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Quantitative determination of esters by saponification in dimethyl sulphoxide
Talaola. 1966, Vol. 13. pp. 1673 to 1677. Per@amon Praur Ltd. Printed in Northern Inland
QUALITATIVE DETERMINATION SAJi’ONIFICATION IN DIMETHYL
OF ESTERS BY SULPHOXIDE”
JOE A. VINSON,JAMESS. FRITZ and CHARLESA. KINGSBURY State University, Ames,
Institute for Atomic Research and Department of Chemistry Iowa Iowa, U.S.A.
(Received 18 hdy 1966. Accepted 12 August 1966) Summary_-A method is given for the quantitative determination of esters which makes use of the unusually rapid rate of their alkaline hydrolysis in aqueous dimethyl sulphoxide medium. Only 5 min heating on a steam-bath is needed for quantitative hydrolysis of most esters, and many react completely in 5 min at room temperature. When hydrolysis is complete, the excess of base is titrated with standard acid, using a visual indicator.
A FAST, simple method for the quantitative determination of esters is needed for routine analysis. Many methods are available for specific types of esters but none incorporates a short reaction time, low temperature and a simple procedure for a wide variety of esters. An excellent review of the literature is given by Hall and Shaeferl who present a coverage of saponification procedures up to 1952. For easily saponifiable esters they recommend refluxing with ethanolic potassium hydroxide for 30 min or less. Hindered esters are saponified at 175’ with a diethylene glycol-potassium hydroxide-phenetole solution.* This method is applicable to rosin esters and less hindered esters. The best method for highly hindered esters is that of Jordan,3 which uses hexanolic sodium hydroxide, with sodium perchlorate as a catalyst. The reaction temperature is approximately 155”, the boiling point of hexanol. Esters which were formerly considered hydrolytically stable can be hydrolysed quantitatively. Alkali-resistant flasks must be used to minimise fixation of the alkali on the glass, and in order to obtain accurate results a blank must be run exactly matching the reaction time used for the sample. The viscous hexanolic base must be pipetted carefully with a uniform drainage time. The present work describes the use of dimethyl sulphoxide (DMSO) as the solvent for saponification with aqueous potassium hydroxide as the base. The procedure given is both fast and simple. A reaction time of only 5 min is required for most esters. Many esters can be saponified quanti~tively at room temperature; others require heating on a steam-bath. In addition to the back-titration procedure which is chiefly used in this work, an ion-exchange method is also described briefly. EXPERIMENTAL Apparatus and reagents Dimethyi suiphoxide@MSO), Maliinckrodt reagent grade. Reaction vessel, 50-migiass-stoppered Erienmeyer flask or a 2%ml 10140 standard taper pear-shaped flask fitted with a reflux condenser. Potassium hydroxide, 0*5&f, carbonate-free aqueous solution standardised against potassium acid phthalate, using phenoiphthaiein as indicator. * This work was performed Contribution No. 1922.
in the Ames Laboratory 1673
of the U.S. Atomic Energy Commission.
1674
J. A. VINSON, J. S. FRITZ and C.
A.
