Robert 1. G r o b and G. Ronald Husk Villanova University Villanova, Pennsylvania 19085
I I
Indentifitation of an Unknown Ester An analytical-organic experiment
This experiment is a part of our overall effort to integrate two of the laboratory courses at Villanova (1). I t is still the belief of our analytical and organic staffs that the efficiency of our approach helps our students overcome the problems of assimilating the large quantity of chemical knowledge and utilizing the many lahoratory techniques necessary for advancement in the field. For the past two years an experiment has been successfully carried out which strives not only to preserve the cooperative, integrated effort, hut at the earliest stage to introduce the student to reactions and teach him basic techniques. We hope that this approach spares him the boredom of a series of "Look and See" demonstrative manipulations. The experiment is based upon the hydrolysis of a simple unknown ester. While the detailed mechanism of the reaction is not presented at this point, enough details are given during the laboratory lecture so that the student understands the basis of the reaction. Details of the mechanism for ester hydrolysis are covered later in the lecture portion of the course. I t is the student's challenge to hydrolyze the ester, separate and purify the resultant alcohol and acid, determine several physical and chemical characteristics, and arrive a t a logical choice for its identity. The student is allowed 15 lahoratory hours in the organic analytical laboratories. I n the organic lahoratory the student is introduced to the technique of temperature control of a reaction by reflux; fractional distillation, both at atmospheric and subatmospheric pressures; extraction; determination of boiling points by distillation and by micro methods; refractive index; recrystallization; determination of melting points; and simple vacuum sublimation. I n the analytical lahoratory the preparation of analytical samples is stressed. The student accurately and precisely determines the neutralization equivalent of the cster, the equivalent weight, and the pK, of the acid. This combined experiment stresses two other important features; that of optimum use of a sample and the planning of laboratory work prior to entering the lab. Regarding the first point; depletion of the ester and/or acid sample quantities because of too many faulty analyses results in the student returning to the organic laboratory and preparing more analytical sample, whereas-poor technique in the organic laboratory results in no analytical samples for the analytical laboratory. The second feature, i.e., poorly planned work, could result in the student not having an experiPresented in the Division of Chemical Education Program st the 157th Meeting of the American Chemical Society, Minnespolis, Minn., April, 1969. Kontes Glass Company, Vineland, N. J.
ment completed when the subsequent laboratory depends upon the first. Experimental Organic Loborotory
General Remarks. In addition to the text (8') supplementary instruction sheets were given to each student. The student was referred to the t a t for general principles and description of basic techniques. Supplementary instructions were kept as general as possible, e.g., students were asked to prepare "100 ml of 6 M sodium hydroxide in 25% by volume methanol in water" rather Standardthan to "dissolve 24 g of sodium hydroxide in taper labamtory equipment (Kem-Kit)' was employed throughout the experiment. Two esters have been used extensively; butyl benzoate and isoamyl benzoate. Results from these studies are described below. Hydrolysis of Ester. Standard technique and apparatus were used. About 30 g of ester was hydrolyzed in 100 ml sodium hydroxide. A mixture of methanol-water was used as the reaotion medium. The effect of methanol on the yields and rate of hydrolysis is shown in Table 1.
. . . ."
Table 1.
Volume Ratio MeOH: HnO
Effect of Methanol on Hydrolysis of Ester
Time of Reactiona (hr)
Weieht of Buhol Isolated (g)
Weight of Benaoic Acid Isolated (g)
It is to be noted that while increase in methanol concentration markedly increases the rate of reaction, the yield of butanol is markedly decreased. Further, at high concentrations of methanol, sodium benzoate becomes insoluble in the reaction mixture, and it is necessary to add water terlowly to maintain a two-phase liquid medium. I t is necessary to reflux the reaction vigorously in order to obtain good mixing of the two-phase systems. S e p a m t i a of Alcohol. The alcohol was extracted from the basic hydrolyzate with 150 ml ether. The ether extract contained nearly a gram of sodium benzortte and considerable methanol. Although the boiling point of methanol is about 50" below that of hutanol, a reasonable separation is only achieved with a fractionating column (see Fig. 1, curves 1 and 2). D i s tillation e m be simplified by removing the'methanal and sodium benzoate by backwashing the ether extract with 1.50 ml of water. Indeed the yield of purified hutanol is increased (see Fig. 1, curves 3 and 4) and, while the distillation curve is not shown, only s. Claisen head is needed for reasonable purification. To the student the most discouraging aspect is the tendency of the alcohols and esters to foam during distillation. Typical data for the two dcohols are shown in Table 2. Separation of Acid. Precipitation and recrystallization of benzoic acid is a well known procedure (2). It is most important to stress desiccation of the benzoic acid. The crude acid may conlain as much as 40% water after suetion-drying for 1 hr. Under similar conditions the recrystdised acid is about 7 % water. Constant weight acid results from desiccat,ionover CeSO, Volume 46, Number 1 1, November 1969
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Table 2.
