Lab 5 - Lecture notes l5 PDF

Preview

Summary

Lecture reading from TA...

Description

Lab 5: Steam Distillation of (S)-(+)-Carvone from Caraway Seeds and (R)-(-) – Carvone from Spearmint Leaves Milan Patel February 24, 2016 Methods and Background The purpose of this lab was to isolate carvone from both caraway seeds and spearmint leaves through the process of steam distillation. In addition, the underline purpose of this lab was also to be able to use a separatory funnel in order to extract carvone from the aqueous mixture with CH2Cl2. After collection of carvone, IR spectrometer was operated to verify isolation of carvone by functional group identification. TLC was also performed to verify isolation of carvone by TLC comparison with authentic samples. And finally, the Baeyer test was subjected to confirm identity of functional groups. The Rf values of the distillate were compared with the Rf values of an authentic sample of S-(+)-carvone and also to examine the relationship between enantiomers of carvones using IR spectroscopy with carbonyl and alkene groups present in the structures. Carvone is a chemical found in essential oils. It is also found in both caraway seeds and spearmint leaves. This two types of carvones are extremely similar. However, they are not identical – they are enantiomers. Enantiomers are compounds that are mirror images of one another. Caraway seeds have the (S)-(+)-Carvone configuration, while spearmint leaves have the (R)-(-)-Carvone configuration. Each enantiomer of carvone is physically identical to the other and contains many of the same properties (as shown in figure 1), such as molecular formula, color, boiling point, melting point and density. The IR spectra and thin layer chromatography analysis are identical for each enantiomer as well.

Structures

(R)-(-)-Carvone

(S)-(+)-Carvone

B.P

231 ºC

231 ºC

Smell

Spearmint

Earthy

Optical rotation

-61°

+61°

Density

0.96

0.96

IR spectrum

Same

same

Figure 1: Enantiomers of Carvone (R and S Configurations) with their propertires Spearmint leaves have (R)-(-)-carvone; caraway seeds have (S)-(+)-carvone.

The difference between the S and R configurations comes in that they are mirror images of one another, and thus, the prime difference between enantiomers is found in that the position of functional groups are not identical as illustrated in figure 1. The difference between the (S)-(+)Carvone and (R)-(-)-Carvone enantiomers can be noted by asymmetric reactions in stereochemical environments, such as the human nose. As a result of this, (R)-(-)-Carvone from spearmint leaves has a characteristic “minty” smell, while (S)-(+)-Carvone has an “earthy” smell. This phenomenon reflects the idea that the human nose has chiral environment and can detect stereochemical differences in compounds. Another difference between the two types of carvones is the direction of the optical activity not the magnitude. Therefore, (S) enantiomer is (+) and R enantiomer is (-). Note that there is no relationship between the direction and the S and R of an enantiomer. Since carvone is chiral it is optically active which means that polarized light waves can either twist counterclockwise or clockwise. In order to isolate the carvone from the caraway seeds and spearmint leaves, steam distillation technique were used. Although, simple and fractional distillation are usually used to separate a mixture of liquids, they are not suitable for isolating liquids with high boiling points and are unstable at high temperature. In this situations, the steam distillation is applicable to substances that are immiscible with water, nonreactive with water, stable at 100 oC, and have a vapor pressure greater than or equal to 5 Torr at 100 oC. Thus, Steam distillation is used to separate volatile organic compounds that are immiscible with water. Carvone, which is nonpolar, is immiscible with water because water gets its polarity from the hydrogen bonds. Water is not linear compound, the lone pairs on the oxygen makes water a bent shape which increases its dipole movement and makes water a polar compound. As a result of immiscibility, the partial pressure of the each compound is independent of the mole fraction of each substance. This can be ascertained from Raoult’s Law, which states that partial pressure (Px) is equal to the mole fraction multiplied by the equilibrium vapor pressure. This means that the total pressure will be higher than the vapor pressure of the volatile component in the mixture, while the boiling point of the immiscible compound is reduced to a level below the boiling point of the lowest boiling point component. Thus, this allows for the immiscible component, carvone, to be collected in the steam distillation process. In addition, the boiling point of a liquid can be defined as the temperature at which the vapor pressure of a liquid will be equal to the atmospheric pressure, which at normal conditions is about 760 Torr. From the pressure-temperature graph (Figure 2), we can determine that the boiling point of the components of our mixture as separate substances. At 760 Torr, the temperature of the water is about 100oC, the temperature of Limonene is about 180oC, and the temperature of Carvone is about 240oC. Steam distillation utilizes this theory to separate components that have high boiling points, at a reasonable lower temperature.

