- Email: [email protected]
Phase-transfer catalysis in organic analysis: ultraviolet spectrophotometric determination of alcohols by formation of dithiocarbonates
203
Annlytrcu Chrmrcu Actu, 230 (1990) 203-206 Elsevter Science Pubhshers B V., Amsterdam - Pnnted m The Netherlands
Short Communication
Phase-transfer catalysis in organic analysis: ultraviolet spectrophotometric determination of alcohols by formation of dithiocarbonates ALBERT Deportment
W.M LEE *, W.H. CHAN *, K.W LEE, L S. AU and W.K CHOI
of Chemlstty, Hong Kong Baptrst College, 224 Waterloo Road, Kowloon (Hong Kong) (Recetved 14th June 1989)
ABSTRACT After m sttu denvattzatron to a drthmcarbonate (xanthate) m a carbon dtsulphtde-aqueous sodmm hydroxtde two-phase system under phase-transfer catalysts condtttons, a pnmary or secondary alcohol can be determmed spectrophotometncally 300 nm
Without a necessary chromophore, aliphatic alcohols cannot be determined by UV spectrophotometry. Based on a recent study of the determination of amines by in situ generation of dithiocarbamates [l], in this communication a simple UV spectrophotometric method for the determination of primary and secondary aliphatic
NaOH aqueous
+
phase
Q+X-
F
Q+OH-
+
NaX
ROC(=S)Sdithiocarbonate
-~-----__----___--_
1
_---___---_---
CS2 organic
alcohols by the formatron of dithtocarbonates (xanthates) under phase-transfer catalysis (PTC) conditions [2] is reported. Traditionally, dithiocarbonates are formed by reaction of the corresponding alkoxides, which are generated with strong base such as metallic sodium or sodium hydride, with carbon disulphide. Re-
l-t
----_----__---
phase
RO-Q+
Q+x-
=
(nBu)
"L-
qN+HSOi
Scheme 1 0003-2670/90/$03
50
0 1990 Elsevter Sctence Pubhshers B.V
Q+OH-
+
ROH
AWM
cently, it was reported that the process can be carried out in a one-pot, two-phase system with the use of a phase-transfer catalyst [3]. The transformation is outlined m Scheme 1. Ditluocarbonates show characteristic absorption at ca. 300 nm [4]. Therefore, after in situ derivatization to dithiocarbonate under FTC conditions, the alcohol concentration can be determined by UV spectrophotometry. Expermental Apparatus and reagents. A Hitachi UV-150 spectrophotometer with a UV150-20 data processor was used for UV measurements. pH measurements were made with an Orion digital Ionanalyser (Model 601A). Deionized water was use throughout. Analytical-reagent grade sodium hydroxide, and carbon disulphide were used as received. Tetrabutylammonium hydrogensulphate was obtained from Fluka. Alcohols were of purity > 98%, and were dtstilled before use. Procedures. For calibration, an accurately weighed sample of the alcohol (ca. 1 ml) was added to a two-phase mixture of 10 ml of carbon disulphide and 20 ml of 50% (w/w) aqueous sodium hydroxide in a lOO-ml round-bottomed flask and 0.2 g of the phase-transfer catalyst, tetrabutylammonium hydrogensulphate, was added. The mixture was stirred magnetically at 550 rpm using a 3/4-m. FTFE-coated stirrmg bar for 2 h at room temperature. An orange emulsion was
TABLE Cahbratlon
formed. Water (40 ml) was pipetted into the icecooled reaction mixture and after stirring for a few minutes, the diluted rmxture separated mto two clear layers. A lo-ml volume of the upper aqueous layer was pipetted into a 250-ml volumetric flask. After dilution to volume, 25 ml of the solution were further diluted to 500 ml m a volumetric flask. Five to seven aliquots (5-60 ml) of this solution were pipetted into lOO-ml round-bottomed flasks and acidified to pH 6 with concentrated hydrochloric acid. They were stirred at room temperature inside a fume-hood for 30 min to decompose the interfering trithiocarbonate anion. Hydrogen sulphide was evolved. Finally, the acid-treated samples were transfer into lOO-ml volumetric flasks and diluted to volume. The absorbances of these final solutions were measured in l-cm quartz cells at 300 or 305 nm against water and plotted against the concentrations of the solutions. Alcohol samples were taken through the same procedure. With cyclohexanol and benzyl alcohol, a yellow gel-hke emulsion was formed after stirring and a modified work-up procedure was adopted. The reaction mixture was poured into 220 ml of water and stirred for 5 mm, resulting in two layers of clear solution. A 40-ml volume of the top, aqueous layer was removed by pipette and diluted to 250 ml in a volumetric flask, then 25 ml of this solution were further diluted to 500 ml. A series of acid-treated standard solutions were prepared from thts stock solution m a similar way.