Alcohol
Alcohols Recovered from Hydrolysis
Boiling Point -("C)-Lit. Expt.
Butanol Isoamyl Alcohol
116 130
115.5 130.0
Table 3.
5 10 15 20 TOTAL VOLUME DISTILLATE. ML. Figure 1. Distillation of butand from hydrolysis of buwl benzoate. Bockwashed lBWl or non-bockwaled (NBWI ether edroct from 5 0 : 5 0 MeOH:H20 or 2 5 : 7 5 Me0H:HpO. Hydrolyrate was stripped of ether and distilled using either simpla Cloisen-head (c.h.1 or froctionoting column If.c.1. o NBW, 5 0 : 5 0 , c.h.; X NBW, 5 0 : 5 0 , f.r; 0 BW, 5 0 : 5 0 , f.e.; A BW, 2 5 : 7 5 , f.c.
Percent Recovery
--
D --
Lit.
48.8 67.0
Ex~t.
1.3991Pn 1.402712 1.4085'6 1.410012
Distillation of Ester
Ester
Micro Boiling Point (OC) . .
Vacuum Boiling Point ("C) . .
Lit. (OC).
Butyl Benzoate Isoamyl Benzoate
249 260
118 (12 mm) 13g139 (18 mm)
249 261
Table 4.
.
Analyses of Equivalent Weight of n-Butyl Benzoote
Range of Results
n
R
n AcceptdAe*
150.0-159.9 160.0-169.9 170.0-179.9 180.0-189.9 190.0-199.9
5 15 48 14 1
153.4 167.3 175.5 183.4 190.0
None None 48 13 None
far 24 hr. The dried, crude bensoic acid can he recovered in greater than 80% yield in all cases. The relative purity of the crude acid is shown by comparison with the recrystallized acid
'Accuracy range acceptable: 0-5y0 true value. True equivalent weight: 178.25. Acceptable data: 2 = 177.1.
Melting point cmde acid, 121.5'C Melting point recrystdlized acid, 121' Recorded melting point, 122' Sublimation of Acid. Vacuum sublimation of s m d quantities of acid is readily carried out. A crude but effective sublimator was made from an 8-in. side-arm test tube, a rubber stopper, and s. closed tube to be used as a. cold-finger condenser. Distilhlia of Ester. Micro bailing points may he determined for the ester. The student is asked to distill the ester as an analytical sample. It is the student's decision whether or not to use vacuum distillation. Typical data areshown in Table 3.
weigh a 4meq sample of the acid (-0.5 g & 10 mg) into a 250ml beaker. Dissolve in 50 ml of water and hest solution to 60-70°C. Since the scid was bensoic acid this heating enhances the solubility (2.2 g/100 ml in 75'C). Titrate warm using a, pH meter equipped with glass and saturated calomel electrodes. Plot the change in pH versus volume of 0.1000 N KOH. On the same graph plot (1) ApHIAV versus mlO.lOOO N KON and (2) AepH/AV* versus mlO.lOOO N KOH. Determine the equivalence point from the graph and calculate the rneq wt. of the acid.
Analytical Laboratory
Geneml Remarks. Accomplishing the analytical portion of this experiment involves several techniques and manipnlst.ions. One important technique is that of sample preparation. The importance of this step cannot he over-emphasized. The results obtainable here, as in any good analytical procedure, will be reliable only to the extent that the srtmple preparation is reliable. Two analytical samples are necessary; an ester sample and an acid sample. The analytical sample of the separated acid must be dried in a desiccrttor and the m e l h g paint determined. The ester must be purified by distillation. The neutralization equivalent of the purified ester is determined by the usual method of hydrolysis (8-4). The equivalent weight of the acid snd its pK. are determined by B potentiometric wid-base titration (6, 6). Neutralization Equivalent Ezperiment. Accurately weigh 4-6 meq (0.8-1.0 g) of the purified ester into a 250-ml round-bottomed flask. I t is important to keep the sample size within the limits set or the experiment may be unsuccessful. Pipet 25.00 ml of 0.5000 N alcoholic potassium hydroxide into the flask. Boil the mixture under s reflux condenser for a minimum of 30 min. If the reaction mixture contains any crystalline solid pour just enough distilled water through the condenser to dissolve (no mare than 50 ml). Reflux an additional 15 min. Add 25 ml additional water through the condenser and allow the reaction mixture to coal. Remove the condenser, add five drops of phenolphthalein indicator and titrate with 0.2500 N sulfuric acid. A blank should be run using everything but the ester. Make the blank correction on all sample titrations. The determination is performed in triplicate; the individual results and the arithmetic mean are reported. meq ester = meq base - meq acid rneq wt. ester = mg samplelmeq ester Equivalent Weight and pK. of Acid Experiment. Accurately
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meg wt. acid = mg sample/ml KOH X 0.1000 N Also determine the pK. of the scid from the graph. This is determined as follows HA=H++A[Ht1[A-I/[HAl = K. solving for [H+] [Ht1 = K.[HAI/lA-I tsking negative log of bath sides of equation pH = PK,
+ log [A-IIIHAI
when acid is half-neutralized log [A-]/[HA] = log 1/1 = 0 therefore pH = PK. Here also, the determinations are performed in triplicate. The individual values m d the arithmetic mean are reported. Results a n d Discussion
Neutralization Equivalent. This experiment has been performed for the past two years. During the first year only two esters were used; i.e., n-butyl benzoate and isoamyl benzoate. This year we issued two other esters~toa limited number of students. These esters were ethyl cinnamate and ethyl phenyla~etate.~As a T h e ~ ewere analytical samples given to students who were not taking the organic laboratory course concurrently.
