EXPERIMENT 3

1. What is the purpose of washing the organic layer with 6M NaOH?− After separating the organic and the aqueous layer, caffeine, an amine, can be recovered by adding a strong base, in this case the 6 M NaOH. Another reason for this addition is to remove any acidic compound in the mixture. Adding NaOH causes a reaction with the acid forming a sodium salt. This shows a higher solubility in aqueous solutions because of its negative charge and high polarity.− Caffeine is an amine. After separation of the organic and the aqueous layer, the amine can be recovered by addition of NaOH. Also, this is done in order to remove an acidic compound from the mixture. The acid and the base react to form a sodium salt, which usually exhibits a higher solubility in aqueous solutions due to its negative charge and higher polarity.

− c. Removal of an amine

Depending on the chain length, amines might or might not be soluble in water i.e., propylamine is miscible with water (log Kow=0.48), triethylamine displays a limited solubility at room temperature (17 g/100 mL, log Kow=1.44), while tributylamine hardly dissolves at all (0.37 g/100 mL, log Kow=4.60). Amines are basic and can be converted to ammonium salts using mineral acids i.e., hydrochloric acid. The resulting salts dissolve in water. However, the solubility of the ammonium salts decreases as the number and size of R-groups increases. Ammonium salts from primary amines are much more soluble in water than salts from tertiary amines due the increased ability to form hydrogen bonds [(H3NEt)Cl: 280 g/100 g H2O, (H2NEt2)Cl: 232 g/100 g H2O, (HNEt3)Cl: 137 g/100 g H2O (all at 25 oC)]. −− After separation of the organic and the aqueous layer, the amine can be recovered by addition of a strong base like NaOH or KOH to the acidic extract i.e., lidocaine synthesis. Note that amides are usually not basic enough to undergo the same protonation (pKa of conjugate acid: ~ -0.5).− additional 5 mL of 2 M NaOH solution to rinse the brown residue out of the flask, and add this 4 rinse to the separatory funnel.− Weak acids: Phenols can be extracted from an organic solvent with a strong base such as 1M sodium hydroxide.

2. Compare the solid products obtained after extraction and purification. Account for the difference between the two solids.− The solid product obtained after extraction was a dark powder. The dark

color is due to the presence of other components, since it is not pure caffeine yet. The solid product obtained after purification was a white powder. By then, the impurities have already been removed. It can be said that the powder is pure caffeine.− The solid obtained after extraction was a dark powder. It is not yet pure caffeine so it has other components that account for its dark color. On the other hand, the solid obtained after purification were white crystals. This is because it’salready mostly caffeine as the impurities were already removed.

− The solid obtained after extraction was a dark powder while the solid obtained after purification werewhite crystals. The solid obtained from extraction is not yet pure caffeine so it has many other components thatmay be the reason of its dark color. The solid obtained after purification is mostly caffeine because theimpurities from the solid after extraction is purified which is why it looks like white crystals

3. What are the other applications of solvent extraction?− Other applications of solvent extraction are: − solvent extraction in precious metal refineries which includes three mechanisms – ion-pair formation, salvation, and compound formation − waste water treatments - metal treatment from acidic and ammoniacal waste solutions, recovery of acids (hydrofluoric, nitric, phosphoric, arsenic); separation of organic pollutants − Solvent Extraction in Biochemical and Pharmaceutical Separations: extraction of carboxylic, amino acids, and penicillin.− Solvent Extraction in Organic and Biofuels Separation: petroleum and petrochemical treatment, separation of isomers, and ethanol processing.− remediation of soils with contaminated hydrocarbons where the dissolved can be removed, − Other applications of solvent extraction include isolation of certain odors or flavors in food, purification of amines and extraction of certain metals. Also, it is applied to DNA purification in which the nucleases that destroy DNA are washed out. − http://www.sciencedirect.com/science/article/pii/B9780444537782100056− http://www.slideshare.net/kumarsachin3801/industrial-application-of-solvent-extraction-technique

