- Email: [email protected]
Some esters and carboxylic acids as donor molecules
J. Inorg. Nucl. Chem., 1961. Vol. 17, pp. 69 to 76. Pergamon P r e ~ Ltd.
SOME ESTERS A N D CARBOXYLIC ACIDS AS DONOR MOLECULES M. ZACKRISSON a n d I. LINDQVIST Institute of Chemistry, University of Oppsala, Uppsala, Sweden (Received 8 June 1960)
AIntract--Infra-red spectra of addition compounds of CHsCOOCtHs with SbCls, SnCI~ and SbCls and of addition compounds of HCOOCtHs with SnCI~ and SbCls have been obtained. The shifts in v(C=O) and v(C--O) have been studied and it is shown that the carbonyl oxygen atom functions as a donor atom. Infra-red spectra of the donor-accepter systems CHsCOOH-SbCIs, CHsCOOH-SnCI4, CH3COOH-SbCla and HCOOH-SbCI5 as pure mixtures or as solutions in 1,2-dichlorethane have also been investigated. In those cases v(C~---O)and v(O--H) have been studied. The spectra indicate that the carbonyl oxygen is the donor atom and that the strong accepter SbClt5 forms adducts with the monomeric acids, but that in interaction with the weaker accepters SnCI4 and SbCI, the hydrogen bonds are retained. ADDITION c o m p o u n d s with esters a n d carboxylic acids as d o n o r molecules a n d with c o m p o u n d s like SbCl 5 a n d SnCl 4 as acceptor molecules were early prepared.t1, z~ It has however n o t been established which o f the oxygen a t o m s functions as d o n o r a t o m , the c a r b o n y l oxygen a t o m or the ether oxygen a t o m . * PFEIFFERt~) has n o t q u e s t i o n e d the d o n o r f u n c t i o n o f the c a r b o n y l oxygen b u t BROWNt3~ e t al. have suggested that the ether oxygen is the d o n o r a t o m in c o m p o u n d s like B F a . R C O O R ' . It m u s t also be t a k e n into a c c o u n t that the same d o n o r molecule m a y react differently with different acceptor molecules. This paper deals with a n a t t e m p t to solve the p r o b l e m b y infra-red spectroscopy. The directions o f the shifts in the c a r b o n y l - b o n d stretching frequencies o f the d o n o r molecules after a d d i t i o n o f some acceptor molecules are studied. EXPERIMENTAL Chemico!s. SbCI6and SnCI4 were purified by distillation at reduced pressure. SbCla was purified by sublimation. CH,COOC2Ht, HCOOC2Ht, CHsCOOH, HCOOH and 1,2-C2H~CIIwere distilled. (b.p. 77°, 54°, 118°, I01 ° and 84°C). In what follows, the mole ratio of donor D to accepter A in a compound or mixture is indicated by the fraction following the molecular formulae--thus D: A x :y. Spectroscopic measurements. The infra-red spectra were recorded on a Perkin-Elmer Model 21 Spectrophotometer equipped with a NaCI prism. The spectra were run with the pure mixtures in thin layers between NaC1 plates and the solutions in 1,2-dichlorethane in a 0-05 mm cell. Ethyl acetate and ethyl formate as donor molecules. A compound SbCls.CHsCOOCtHs has been reported, c~ It is crystalline at room temperature but is very hygroscopic and cannot be ground in air. It was thus not possible to obtain infra-red spectra of the crystals without extra arrangements to avoid moisture. The compound was soluble, however, in an excess of ethyl a~tate and this mixture was studied. The mixtures of SnCI4 and SbCls with ethyl acetate are liquid at room temperature. No compounds have been isolated but the spectra indicate donor-accepter interaction. * The authors are indebted to the referee for drawing their attention to two recent papers which present evidenceof the same type for the donor function of the carbonyl oxygen in esters: (H. F. L^pPZRT,paper at International Conference on Co-ordination Chemistry, London, April 1959; abstract (on BXs-ethyl acetate adducts) published in Chem. Soc. Spec. Publ. No 13, p. 179; D. S. BYs'rRov and H. N. FILIMONOV, Dokl. Akad. Nauk, SSSR 131, 338 (1960). ~t~A. ROaL'NHEIMand W. LOt~WENST^MM,Dtsch. Chem. Ges. Bet. 35, 1116 (1902). itl p. P~IvVlm,Ltebigs Ann. 376, 285 (1910). ~s~H. C. BROWN,H. I. SCHLESINGERand A. B. BURO,J. Amer. Chem. Soc. 61,673 (1939). 69
70
M. ZACrdUSSONand I. LINDQVIST
