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Spectra and structure of binary azeotropes
Spectrochimica Acta Part A 66 (2007) 976–978
Spectra and structure of binary azeotropes III. Acetone–n-hexane M.R. Jalilian ∗ Department of Chemistry, Faculty of science, Al-Zahra University, Vanak, Tehran, Iran Received 27 February 2006; accepted 1 May 2006
Abstract Acetone and n-hexane form an azeotrope with the mole ratio 2:1. As a result of this phenomenon, some characteristic vibrational modes in FT-IR and some chemical shifts in 1 H NMR spectra changes. The amount of these changes is an indication of the extension of interaction between two components and their orientation in unit structure of the cluster. FT-IR and 1 H NMR spectra of pure substances and their azeotrope were recorded and spectral changes analyzed. Based on mole ratio of constituents, boiling point depressions, spectral changes in fundamental frequencies, and chemical shifts, unit structure of the azeotrope were deduced. © 2006 Published by Elsevier B.V. Keywords: Acetone; n-Hexane; Azeotrope; Unit structure; Cluster; FT-IR; 1 H NMR spectra; Frequency changes; Chemical shift changes
1. Introduction Acetone and n-hexane form a homogenous minimum boiling point azeotrope. No IR data have been reported for this azeotrope, also 1 H NMR spectrum has not been investigated previously. The FT-IR and 1 H NMR spectra of pure liquids and azeotrope, contains useful information on intermolecular forces and orientation of the constituents of the azeotrope in the unit structure of the cluster. There have been many investigations of acetone–n-hexane mixtures [1–18], but there has not been any specific attempt to determine unit structure of the foregoing azeotrope and/or its IR and NMR spectra. During the last decades, FT-IR and FT-NMR has appeared to be powerful tools, not only for analytical purposes, but also for gaining insight into the structure of molecules and/or their complexes. Both vibrational frequencies and chemical shifts are solvent (environment) dependent [19]; from these shifts, information can be obtained about possible molecular interactions and accompanying electronic displacements. It has been shown that the chemical shifts as well as vibrational frequencies depend highly on the nature of the solvent or accompanying molecules ∗
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[20–22]. Therefore, FT-IR spectra from 600 to 4000 cm−1 and 1 H NMR spectra of the pure acetone, pure n-hexane and their azeotrope were recorded and analyzed. The carbonyl group in acetone is able to attract proton donating solvents which causes electronic displacements in the molecules. This effect can be determined by IR spectroscopy, as well as 1 H NMR spectroscopy. Changes are found in the carbonyl stretching frequency ν(CO) in infrared and Raman spectroscopy [23,24]. ν(CO) and thus the strength of the CO bond, decrease with increasing attraction between oxygen and hydrogen atoms. However, this effect is accompanied by some changes in strength of the CC and CH bonds in the acetone [23]. The present work is an attempt to correlate the proton resonance and IR vibrational data of acetone and n-hexane in their azeotrope to determine the unit structure of this minimum boiling azeotrope. 2. Experimental The chemical acetone and n-hexane were purchased from Merck. Acetone, 99.9%, with less than 0.1% water and n-hexane were distilled for further purification and measuring their boiling points at laboratory pressure (733 mmHg). The binary azeotrope were prepared by adding appropriate quantities of the liquids by volume using micropipettes, mixture
M.R. Jalilian / Spectrochimica Acta Part A 66 (2007) 976–978
977
Table 1 Vibrational frequencies of acetone, n-hexane and their azeotrope together with frequency shifts in azeotrope with respect to corresponding band in pure substances Vibrational mode
ν¯ (cm−1 ) acetone
ν¯ (cm−1 ) n-hexane
ν¯ (cm−1 ) azeotrope
¯ν (cm−1 )
C O str. CH3 s.def. C C a.str. CH3 a.rock. CH3 s.rock. CH3 str. CH3 str. CH3 a.def. CH2 rock. CH2 rock.