Figure 2: Vapor PressureTemperature Diagram for Water, Carvone, and Limonene

After the sample gets isolated in the experiment, TLC and IR spectroscopy as well as Baeyer test are used to confirm the presence of carvone in the obtained samples of caraway seeds and spearmint leaves. It is important to note only for this experimental purpose that each group was assigned either caraway seeds or spearmint leaves to perform the experiment on and compare the results with other group, who worked with different sample than ours, in the end. Our group was assigned to investigate Caraway seeds by the Teaching Assistant and subsequently obtained data including IR spectroscopy print out and TLC calculations pertaining to the spearmint leaves from another lab group. In TLC, the sample of isolated carvone will be compared to a sample of pure carvone. The retention factors between both samples will be compared in order to determine whether carvone was indeed isolated. Baeyer test will be done to check for carbon carbon double bonds (C=C) and since our structure contains this functional group, we conducted this test. This is done by dipping the final TLC plate into a mixture of Potassium Permanganate which will change the color from purple to yellowish. Additionally, IR spectroscopy will be used to confirm whether the fictional groups present in the isolated sample match the expected functional groups to be seen in the IR spectra of pure carvone. Those functional groups include carbonyl groups and alkene carbons in carvone. The overall results of this lab indicates that carvone was indeed isolated from the caraway seeds and spearmint leaves. This is evident in the IR spectra with the presence of a characteristic peaks. Furthermore, the TLC analysis and Baeyer test yielded similar retention factors for both substances, thus indicating their similarity in terms of polarity. This make sense because both enantiomers have the same structure and functional groups. Thus, they should also have the same polarity.

Experimental Procedure

Before the experimental procedure started, the TA assigned our lab group to isolate the sample of carvone from the caraway seeds. The data for the carvone from spearmint leaves would be obtained from another lab group that was designated to isolate it. In order to perform the steam distillation, 5 grams of caraway seeds were balanced and obtained. Then, this organic substance was placed in a 500 mL round bottom flask. Next, the steam distillation apparatus was prepared as shown in Figure 3. Still head, a vacuum adapter, West condenser, and then a Claisen adapter was connected to the still head, as well as a thermometer and adapter at the top opening of the still head. Keck clips were securely attached to each junction between the glassware. Then, two rubber hoses were attached to West condenser and on end was attached to a water pipe, and the other to a sink drain. The water was turned on slowly to prevent any sudden rush of water. A graduated cylinder was used to obtain the carvone sample by placing it under the Figure 3: Steam Apparatus vacuum adapter. At theDistillation top of the Claisen adapter, a separatory funnel filled with 150 mL of very hot water was attached. Next, the round bottom flask with the caraway seeds had 150 mL of warm water added in order to speed up boiling. The flask was finally attached to the bottom of the Claisen adapter. The substance was then heated using a heating mantle until distillation started to occur. Once, the distillation rate became steady, the stopcock of the separatory funnel was opened to admit replacement water at the rate that it distills out of the flask. So that the volume of water in the flask remains nearly constant. When the volume of the distillate increased by 10 mL, the temperature was recorded every 10 mL intervals. This was performed until approximately 75 mL of distillate was collected in the graduated cylinder. When the distillate was collected, a new separatory funnel was obtained and filled with collected 75 mL of distillate and 7 mL of CH2Cl2. The separatory funnel was then gently inverted about 10 times. The bottom organic layer, which contained methylene chloride, was then extracted from the separatory funnel and put into a 50 mL Erlenmeyer flask. This extraction procedure was repeated two more times. The final solution was then mixed with approximately 1 g of anhydrous Na2SO4, which was filtered with a gravity filtration system into another Erlenmeyer flask. The isolated carvone solution was then subjected to TLC analysis. A solvent front and base line was drawn on the TLC plate. The plate also had 3 dot on it. From left to right the dots represented, respectively, the experimental sample, a co-spot sample that contained both the experimental sample and authentic carvone sample, and finally, an authentic carvone sample