KINGSBURY
Sodium hydroxide, 1OM and O.lM, carbonatefree aqueous solutions. The O+lM solution was standardised against potassium acid phthalate. Hydrochloric acid, 0.1 M, standardised against standard sodium or potassium hydroxide. Ion-exchange column, a conventional column 2.25 cm* cross-section x 30 cm packed with Dowex 50W X8 resin (20-50 mesh) in the hydrogen form. Buck-titration procedure Weigh accurately approximately 10 meq of the ester into a lOO-ml volumetric flask and dilute to 100 ml with dimethyl sulphoxide. Transfer lO-ml aliquots by pipette into 25-ml pear-shaped flasks or 50-ml Erlenmeyer flasks. Add exactly 8 ml of 0*5M potassium hydroxide from a burette or pipettes together with 30 ml of dimethyl sulphoxide. Either stir the reaction mixture magnetically at room temperature in the Erlenmeyer flask or heat in the pear-shaped flask with condenser on a steam-bath until saponification is complete (l-15 min). Dilute the reaction mixture with lOOmI of distilled water, add Cresol Red indicator and titrate with standard O.lM hydrochloric acid. Determine a blank with base and DMSO for the same temperature and reaction time used for the ester sample. Zon-exchange procedure Weigh accurately 2.5 meq of the ester into a lO_ml volumetric flask and dilute to the mark with DMSO. Transfer 2-ml aliquots by pipette into 50-ml Erlenmeyer flasks and add to each @5 ml of 1OM sodium hydroxide along with 1 ml of DMSO. Stir the reaction mixture magnetically for 15 min at room temperature. A precipitate of the acid salt appears in l-5 min. Dilute the contents to 40 ml with a methanol-water solution (1 + 1), and add to the ion-exchange column containing 6 g of air-dried cation-exchange resin, previously washed with methanol-water solution (1 + 1). Continue elution with methanol-water mixture until 100 ml of effluent is collected. Boil the solution to remove carbon dioxide and titrate the carboxylic acid with standard @lM sodium hydroxide to
the phenolphthaleinend-point. Collect additional 50-mlfractions of effluentuntil the titre becomes equal to the blank (approximately0.07ml). Calculatethe amount of ester from the volumeof sodium hydroxide required to titrate the carboxylic acid. THEORY Since the advent of commercially available dimethyl sulphoxide, much work has been done using this amazing solvent. It has been found that the rates of many reactions are accelerated in going from alcoholic or protic solvents to dipolar aprotic solvents such as dimethylformamide and dimethyl sulphoxide. Parkei’ has stated that any bimolecular reaction of a small anion passing through a large polarisable transition state will be considerably accelerated in the change from protic to dipolar aprotic solvents. This remarkable effect is due to the much decreased solvation of small anions in DMSO compared to that in protic solvents. There is no significant contribution to solvation by hydrogen bonding in DMSO compared to water and alcohols; the very low solubility of alkali metal hydroxides in DMS05 is evidence that the hydroxide ion is poorly solvated in DMSO. The first work on ester hydrolysis in DMSO was published by Tommi1a.B Roberts has published data on the alkaline hydrolysis of several esters in DMS0.7*8 Ethyl benzoate was hydrolysed at 25” and relative rates of 1, 320, 160 and 54 were obtained in 85 % aqueous ethanol, 85 % aqueous DMSO, 80% aqueous DMSO and 65 % aqueous DMSO, respectively. Thus the amount of DMSO has a profound effect on the rate of hydrolysis and is a much better solvent to use for saponification. Several other esters were found to have considerably faster saponification rates in 85 % DMSO than in 85 % ethanol. Anhydrous dimethyl sulphoxide cannot be used for saponification owing to the very low solubility of potassium hydroxide. In our work SO-90% v/v solutions of DMSO in water were used as the solvent. Most esters are soluble in pure DMSO at room temperature. A few hindered or long-chained esters were found to require heating for dissolution and saponification.
Quantitative
determination
of esters by saponification
RESULTS
AND
1675
in dimethyl sulphoxide
DISCUSSION
Back-titration procedure Table I shows results for saponification of a wide variety of esters ranging from simple esters to some that are difficult to saponify. The relative standard deviation calculated for all the esters used was 0.29 %. As expected the time for complete reaction varied with the ester. A kinetic run on ethyl acetate showed it to be completely saponified in 1 min at room temperature in 83.5 % aqueous dimethyl sulphoxide. In the 83.5 ‘A aqueous ethanol the reaction TABLE ~.-QUANT~ATIW
Ester Ethyl acetate n-B&y1 acetate Ethyl chloroacetate Methyl benzoate Ethyl-j+hydroxybenzoate Phenvl benzoate Dietdyl phthalate 1-Boronyl acetate Ethyl lactate Ethyl suberate Dieihyleneglycol adipate Di-2-ethvlhexvl sebacate Di-(2,2-dimethylhexyl)2,2,6,6-tetramethylpimelate
SAPONIFICATION OF ESTER SAMPLES OF 98-100%
Temperature, “C
Reaction time,
min
25 2.5 25 25 100 25 100 100 100 25 100 100
5 5 5 5 5 5 5 5 5 5 5 15
155
10
PURITY
Found, % 99.5 98.7 98.4 98.6 100.0 99.1 98.7 98.4 99.3 98.5 98.5 99.3
* f f f + + f f f + & *
o.o* 04 0.2 0.8 0.0 0.3 0.5 0.0 0.4 0.2 0.1 0.1
99.0 & 0.2
* Average deviation of 3 or more determinations.