1714.4 1363.4 1222.7 1093.4 906.4 – – – – –
– – – – – 2966 2933.2 1467.6 904.5 725.1
1716.4 1361.5 1220.7 1091.5 902.5 2960.2 2927.4 1459.9 902.5 727
+2 −1.9 −2 −1.9 −3.9 −5.8 −5.8 −7.7 −2 +1.9
Only those frequencies which have been observable and/or have measurable frequency shift are listed.
were fractionally distilled and a center fraction boiling at 45 ◦ C was chosen from the same multiplate column. The mid-infrared spectra of pure acetone, pure n-hexane, and azeotrope was recorded on a Nicolet model 800 Fourier transform interferometer equipped with a high intensity Globar source, Ge/KBr beam splitter, and DTGS detector. A resolution of 0.09 cm−1 was utilized and 500 interferograms were taken of both the sample and empty reference cell. The H NMR spectra of acetone, n-hexane, and their azeotrope were obtained on a Bruker 500 MHZ model FT-NMR. 3. Results and discussion Acetone and n-hexane form a homogenous azeotrope with the mole ratio 2:1. Pure acetone boils at 51.5 ◦ C and n-hexane boils at 64 ◦ C. The azeotrope boils at 45 ◦ C (at 733 mmHg) showing a minimum boiling azeotrope. 6.5 ◦ C decrease in boiling point of acetone and a 19 ◦ C decrease in boiling point of n-hexane shows positive deviation from ideal behavior with a maximum vapor pressure in the mixture. Boiling point depression for n-hexane is three times more than depression for acetone, showing that interaction forces between azeotrope molecules are less than that of original constituents. It is obvious that acetone with high dipole moment (9.3 × 10−30 C m) has very strong attractive intermolecular forces with neighboring ones. Upon vaporization or dilution in an inert solvent, the acetone molecules separate from each
Fig. 1. FT-IR spectra of acetone (—), n-hexane (· · ·) as well as their azeotrope (- - -) at room temperature in the region 600–4000 cm−1 .
Table 2 Proton chemical shift data for acetone, n-hexane, and their azeotrope Substance
Proton in
δ (ppm)
δ (ppm) in azeotrope
δ (ppm)
n-Hexane
CH3 CH2
0.8924 1.2624
0.8892 1.3037
−0.0032 0.0413
Acetone
CH3
2.1010
2.0606
−0.0404
other and attraction forces between electron deficient carbon of one carbonyl and electronegative oxygen atom of the adjacent molecule decreases and C O moiety becomes shorter in the length, consequently C O stretching mode shifts toward higher frequencies. Whereas CH3 bending modes shift to the lower frequency because strong attraction forces between methyl group of one molecule and oxygen of the other molecule have been diminished, and these modes need less energy to take place [25]. Observed FT-IR spectra of C O (Fig. 1) show that above anticipation is correct. Upon azeotrope formation C O stretching mode shows blue shift, while, CH3 deformation modes show red shift (Table 1). Assuming new attraction forces between methylene groups of n-hexane and lone pair electrons on the oxygen atoms of acetone molecules, we expect red shift for all bending modes of hexane methylene groups. Here, a holding force has been established between CH2 groups and oxygen, therefore, CO bending mode of carbonyl moiety facilitates bending modes of mentioned groups (Table 2).
Fig. 2. 1 H NMR spectra of acetone (A), n-hexane (B), and corresponding azeotrope (C).
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M.R. Jalilian / Spectrochimica Acta Part A 66 (2007) 976–978
Fig. 3. Proposed structure for unit structure of the acetone–n-hexane azeotrope.
