fig. 3. Infrared absorption curves of 3-dodecyloxy-],2-propanediol ( A ) a n d 3-octadecyloxy-2-dodecyloxy-propanol (B) at various concentrations.

Bands in the range of1100-1000 cm- 1

Considerable caution is necessary in correlating bands in this range. The C--O stretching frequency of primary alcohols, being the most prominent band in this region44), is quite sensitive with respect to variations in the environment of the hydroxy group, the occurrence of hydrogen bonding, and the presence of conformationally different species. In addition, C--C skeletal vibrations of the long-chain moieties, as well as C---O skeletal vibrations coupled with ~-CHz deformations of ether and ester groups appear in the same frequency range ~1).

XI XXI XV

2-Octadecyloxy-ethanol 3-Octadecyloxy-propanol 3-Dodecyloxy-1.2-propanediol

3605 -

3602 3606

V o t f ~ree

3573

3589 3531 3570

3505 3529

VOH bond

74

97 77

AVOH

Intramolecular hydrogen bonding b

3455 (0.50 M); ~ 3495 (0.25 M) 3500 (0.50 M); 3520, 3470 (0.25 M); 3465 (sh, 0.125 M, 0.062 M) 3465 (0.50 M); 3485 (0.25 M, 0.125 M) 3420 (0.50 M); 3465 (sh, 0.25 M, 0.125 M) 3560 (sh), ~3400 (0.25 M, 0.125 M); 3435 (0.062 M); 3415, 3470 (sh, 0.031 M); 3510 (sh, 0.016 M, 0.008 M) d 3465, 3505 (sh) (0.50 M); 3520, 3480 (sh) (0.25 M); 3530 (sh, 0.125 M--0.016 M)

VOH bond

Intermolecular hydrogen bonding e

a The spectra were taken at 25°C in carbon disulfide solution. b At concentrations higher than 0.125 M the band can be broadened due to intermolecular hydrogen bonding. ~' The bands are broad and present only at the molar concentrations given. d At a concentration of 0.50 M a very broad band occurs.

3-Octadecyloxy-2-dodecyloxy-propanol XX

VII V

NO.

1-0-Octadecanoyl-1.2-ethanediol 1-0-Octadecanoyl-1.3-propanediol

Compound

TABLE 3 O - - H Absorption frequencies [cm-1] of some hydroxylipids a

©

.r.

> Z

7' Z

>

INFRARED SPECTRA OF ETHERS AND ESTERS

127

The highest C - - O frequencies o f alcoholic groups observed are those o f the m o n o e s t e r s o f 1,2-ethanediol (VII, 1080 cm -1) a n d 1 , 3 - p r o p a n e d i o l (V, 1075 c m - l ) , a n d o f 1,3-propanediol m o n o e t h e r (XXI, 1078 c m - 1 ) . These c o m p o u n d s also exhibit the highest A VoH values (table 3). Evidently, h y d r o g e n b o n d i n g has much stronger influence on the C - - O b a n d t h a n any inductive effect. G l y c o l m o n o e t h e r XI which, as was d e m o n s t r a t e d , is not c a p a b l e o f f o r m i n g s t r o n g i n t r a m o l e c u l a r bridges shows a C - O frequency o f 1056 c m - 1 which is a l m o s t identical with t h a t of o c t a d e c a n o l (1054 c m - 1, in CS2). The a p p e a r a n c e o f the strong b a n d at 1 056 c m - 1 in the spectra o f alkyl glycerol-(1) ethers (XV), alkyl glycerol-(2) ethers ( X I X ) a n d 1,3-prop a n e d i o l monoesters (V) is p r o b a b l y associated with the C - O v i b r a t i o n o f " f r e e " (or i n t e r m o l e c u l a r l y b o n d e d ) h y d r o x y groups. In the case o f c o m p o u n d s XV (1045 c m - 1) a n d X X ( 1 0 4 l cm -1) an e l e c t r o n - a t t r a c t i n g effect o f the g r o u p s a d j a c e n t to the p r i m a r y h y d r o x y g r o u p s m a y explain the lower b a n d positions. The C - - O frequencies o f the s e c o n d a r y alcoholic g r o u p s in the spectra o f XV a n d X V I are o b s c u r e d by the stronger C - - O b a n d s o f the ether groups.

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29) 30) 31) 32) 33) 34) 35) 36) 37) 38) 39) 40) 41) 42) 43) 44)

1770

Inc., and

Soc.