Lab Final Study Guide
Lab Final Study Guide CHEM231
Popular in Organic Chemistry I
Popular in Chemistry
This 9 page Study Guide was uploaded by Udoka Oji on Saturday July 2, 2016. The Study Guide belongs to CHEM231 at Brown University taught by Dr. Bonnie Dixon in Summer 2016. Since its upload, it has received 22 views. For similar materials see Organic Chemistry I in Chemistry at Brown University.
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Date Created: 07/02/16
Lab Saturday, July 2, 20164:13 PM Golden Rules of Organic Chemistry Lab • Never heat a closed system • Always wear closed-toed shoes • Always cover skin (where a lab coat and long pants) • Always wear goggles • Always wear gloves • No flames, most organic solvents and materials used in our labs were flammable Experiment 1: Recrystallization of Impure Solids • Safety: Never add decolorizing carbon to a boiling solution • Solution recrystallization: dissolving the solid in an appropriatesolvent at an elevated temperatureand allowing the crystals to reform on cooling, so that any impurities remain in the solution • Steps of solution recrystallization ○ Selection of an appropriate solvent Desirable solvents: □ Compound should be reasonably soluble in the hot solvent, and insoluble or nearly insoluble in the cold solvent (favorable temperaturecoefficient) □ Impurities should be insoluble in the solvent at all temperaturesor must remain moderately soluble in the cold solvent □ Boiling point of the solvent should be low enough so that it can be readily removed from the crystals □ Boiling point of the solvent should generally be lower than the melting point of the solid being purified □ The solvent should not react chemically with the substance being purified Occasionally a mixture of solvents is required for satisfactory recrystallization ○ Dissolution of the solid to be purified in the solvent near or at its boiling point Too much solvent will decrease the recovery of the solute, so when more solvent is needed,add in small amounts ○ Decoloration with an activated form of carbon, if necessary, to remove colored impurities and filtration of the hot solution to remove insoluble impurities and the decolorizingcarbon ○ Formation of crystalline solid from the solution as it cools First allow boiling solution to cool to room temperature,then put in ice, to avoid impurities crashing out quickly as well Failure of crystallization to occur is a sign of supersaturation □ Can be fixed by seeding (adding a crystal of the original solid to the solution, to induce crystallization) ○ Isolation of the purified solid by filtration Vacuum filtration ○ Drying the crystals Air or oven drying Air or oven drying • Quiz questions ○ If your recrystallized solid is impure, describe two ways the melting point range will differ from if it were pure? The melting point range will be lower The melting point range will be wide ○ If you recrystallize 1.87 g of an unknown solid and recover 1.23 g of crystals, what is the percent recovery? ○ Provide 2 properties that are desirable for a recrystallization solvent? Boiling point lower than melting point of solute Will not react with solute Experiment 2: Extraction of Trimyristin • Safety: Diethyl ether is highly volatile, try to keep the reflux ring in the lower third of the condenser • Heating under reflux means that a reaction mixture is heated to its boiling point in a flask equipped with a reflux condenser to allow continuousreturn of the volatile materials to the flask • Volatile solutions are usually slow heated to boiling point on indirect heat • The volatile componentsof the mixture vaporize and reliquefyin the condenser, and the condensate returns to the boiling flask, where the process continues ○ A reflux ring appears in the condenser, and the condenser appears dry above the ring, as a sign that reflux is happeningcorrectly • If any reactants must be added to the reaction during reflux, set up the apparatusto include a separatory funnel • This experiment is an example of solid-liquid extraction ○ Common organic solvents such as diethyl ether, dichloromethane,ethanol, and acetone are used for extracting natural products ○ Trimyristin is a triester of glycerol and the fatty acid myrisitic acid Myrisitic acid is a saturate fats ○ Trimyristin is found in nutmeg (~40%) • Quiz questions ○ What organic functional group is present in trimyristin? Ketone Ester ○ What solvent will be used to extract the trimyristin from nutmeg today? Diethyl ether ○ Instructionscaution that the ring of condensation be kept in the lower third of the condenser. Why is this caution given? In order to guarantee that vapor doesn't enter the room In order to guarantee that vapor doesn't enter the room Experiment 3: Base/Base Extraction • Safety: Do not allow chemicals to come in contact with skin • Guidelines to using separatory funnels: ○ Filling separatory funnels Stopcock should be closed and a clean beaker should be placed under the funnel before any liquids are added Funnel should never be more than three-quartersfull The upper opening should be stopped with a stopper ○ Holding the separatory funnel Hold funnel in hands so that index finger is tightly holding the stopper for shaking ○ Shaking separatory funnels Gently shake to avoid emulsions Funnel should be vented every few seconds to prevent the buildup of pressure ○ Layer identification To ascertain the aqueous layer from the organic layer, separate them and add a drop of water to each watching to see how the droplet interacts with the fractions To be safe always save both layers until there is no doubt about the identity of each and the desired product has been isolated • This experiment is an example of liquid-liquidextraction ○ Selecting an appropriate extracting solvent requires a few key ideas The extracting solvent must not react in a chemically irreversible way with the componentsof the mixture The extracting solvent must be immiscible, or nearly so, with the original solution The extracting solvent must selectively remove the desired