As it was pointed out in our previous works [25,26], correlation between peak position of FT-IR and H NMR spectra is a great approach for determination of the situation and orientation of molecules in mixtures, especially in azeotropes. The benefit of 1 H NMR is based upon the relatively significant change in the diamagnetic shielding of proton upon the formation of a hydrogen bond or location of a proton in the vicinity of a high electron density media, which causes a significant decrease in the effective electron density near the proton in a covalent bond. When an electronegative element, such as oxygen, goes in the neighborhood of protons of methylene groups of n-hexane, the oxygen atom deshields the hydrogen atoms causing the δ (1 H) chemical shift to occur at higher ppm values than the δ (1 H) chemical shift exhibited by n-hexane. In the case of acetone, we predict shielding for protons, because lone pair electrons on the adjacent acetone molecule have been vanished, therefore protons of CH3 groups on acetone are not deshielded anymore, and as the result of azeotrope formation, electron clouds are centered more than before around proton nucleus. The 1 H NMR spectra of acetone, n-hexane, and their azeotrope are shown in Fig. 2. The displacement of chemical shifts (Table 2) exactly confirms the foregoing discussion. 4. Conclusion Based on boiling point depression, mole ratio of components in the azeotrope, blue shift for CO stretching mode, red shift of all CH2 , as well as CH3 groups of n-hexane in the azeotrope, and 1 H NMR chemical shift displacements in the azeotrope, we suggest the following structure (Fig. 3) as the unit structure of the
acetone–n-hexane azeotrope in the cluster. This structure provides all mentioned changes in the FT-IR and H NMR spectra. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
G. Ryland, J. Am. Chem. Soc. 22 (1899) 38. F.Z. Zawidzki, Z. Phys. Chem. 35 (1900) 129. G. Ryland, Chem. News 81 (1900) 15. M. lecat L’Azeotropisme, Lamartin, Bruxelles. (1918). M.Z. Postemer, Elektrochem 40 (1934) 493. M. Postemer, Monatsh. Chem. 65 (1934) 1. M. Lecat, Tables Azeotropiques, vol. 1 L’Autear, Bruxelles (1949). Hermann, Hartmann, Schlafer, H. Ludwig, Z. Elektochem. 54 (1950) 337. V.M. Chulanvski, Dokl. Akad. Nauk. S.S.S.R. 101 (1955) 457. A.E. Lutski, D.S. Bidnaya, Opt. Spektrosk. 2 (1957) 160. T.V. Yakovleva, A.G. Maslennikova, A.A. Perov, Opt. Spektrosk. 10 (1) (1961) 131. G.S. Denisov, Opt. Spektrosk. 11 (1961) 428. Dertooz, M. Nasielski, J. Bull. Soc. Chim. Belges. 70 (1961) 794. G.E. Maciel, G.C. Ruben, J. Am. Chem. Soc. 85 (23) (1963) 3903. L.H. Horsley, Azeotropic Data III, American Chemical Society, Washington, DC, 1973. D. Petkovic, M. Pavlovic, S. Milank, A.V. Glos, Hem. Drus. Beograd. 43 (3) (1987) 31. R.A. Nyquist, Appl. Spectrosc. 44 (3) (1990) 433. J.M. Gmelhing, J. Krafczyk, K. Fischer, Fluid Phase Equilibr. 103 (1995) 51. P. Laszlo, in: J.W. Emsely, J. Feeney, L.H. Sute liffe (Eds.), Progress in NMR Spectroscopy, vol. III, 1967, p. 231. W.H. de Jeu, H. Angad Gaur, J. Smidt, Rec. Trav. Chim. 4 (1965) 1621. H.A. Christ, P. Diehl, Helv. Phys. Acta 36 (1963) 170. G.E. Maciel, J.J. Natterstad, J. Chem. Phys. 42 (1965) 2752. L.J. Bellamy, R.L. Williams, Trans. Faraday Soc. 55 (1959) 14. P.G. Puranik, Proc. Ind. Acad. Sci. 37A (1953) 499. M.R. Jalilian, L. Alibabaei, Spectrochim. Acta, Part A 62 (2005) 322. M.R. Jalilian, Spectrochim. Acta, Part A, in press.