4. What are the different phase changes that occur during purification using sublimation?− Sublimation, which can be used to purify substances, is a phase transition

without entering into an intermediate liquid phase. The different phase change processes that occur are sublimation and depositon. At a high tempereature, the solid caffeine changes to its gaseous state. Upon contact with the cool surface of the beaker, a lower temperature causes the gaseous caffeine to undergo deposition. It cools back down and changes back to its solid form, depositing on the sides of the beaker and on the filter paper.− Sublimation is a phase transition process from a solid to a gas without ever entering an intermediate liquid phase− The different phase changes that occur during purification using sublimation are sublimation and deposition.Sublimation occurs when the high temperature changes the solid caffeine to its gaseous state. And as the gaseous caffeine interacts with the cooler surface of the of the beakers, the

caffeine cools, depositing itself onthe sides of the beaker and on the filter paper. − Firstly, caffeine can sublime from solid state to gas state and then quickly change from gas state to solidstate. The solid state of pure caffeine can be extracted from the sides of the beak− Sublimation, like distillation, can be used to purify substances. That is because the process can separate materials that will sublime at a certain temperature from those other materials that will not sublime at that temperature. The material that goes through the phase change process, the sublimation in this case, is purified because it is separated from materials that were formerly mixed with it.− There is a small crystal of iodine in the test tube shown below to the left. Iodine needs to be heated just slightly above room temperature to sublime (in the center pictures below) and will change back to the solid form when it cools back down to room temperature. It is hard to see but iodine has crystallized in the cooler upper right portion of the test tube shown below on the right. The overall process of a solid to gas to solid is often referred to as sublimation. Thus, the term, sublimation, applies to two different phenomena, which is very unscientific. One of those is changing from a solid to a gas. The other is changing from a solid to a gas and back to a solid.− Further purification of caffeine is possible through sublimation if impurities are nonvolatile. Crude material is placed on the bottom of a sublimation chamber below a cool surface test tube. Impure caffeine extract is then heated to sublimation. A vacuum is applied to the airtight environment by sucking all of the air out of the chamber through the vacuum/gas line as shown in Figure 2.− (1)(1)left parenthesis, 1, right parenthesis Cooling water goes in test tube.− (2)(2)left parenthesis, 2, right parenthesis Cooling water goes out of test tube, generating a cold surface.− (3)(3)left parenthesis, 3, right parenthesis Vacuum/gas line creates a near zero pressure environment. It constantly removes gas it is exposed to from the system.− (4)(4)left parenthesis, 4, right parenthesis Crude caffeine material is placed inside the base of the sublimation apparatus.− (5)(5)left parenthesis, 5, right parenthesis When heated, sublimed caffeine gas rises from the base of the apparatus, forming solid crystals when it makes contact with the cold surface test tube.

− (6)(6)left parenthesis, 6, right parenthesis Solid non-volatile impurities are left at the base of the apparatus after sublimation.− (7)(7)left parenthesis, 7, right parenthesis External heating drives the sublimation process. The gaseous caffeine will be separated from the less volatile impurities and then forms crystal caffeine deposits along the cool surface. The caffeine deposits are then collected for the next step in processing.− https://www.khanacademy.org/test-prep/mcat/physical-sciences-practice/physical-sciences-practice-tut/e/purification-of-caffeine

5. Give two advantages of sublimation over recrystallization as a purification technique− (1) Sublimation is mostly used for micro scale purifications of solids, since

the loss of product is minimal− (2) It can be used for any heat sensitive compound (but in high vacuum, low temperature can affect sublimation)− (3) Solvents are not involved− (4) Occlusion of solvent in the crystals is prevented.− One, no solvent is needed. Two, there is less waste and the loss of product is typically very minimal. Three, it also avoids the occlusion of solvent in the crystals. Furthermore, this technique is appropriate for any heat sensitive compound (but under high vacuum, sublimation can be affected under low temperatures).−  There are many advantages for performing sublimation over other purification methods. This process is principally used for micro scale purifications of solids because the loss of product is typically very minimal. Furthermore, this technique is appropriate for any heat sensitive compound (but under high vacuum, sublimation can be affected under low temperatures). Thirdly, unlike recrystallization, solvents are not involved at all in the process, and most traces of any solvent are effectively eliminated. However sublimation is only favored over crystallization, when the substance weighs less than 100mg, and has the correct properties. Based on the theory of this phase change, the process is highly dependent on the different vapor pressures of not only the desired compound, but also on the vapor pressures of the impurities that are present in the crude.