was only 35 % complete in 5 min. Nominally hindered esters such as diethyl phthalate are hydrolysed in 5 min on the steam-bath. A hindered ester such as di-2-ethylhexyl sebacate was 63.5 % hydrolysed after 3 min in DMSO on the steam-bath (100 % in 15 min), but only 9.2 % in the same period of time in ethanol. The most highly hindered ester successfully analysed, di-(2,2-dimethylhexy1)2,2,6,6-tetramethylpimelate, required a high temperature, and the blank was quite large, indicating that the alkali was reacting with the glass vessel. Jordan’s methods required about 1 hr for saponification of this compound. However, we were unable to obtain any reasonable results in DMSO for di-(2,2-dimethylhexyl)-2,2,8,&tetraethylazelate which Jordan was able to determine accurately, using a 7-hr reaction time. Long heating at high temperature appears to partly decompose dimethyl sulphoxide. A study of the effect of substances likely to interfere with the quantitative determination of esters showed remarkably few interferences. Probably this fortunate situation is a result of the low temperature and short reaction times used in the DMSO saponification. Thus in the alkaline hydrolysis of ethyl chloroacetate no baseconsuming replacement of halogen by hydroxyl occurs under the mild hydrolytic conditions employed; only the theoretical amount of alkali required for saponification of the ester was consumed. Reactive compounds such as acetone and acetonitrile, which can undergo condensation readily in basic solution, cause no interference with the esters tried (Table II). Butyraldehyde causes essentially no interference when the ester has no a-hydrogen for
1676
J. A. VINSON, J. S. FRITZ and C. A. TABLE II.-EFFECT
OF POSSIBLE
INTERFERING SUBSTANCES ON THE ANALYSJS OF ESTERS BY SAPONIFICATION
Temperature, C”
Ester Ethyl naphthoate Ethyl naphthoate Phenyl benzoate Ethyl naphthoate Ethyl acetate Ethyl acetate Ethyl acetate
KINGSBURY
loo 100 100 100 25 25 25
Compound added* A&amide Acetonitrile Acetone Butyraldehyde Acetone Acetonitrile Butymldehyde
Found, % 98-9 98.0 98.9 99.5 98.5
99.4 120
* Added in IO-fold molar excess over ester. All reaction times were 5 min.
cross-condensation. Evidently self-condensation of the aldehyde is either slow or does not consume base under the conditions used. Amides are diflicult to saponify even in aqueous DMSO. Extended heating with base failed to give quantitative results for acetamide. From the single result in Table II it appears that esters saponifiable at room temperature can be determined without interference from aliphatic amides. The recommended procedure employs about a 4-fold excess of base. The moderately high blanks caused by this amount of base could be reduced by using a lower ratio of base to ester, although this would probably necessitate a slightly longer reaction time for some esters. It should be noted that phenolphthalein, the usual indicator in saponification procedures, gives a poor end-point in the DMSO method. However, Cresol Red indicator gives a sharp colour change from red to yellow. This change corresponds with the steepest part of the titration curve obtained by using a pH meter. Ion-exchange procedure
Results for some esters analysed by the ion-exchange procedure are shown in Table III. All esters were saponified for 15 min at room temperature. The results are very good with the exception of phenyl benzoate, in which some phenol is evidently formed on the column and titrated. Only one ester group is hydrolysed in the hindered diester, di-2-ethylhexyl sebacate, owing to the insolubility of the half-ester sodium salt which precipitates before further hydrolysis can take place. The ion-exchange method suffers from being too time-consuming; an ordinary TABLEIIL-ANALYSIS
OF ESTERS BY THE ION-EXCHANGE PROCEDuRE
Ester
Found, %
Ethyl acetate Methyl isobutyrate Diethyl phthalate Cyclohexyl pivalate Phenyl benzoate Di-2-ethylhexyl sebacate
99.5 99.6 995 99.4 111.9 49.6*
* Based on 2 ester groups; on 1 ester group.
99,2 % found based
Quantitative
determination
of esters by saponification
in dimethyl sulphoxide
analysis requires 45-60 min. It is difficult to remove the last traces acid from the column, and several washings are required. However, is generally
applicable
as a quantitative
semi-micro
1677
of carboxylic the procedure
method.