Spectra and structure of binary azeotropes III. Acetone–n-hexane M.R. Jalilian ∗ Department of Chemistry, Faculty of science, Al-Zahra University, Vanak, Tehran, Iran Received 27 February 2006; accepted 1 May 2006
Abstract Acetone and n-hexane form an azeotrope with the mole ratio 2:1. As a result of this phenomenon, some characteristic vibrational modes in FT-IR and some chemical shifts in 1 H NMR spectra changes. The amount of these changes is an indication of the extension of interaction between two components and their orientation in unit structure of the cluster. FT-IR and 1 H NMR spectra of pure substances and their azeotrope were recorded and spectral changes analyzed. Based on mole ratio of constituents, boiling point depressions, spectral changes in fundamental frequencies, and chemical shifts, unit structure of the azeotrope were deduced. © 2006 Published by Elsevier B.V. Keywords: Acetone; n-Hexane; Azeotrope; Unit structure; Cluster; FT-IR; 1 H NMR spectra; Frequency changes; Chemical shift changes
1. Introduction Acetone and n-hexane form a homogenous minimum boiling point azeotrope. No IR data have been reported for this azeotrope, also 1 H NMR spectrum has not been investigated previously. The FT-IR and 1 H NMR spectra of pure liquids and azeotrope, contains useful information on intermolecular forces and orientation of the constituents of the azeotrope in the unit structure of the cluster. There have been many investigations of acetone–n-hexane mixtures [1–18], but there has not been any specific attempt to determine unit structure of the foregoing azeotrope and/or its IR and NMR spectra. During the last decades, FT-IR and FT-NMR has appeared to be powerful tools, not only for analytical purposes, but also for gaining insight into the structure of molecules and/or their complexes. Both vibrational frequencies and chemical shifts are solvent (environment) dependent [19]; from these shifts, information can be obtained about possible molecular interactions and accompanying electronic displacements. It has been shown that the chemical shifts as well as vibrational frequencies depend highly on the nature of the solvent or accompanying molecules ∗
Tel.: +98 21 8044054; fax: +98 21 8047861. E-mail address: [email protected].
1386-1425/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.saa.2006.05.009
[20–22]. Therefore, FT-IR spectra from 600 to 4000 cm−1 and 1 H NMR spectra of the pure acetone, pure n-hexane and their azeotrope were recorded and analyzed. The carbonyl group in acetone is able to attract proton donating solvents which causes electronic displacements in the molecules. This effect can be determined by IR spectroscopy, as well as 1 H NMR spectroscopy. Changes are found in the carbonyl stretching frequency ν(CO) in infrared and Raman spectroscopy [23,24]. ν(CO) and thus the strength of the CO bond, decrease with increasing attraction between oxygen and hydrogen atoms. However, this effect is accompanied by some changes in strength of the CC and CH bonds in the acetone [23]. The present work is an attempt to correlate the proton resonance and IR vibrational data of acetone and n-hexane in their azeotrope to determine the unit structure of this minimum boiling azeotrope. 2. Experimental The chemical acetone and n-hexane were purchased from Merck. Acetone, 99.9%, with less than 0.1% water and n-hexane were distilled for further purification and measuring their boiling points at laboratory pressure (733 mmHg). The binary azeotrope were prepared by adding appropriate quantities of the liquids by volume using micropipettes, mixture
M.R. Jalilian / Spectrochimica Acta Part A 66 (2007) 976–978
977
Table 1 Vibrational frequencies of acetone, n-hexane and their azeotrope together with frequency shifts in azeotrope with respect to corresponding band in pure substances Vibrational mode
ν¯ (cm−1 ) acetone
ν¯ (cm−1 ) n-hexane
ν¯ (cm−1 ) azeotrope
¯ν (cm−1 )
C O str. CH3 s.def. C C a.str. CH3 a.rock. CH3 s.rock. CH3 str. CH3 str. CH3 a.def. CH2 rock. CH2 rock.