6. Give at least two limitations of sublimation as a purification technique.− (1) It ypically requires that the compound being separated is volatile, while the rest (in the mixture) are not− (2) Following this, the vapor pressure of the target substance has to be different than the vapor pressure of the impurities− (3) Compounds must have high vapor pressures. Sublimation cannot be used when compounds have low vapor pressures.− One, sublimation is less effective than recrystallization in cases where the impurities have vapour pressures that are similar to the compound being purified. Two, it cannot be used for compounds with very low vapour pressures.

− It is not as selective as crystalization, it typically requires a vacuum and usually requires that the compound you are trying to separate is volatile, while everything else in the mixture is not volatile.−  the vapor pressure of the substance must be different than the vapor pressure of the impurities, otherwise both substances will be transported into the gas phase in almost equal amounts and you will not achieve a good purification.− So very large molecules or ionic substances probably can't be purified in this way. Second, the vapor pressure of the target substance has to be much different than the vapor pressure of the impurities, otherwise both substances will be transported into the gas phase in roughly equal amounts and you will not achieve a good purification.− compound you are trying to separate is volatile, while everything else in the mixture is not volatile. 

http://www.linfield.edu/assets/files/chem/Courses/CHEM%20321/2014-week3-4-caffeine-chem321l-53ebfd7b679e5.pdfhttp://www.chem.ucalgary.ca/courses/351/laboratory/351expt_06_caffeine_exp.pdf

EXPERIMENT 4

1. Why is the chromatogram developed in an essentially closed system?− The chromatogram is developed in a closed system, since the solvents are highly volatile and vaporize easily. This prevents the solvent from evaporating. It is also for maintaining a balanced solvent vapor in the container. The plate must have the same exposure and saturation to the vapors. An open system can create a gradient. Another reason is to prevent other gaseous substances from entering and mixing with the solvents.− The chromatogram needs to be in a closed system as the solvents are highly volatile, meaning they easily vaporize. Also, this allows the container to be saturated with the vapour of the solvent. The closed system also ensures that no other gaseous substances may enter the chromatogram and mix with the solvents or stain the chromatography paper.−  The chromatogram is developed in a closed system in order to prevent the solvent to evaporate. Most solvents used in the chromatograph are toxic and flammable. It is also put in a close system to reduce the chance of outside factors affect the chromatograph. − plate above the solvent pool has the same exposure to solvent vapors. The assumption is the solvent vapor is the same throughout the container. If you had an open system, you would get a gradient and the vapor level would change. − the closed system maintained the column chromatography efficiency by preventing the formation of column bubbles. The system was efficient in reducing solvent evaporation as well as preventing water condensation at the column outlet. Since free solvent vapors were eliminated, the system provided an additional safety factor when a flammable solvent, was employed.While the columns were used for solvent cleanup, a small modification would transform the system into a solute purification apparatus.

2. What are the considerations in choosing a chromatographic solvent?− Polarity of the solvent and solute must be considered. The solvent must have a different polarity and chemical properties than the solute.− The chromatographic solvent has to be able to bring out different Rf values for the compounds. − It must also separate the components into different solubilites.− The polarity of the chromatographic solvent must be noted. The solvent and the solute should have different polarities and chemical properties. They should be able to separate compounds with different Rf.− The solvent must also be able to dissolve the components of the substance into varying solubilities.− Whether it will bring out different Rf values of the samples used. If all of the compounds have the same Rf values then the separation of pigments would have failed due to the fact that all of them are in the same spot.− Polarity of the compounds that will be separated.− by using the polarity property of the mixture, we can choose a development solvent which will allow us to separate the substance based on their rate of travel up the chormatographic plate. Depending on the polarity of the substance present, they will travel at different rates in the presence of different polarity solvent systems.