Ac/cnowZedgements-The authors express appreciation to D. Jordan and the Continental Oil Company for a generous supply of highly hindered esters and are grateful to C. E. Thompson, of the Petroleum Products Division of that Company, who synthesised these hindered esters. We also thank Alan G. Bemis for suggesting this method and Frank E. Gainer for valuable experimental assistance. R&sum&On donne une mtthode de dosage des esters faisant appel a la vitesse particulierement rapide de leur hydrolyse alcaline en milieu dimethylsulfoxyde aqueux. Pour l’hydrolyse quantitative, la majeure partie des esters ne necessite que 5 mn de chauffage au bain de vapeur, et nombre d’entre eux reagissent completement en 5 mn a temperature ordinaire. Lorsque l’hydrolyse est totale, on dose l’exces de base avec un acide tit& utilisant un indicateur visuel. Zusammenfasflmg-Eine Methode zur quantitativen Bestimmung von Estem wird angegeben, die die ungewijhnlich hohe Geschwindigkeit ihrer alkalischen Hydrolyse in w5iBrigem Dimethylsulfoxyd ausntltzt. Nur fiinf Minuten Erhitzen auf einem Dampfbad reicht aus, die meisten Ester quantitativ zu verseifen, und viele reagieren in fiinf Minuten bei Zimmertemperatur. Wenn die Hydrolyse vollstlndig ist, wird die iiberschtlssige Base mit eingestellter Saure und einem visuellen Indikator titriert. REFERENCES 1. R. T. Hall and W. T. Shaefer in J. T. Mitchell et al., Organic Analysis, Vol. II, pp. 20-70. Interscience, New York, 1954. 2. W. E. Shaefer and W. J. Balling, Anal. Gem., 1951,23,1126. 3. D. E. Jordan, ibid., 1964,36,2134. 4. A. J. Parker, J. C/rem. Sec., 1961,132s. 5. Idem, Quart. Rev. London, 1962,16,163. 6. E. Tommila, and M. L. Murto, Actu Chem. Scund. 1963,17,1947. 7. D. D. Roberts, J. Org. Chem. 1964,29,2039. 8. Idem, ibid., 1964, 29,2714.
QUALITATIVE DETERMINATION SAJi’ONIFICATION IN DIMETHYL
OF ESTERS BY SULPHOXIDE”
JOE A. VINSON,JAMESS. FRITZ and CHARLESA. KINGSBURY State University, Ames,
Institute for Atomic Research and Department of Chemistry Iowa Iowa, U.S.A.
(Received 18 hdy 1966. Accepted 12 August 1966) Summary_-A method is given for the quantitative determination of esters which makes use of the unusually rapid rate of their alkaline hydrolysis in aqueous dimethyl sulphoxide medium. Only 5 min heating on a steam-bath is needed for quantitative hydrolysis of most esters, and many react completely in 5 min at room temperature. When hydrolysis is complete, the excess of base is titrated with standard acid, using a visual indicator.
A FAST, simple method for the quantitative determination of esters is needed for routine analysis. Many methods are available for specific types of esters but none incorporates a short reaction time, low temperature and a simple procedure for a wide variety of esters. An excellent review of the literature is given by Hall and Shaeferl who present a coverage of saponification procedures up to 1952. For easily saponifiable esters they recommend refluxing with ethanolic potassium hydroxide for 30 min or less. Hindered esters are saponified at 175’ with a diethylene glycol-potassium hydroxide-phenetole solution.* This method is applicable to rosin esters and less hindered esters. The best method for highly hindered esters is that of Jordan,3 which uses hexanolic sodium hydroxide, with sodium perchlorate as a catalyst. The reaction temperature is approximately 155”, the boiling point of hexanol. Esters which were formerly considered hydrolytically stable can be hydrolysed quantitatively. Alkali-resistant flasks must be used to minimise fixation of the alkali on the glass, and in order to obtain accurate results a blank must be run exactly matching the reaction time used for the sample. The viscous hexanolic base must be pipetted carefully with a uniform drainage time. The present work describes the use of dimethyl sulphoxide (DMSO) as the solvent for saponification with aqueous potassium hydroxide as the base. The procedure given is both fast and simple. A reaction time of only 5 min is required for most esters. Many esters can be saponified quanti~tively at room temperature; others require heating on a steam-bath. In addition to the back-titration procedure which is chiefly used in this work, an ion-exchange method is also described briefly. EXPERIMENTAL Apparatus and reagents Dimethyi suiphoxide@MSO), Maliinckrodt reagent grade. Reaction vessel, 50-migiass-stoppered Erienmeyer flask or a 2%ml 10140 standard taper pear-shaped flask fitted with a reflux condenser. Potassium hydroxide, 0*5&f, carbonate-free aqueous solution standardised against potassium acid phthalate, using phenoiphthaiein as indicator. * This work was performed Contribution No. 1922.