Ethyl formate gave with SbC16 a very slightly soluble product with high melting point. This product was not further studied. SnCI,:HCOOC~H5 1:2 formed as hygroscopic crystals soluble in an excess of ethyl formate. SbCls: HCOOCsH6 1 : 1 was a liquid at room temperature. The carbonyl frequencies are situated in formates about 1720 cm -1 and in acetates close to 1740 cm-~. c,~ Studies of the frequency near 1200 cm -1 in formates and near 1250 cm -~ in acetates indicate the correlation of those bands with the C - - O stretching vibration. ",5) The spectra of the pure esters and the mixtures are given in Figs. 1 and 2. The spectra of the mixtures show in all cases decreases in the C - - O stretching mode and increases in the C---O stretching mode referred to the spectra of the pure esters. Because the composition of the mixtures did not correspond to addition compounds there would be no sense in comparing the magnitude of the shifts or in discussing the splitting of the bands. Acetic acid and formic acid as donor molecules. Addition compounds SbCIs.CH3COOH ~1~ and SbCla.CHsCOOW e~ have been reported. The compound with SbCls is crystalline and hygroscopic and was not studied. The compound with SbCla is liquid at room temperature. SnCI~ and acetic acid gave two liquid phases at mole ratio 1 :I, but at 1:2 they were miscible. SbCl5 gave hygroscopic crystals with formic acid and SnCI, gave two very viscous liquid phases which were not studied. SbCI::HCOOH 1:1 is a viscous liquid. The carbonyl frequencies are situated in the range 1705-1725 cm ~ "~ in saturated carboxylic acids. The spectra of SnCI4:CHsCOOH 1:2 and SbCla:CH3COOH 1 : 1 show a decrease in this frequency referred to the liquid acid. (Fig. 3). SbC13 did not change the spectrum of the formic acid. The spectra of carboxylic acids have peaks near 1400 cm -~ and 1300 cm -~ which are proposed to arise from C - - O vibration but coupled with OH in a plane deformation vibration. (7~ It is thus not possible to study the C - - O stretching vibrations in the acids in the same way as in the esters. The O - - H bond stretching frequency in the pure acids has a broad shape typical of hydrogen bonded OH-groups. ~8~ This shape is altered gradually and the center of the absorption shifts to higher frequencies. However there is no sharp peak indicating free OH-groups. (Fig. 3). The compounds with SbCIs and the acids were soluble in 1,2-dichlorethane. Spectra of such solutions (10 mole per cent) were obtained and for comparison also solutions of the earlier studied liquids. (Figs. 4 and 5. The frequencies given by WILMSStJgSTcy'I°> for the monomeric acids are indicated at the bottom in the figures). The range about 2950 cm -~ where the solvent absorbs is excluded in the figures. (Unfortunately neither the crystals with SbCl~ nor the liquids with SnCll and SbCIs were soluble in CCI~). In the spectra of the adducts with SbCIs, the C O band is split and the lower frequencies show a decrease referred to the polymeric acid. All shifts are negative referred to the monomeric acids. The main peak in the O - - H region is in both cases sharp and situated near the frequencies for the monomeric acids. It thus seems as if the hydrogen bonds are almost completely broken in the adduct formation. (The shoulders about 2500 cm -~ however may indicate a very weak association). The solution of SnCI~:CHaCOOH 1:2 in 1,2-dichlorethane gave about the same spectrum as the pure liquid mixture. The small shift in the C- :O stretching mode obtained with SbCI3 in acetic acid is not to be seen in the solution in 1,2-dichlorethane, but there is still a small shift in the O-- H stretching mode. DISCUSSION T h e shifts in the c a r b o n y l b o n d s t r e t c h i n g f r e q u e n c y in a d d i t i o n c o m p o u n d s with k e t o n e s on-is1 a n d a l d e h y d e s ~141 a n d d i f f e r e n t a c c e p t o r m o l e c u l e s h a v e b e e n s t u d i e d co L. J. BELLAMV,The lnfra-redSpectra of Complex Molecules. John Wiley, London, New York 0954). ~51j. K. W]LMSHURST,J. Mol. Spectrosc. I, 201 (19571. tl~ Gmelins Handbuch der anorganischen Chemic Antimon B(8. Auflag¢ p. 436. Verlag Chemic Liibcck (19491. ~71C. G. PIMEWrELand A. L. MCCLELLAN,The Hydrogen Bond, p. 122. Freeman & Co., San Francisco; London (I 960). ce) C. G. PIMEWrELand A. L. McCLrLLAN, The Hydrogen Bond, p. 102. Freeman & Co., San Francisco; London (19601. ~l~j. K. W[LMSHt~RST,J. Chem. Phys. 25, 1171 (1956). ct0~j. K. WILMSHURS'r,J. Chem. Phys, 25, 478 (19561. ~ ) B. P. S0sz and P. CHAI.ANDON,Heir. Chim. Acta 41, 1332 (1958). ~s~ M. Z^CKR~SSONand K. I. AI.D[N, Acta Chem. Scand. 14, 994 (19601. (:s~ A. TEgENIIq,W. FILIMONOVand D. BYSTROV,Z. Elektrochem. 62, 181 0958). ~to L. PAOLON~and G. B. I~ ,,R[N~-BgTr6LO,Gazz. Chim. Ital. 89, 1972 (19591.