Table 5.
Analyses of Equivalent Weight of lsoomyl Benzoate
Accuracy ran@ acceptable: 0-5'% true-value. True equivalent weight: 192.28. Acceptable data: X = 190.5. Table 6. Analvses of Eauivalent Weight of Ethvl ~innamdteand ~thyl~henylacehe
Range of Results
n
R
~cie~table*
nimate Ethyl Phenylacetate
True Eauivalent Weieht: 176.23 160.0-169.9 6 164.7
6
data was that his relative average deviation could not exceed 10 ppt. Low equivalent weight values could usually be attributed to poor standardization technique for the KOH. High values were usually a result of using wet samples of the acid. Table 8 lists the student data for the pK. of their acids. I n order to obtain a more precise value for the pK. it is necessary to expand the coordinates of the graph so that the plotting is as accurate as the pH and buret readings. This is illustrated in Figure 2. Here we have plotted pH versus ml of 0.1 N KOH added and ApH/Aml versus ml of 0.1 N KOH. Data with greater than 5y0 absolute error was unacceptable. The greatest cause for error in this determination was inaccurate standardization of the pH meter. I n no case was any significant change noted in the calibration of the pH meter before and after use for a titration. Student criterion for precision of data was a maximum relative average deviation of 50 ppt. Summary and Recommendations
True Equivalent Weight: 164.22 Accuracy range acceptable: 0-5% true value.
result the largest amount of data is available on the benzoate esters. Table 4 shows the data obtained with the n-butyl benzoate ester. Data which were greater than *5%of the truevalue were not acceptable, i.e., the student had to repeat the experiment until acceptable data were obtained. The criterion used by the student to check the precision of his data was that his average deviation should not be greater than three equivalent weight units. The large number of low results was usually due to improper standardization of either the acid or base solutions. High results were explainable by insufficient purification of ester sample. Tahle 5 lists the data obtained with the isoamyl benzoate ester. The same criterion was used for the judgment of data as presented above for n-butyl benzoate. Table 6 lists the data for both ethyl cinnamate and ethyl phenylacetate. Equivalent Weight and pK,. Here, as with the equivalent weight of ester experiment, most of the data comes from the benzoate esters. Tahle 7 lists the data for the equivalent weights of the various acids. Data which were greater than 10 ppt relative error were not acceptable (the student had to repeat the laboratory work until acceptahle data were obtained). The criterion used by the student to check the precision of his
Ester hydrolysis has been successfully used as an introductory experiment to acquaint junior level students with basic techniques in both the analytical and organic laboratories. Data obtained from two separate classes show that precise and accurate results can be obtained by the competent student. The students are thus able to come to a logical conclusion for the identity of their unknown ester. A typical set of data is shown below in Table 9 for isoamyl benzoate, as presented to the instructor a t the end of the experiment. A listing of commercially available esters which fit the Table
7. Analyses of Equivalent Weight of Acids Range of Results
None 58 None None 6 3
" Accuracy range acceptable: 0-10 ppt relative error. Table 8.
Benzoic Acid
5-
True pK,: trans-Cinnamie Acid Phenylaeetic Acid
Determination of pK. of Acid
a
n
R
4.004.09 14 4.10-4.19 11 4.204.29 23 4.304.39 13 4.404.49 18 4.504.62 18 4.21 Acceptable Data: 4.334.53 6
True pK,: 4.44 4.214.39 True pK,:
ML. KOH Titrotion curves for benroic acid.