component from the solution being extracted. That is, the partition coefficient K of the component being removed must be high, while the partition coefficientsof all other componentsshould be low The extracting solvent should be readily separable from the solute • Quiz questions ○ Which of the following will result in extraction and isolation of more solute: extraction with three 10 - mL portionsof ether or with one 30-ml portion? Extraction with three 10-mL portionsof ether ○ What is the piece of glassware used to perform the extraction called? Separatory funnel ○ Why is it important to frequentlyvent a separatory funnel during the extraction process? To prevent the buildup of gas, so that the stopper does not blow off the funnel Experiment 4: Separation of Spinach • Safety: No flames • Chromatography:the separation of colored bands of different componentsof a mixture • Chromatographyoperates on the principle that the componentsof a mixture will distribute unequallybetween two immiscible phases, which is the basis for separation ○ The mobile phase: a liquid or gas that flows continuously ○ The stationaryphase: a solid or liquid ○ The individual componentsof the mixture have different affinities for the mobile and stationaryphase, once a dynamic equilibrium Is established for each component,they will bind to the stationary phase • The componentsdivide into separate regions termed migratory bands • The component that interacts or binds more strongly to the stationary phase moves more slowly in the direction of the flow of the mobile phase • Different types of chromatography: ○ Thin-layer ○ Column ○ Gas-liquid partition chromatographyor gas chromatography • In this experiment we perform thin-layer chromatography,a form of solid - liquid adsorption chromatography ○ The stationaryphase is a solid adsorbent bound to rectangular glass or plastic plate Silica gel or Alumina (both are polar) ○ A spot of mixture is placed on the plate and the plate is placed in the developing chamber with the spot about a centimeter above the eluting solvent ○ The eluting power of eluting solvents differs dependingon the solid phase ○ Different types of moleculeshave different affinities for polar stationary phases ○ The retention factor for a compound is a physical constant for a given set of chromatographicconditions ○ For colorless compounds,there are some techniques for detecting their presence: Ultravioletlight Spraying the plate with a variety of reagents that will react with the componentsto produce colored or dark spots Exposing the plate to iodine vapor in a closed chamber to spots brown • Quiz questions ○ What function does a piece of filter paper serve in the developingchamber? To help maintain solvent equilibrationin the chamber ○ What is the purpose of chromatography? To determine the componentsin a sample ○ What are the characteristic hazards of the solvents used in TLC? They are volatile so flames should not be used Experiment 5: Column Chromatography • Safety: • Column chromatographyis another form of solid - liquid adsorption ○ Theology is similar to that of thin - layer chromatography • In normal-phase column chromatographyis performed,a polar stationary phase such as alumina or silica gel is used in combination with organic solvents as the mobile phase • In reverse-phase column chromatographyglass beads coated with nonpolar hydrocarbon film are used as the polar stationary phase while mixtures of water and organic solvents are used as the mobile phase • The most efficient way to determine the optimal solvent system for a specific column is to perform a series of trial separations using TLC • In this experiment,we performed a normal-phase column chromatography using silica gel as our stationary phase • Uniform packing of the adsorbent is essential so the solvent will move down the column with a horizontalfront • In simple elution, a single solvent is passed through the column during the entire course of the separation ○ Good for mixtures with only two or three compoundshaving similar polarities • In fractional elution, increasingly more polar solvents are used to elute the mixture from • In fractional elution, increasingly more polar solvents are used to elute the mixture from the column • Quiz questions ○ What is the basis of separation in LC? The polarity of the stationary phase vs. the polarity of the mobile phase and the affinity for each compound for each phase ○ Which phase is more polar in our column chromatographyexperiment? The stationaryphase (silica gel) ○ How does the mobile phase move through the stationary phase? The eluant is placed at the top of the column and gravity allows it to slide down the column Experiment 6: Simple and Fractional Distillation • Safety: Exam glassware for cracks that could cause them to break during heating, use grease so that the glassware does not stick together,and use a lot of clips! • Equilibrium vapor pressure is the magnitude of vapor pressure • To reach the boiling point of a liquid, the equilibrium vapor pressure of the sample must equal the total pressure of the space above it ○ This happensbecause the air in the space above the vessel is replaced by the vapors of the liquid • The normal boiling point of a liquid is measured at 760 torr • A pure liquid generally boils at a constant temperatureor over a narrow temperature range, provided the total pressure in the system remains constant ○ Impurities cause mixtures to boil across wider ranges • Simple distillation allows separation of distillates from less-volatile substances that remain as pot residue at the completion of the distillation • Ideally, only a single component of the mixture will be volatile, so the distillate will be a pure compound • Difference between the boiling points of each pure substance is greater than 40-50°C • Fractional distillation runs on the basis that the vapor phase is richer in the more volatile component than is the boiling liquid with which the vapor is in equilibrium • When looking at a temperature-compositiondiagram, the lower