in the Ames Laboratory 1673
of the U.S. Atomic Energy Commission.
1674
J. A. VINSON, J. S. FRITZ and C.
A.
KINGSBURY
Sodium hydroxide, 1OM and O.lM, carbonatefree aqueous solutions. The O+lM solution was standardised against potassium acid phthalate. Hydrochloric acid, 0.1 M, standardised against standard sodium or potassium hydroxide. Ion-exchange column, a conventional column 2.25 cm* cross-section x 30 cm packed with Dowex 50W X8 resin (20-50 mesh) in the hydrogen form. Buck-titration procedure Weigh accurately approximately 10 meq of the ester into a lOO-ml volumetric flask and dilute to 100 ml with dimethyl sulphoxide. Transfer lO-ml aliquots by pipette into 25-ml pear-shaped flasks or 50-ml Erlenmeyer flasks. Add exactly 8 ml of 0*5M potassium hydroxide from a burette or pipettes together with 30 ml of dimethyl sulphoxide. Either stir the reaction mixture magnetically at room temperature in the Erlenmeyer flask or heat in the pear-shaped flask with condenser on a steam-bath until saponification is complete (l-15 min). Dilute the reaction mixture with lOOmI of distilled water, add Cresol Red indicator and titrate with standard O.lM hydrochloric acid. Determine a blank with base and DMSO for the same temperature and reaction time used for the ester sample. Zon-exchange procedure Weigh accurately 2.5 meq of the ester into a lO_ml volumetric flask and dilute to the mark with DMSO. Transfer 2-ml aliquots by pipette into 50-ml Erlenmeyer flasks and add to each @5 ml of 1OM sodium hydroxide along with 1 ml of DMSO. Stir the reaction mixture magnetically for 15 min at room temperature. A precipitate of the acid salt appears in l-5 min. Dilute the contents to 40 ml with a methanol-water solution (1 + 1), and add to the ion-exchange column containing 6 g of air-dried cation-exchange resin, previously washed with methanol-water solution (1 + 1). Continue elution with methanol-water mixture until 100 ml of effluent is collected. Boil the solution to remove carbon dioxide and titrate the carboxylic acid with standard @lM sodium hydroxide to
the phenolphthaleinend-point. Collect additional 50-mlfractions of effluentuntil the titre becomes equal to the blank (approximately0.07ml). Calculatethe amount of ester from the volumeof sodium hydroxide required to titrate the carboxylic acid. THEORY Since the advent of commercially available dimethyl sulphoxide, much work has been done using this amazing solvent. It has been found that the rates of many reactions are accelerated in going from alcoholic or protic solvents to dipolar aprotic solvents such as dimethylformamide and dimethyl sulphoxide. Parkei’ has stated that any bimolecular reaction of a small anion passing through a large polarisable transition state will be considerably accelerated in the change from protic to dipolar aprotic solvents. This remarkable effect is due to the much decreased solvation of small anions in DMSO compared to that in protic solvents. There is no significant contribution to solvation by hydrogen bonding in DMSO compared to water and alcohols; the very low solubility of alkali metal hydroxides in DMS05 is evidence that the hydroxide ion is poorly solvated in DMSO. The first work on ester hydrolysis in DMSO was published by Tommi1a.B Roberts has published data on the alkaline hydrolysis of several esters in DMS0.7*8 Ethyl benzoate was hydrolysed at 25” and relative rates of 1, 320, 160 and 54 were obtained in 85 % aqueous ethanol, 85 % aqueous DMSO, 80% aqueous DMSO and 65 % aqueous DMSO, respectively. Thus the amount of DMSO has a profound effect on the rate of hydrolysis and is a much better solvent to use for saponification. Several other esters were found to have considerably faster saponification rates in 85 % DMSO than in 85 % ethanol. Anhydrous dimethyl sulphoxide cannot be used for saponification owing to the very low solubility of potassium hydroxide. In our work SO-90% v/v solutions of DMSO in water were used as the solvent. Most esters are soluble in pure DMSO at room temperature. A few hindered or long-chained esters were found to require heating for dissolution and saponification.