71
Some esters and carboxylic acids as donor molecules
v(C-0)
t~ t~
Lc
tO o~
E ¢-
o I--
d
i
l
1800
I
I
1600
i
I
1400
I
I
120q
c~
FtG. 1.--Infra-red spectra in the range 5-8.5 L of: (a) CH3COOC2Hs, (b) SbCI3: C H a C O O C 2 H s : 2 , (c) SnCI4:CHsCOOCzH5 1:2, (d) SbCIs: C H a C O O C z H 5 1:2.
I
1
1800
I
~
1600
I
I
1400
t
I
1200
FIG. 2.--Infra-red spectra in the range 5 - 9 / ~ of: (a) H C O O C a H s , (b) SbCIs: H C O O C s H 5 1 : I, (c) SnC14:HCOOC2Hs 1:4.
cm-1
72
M . ZACBmmSON a n d I. LINDQVmT
o
"~'~
i.
o ~
v(c=0)
v (O-'H)
./
E ¢-
p
I
4000
I
I
3000
I
I
2000
I
I
1600 cm "1
FIG. 3.--Infra-red spectra in the range 2-6-5 # of: (a) C H s C O O H , Co) SbCIs: C H s C O O H 1 : 1 ; (c) SnCI 4: C H s C O O H I : 2.
~arlier by infra-red spectroscopy. In tho~e cases the adduct formation has always caused negative shifts. The same effect has been shown by Raman spectroscopy for hydrogen bond formation with alcohols and phenols, c~-1~ In the hydrogen bond formation with esters, the decrease in frequency for the C = O stretching mode is accompanied by an increase for the C--O stretching mode. tm These two effects are analogous to the decrease in the P--O bond frequency and the ~15~G. P. PURANIK, J. Chem. Phys. 26, 601 (1957). ts6~ G. P. PURANIX, Peoc. Indian. Acad. Sci. 37 A 499 (1959). c17~G. P. PURAbaX, Proc. Indian. Acad. Sci. 38 A 233 0953).
Some esters and carboxylic acids as donor molecules
o
v(0-H)
el I
4000
I
I
3000
I
I
2000
i
I
1600 cm "1
FIG. 4.--Infra-red spectra in the range 2-6-5 ~ of solutions in 1,2-dichlorethanc (10 tool© cent) of: (a) CHsCOOH, (b) SbCIs:CH=COOH 1:1, (c) SnCI4:CHsCOOH ½: l, (d) SbCls: CHsCOOH l : l, (¢) Frequencies of monomeric CHsCOOH given by WILIb~HUI~T{l).
73
74
M. ZACK~RBSONand I. LINDQVIST
increase in the P--CI bond frequencies found in POCI 3 due to interaction with a ~ e p t o r molecules.~ls~ If the ~ r b o n y l oxygen atom of an ester is the donor atom, a decrease in the C---q3
v(-
I
c-
.o q)
o I"-
I
4000
I
I
3000
I
1
2000
I
1600~m "1
FIG. 5.--Infra-red spectra in the range 2-6.5 p o f solutions in 1,2-dichlorethane (10 mole per cent) of: (a) H C O O H , (b) SbCIs: H C O O H 1 : 1, (c) Frequencies o f m o n o m e r i c H C O O H given by WILMSHURST110).
stretching mode can thus be expected, possibly accompanied by an increase in the C---O stretching mode. I f the ether oxygen atom is the donor atom, the formation of an adduct should cause electron withdrawal from the carbon atom in the C-----Obond. It can be assumed that the donor-acceptor interaction in this case should have the same effect on the (xs) P.-O. I~NELL, I. LINDQVISTa n d M. ZACKmSSON, Acta Chem. Scand. 13, 1159 (1959).