~cceptablem
118.0-120.8 13 119.9 58 122.5 120.9-123.7 13 128.2 123.8-129.5 129.6-135.6 6 133.4 True Equivalent Weight: 122.12 trans-Cinnamic 146.6-149.6 6 148.3 Acid True Equivalent Weight: 148.17 Phenylaeetic 134.2-136.7 3 135.3 Acid True Eouivalent Weieht: 136.16
Range of Results
Figvro 2.
I?
Bensoio Acid
,3022 GRAM BENZOIC ACID
.z
n
3
nAccepe able* 14 11 23 13 13 None 6
4.29
3
4.31
Accuracy range acceptable: 0-570 absolute error. Volume 46, Number 1 1 , November 1969
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Table 9. Student Data Sheet he following data is consistent, suggesting my unknown ester is Isoamyl Benzoate lhtor
Boiling Point: micro other Refractive Index Neutralizat,ion Eauivalent Alcohol Boiling Point Refractive Index Molecular Weight Acid Neutralizt~tionEquivalent P&
w ~ ~".,"".""w" ~ r i --r".
r n ~ Tt ilarntrrra ~ l
260°C 260.7T 138-139 (18 mm) 1.4969 (12O) 1.4930(25") 191. 8 192.28 130°C 1.4100 (12') 88.0
130PC 1.4085 (15') 88.17
121.8 4.27 121.9"C
122.12 4.21 122°C
Melting Point Comments Based on the ester chosen, I was able to recover 85% of the aeid and 67% of the alcohol from the hydrolyyis.
description of above conditions is shown in Table 10. The listing in Table 10 is by no mcans exhaustive, but shows that suitablevariations in esters of three acids are available so that the "unknown" quality of the experiment can be reasonably preserved. Extension to other ester systems is contemplated through utilization of student preparations either later in this course or in advanced courses. While the experiment has becn employed in a situation where the laboratories are taught concurrently, it can easily he modified to he used independently for each laboratory. I n order to gain maximum utility of the integrated approach in this experiment it is necessary to choose esters with the following characteristics Table 10.
178.25 178.25 192.28 192.28 206.31 204.29 218.32 230.33 192.28
Distillation Products, Rochester, New Ynrk. W and K Laboratories, Plainview, New York. a
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Acknowledgment
The authors wish to thank all the staff members who have contributed time and suggestions for the operation of this laboratory course. Our special thanks go to the students who actually performed the experiments and whose data we have presented. Literature Cited (1) GROB,R. L., J. CHEM.EDUC.44, 319 (1967). T. L., "Laboratory Practice (2) ROBERTSON, G. R., AND JACOBS, in Organic Chemistry" (4th ed.), The Macmillan Co., New York, 1962. (3) KOEITSTORFER, J., Z. Anal. Chem., 18, 199, 431 (1879). W. E., AND BALLING, W. J., Anal. Chem.,23,1126 (4) SCHAEFER, (1951). A. L., "Quantitstive Analysis (5) DAY,R. A,, A N D UNDERWOOD, Laboratory Mmual," (2nd ed.), Prentiee-Hall Inc., Engle wood Cliffs, New Jersey, 1967, pp. 152-7. (6)FrscHEn, R. B., A N D PETERS,D. G., "Quantitative Chemical Analysis'' (3rd ed.), W. B. Saunders Co., Philadelphia, Pa., '1968,pp. 714-21.
Commerciallv Available Esters B.P. Ester ("C)
n-Butyl Benzoate iso-Butyl Benzoate Amyl Benzoate Isoamyl Benzoate Hexyl Benzoate n-But.yl Cinnamate n-Amyl Cinnsmste Cyelobexyl Cinnamate Butyl Phenylacetate
The ester should have a boiling point compatible with a simple method of purification, e.g., recrystallization or vacuum distillstion. I t must have same solubility in methenol-water. The alcohol derived from the ester should have a favorable distribution coefficient between methsnal-water and ether. Its boiling paint fihould be law enough for simple or vacuum distillation (most CI, Cs, and Ca alcohols fall into this category). The acid derived from the ester should be a sublimeable solid which can be recrystallized from s. simple solvent system, e.g., water or alcohal-water. I t should have a. pK, in the range of 3-6 (benzoic acid, most mono-substituted, e.g., C1, Br, CHa-, NO2- benzoic acid, phenylacetic acid, and cinnilrnic acid seem suitable).
261 159.5 (20 mm) 90-100 (0.5mm) 104-106 (0.5mm) 114-11.5 (0.4Rmm) 143. 1.5
B.P. ROH ("C) 138 130.5 157.2 117.71 137 161.5 117.71
M.P. RCOOH ("C)
Source
-
DPo DP K and Kb I)P DP K and K K and K K and K K and K
