Esterification for the introductory
W. H. Puterbaugh, C. H. Vanrelow, K. Nelson, and E. J. Shrawder
Thiel College Greenville, Pennn/lvonio
I
Organic Laboratory Course A modified Dean-Stark trap
The process of esterification, or its revernal, is often used in the physical chemistry course as a laboratory experiment to demonstrate attainment of equilibrium from various directions, or as an easily followed kinetic study. A survey of esterification experiments in some of the widely used organic laboratory manuals shows that while the equilibrium nature of the reaction is usually discussed, the actual experiments chosen for laboratory practice do not seem to give the student the fullest possible appreciation of the phenomena in physical terms. Most of the manuals call for the use of a large excess of alcohol. Another technique which can be used to advantage in systems where water is one of the products of the reaction is that of removing the water as it is formed by azeotropic distillation. Bremster, VenderWerf, and McEwen,' and Robertson and Jacobs,? provide alternate esterificat,ion experiments in which this principle is used, but the procedures seem more time-consuming and less convenient than that described here. A simple technique which makes possible continuous aze~tropicremoval of water along with solvent recycling employs a trap of the type described by Dean and Stark,3 and is widely used in industrial practice in the preparation of polyesters. A variety of inert solvents will form an azeotropic system. The solvent chosen should form an azeotrope with water which is the lo~vest boiling of all the possible components in the mixture. Also the boiling point of the solvent must permit easy separation from the desired product. Benzene which forms an azeotrope of 8.8% water boiliug at 69.3' is particularly couvenient. The preparation of y-chloropropyl acetate in 93-95% yield by t,his method has heen descrihed in detail.* Such an experiment has not been suggested for the beginning organic course probably because of the relat,irely high cost of purchasing such traps. At a negligible cost of materials, however, traps can be made from a standard 20 X 150 mm Pyrex test tube and 12 mm od Pyrex tubing. A side arm is sealed onto the test tubr approximately 50 mm from the top. A right auglr bend is made in the side arm about 100 mm from 'RREWSTER,R. Q., VANDERWERF, C. A.,
AND
MCEWEN,
W. E., "IJnit,ii;ed Experiments in Organic Chemistry," D. Van h'ostrnnd Company, Princetan, N. J., 1960, p. 10H.
= R O B E R T S ~(:.NR , . , AN!) JACOBS, T. L., " T ~ h o r ~ t o rPractice y in Organic Chemistry,'' 4th ed., The M.lnemil!an Company, h'ew I'ork, 1962, p. 2%. I)EAN, 1':. \\-., A N D STARK,I). TI., h d . m d En#. Chem., 12, 4S6
(lR2O).
HORNING. E. C., ed., "0rg:tnia Synt,hesis," Co!l. Vnl. 3, John Wile? and Sons, Sea York, 1955, p. 203. Presented at the April, 1960 Meeting of the Pennsylvanin Assoriation of College Chemist,r? Tearhers, Gett?shurg, Pn.
its joint with the test tube, and the side arm is then cut and polished about 50 mm below the bend. The experiment is set up by attaching a round bottom flask to the side arm, and an upright condenser to the test tube. The flask is charged with the reactants and solvent, and the trap is filled to the side arm return with solvent. The mixture is heated to reflux; and as the reaction occurs, the condensate from the refluxing mixture separates, a i d the heavier water layer is retained in the bottom of the trap, while the solvent is returned by overflow to the reaction flask. As a senior project, a student determined optimum conditions for the preparation of n-amyl acetate by this method in terms of mole ratios of reactants, amount of catalyst, and reaction time. The study indicated that best results are obtained on a convenient scale by refluxing a mixture of 0.25 mole of acetic acid, 0.20 mole of n-amyl alcohol, 30 ml of benzene, and 0.15 g of p-toluenesulfonic acid (catalyst) for one hour. At the conclusion of this time, water evolution is essentially complete. Usually, about l(t15% more than the theoretical amount of water is given off, due to a combination of water present in the commercial reageuts and to some entrainment in the vapor of the alcohol or acid. The reaction mixture is worked up by extracting with sodium bicarbonate solution to remove excess acid, washing with water, and then with saturated sodium chloride solution. The organic layer is distilled through a fractionating column to remove the bulk of the benzene (and any residual water as an azeotrope), and when most of this has distilled, the residue is transferred to a small distillation flask containing a small plug of steel wool in the neck aud the product is isolated by simple distillation. For 100 student preparations, the average yield of n-amyl acetate, bp 141-14Fo, was 71%. The quality of the preparations may be easily checked by taking an infrared spectrum. The most likely contaminants are n-amyl alcohol (which boils withiu 10' of the ester) and benzene. These may he readily detected in the presence of n-amyl acetate, since the alcohol shows the typical OH band near 3500 em-', and benzene has absorptions close to 1970 cnx-' and 1820 cnx-', regions where the pure ester does not ahsorb. A study also was made of the yields of esters which could be obtained under similar reaction conditions from a variety of common alcohols and acids. The results are given in the table. Longer reaction time was needed with a secondary alcohol. The yield of sec-butyl acetate after one hour was 26%; after 2 hours, 40%. No water evolut,ion or ester product was obtained with tert-butyl alcohol even after 4 hours reaction time. While n-hutyl, iso-butyl, Volume 40, Number 7, July 1963
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349
n-propyl, and iso-propyl acetates can be prepared in good yields by this procedure, difficulty was encountered in getting clean separation of these lower boiling esters from the starting alcohol and the benzene by Yields of Esters from Alcohols and Acids Using Azeotropic Esterification Method
Alcohol n-Amy1 is+Amyl n-Butyl n-Butyl n-Butyl is+Butyl see-Butyl tert-Butyl n-Propyl iso-Pmpyl
350
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Acetic Acetio Acetic Acetic Acetic Acetic
Journal o f Chemical Education
Reaction time hours
Ester yield
1
75
%b
simple distillation. This was indicated by the wider boiling ranges and by infrared examination of the products. Student response to the experiment has been good, and a number alwavs comment that this was one reaction they could actually "see" going. There are several variations which can be used, if dedred, to illustrate further principles. The reaction may be followed kinetically by having the students calibrate the trap and measure water evolved versus time. The effect of catalyst can be demonstrated by having the students reflux the charge without catalyst for some time, then add catalyst and observe the rapid increase in water evolution. The effect of structure on the reactivity of alcohols toward esterification may be shown by giving different sections of the class a primary, secondary, or tertiary alcohol as starting material and have them observe and report relative reaction rate and yield after a specified period of heating.