curve is the liquid line and the lower curve is the vapor line • Requirementsfor a successful fractional distillation ○ Intimate and extensive contact between the liquid and vapor phases in the column ○ Maintenance of the proper temperaturegradient along the column ○ Sufficient length of the column ○ Sufficient difference in the boiling points of the componentsof the liquid mixture 20 - 30 °C • HETP stands for height equivalent to a theoretical place and is the vertical length of a column that is necessary to obtain a separation efficiency of one theoretical plate column that is necessary to obtain a separation efficiency of one theoretical plate • Quiz questions ○ What is the purpose of distillation? To remove liquids of differentcompositions, one typically more volatile than the other ○ What criteria is used for deciding whether you can do a simple distillation or fractional distillation? If the liquids have boiling points that differ by 40 - 50 °C ○ What must you do to the distillation system to assure that the apparatuswill not explode? Always ensure heated systems are open Experiment 7: GC Analysis for Distillate • Safety: None! This is a dry lab • Gas chromatographyis used to separate mixtures of volatile compoundswhose boiling points may differ by as little as 0.5 °C • A sample of the mixture is injected into a chamber where it is immediately vaporized and carried by a flowing inert gas, such as helium or nitrogen,called the carrier gas ○ The gaseous mixture is the mobile phase ○ A column packed with a finely divided solid support that has been coated with a viscous, high-boilingliquid which is the stationary phase • Retention time of a component is the elapsed time required for the compound to pass from the point of injection to the detector • For experimental factors that influence retention time: ○ Nature of the stationary phase ○ Length of the column ○ Temperature of the column ○ Flowrate of the inert carrier gas • Two types of common detectors ○ Thermal conductivity detector: operates on the basis of differences in the thermal conductivity of the mobile phase as a function of its composition ○ Flame ionization detector: functions by detectingthe number of ions produced by passing the mobile phase through a hydrogen flame More sensitive • When analyzing the peaks on the gas chromatogram, values must be corrected ○ The peak areas, for example are related to the mole fraction of the component in the mobile phase, but are not related quantitativelybecause the response of the detector varies with the class of the compound Correct measured areas using a response factor ○ To analyze the mixture of substances quantitativelywhen weight factors are known, the peak area for each component is simple multiplied by the weight factor for that particular compound Corrected areas are used to calculate the percentage composition of the mixture Calculation of the composition of a mixture on a mole percentage basis requires the ○ Calculation of the composition of a mixture on a mole percentage basis requires the use of mole factors, M f • Quiz questions ○ What information from a gas chromatogram is used to quantitativelyanalyze the composition of a mixture? Area under the peaks ○ What property of the components of a mixture is largely responsible for the order in which they are observed on a gas chromatogram? Boiling point ○ List two ways one can shorten the time required to obtain a gas chromatogram Increase flowrate and increase temperature Experiment 8: An S 2 RNaction - Preparationof 1-Bromobutane • Safety: Examine glassware for cracks and chips, wear gloves, always • add acid to water • In an S 2 mechanism, the rate-determiningstep is bimolecular, two reactants are involved N ○ The reaction is second-order • The nucleophile directly attacks the substrate, with the angle of approach being 180 ° to the C-L bond, this is backside attack • S 2Nprocessescompetewith E2 processes - - - • Substitution is favored with weakly basic and highly polarizable nucleophiles such as I , Br , Cl , H2O, CH 3O 2- • Elimination is favored when strongly basic and only slightly polarizable nucleophiles such as RO , - - - - H2N , H , and HO are used • Polarizability measures the ease with which the electron cloud is distorted • Bulky nucleophiles favor elimination Experiment 9: Elimination with Alcoholic Potassium Hydroxide • Safety: Highly caustic solutions, where gloves! • Dehydrohalogenationis a commonelimination process in which a carbon - hydrogen bond and a carbon - halogen bond are broken to form a carbon - carbon π bond carbon - halogen bond are broken to form a carbon - carbon π bond • Dehydration is a commonelimination process in which an alcohol functional group and a hydrogen atom are removedto form a carbon-carbon π bond • In E2, the leaving group and the hydrogen atom must be anti-periplanar, or 180° ○ If this is not the case, the process produces substitution rather than elimination • In this experiment 2 - Methyl-2 butene is the major product, with the bond forming on the more substituted carbon ○ This is the Zaitsev's product • The less substituted product could become the major product if steric factors inhibit the nucleophile Experiment 10: Relative Rates of Free-Radical Chain Bromination • Safety: • In free radical halogenation, there are three steps ○ Initiation: the thermal or photochemicalhomolysisof molecular halogens to form their radicals The energy required for this is the bond dissociation energy ○ Propagation:production of the substitution product R-X, and regenerationof the halogen radical to continue the chain reactions ○ Termination • Free-Radical Halogenation results in the halogen binding to the less substituted carbon across the π bond • Two factors controlling the energies of the transition state: ○ Statistical factors: the number of hydrogen atoms whose replacement will give a specific constitutional isomer ○ Energy factor: the strength of the type of C-H bond being broken Relative reactivity
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