Quantitative
determination
of esters by saponification
RESULTS
AND
1675
in dimethyl sulphoxide
DISCUSSION
Back-titration procedure Table I shows results for saponification of a wide variety of esters ranging from simple esters to some that are difficult to saponify. The relative standard deviation calculated for all the esters used was 0.29 %. As expected the time for complete reaction varied with the ester. A kinetic run on ethyl acetate showed it to be completely saponified in 1 min at room temperature in 83.5 % aqueous dimethyl sulphoxide. In the 83.5 ‘A aqueous ethanol the reaction TABLE ~.-QUANT~ATIW
Ester Ethyl acetate n-B&y1 acetate Ethyl chloroacetate Methyl benzoate Ethyl-j+hydroxybenzoate Phenvl benzoate Dietdyl phthalate 1-Boronyl acetate Ethyl lactate Ethyl suberate Dieihyleneglycol adipate Di-2-ethvlhexvl sebacate Di-(2,2-dimethylhexyl)2,2,6,6-tetramethylpimelate
SAPONIFICATION OF ESTER SAMPLES OF 98-100%
Temperature, “C
Reaction time,
min
25 2.5 25 25 100 25 100 100 100 25 100 100
5 5 5 5 5 5 5 5 5 5 5 15
155
10
PURITY
Found, % 99.5 98.7 98.4 98.6 100.0 99.1 98.7 98.4 99.3 98.5 98.5 99.3
* f f f + + f f f + & *
o.o* 04 0.2 0.8 0.0 0.3 0.5 0.0 0.4 0.2 0.1 0.1
99.0 & 0.2
* Average deviation of 3 or more determinations.
was only 35 % complete in 5 min. Nominally hindered esters such as diethyl phthalate are hydrolysed in 5 min on the steam-bath. A hindered ester such as di-2-ethylhexyl sebacate was 63.5 % hydrolysed after 3 min in DMSO on the steam-bath (100 % in 15 min), but only 9.2 % in the same period of time in ethanol. The most highly hindered ester successfully analysed, di-(2,2-dimethylhexy1)2,2,6,6-tetramethylpimelate, required a high temperature, and the blank was quite large, indicating that the alkali was reacting with the glass vessel. Jordan’s methods required about 1 hr for saponification of this compound. However, we were unable to obtain any reasonable results in DMSO for di-(2,2-dimethylhexyl)-2,2,8,&tetraethylazelate which Jordan was able to determine accurately, using a 7-hr reaction time. Long heating at high temperature appears to partly decompose dimethyl sulphoxide. A study of the effect of substances likely to interfere with the quantitative determination of esters showed remarkably few interferences. Probably this fortunate situation is a result of the low temperature and short reaction times used in the DMSO saponification. Thus in the alkaline hydrolysis of ethyl chloroacetate no baseconsuming replacement of halogen by hydroxyl occurs under the mild hydrolytic conditions employed; only the theoretical amount of alkali required for saponification of the ester was consumed. Reactive compounds such as acetone and acetonitrile, which can undergo condensation readily in basic solution, cause no interference with the esters tried (Table II). Butyraldehyde causes essentially no interference when the ester has no a-hydrogen for
1676
J. A. VINSON, J. S. FRITZ and C. A. TABLE II.-EFFECT
OF POSSIBLE
INTERFERING SUBSTANCES ON THE ANALYSJS OF ESTERS BY SAPONIFICATION
Temperature, C”
Ester Ethyl naphthoate Ethyl naphthoate Phenyl benzoate Ethyl naphthoate Ethyl acetate Ethyl acetate Ethyl acetate
KINGSBURY
loo 100 100 100 25 25 25
Compound added* A&amide Acetonitrile Acetone Butyraldehyde Acetone Acetonitrile Butymldehyde
Found, % 98-9 98.0 98.9 99.5 98.5
99.4 120
* Added in IO-fold molar excess over ester. All reaction times were 5 min.