S o m e esters and carboxylic acids as d o n o r molecules
75
C ~ O bond frequency as a substitution of a more electronegative CFa-group for a CH3-grou p in the alcohol residue of the ester. It has been shown that such a substitution is followed by an increase in the C = O stretching mode. The actual frequencies are for example 1739 cm -1 in C3HTCOOC~H 5 and 1767 cm -1 in C3HTCOOCH 2CFz.C19} In adduct formation, electrons are withdrawn first from the donor atom and as a secondary effect from the adjacent atom. According to a description of inductive effects on chemical bondg recently published, c~°l a weakening of a bond can usually be expected when electrons are withdrawn from the most electronegative atom of the bond and a strengthening when electrons are withdrawn from the least electronegative atom. If the carbonyl oxygen of an ester is the donor atom, a weakening of the C = O bond thus can be expected and as a secondary effect a strengthening of the adjacent C - - O bond, in the spectra indicated as a decrease in the C - - O stretching mode and an increase in the C - - O stretching mode. Opposite shifts could be expected if the ether oxygen atom is the donor atom. The results presented here, the decrease in the C - - O stretching mode and the increase in the C - - O stretching mode, clearly indicate that, with the acceptor molecules studied, the carbonyl oxygen in the esters is the donor atom. This conclusion is thus based both upon analogy with earlier experiments and upon the discussion of inductive effects on chemical bonds. The discussion of the effects found with the acids is complicated by the existence of intermolecular hydrogen bonds. Chains of molecules are shown to be present in liquid formic acid, but the structure of liquid acetic acid is not clarified, t~l~ The spectra of the acids in the concentrated solutions in 1,2-dichlorethane show that hydrogen bonds are still present, but it is not important whether the molecules are chains or cyclic dimers, as in the gaseous state. The negative shifts in the C-----O stretching mode show that the carbonyl oxygen is the donor atom in the acids too. The withdrawal of electrons from the carbonyl oxygen in adduct formation ought to weaken the hydrogen bond to this atom and to strengthen the adjacent O - - H bond of the system O . . . . H - - O . t~9} This effect should be more obvious in interaction with strong acceptor molecules. The small shifts with SbCI 3, earlier known as a weak acceptor molecule, ~12,1s) indicate also in this case very weak interaction. SnC14 as a stronger acceptor give larger shifts which remained even at dilution with a solvent. The spectra, however, still indicate association with hydrogen bonds. SbC15, evidently the strongest acceptor studied here, causes such a strong interaction that the hydrogen bonds are broken. It thus seems as if SbCI5 forms an adduct mainly with a monomeric acid, while the weaker acceptor molecules form adducts mainly with the dimeric or polymeric acid. HASSELet al. have found that the oxygen in acetone forms two bonds with the very weak acceptor molecule Br~t ~ and that in the compound 2CHaOH.Br 2 both hydrogen bridges and halogen bridges to the same oxygen atom are present, taa~ It is suggested tt*~ G. RAPPAPORT, M. HXUPTSCHEIN,J. F. O'BRIEN and R. FILLER,./'. Arner. Chem. Soc. 75, 2695 (1953). ~ze~I. LINDQVlST,Nova Acta Reg. Soc. Sci. Ups. (IV) 17. No. 11 (1960). ~2~ C. G. PlUENTELand A. L. MCCLELLAN, The Hydrogen Bondp. 16. Freeman & Co. San Francisco; London (1960). ~n~ O. HASSELand K. O. STR~UME, Acta Chem. Scand. 13, 275 (1959). ~zs~ O. HASSI~L,Svensk Kern. Tidskrift 72, 88 (1960).
76
M. Z~CKR~ON and I. I.~DQV~ST
that this is possible because o f the two lone pair electrons available on the oxygen atom. Our results support such a suggestion provided that the bonds are not too strong. It is of course possible that the solid compounds should give other results and spectroscopic studies o f solid compounds and particularly structure determinations would be o f great interest.
Acknowledgement--A grant from the Swedish Natural Science Research Council is gratefully acknowledged. We wish to thank Professor G. H~.c~3for all facifitiesput at our disposal and Mr. S. 30HANSSONfor helpful assistance.