1 data for primary
Alcohol
and secondary
Lmear range
alcohols h (nm)
Slope a (X104)
Standard devlatlon
300 305 300 300 305 300 300 300
1 340 1 390 1 399 0 731 1 070 1 494 1 378 1.221
0.27 1 25 0 49 0 31 0 09 0 52 0 88 0.62
(pg ml-‘) Ethanol (absolute) 1-Propanol l-Butanol Benzyl alcohol 2-Propanol 2-Butanol 2-Pentanol Cyclohexanol * Molar absorptlwty
128-10 133-10 1 32-13 3 31-39 088-105 131-13 1.29-15 149-17 (1 mol-’
2 6 2 8 0 4 9
cm-‘),
LEEETAL
Correlation coefficient
Intercept (absorbance)
0 9999 0 9995 0 9997 0 9992 0 9994 0 9993 0 9997 0.9999
0 024 -0012 0 055 0 022 0 032 0 030 0 022 0 009
(%)
mean of at least three sets of measurements
b 5 points
b b c d = c d d
’ 6 pomts.
d 7 pomts
PHASE-TRANSFER
CATALYSIS
IN ORGANIC
205
ANALYSIS
0.0 200
250
300
Wavelength
350
400
tnm)
Fig. 1 IJV absorption
spectra of denvatlzed l-butanol after acid treatment I-Butanol 3 545 x 10m5; (c) 7.089 x lo-‘, (d) 1.063 x 10e4, (e) 1418 X 10m4, (f) 1772 X 10m4 M
Results and discumon Without the FTC, sodium hydroxide is not strong enough to deprotonate the alcohols. In the presence of the F’TC, 50% sodium hydroxide was used even though a 20% solution can still effect the reaction but with a longer reaction time (4 h instead of 2 h). The reaction involves a two-phase system, and therefore the stirring rate should be well controlled. In general, stirring for 2 h at 550 t-pm is sufficient. The method worked well with a wide variety of primary and secondary alcohols. Typical calibration graphs showed that Beer’s law is obeyed and the linearity and reproducibility were good. The results are summarized in Table 1. Spectra of the
concentration
(a) 1772 X 10m5,
(b)
product from 1-butanol at various concentration are shown in Fig. 1. Trithiocarbonate anion (CS:-) was formed m the reaction mixture [5,6]. Its absorption at 330 nm interfered with the measurement of the alcohol-derived dithiocarbonates at 300 nm. Acid treatment was used to overcome this interference problem. Trithiocarbonate is known to be decomposed at pH 6 whereas dithiocarbonates are stable at this pH [7,8].
REFERENCES 1 A.W.M. Lee, W H Chan, C M.L Anal Glum. Acta, 218 (1989) 157
Chm
and
K T
Tang,
206 2 C.M Starks (Ed ), Phase Transfer Catalysis - New ChemIstry, Catalysts and Apphcatlon, Amencan Chenucal Society, Washmgton, DC, 1987 3 A W M. Lee, W.H. Chan, H.C Wong and M.S. Wong, Synth Commun , (1989) 547. 4 M L Shankaranarayana and C C Patel, Acta Chem. Stand., 19 (1965) 1113.