1714.4 1363.4 1222.7 1093.4 906.4 – – – – –
– – – – – 2966 2933.2 1467.6 904.5 725.1
1716.4 1361.5 1220.7 1091.5 902.5 2960.2 2927.4 1459.9 902.5 727
+2 −1.9 −2 −1.9 −3.9 −5.8 −5.8 −7.7 −2 +1.9
Only those frequencies which have been observable and/or have measurable frequency shift are listed.
were fractionally distilled and a center fraction boiling at 45 ◦ C was chosen from the same multiplate column. The mid-infrared spectra of pure acetone, pure n-hexane, and azeotrope was recorded on a Nicolet model 800 Fourier transform interferometer equipped with a high intensity Globar source, Ge/KBr beam splitter, and DTGS detector. A resolution of 0.09 cm−1 was utilized and 500 interferograms were taken of both the sample and empty reference cell. The H NMR spectra of acetone, n-hexane, and their azeotrope were obtained on a Bruker 500 MHZ model FT-NMR. 3. Results and discussion Acetone and n-hexane form a homogenous azeotrope with the mole ratio 2:1. Pure acetone boils at 51.5 ◦ C and n-hexane boils at 64 ◦ C. The azeotrope boils at 45 ◦ C (at 733 mmHg) showing a minimum boiling azeotrope. 6.5 ◦ C decrease in boiling point of acetone and a 19 ◦ C decrease in boiling point of n-hexane shows positive deviation from ideal behavior with a maximum vapor pressure in the mixture. Boiling point depression for n-hexane is three times more than depression for acetone, showing that interaction forces between azeotrope molecules are less than that of original constituents. It is obvious that acetone with high dipole moment (9.3 × 10−30 C m) has very strong attractive intermolecular forces with neighboring ones. Upon vaporization or dilution in an inert solvent, the acetone molecules separate from each
Fig. 1. FT-IR spectra of acetone (—), n-hexane (· · ·) as well as their azeotrope (- - -) at room temperature in the region 600–4000 cm−1 .
Table 2 Proton chemical shift data for acetone, n-hexane, and their azeotrope Substance
Proton in
δ (ppm)
δ (ppm) in azeotrope
δ (ppm)
n-Hexane
CH3 CH2
0.8924 1.2624
0.8892 1.3037
−0.0032 0.0413
Acetone
CH3
2.1010
2.0606
−0.0404
other and attraction forces between electron deficient carbon of one carbonyl and electronegative oxygen atom of the adjacent molecule decreases and C O moiety becomes shorter in the length, consequently C O stretching mode shifts toward higher frequencies. Whereas CH3 bending modes shift to the lower frequency because strong attraction forces between methyl group of one molecule and oxygen of the other molecule have been diminished, and these modes need less energy to take place [25]. Observed FT-IR spectra of C O (Fig. 1) show that above anticipation is correct. Upon azeotrope formation C O stretching mode shows blue shift, while, CH3 deformation modes show red shift (Table 1). Assuming new attraction forces between methylene groups of n-hexane and lone pair electrons on the oxygen atoms of acetone molecules, we expect red shift for all bending modes of hexane methylene groups. Here, a holding force has been established between CH2 groups and oxygen, therefore, CO bending mode of carbonyl moiety facilitates bending modes of mentioned groups (Table 2).
Fig. 2. 1 H NMR spectra of acetone (A), n-hexane (B), and corresponding azeotrope (C).
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M.R. Jalilian / Spectrochimica Acta Part A 66 (2007) 976–978
Fig. 3. Proposed structure for unit structure of the acetone–n-hexane azeotrope.