SOME ESTERS A N D CARBOXYLIC ACIDS AS DONOR MOLECULES M. ZACKRISSON a n d I. LINDQVIST Institute of Chemistry, University of Oppsala, Uppsala, Sweden (Received 8 June 1960)
AIntract--Infra-red spectra of addition compounds of CHsCOOCtHs with SbCls, SnCI~ and SbCls and of addition compounds of HCOOCtHs with SnCI~ and SbCls have been obtained. The shifts in v(C=O) and v(C--O) have been studied and it is shown that the carbonyl oxygen atom functions as a donor atom. Infra-red spectra of the donor-accepter systems CHsCOOH-SbCIs, CHsCOOH-SnCI4, CH3COOH-SbCla and HCOOH-SbCI5 as pure mixtures or as solutions in 1,2-dichlorethane have also been investigated. In those cases v(C~---O)and v(O--H) have been studied. The spectra indicate that the carbonyl oxygen is the donor atom and that the strong accepter SbClt5 forms adducts with the monomeric acids, but that in interaction with the weaker accepters SnCI4 and SbCI, the hydrogen bonds are retained. ADDITION c o m p o u n d s with esters a n d carboxylic acids as d o n o r molecules a n d with c o m p o u n d s like SbCl 5 a n d SnCl 4 as acceptor molecules were early prepared.t1, z~ It has however n o t been established which o f the oxygen a t o m s functions as d o n o r a t o m , the c a r b o n y l oxygen a t o m or the ether oxygen a t o m . * PFEIFFERt~) has n o t q u e s t i o n e d the d o n o r f u n c t i o n o f the c a r b o n y l oxygen b u t BROWNt3~ e t al. have suggested that the ether oxygen is the d o n o r a t o m in c o m p o u n d s like B F a . R C O O R ' . It m u s t also be t a k e n into a c c o u n t that the same d o n o r molecule m a y react differently with different acceptor molecules. This paper deals with a n a t t e m p t to solve the p r o b l e m b y infra-red spectroscopy. The directions o f the shifts in the c a r b o n y l - b o n d stretching frequencies o f the d o n o r molecules after a d d i t i o n o f some acceptor molecules are studied. EXPERIMENTAL Chemico!s. SbCI6and SnCI4 were purified by distillation at reduced pressure. SbCla was purified by sublimation. CH,COOC2Ht, HCOOC2Ht, CHsCOOH, HCOOH and 1,2-C2H~CIIwere distilled. (b.p. 77°, 54°, 118°, I01 ° and 84°C). In what follows, the mole ratio of donor D to accepter A in a compound or mixture is indicated by the fraction following the molecular formulae--thus D: A x :y. Spectroscopic measurements. The infra-red spectra were recorded on a Perkin-Elmer Model 21 Spectrophotometer equipped with a NaCI prism. The spectra were run with the pure mixtures in thin layers between NaC1 plates and the solutions in 1,2-dichlorethane in a 0-05 mm cell. Ethyl acetate and ethyl formate as donor molecules. A compound SbCls.CHsCOOCtHs has been reported, c~ It is crystalline at room temperature but is very hygroscopic and cannot be ground in air. It was thus not possible to obtain infra-red spectra of the crystals without extra arrangements to avoid moisture. The compound was soluble, however, in an excess of ethyl a~tate and this mixture was studied. The mixtures of SnCI4 and SbCls with ethyl acetate are liquid at room temperature. No compounds have been isolated but the spectra indicate donor-accepter interaction. * The authors are indebted to the referee for drawing their attention to two recent papers which present evidenceof the same type for the donor function of the carbonyl oxygen in esters: (H. F. L^pPZRT,paper at International Conference on Co-ordination Chemistry, London, April 1959; abstract (on BXs-ethyl acetate adducts) published in Chem. Soc. Spec. Publ. No 13, p. 179; D. S. BYs'rRov and H. N. FILIMONOV, Dokl. Akad. Nauk, SSSR 131, 338 (1960). ~t~A. ROaL'NHEIMand W. LOt~WENST^MM,Dtsch. Chem. Ges. Bet. 35, 1116 (1902). itl p. P~IvVlm,Ltebigs Ann. 376, 285 (1910). ~s~H. C. BROWN,H. I. SCHLESINGERand A. B. BURO,J. Amer. Chem. Soc. 61,673 (1939). 69
70
M. ZACrdUSSONand I. LINDQVIST