AWM
LEEETAL
5 E. Werthelm, J Am. Chem. Sot , 48 (1926) 826 6 A W.M Lee, W.H. Chan and H.C Wong, Synth. Commun., 18 (1988) 1531 7 G Ingram and B A Toms, J. Chem Sot , (1957) 4328 8 W.H Chan, A W M. Lee, K.S Lam and C.L Tse, Analyst, 114 (1989) 233
Annlytrcu Chrmrcu Actu, 230 (1990) 203-206 Elsevter Science Pubhshers B V., Amsterdam - Pnnted m The Netherlands
Short Communication
Phase-transfer catalysis in organic analysis: ultraviolet spectrophotometric determination of alcohols by formation of dithiocarbonates ALBERT Deportment
W.M LEE *, W.H. CHAN *, K.W LEE, L S. AU and W.K CHOI
of Chemlstty, Hong Kong Baptrst College, 224 Waterloo Road, Kowloon (Hong Kong) (Recetved 14th June 1989)
ABSTRACT After m sttu denvattzatron to a drthmcarbonate (xanthate) m a carbon dtsulphtde-aqueous sodmm hydroxtde two-phase system under phase-transfer catalysts condtttons, a pnmary or secondary alcohol can be determmed spectrophotometncally 300 nm
Without a necessary chromophore, aliphatic alcohols cannot be determined by UV spectrophotometry. Based on a recent study of the determination of amines by in situ generation of dithiocarbamates [l], in this communication a simple UV spectrophotometric method for the determination of primary and secondary aliphatic
NaOH aqueous
+
phase
Q+X-
F
Q+OH-
+
NaX
ROC(=S)Sdithiocarbonate
-~-----__----___--_
1
_---___---_---
CS2 organic
alcohols by the formatron of dithtocarbonates (xanthates) under phase-transfer catalysis (PTC) conditions [2] is reported. Traditionally, dithiocarbonates are formed by reaction of the corresponding alkoxides, which are generated with strong base such as metallic sodium or sodium hydride, with carbon disulphide. Re-
l-t
----_----__---
phase
RO-Q+
Q+x-
=
(nBu)
"L-
qN+HSOi
Scheme 1 0003-2670/90/$03
50
0 1990 Elsevter Sctence Pubhshers B.V
Q+OH-
+
ROH
AWM
cently, it was reported that the process can be carried out in a one-pot, two-phase system with the use of a phase-transfer catalyst [3]. The transformation is outlined m Scheme 1. Ditluocarbonates show characteristic absorption at ca. 300 nm [4]. Therefore, after in situ derivatization to dithiocarbonate under FTC conditions, the alcohol concentration can be determined by UV spectrophotometry. Expermental Apparatus and reagents. A Hitachi UV-150 spectrophotometer with a UV150-20 data processor was used for UV measurements. pH measurements were made with an Orion digital Ionanalyser (Model 601A). Deionized water was use throughout. Analytical-reagent grade sodium hydroxide, and carbon disulphide were used as received. Tetrabutylammonium hydrogensulphate was obtained from Fluka. Alcohols were of purity > 98%, and were dtstilled before use. Procedures. For calibration, an accurately weighed sample of the alcohol (ca. 1 ml) was added to a two-phase mixture of 10 ml of carbon disulphide and 20 ml of 50% (w/w) aqueous sodium hydroxide in a lOO-ml round-bottomed flask and 0.2 g of the phase-transfer catalyst, tetrabutylammonium hydrogensulphate, was added. The mixture was stirred magnetically at 550 rpm using a 3/4-m. FTFE-coated stirrmg bar for 2 h at room temperature. An orange emulsion was
TABLE Cahbratlon
formed. Water (40 ml) was pipetted into the icecooled reaction mixture and after stirring for a few minutes, the diluted rmxture separated mto two clear layers. A lo-ml volume of the upper aqueous layer was pipetted into a 250-ml volumetric flask. After dilution to volume, 25 ml of the solution were further diluted to 500 ml m a volumetric flask. Five to seven aliquots (5-60 ml) of this solution were pipetted into lOO-ml round-bottomed flasks and acidified to pH 6 with concentrated hydrochloric acid. They were stirred at room temperature inside a fume-hood for 30 min to decompose the interfering trithiocarbonate anion. Hydrogen sulphide was evolved. Finally, the acid-treated samples were transfer into lOO-ml volumetric flasks and diluted to volume. The absorbances of these final solutions were measured in l-cm quartz cells at 300 or 305 nm against water and plotted against the concentrations of the solutions. Alcohol samples were taken through the same procedure. With cyclohexanol and benzyl alcohol, a yellow gel-hke emulsion was formed after stirring and a modified work-up procedure was adopted. The reaction mixture was poured into 220 ml of water and stirred for 5 mm, resulting in two layers of clear solution. A 40-ml volume of the top, aqueous layer was removed by pipette and diluted to 250 ml in a volumetric flask, then 25 ml of this solution were further diluted to 500 ml. A series of acid-treated standard solutions were prepared from thts stock solution m a similar way.