(this has been illustrated in Figure 4). A capillary tubes were used to place samples of each compound on the corresponding dot. Each, TLC plate was placed in a solvent system of ethyl acetate and hexane with the ration of 1:9. Then, the Baeyer test was performed by exposing the plate to the KMnO4 solution and heating the plate with a heat gun and results were recorded. Finally, left over carvone sample was then placed under the hood to evaporate Methylene Chloride till about only 0.5 mL of liquid was left. Next, data collection was conducted with IR spectroscopy. The IR was completed for the caraway seeds sample. This elucidated the functional groups present in the isolated extract and the graphs were printed out for data collection.

Data Acquisition and Presentation Note: The data for the caraway seeds sample was acquired by our group. The data for the spearmint leaves sample was obtained from another lab group. This was done because each lab group was assigned one enantiomer of carvone only and was instructed to obtain the other data from a group with the other enantiomer.

Table 1: Distillation Heating Table – Caraway Seeds Volume (mL)

Temperature (oC)

10

85

20

86

30

87

40

87

50

87

60

87

70

87

75

87

Table 2: Distillation Heating Table – Spearmint Leaves Volume (mL)

Temperature (oC)

10

100

20

101

30

101

40

101

50

102

60

102

70

101

75

101

Calculation for Retention Factor for TLC of Carvone Retention Factor (R f )=

Distance Travelledby Substance(dx) Distance Travelled by Solvent (ds)

Figure 4: TLC Plates for Carvone after performing Baeyer test (Left: Caraway Seeds; Right: Spearmint Leaves). This figure is not up to scale.

Caraway Seeds Rf (Our Sample) = 2.7 cm /4.77 cm = 0.56

Spearmint Leaves Rf (Co-Spot) =2.61 cm /4.77 cm=0.55

Rf (Our Sample) = 2.31 cm /4.61 cm = 0.50

Rf (Authentic Sample) =2.61 cm /4.77 cm=0.55

Rf (Co-Spot) =2.31 cm /4.61 cm =0.50 Rf (Authentic Sample) =2.31 cm/4.61 cm=0.50

Table 3: IR Spectra Functional Group Stretches for Caraway Seeds and Spearmint Leaves Functional Groups

Spearmint Leaves

Caraway Seeds Wavenumber (cm )

Wavenumber (cm-1)

1643.26

1643.26*

1672.18

1672.18*

-1

Carbon-Carbon Double Bond (C=C) Carbon-Oxygen Double Bond (C=O)

*There were two distinct peaks, but wavenumber was not included. However, the peaks were approximately at the same position compared to the Caraway Seeds’ IR spectra.

Conclusion The purpose of this lab was to isolate the (S)-(+)-Carvone and (R)-(-)-Carvone enantiomers from caraway seeds and spearmint leaves respectively using steam distillation followed by extraction procedure and finally to verify the identity by using IR graphs, TLC plates and Baeyer test. Based upon the analysis of TLC retention factors and functional group identification from IR spectra, the presence of carvone was determined to be successfully isolated from the sample. Despite the data obtained, the enantiomers could not be determined with absolute certainty without the use of a polarimetry analysis. However, while it is not recommended to smell contents in a chemistry laboratory, the human nose could potentially be used to confirm the identification of enantiomers because each has a unique scent. The is due to the spatial arrangement of atoms for each carvone, meaning each structure differs at the chiral center which makes each carvone to be unique in certain properties such as smell and optical activity. In addition, the human nose has enzyme-binding site, which are also chiral and can detect the minty smell of (R)-(-)-Carvone, and the earthy smell of (S)-(+)-Carvone. Due to the specificity of an atom’s position, smell receptors only bind to a specific molecule. Overall, the isolation of carvone from both caraway seeds and spearmint leaves was successful. During steam distillation, the data from volume vs. temperature tables (Table 1 and 2) indicated that the temperature of the vapor of mixture was significantly lower (~87 oC for Caraway seeds and ~101 oC for spearmint leaves) and stayed constant over the collection of 75 mL. This would indicate that the vapor pressure of the system was high enough to cause the vaporization of both