3. Compare and contrast between normal phase and reverse phase chromatography.− Normal phase chromatography makes use of a polar stationary phase and a non-polar mobile phase (solvent). Reverse phase chromatography is practically the same, however, the stationary phase is non-polar and the mobile phase (solvent) is polar. − In the normal phase, a non-polar solvent (mobile phase) and a polar stationary phase are used. While, in the reverse phase, a polar solvent (mobile phase) and a non-polar stationary phase are used. − Normal – stationary phase is polar and mp is non-polar, reverse is practically the same stationary is non-polar and mp is polar− Reversed phase  has a non-polar stationary phase and an aqueous, moderately polar mobile phase. In reversed phase retention time is longer for molecules which are more non-polar, while polar molecules elute more readily. An investigator can also increase retention time by adding a polar solvent to the mobile phase, or decrease retention time by adding a more hydrophobic solventNormal phase chromatography separates analytes based on polarity. The polar analyte associates with and is retained by the polar stationary phase. Adsorption strengths increase with increased analyte polarity, and the interaction between the polar analyte and the polar stationary phase (relative to the mobile phase) increases the elution time. The interaction strength depends not only on the functional groups in the analyte molecule, but also on steric factors.

4. Explain how iodine crystals help to visualize the other pigments.− Iodine reacts with many organic materials to form complexes that are colored. In the experiment, iodine binds to the pigment compounds and intensifies their color for easier visualization. Iodine, through weak

electronic attraction, collects on the spots. Some yellow or brown spots start to form on most organic compounds. It works well for unsaturated and aromatic compounds. It is, however, not permanent.− Iodine binds to some pigment organic compounds and give them more color so that the pigments are easier to see.By weak electronic attraction, iodine will collect on the spots and form a yellow or brown stain on most organic compounds except saturated hydrocarbons and alkyl halides.− Iodine binds to some organic compounds that are pigments and give it more color so that the pigmentsare easier to see.

5. Suggest one method that can be used to visualize highly conjugated compounds but are not colored.− One method that can be used for visualization is ultraviolet light detection. When placed under UV light, most conjugated compounds (even when not colored) appear as violet or purple spots on a light green background. Compounds that have benzene rings or conjugated systems absorb UV light. This works well on compounds with extended conjugation like aromatic compounds. Pink on light green background.− Although several techniques are available, one of the simplest and most used methods is the I2 (Iodine) visualization technique. In this process, the TLC plate is placed in an iodine chamber. Most organic compounds adsorb yellow-brown I2 molecules. Thus, spots of chromatographed compounds appear as yellow brown spots after the plate has been treated with I2. When the plate is removed from the I2 chamber, the I2 will gradually diffuse away from the spots, so the plate should be immediately marked to indicate the positions of spots. Another method used to visualize the spots is to observe the plate under UV light. Most conjugated compounds appear as violet or purple spots under UV light, if the coating on the plates contains a UV absorber.− Ultraviolet Light Detection: A nondestructive visualization technique that will show any

compounds that absorb UV light. Compounds containing benzene rings or conjugated

systems usually absorb UV light. Commercial TLC plates have phosphor in the

adsorbent, which fluoresces in short-wave UV light. If a compound is present on the plate

it blocks the glow and appears as a dark spot. (This is technically true only for

compounds that quench the fluorescence). Some organic compounds also fluoresce

themselves, and will show up as bright spots under short-wave UV light.− Iodine vapor: Iodine vapor is also a non-destructive visualization technique. A few

crystals of iodine are placed in a closed chamber, such as a capped jar containing silica

gel, and the slide is placed into the chamber to collect on the spots by a weak electronic

attraction. Iodine forms a yellow or brown complex with most organic compounds

containing double bonds, even isolated ones. The reaction is reversible, so that I2 staining

can be followed by another chemical stain if the plate is allowed to sit in air for several

minutes so that the iodine can sublime off the plate.− Please note that while these two methods will show any compounds containing double

bonds, any compounds without double bonds will not show up on your plate (unless they

are themselves colored, in which case you will be able to see them with your naked eye).

Other TLC visualization methods exist for these compounds, but they are usually strong

oxidizers and for that reason not safe for use in an undergraduate lab.−6. Given a mixture that contains the following compounds: 2-chloropentane,

pentanoic acid, cyclopentane and pentanal. Arrange the following compounds in increasing Rf if the mixture is ran using paper chromatography with the same solvent system used in the experiment.

Pentanoic acid <pentanal< 2-chloropentane <cyclopentanehttp://www.columbia.edu/cu/chemistry/ugrad/hssp/EXP_8.htmlhttp://chemistry.barnard.edu/sites/default/files/inline/experiment3.pdf