1 data for primary
Alcohol
and secondary
Lmear range
alcohols h (nm)
Slope a (X104)
Standard devlatlon
300 305 300 300 305 300 300 300
1 340 1 390 1 399 0 731 1 070 1 494 1 378 1.221
0.27 1 25 0 49 0 31 0 09 0 52 0 88 0.62
(pg ml-‘) Ethanol (absolute) 1-Propanol l-Butanol Benzyl alcohol 2-Propanol 2-Butanol 2-Pentanol Cyclohexanol * Molar absorptlwty
128-10 133-10 1 32-13 3 31-39 088-105 131-13 1.29-15 149-17 (1 mol-’
2 6 2 8 0 4 9
cm-‘),
LEEETAL
Correlation coefficient
Intercept (absorbance)
0 9999 0 9995 0 9997 0 9992 0 9994 0 9993 0 9997 0.9999
0 024 -0012 0 055 0 022 0 032 0 030 0 022 0 009
(%)
mean of at least three sets of measurements
b 5 points
b b c d = c d d
’ 6 pomts.
d 7 pomts
PHASE-TRANSFER
CATALYSIS
IN ORGANIC
205
ANALYSIS
0.0 200
250
300
Wavelength
350
400
tnm)
Fig. 1 IJV absorption
spectra of denvatlzed l-butanol after acid treatment I-Butanol 3 545 x 10m5; (c) 7.089 x lo-‘, (d) 1.063 x 10e4, (e) 1418 X 10m4, (f) 1772 X 10m4 M
Results and discumon Without the FTC, sodium hydroxide is not strong enough to deprotonate the alcohols. In the presence of the F’TC, 50% sodium hydroxide was used even though a 20% solution can still effect the reaction but with a longer reaction time (4 h instead of 2 h). The reaction involves a two-phase system, and therefore the stirring rate should be well controlled. In general, stirring for 2 h at 550 t-pm is sufficient. The method worked well with a wide variety of primary and secondary alcohols. Typical calibration graphs showed that Beer’s law is obeyed and the linearity and reproducibility were good. The results are summarized in Table 1. Spectra of the
concentration
(a) 1772 X 10m5,
(b)
product from 1-butanol at various concentration are shown in Fig. 1. Trithiocarbonate anion (CS:-) was formed m the reaction mixture [5,6]. Its absorption at 330 nm interfered with the measurement of the alcohol-derived dithiocarbonates at 300 nm. Acid treatment was used to overcome this interference problem. Trithiocarbonate is known to be decomposed at pH 6 whereas dithiocarbonates are stable at this pH [7,8].
REFERENCES 1 A.W.M. Lee, W H Chan, C M.L Anal Glum. Acta, 218 (1989) 157
Chm
and
K T
Tang,
206 2 C.M Starks (Ed ), Phase Transfer Catalysis - New ChemIstry, Catalysts and Apphcatlon, Amencan Chenucal Society, Washmgton, DC, 1987 3 A W M. Lee, W.H. Chan, H.C Wong and M.S. Wong, Synth Commun , (1989) 547. 4 M L Shankaranarayana and C C Patel, Acta Chem. Stand., 19 (1965) 1113.
AWM
LEEETAL
5 E. Werthelm, J Am. Chem. Sot , 48 (1926) 826 6 A W.M Lee, W.H. Chan and H.C Wong, Synth. Commun., 18 (1988) 1531 7 G Ingram and B A Toms, J. Chem Sot , (1957) 4328 8 W.H Chan, A W M. Lee, K.S Lam and C.L Tse, Analyst, 114 (1989) 233