As it was pointed out in our previous works [25,26], correlation between peak position of FT-IR and H NMR spectra is a great approach for determination of the situation and orientation of molecules in mixtures, especially in azeotropes. The benefit of 1 H NMR is based upon the relatively significant change in the diamagnetic shielding of proton upon the formation of a hydrogen bond or location of a proton in the vicinity of a high electron density media, which causes a significant decrease in the effective electron density near the proton in a covalent bond. When an electronegative element, such as oxygen, goes in the neighborhood of protons of methylene groups of n-hexane, the oxygen atom deshields the hydrogen atoms causing the δ (1 H) chemical shift to occur at higher ppm values than the δ (1 H) chemical shift exhibited by n-hexane. In the case of acetone, we predict shielding for protons, because lone pair electrons on the adjacent acetone molecule have been vanished, therefore protons of CH3 groups on acetone are not deshielded anymore, and as the result of azeotrope formation, electron clouds are centered more than before around proton nucleus. The 1 H NMR spectra of acetone, n-hexane, and their azeotrope are shown in Fig. 2. The displacement of chemical shifts (Table 2) exactly confirms the foregoing discussion. 4. Conclusion Based on boiling point depression, mole ratio of components in the azeotrope, blue shift for CO stretching mode, red shift of all CH2 , as well as CH3 groups of n-hexane in the azeotrope, and 1 H NMR chemical shift displacements in the azeotrope, we suggest the following structure (Fig. 3) as the unit structure of the
acetone–n-hexane azeotrope in the cluster. This structure provides all mentioned changes in the FT-IR and H NMR spectra. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
G. Ryland, J. Am. Chem. Soc. 22 (1899) 38. F.Z. Zawidzki, Z. Phys. Chem. 35 (1900) 129. G. Ryland, Chem. News 81 (1900) 15. M. lecat L’Azeotropisme, Lamartin, Bruxelles. (1918). M.Z. Postemer, Elektrochem 40 (1934) 493. M. Postemer, Monatsh. Chem. 65 (1934) 1. M. Lecat, Tables Azeotropiques, vol. 1 L’Autear, Bruxelles (1949). Hermann, Hartmann, Schlafer, H. Ludwig, Z. Elektochem. 54 (1950) 337. V.M. Chulanvski, Dokl. Akad. Nauk. S.S.S.R. 101 (1955) 457. A.E. Lutski, D.S. Bidnaya, Opt. Spektrosk. 2 (1957) 160. T.V. Yakovleva, A.G. Maslennikova, A.A. Perov, Opt. Spektrosk. 10 (1) (1961) 131. G.S. Denisov, Opt. Spektrosk. 11 (1961) 428. Dertooz, M. Nasielski, J. Bull. Soc. Chim. Belges. 70 (1961) 794. G.E. Maciel, G.C. Ruben, J. Am. Chem. Soc. 85 (23) (1963) 3903. L.H. Horsley, Azeotropic Data III, American Chemical Society, Washington, DC, 1973. D. Petkovic, M. Pavlovic, S. Milank, A.V. Glos, Hem. Drus. Beograd. 43 (3) (1987) 31. R.A. Nyquist, Appl. Spectrosc. 44 (3) (1990) 433. J.M. Gmelhing, J. Krafczyk, K. Fischer, Fluid Phase Equilibr. 103 (1995) 51. P. Laszlo, in: J.W. Emsely, J. Feeney, L.H. Sute liffe (Eds.), Progress in NMR Spectroscopy, vol. III, 1967, p. 231. W.H. de Jeu, H. Angad Gaur, J. Smidt, Rec. Trav. Chim. 4 (1965) 1621. H.A. Christ, P. Diehl, Helv. Phys. Acta 36 (1963) 170. G.E. Maciel, J.J. Natterstad, J. Chem. Phys. 42 (1965) 2752. L.J. Bellamy, R.L. Williams, Trans. Faraday Soc. 55 (1959) 14. P.G. Puranik, Proc. Ind. Acad. Sci. 37A (1953) 499. M.R. Jalilian, L. Alibabaei, Spectrochim. Acta, Part A 62 (2005) 322. M.R. Jalilian, Spectrochim. Acta, Part A, in press.