Boron Trifluoride-Methanol Transesterification as a Means of Characterizing Alcohol Sulfate Detergents C. B. Puchalsky Uniroyal Chemical, Division of Uniroyal, Inc., Naugatuck, Conn.
AFTERIDENTIFICATION of a detergent as an alcohol sulfate, conveniently done by infrared spectroscopy, much important information is still lacking about its hydrophobic portion. A hydrolysis step is necessary to release the bound alcohols. Acid hydrolysis proceeds more rapidly than basic for these esters, but is still time-consuming and incomplete. For example, 5 % aqueous sulfuric acid containing half its volume of ethanol to displace the hydrolysis equilibrium, required 5 hours of reflux for 50 % completion. More severe hydrolysis conditions ( I ) may cause dehydration of alcohols to olefins, and rearrangement, particularly of branched alcohols. The procedure described here is a variant of the well known boron trifluoride-methanol transesterification method for preparing methyl esters from higher molecular weight esters, or from acids. A large excess of methanol replaces the bound alcohols which are recovered by extraction. The stability of the boron trifluoride-alcoholate intermediates is complex and not yet fully understood (2, 3). Experiments involving the reaction of boron trifluoride-methanol with straight and branched-chain primary and secondary alcohols showed no decomposition or rearrangement, under the conditions of the test. Tertiary alcohols are decomposed rapidly. A short alkaline reflux effectively decomposed any borate esters formed during transesterification.
CI 2
Figure 1. mixture
Chromatogram of standard alcohol
EXPERIMENTAL Apparatus. Programmed temperature gas chromatograph, with a 5-foot column containing 5% Carbowax 20M and 1% Igepal CO-880 on 70/80 mesh Chromosorb G was used. Reagents. 14% Boron trifluoride-methanol, Analabs Inc., Hamden, Conn., was employed. Procedure. Free alcohols present in the sample should be removed prior to treatment by extraction with petroleum ether. A small amount of ethanol added to the aqueous phase will reduce foaming. Weigh accurately about 0.25 g of the well-dried sample into a 125-ml Erlenmeyer flask. Add 10 ml of 14% boron trifluoride-methanol, several boiling chips, and insert a reflux condenser. Apply a pre-heated hot plate and boil the solution for ten minutes. Then, without cooling the solution, add 50 ml of 1N sodium hydroxide solution. Reflux the basic solution for 15 minutes more. Remove the hot plate and chill the flask and contents with ice water to below room temperature. Put 25 ml of petroleum ether through the condenser without disconnecting the apparatus to recover alcohols trapped in the condenser. Shake out the phases in a separatory funnel, and collect the petroleum ether phase in a pre-weighed beaker, containing a filter paper cone to remove droplets. Repeat the petroleum ether extraction twice more with 25-ml portions. Bring the pooled extract cautiously to dryness, cool, and reweigh. (1) J. D. Knight and R. House, J. Amer. Oil Chem. SOC.,36, 195
START,
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Figure 2. Chromatogram of Tergitol-08 alcohols, solid line due to BF3-CH30H product; dotted lines are additional peaks of aqueous hydrolyzate
Inject several microliters into the prepared gas chromatograph. Typical conditions are a starting temperature of 150 “C,programmed at 12 “C per minute to 250 “C,using a flow rate of 25 ml per minute.
(1959). (2) H. S. Booth and D. R. Martin, “Boron Trifluoride and Its Derivatives,” p 61, John Wiley & Sons, New York, 1949. (3) W. Gerrard, “Organic Chemistry of Boron,” p 209, Academic Press, New York, 1961.
RESULTS AND DISCUSSION
Dodecanol, 2-dodecanol, and 4 methyl-4-nonanol were analyzed by the preceding procedure. Both dodecanol and 2ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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__--Figure 3. Chromatogram of Tergitol-4 alcohols Figure 5. Chromatogram of Sipex UB alcohols
Figure 4. Chromatogram of Tergitol-7 alcohols
dodecanol were recovered unchanged, as shown by infrared and gas chromatography. The recoveries were 93 and 95%, respectively. A tertiary alcohol, 4 methyl-Cnonanol showed complete loss of the hydroxyl group (infrared) and a weight recovery of about 50%. Dodecyl sodium sulfate, Eastman White Label, (1.8% water, 4.7 free dodecanol), when transesterified by this procedure, gave recoveries of 90 and 92 % on duplicate analysis. The recovered alcohols were shown to be essentially all ndodecanol by gas chromatography and infrared. Heating with boron trifluoride-methanol for only 2 and 5 minutes showed recoveries of 13 and 78 %, respectively. Samples of Tergitol-08,-4, and -7 (Union Carbide Corp.) were analyzed, and also refluxed with 5 % sulfuric acid in water-ethanol for 6 hours. See Figures 1-4. Tergitol-08 is
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described as the sodium sulfate derivative of 2 ethyl-1-hexanol, Tergitol-4 as the sodium sulfate derivative of 7-ethyl-2-methy1, 4-undecanol, while Tergitol-7 is the sodium sulfate derivative of 3,9-diethyl tridecanold. In all cases the samples transesterified by the boron trifluoride-methanol reagent were identical to the aqueously hydrolyzed samples, or had fewer peaks by gas chromatography. A sample of Sipex UB, identified as the sodium salt of lauryl sulfate, was analyzed three times by the procedure. Four peaks were obtained on each chromatogram (see Figure 5 ) that had retention times equivalent to decanol, dodecanol, tetradecanol, and hexadecanol. The area per cent values for these three runs agreed to better than 3=0.4%, absolute, on all peaks. The recoveries were 86,86, and 88 of theoretical.
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CONCLUSION
The method described in this report is not limited to alcohol sulfates, but is applicable to any ester undergoing transesterification in a reasonable time. The majority of carboxylic acid esters are transesterified completely in 10 minutes, or less. RECEIVED for review February 16,1970. Accepted March 23, 1970.