Ethyl formate gave with SbC16 a very slightly soluble product with high melting point. This product was not further studied. SnCI,:HCOOC~H5 1:2 formed as hygroscopic crystals soluble in an excess of ethyl formate. SbCls: HCOOCsH6 1 : 1 was a liquid at room temperature. The carbonyl frequencies are situated in formates about 1720 cm -1 and in acetates close to 1740 cm-~. c,~ Studies of the frequency near 1200 cm -1 in formates and near 1250 cm -~ in acetates indicate the correlation of those bands with the C - - O stretching vibration. ",5) The spectra of the pure esters and the mixtures are given in Figs. 1 and 2. The spectra of the mixtures show in all cases decreases in the C - - O stretching mode and increases in the C---O stretching mode referred to the spectra of the pure esters. Because the composition of the mixtures did not correspond to addition compounds there would be no sense in comparing the magnitude of the shifts or in discussing the splitting of the bands. Acetic acid and formic acid as donor molecules. Addition compounds SbCIs.CH3COOH ~1~ and SbCla.CHsCOOW e~ have been reported. The compound with SbCls is crystalline and hygroscopic and was not studied. The compound with SbCla is liquid at room temperature. SnCI~ and acetic acid gave two liquid phases at mole ratio 1 :I, but at 1:2 they were miscible. SbCl5 gave hygroscopic crystals with formic acid and SnCI, gave two very viscous liquid phases which were not studied. SbCI::HCOOH 1:1 is a viscous liquid. The carbonyl frequencies are situated in the range 1705-1725 cm ~ "~ in saturated carboxylic acids. The spectra of SnCI4:CHsCOOH 1:2 and SbCla:CH3COOH 1 : 1 show a decrease in this frequency referred to the liquid acid. (Fig. 3). SbC13 did not change the spectrum of the formic acid. The spectra of carboxylic acids have peaks near 1400 cm -~ and 1300 cm -~ which are proposed to arise from C - - O vibration but coupled with OH in a plane deformation vibration. (7~ It is thus not possible to study the C - - O stretching vibrations in the acids in the same way as in the esters. The O - - H bond stretching frequency in the pure acids has a broad shape typical of hydrogen bonded OH-groups. ~8~ This shape is altered gradually and the center of the absorption shifts to higher frequencies. However there is no sharp peak indicating free OH-groups. (Fig. 3). The compounds with SbCIs and the acids were soluble in 1,2-dichlorethane. Spectra of such solutions (10 mole per cent) were obtained and for comparison also solutions of the earlier studied liquids. (Figs. 4 and 5. The frequencies given by WILMSStJgSTcy'I°> for the monomeric acids are indicated at the bottom in the figures). The range about 2950 cm -~ where the solvent absorbs is excluded in the figures. (Unfortunately neither the crystals with SbCl~ nor the liquids with SnCll and SbCIs were soluble in CCI~). In the spectra of the adducts with SbCIs, the C O band is split and the lower frequencies show a decrease referred to the polymeric acid. All shifts are negative referred to the monomeric acids. The main peak in the O - - H region is in both cases sharp and situated near the frequencies for the monomeric acids. It thus seems as if the hydrogen bonds are almost completely broken in the adduct formation. (The shoulders about 2500 cm -~ however may indicate a very weak association). The solution of SnCI~:CHaCOOH 1:2 in 1,2-dichlorethane gave about the same spectrum as the pure liquid mixture. The small shift in the C- :O stretching mode obtained with SbCI3 in acetic acid is not to be seen in the solution in 1,2-dichlorethane, but there is still a small shift in the O-- H stretching mode. DISCUSSION T h e shifts in the c a r b o n y l b o n d s t r e t c h i n g f r e q u e n c y in a d d i t i o n c o m p o u n d s with k e t o n e s on-is1 a n d a l d e h y d e s ~141 a n d d i f f e r e n t a c c e p t o r m o l e c u l e s h a v e b e e n s t u d i e d co L. J. BELLAMV,The lnfra-redSpectra of Complex Molecules. John Wiley, London, New York 0954). ~51j. K. W]LMSHURST,J. Mol. Spectrosc. I, 201 (19571. tl~ Gmelins Handbuch der anorganischen Chemic Antimon B(8. Auflag¢ p. 436. Verlag Chemic Liibcck (19491. ~71C. G. PIMEWrELand A. L. MCCLELLAN,The Hydrogen Bond, p. 122. Freeman & Co., San Francisco; London (I 960). ce) C. G. PIMEWrELand A. L. McCLrLLAN, The Hydrogen Bond, p. 102. Freeman & Co., San Francisco; London (19601. ~l~j. K. W[LMSHt~RST,J. Chem. Phys. 25, 1171 (1956). ct0~j. K. WILMSHURS'r,J. Chem. Phys, 25, 478 (19561. ~ ) B. P. S0sz and P. CHAI.ANDON,Heir. Chim. Acta 41, 1332 (1958). ~s~ M. Z^CKR~SSONand K. I. AI.D[N, Acta Chem. Scand. 14, 994 (19601. (:s~ A. TEgENIIq,W. FILIMONOVand D. BYSTROV,Z. Elektrochem. 62, 181 0958). ~to L. PAOLON~and G. B. I~ ,,R[N~-BgTr6LO,Gazz. Chim. Ital. 89, 1972 (19591.