the water and carvone from both samples. Thereby, showing the theory of steam distillation discussed in the beginning of the paper. Subsequently, TLC analysis was conducted with the solvent system being 1:9 ethyl acetate/hexanes. The TLC plates had three different spots in order to fully compare the level of polarity of pure carvone with the experimentally isolated carvone. The result showed that the carvone was successfully isolated by calculating the retention factors for each spot. The retention factors were almost same for the experimental sample, co-spot and authentic sample as shown in the calculations. The very similar retention factor values for each spot conclude that they all had the similar polarity. Though there was a slight difference between the samples, they are reasonably close enough to merit that they have similar polarity. The carvone is an organic molecule that is the reason behind using the higher amount of hexanes in the solvent mixture. Hexanes is also nonpolar that means the carvone will travel up the plate along with the mobile phase since the mobile phase is nonpolar as well as carvone. This means that carvone was likely isolated from caraway seeds and spearmint leaves, but may have the presence of a polar solvent such as water, which would lower the retention factor of the carvone isolated from the spearmint. Yet another test, Baeyer test, was done to support our data which involved dipping the final TLC plate into KMnO4 in order to see if the color changes from purple to yellowish. This test for the unsaturation number and in the presence of carbon-carbon double bonds, it oxidizes the bonds and turns them into diols hence producing the different color. This test also helped us to clearly see and locate the positions of three spots because they were not visible on the normal TLC plate. IR spectroscopy also yielded insight on the functional groups present in the carvone samples obtained from the caraway seeds and spearmint leaves. The hypothetical functional groups peaks for carvone include a carbonyl stretch as well as alkene stretch. In the IR for caraway seeds there was a strong peak at 1672.18 cm-1 which was likely representative of the carbonyl carbon (C=O) stretch. In addition to that, there seem to be a weak peak at 1643.26 cm-1 which could be representing of carbon carbon double bond (C=C) peak. The IR for spearmint leaves which we got from our neighboring group had very similar peaks compared to Carvone from caraway seeds. Despite not having the wavenumbers on the IR graph we could visualize that the graphs are very similar to each other. Thus, IR for spearmint leaves was also characteristic with strong peaks representing carbonyl carbon and alkene carbons. The graphs may look little different from each other because the isolations for each carvone was performed by different groups. However, ultimately it was found that the IR spectrum of the experimental samples strongly resembled the theoretical IR spectra for pure carvone. The IR spectra for the pure carvone is shown in Figure 5. Comparing the IR spectra for a pure carvone to the carvones isolated experimentally, we were able to conclude that the steam distillation and extraction was successful to isolate enantiomers of carvones. We were also able to come to the conclusion that we obtained the extracts of two different enantiomers by wafting the samples and smelling the difference between caraway seeds (smells earthy) and spearmint leaves (smells minty). While performing this experiment, one possible error is the evaporation of the extract obtained from steam distillation followed by extraction. After collecting the TLC plates, the extract is supposed to be placed under the hood to evaporate the extra methylene chloride. Sometimes

more than necessary liquid evaporates, which results in the evaporation of carvone before obtaining the IR spectra. As a consequence, one could obtain misguided graphs and hence give different wavenumbers than expected. However, this was not the case since the IR spectra subjected to both a carbonyl (for this molecule it would be Ketone to be specific) and an alkene peak.

Figure 5: Theoretical IR Spectra for pure Carvone

Finally, it could be concluded from the overall experiment that the isolation of carvone from caraway seeds and spearmint leaves was successful. This was supported by the results from TLC analysis with the aid of Baeyer test and IR spectroscopy. The results from pure carvone were compared to the experimentally isolated carvone, and both compounds matched reasonably. Despite this, the enantiomers of carvone could only be determined by smell, and though each enantiomer reacted differently in the chiral environment of the nose, both the (R)-(-)-Carvone and (S)-(+)-Carvone forms had nearly identical properties in terms of IR and TLC. Thus, steam distillation of carvone was successful. In addition, now we are able to differentiate between simple distillation, fractional distillation and steam distillation by deciding the use of each te...