cross-condensation. Evidently self-condensation of the aldehyde is either slow or does not consume base under the conditions used. Amides are diflicult to saponify even in aqueous DMSO. Extended heating with base failed to give quantitative results for acetamide. From the single result in Table II it appears that esters saponifiable at room temperature can be determined without interference from aliphatic amides. The recommended procedure employs about a 4-fold excess of base. The moderately high blanks caused by this amount of base could be reduced by using a lower ratio of base to ester, although this would probably necessitate a slightly longer reaction time for some esters. It should be noted that phenolphthalein, the usual indicator in saponification procedures, gives a poor end-point in the DMSO method. However, Cresol Red indicator gives a sharp colour change from red to yellow. This change corresponds with the steepest part of the titration curve obtained by using a pH meter. Ion-exchange procedure
Results for some esters analysed by the ion-exchange procedure are shown in Table III. All esters were saponified for 15 min at room temperature. The results are very good with the exception of phenyl benzoate, in which some phenol is evidently formed on the column and titrated. Only one ester group is hydrolysed in the hindered diester, di-2-ethylhexyl sebacate, owing to the insolubility of the half-ester sodium salt which precipitates before further hydrolysis can take place. The ion-exchange method suffers from being too time-consuming; an ordinary TABLEIIL-ANALYSIS
OF ESTERS BY THE ION-EXCHANGE PROCEDuRE
Ester
Found, %
Ethyl acetate Methyl isobutyrate Diethyl phthalate Cyclohexyl pivalate Phenyl benzoate Di-2-ethylhexyl sebacate
99.5 99.6 995 99.4 111.9 49.6*
* Based on 2 ester groups; on 1 ester group.
99,2 % found based
Quantitative
determination
of esters by saponification
in dimethyl sulphoxide
analysis requires 45-60 min. It is difficult to remove the last traces acid from the column, and several washings are required. However, is generally
applicable
as a quantitative
semi-micro
1677
of carboxylic the procedure
method.
Ac/cnowZedgements-The authors express appreciation to D. Jordan and the Continental Oil Company for a generous supply of highly hindered esters and are grateful to C. E. Thompson, of the Petroleum Products Division of that Company, who synthesised these hindered esters. We also thank Alan G. Bemis for suggesting this method and Frank E. Gainer for valuable experimental assistance. R&sum&On donne une mtthode de dosage des esters faisant appel a la vitesse particulierement rapide de leur hydrolyse alcaline en milieu dimethylsulfoxyde aqueux. Pour l’hydrolyse quantitative, la majeure partie des esters ne necessite que 5 mn de chauffage au bain de vapeur, et nombre d’entre eux reagissent completement en 5 mn a temperature ordinaire. Lorsque l’hydrolyse est totale, on dose l’exces de base avec un acide tit& utilisant un indicateur visuel. Zusammenfasflmg-Eine Methode zur quantitativen Bestimmung von Estem wird angegeben, die die ungewijhnlich hohe Geschwindigkeit ihrer alkalischen Hydrolyse in w5iBrigem Dimethylsulfoxyd ausntltzt. Nur fiinf Minuten Erhitzen auf einem Dampfbad reicht aus, die meisten Ester quantitativ zu verseifen, und viele reagieren in fiinf Minuten bei Zimmertemperatur. Wenn die Hydrolyse vollstlndig ist, wird die iiberschtlssige Base mit eingestellter Saure und einem visuellen Indikator titriert. REFERENCES 1. R. T. Hall and W. T. Shaefer in J. T. Mitchell et al., Organic Analysis, Vol. II, pp. 20-70. Interscience, New York, 1954. 2. W. E. Shaefer and W. J. Balling, Anal. Gem., 1951,23,1126. 3. D. E. Jordan, ibid., 1964,36,2134. 4. A. J. Parker, J. C/rem. Sec., 1961,132s. 5. Idem, Quart. Rev. London, 1962,16,163. 6. E. Tommila, and M. L. Murto, Actu Chem. Scund. 1963,17,1947. 7. D. D. Roberts, J. Org. Chem. 1964,29,2039. 8. Idem, ibid., 1964, 29,2714.