71
Some esters and carboxylic acids as donor molecules
v(C-0)
t~ t~
Lc
tO o~
E ¢-
o I--
d
i
l
1800
I
I
1600
i
I
1400
I
I
120q
c~
FtG. 1.--Infra-red spectra in the range 5-8.5 L of: (a) CH3COOC2Hs, (b) SbCI3: C H a C O O C 2 H s : 2 , (c) SnCI4:CHsCOOCzH5 1:2, (d) SbCIs: C H a C O O C z H 5 1:2.
I
1
1800
I
~
1600
I
I
1400
t
I
1200
FIG. 2.--Infra-red spectra in the range 5 - 9 / ~ of: (a) H C O O C a H s , (b) SbCIs: H C O O C s H 5 1 : I, (c) SnC14:HCOOC2Hs 1:4.
cm-1
72
M . ZACBmmSON a n d I. LINDQVmT
o
"~'~
i.
o ~
v(c=0)
v (O-'H)
./
E ¢-
p
I
4000
I
I
3000
I
I
2000
I
I
1600 cm "1
FIG. 3.--Infra-red spectra in the range 2-6-5 # of: (a) C H s C O O H , Co) SbCIs: C H s C O O H 1 : 1 ; (c) SnCI 4: C H s C O O H I : 2.
~arlier by infra-red spectroscopy. In tho~e cases the adduct formation has always caused negative shifts. The same effect has been shown by Raman spectroscopy for hydrogen bond formation with alcohols and phenols, c~-1~ In the hydrogen bond formation with esters, the decrease in frequency for the C = O stretching mode is accompanied by an increase for the C--O stretching mode. tm These two effects are analogous to the decrease in the P--O bond frequency and the ~15~G. P. PURANIK, J. Chem. Phys. 26, 601 (1957). ts6~ G. P. PURANIX, Peoc. Indian. Acad. Sci. 37 A 499 (1959). c17~G. P. PURAbaX, Proc. Indian. Acad. Sci. 38 A 233 0953).
Some esters and carboxylic acids as donor molecules
o
v(0-H)
el I
4000
I
I
3000
I
I
2000
i
I
1600 cm "1
FIG. 4.--Infra-red spectra in the range 2-6-5 ~ of solutions in 1,2-dichlorethanc (10 tool© cent) of: (a) CHsCOOH, (b) SbCIs:CH=COOH 1:1, (c) SnCI4:CHsCOOH ½: l, (d) SbCls: CHsCOOH l : l, (¢) Frequencies of monomeric CHsCOOH given by WILIb~HUI~T{l).
73
74
M. ZACK~RBSONand I. LINDQVIST
increase in the P--CI bond frequencies found in POCI 3 due to interaction with a ~ e p t o r molecules.~ls~ If the ~ r b o n y l oxygen atom of an ester is the donor atom, a decrease in the C---q3
v(-
I
c-
.o q)
o I"-
I
4000
I
I
3000
I
1
2000
I
1600~m "1
FIG. 5.--Infra-red spectra in the range 2-6.5 p o f solutions in 1,2-dichlorethane (10 mole per cent) of: (a) H C O O H , (b) SbCIs: H C O O H 1 : 1, (c) Frequencies o f m o n o m e r i c H C O O H given by WILMSHURST110).
stretching mode can thus be expected, possibly accompanied by an increase in the C---O stretching mode. I f the ether oxygen atom is the donor atom, the formation of an adduct should cause electron withdrawal from the carbon atom in the C-----Obond. It can be assumed that the donor-acceptor interaction in this case should have the same effect on the (xs) P.-O. I~NELL, I. LINDQVISTa n d M. ZACKmSSON, Acta Chem. Scand. 13, 1159 (1959).
S o m e esters and carboxylic acids as d o n o r molecules
75
C ~ O bond frequency as a substitution of a more electronegative CFa-group for a CH3-grou p in the alcohol residue of the ester. It has been shown that such a substitution is followed by an increase in the C = O stretching mode. The actual frequencies are for example 1739 cm -1 in C3HTCOOC~H 5 and 1767 cm -1 in C3HTCOOCH 2CFz.C19} In adduct formation, electrons are withdrawn first from the donor atom and as a secondary effect from the adjacent atom. According to a description of inductive effects on chemical bondg recently published, c~°l a weakening of a bond can usually be expected when electrons are withdrawn from the most electronegative atom of the bond and a strengthening when electrons are withdrawn from the least electronegative atom. If the carbonyl oxygen of an ester is the donor atom, a weakening of the C = O bond thus can be expected and as a secondary effect a strengthening of the adjacent C - - O bond, in the spectra indicated as a decrease in the C - - O stretching mode and an increase in the C - - O stretching mode. Opposite shifts could be expected if the ether oxygen atom is the donor atom. The results presented here, the decrease in the C - - O stretching mode and the increase in the C - - O stretching mode, clearly indicate that, with the acceptor molecules studied, the carbonyl oxygen in the esters is the donor atom. This conclusion is thus based both upon analogy with earlier experiments and upon the discussion of inductive effects on chemical bonds. The discussion of the effects found with the acids is complicated by the existence of intermolecular hydrogen bonds. Chains of molecules are shown to be present in liquid formic acid, but the structure of liquid acetic acid is not clarified, t~l~ The spectra of the acids in the concentrated solutions in 1,2-dichlorethane show that hydrogen bonds are still present, but it is not important whether the molecules are chains or cyclic dimers, as in the gaseous state. The negative shifts in the C-----O stretching mode show that the carbonyl oxygen is the donor atom in the acids too. The withdrawal of electrons from the carbonyl oxygen in adduct formation ought to weaken the hydrogen bond to this atom and to strengthen the adjacent O - - H bond of the system O . . . . H - - O . t~9} This effect should be more obvious in interaction with strong acceptor molecules. The small shifts with SbCI 3, earlier known as a weak acceptor molecule, ~12,1s) indicate also in this case very weak interaction. SnC14 as a stronger acceptor give larger shifts which remained even at dilution with a solvent. The spectra, however, still indicate association with hydrogen bonds. SbC15, evidently the strongest acceptor studied here, causes such a strong interaction that the hydrogen bonds are broken. It thus seems as if SbCI5 forms an adduct mainly with a monomeric acid, while the weaker acceptor molecules form adducts mainly with the dimeric or polymeric acid. HASSELet al. have found that the oxygen in acetone forms two bonds with the very weak acceptor molecule Br~t ~ and that in the compound 2CHaOH.Br 2 both hydrogen bridges and halogen bridges to the same oxygen atom are present, taa~ It is suggested tt*~ G. RAPPAPORT, M. HXUPTSCHEIN,J. F. O'BRIEN and R. FILLER,./'. Arner. Chem. Soc. 75, 2695 (1953). ~ze~I. LINDQVlST,Nova Acta Reg. Soc. Sci. Ups. (IV) 17. No. 11 (1960). ~2~ C. G. PlUENTELand A. L. MCCLELLAN, The Hydrogen Bondp. 16. Freeman & Co. San Francisco; London (1960). ~n~ O. HASSELand K. O. STR~UME, Acta Chem. Scand. 13, 275 (1959). ~zs~ O. HASSI~L,Svensk Kern. Tidskrift 72, 88 (1960).
76
M. Z~CKR~ON and I. I.~DQV~ST
that this is possible because o f the two lone pair electrons available on the oxygen atom. Our results support such a suggestion provided that the bonds are not too strong. It is of course possible that the solid compounds should give other results and spectroscopic studies o f solid compounds and particularly structure determinations would be o f great interest.
Acknowledgement--A grant from the Swedish Natural Science Research Council is gratefully acknowledged. We wish to thank Professor G. H~.c~3for all facifitiesput at our disposal and Mr. S. 30HANSSONfor helpful assistance.