Organic Chemistry Midterm 1 study guide
Organic Chemistry Midterm 1 study guide CHEM 3221
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This 8 page Study Guide was uploaded by Aishwarya Juttu on Wednesday October 12, 2016. The Study Guide belongs to CHEM 3221 at University of Houston taught by Lab TA in Fall 2016. Since its upload, it has received 5 views. For similar materials see Organic Chemistry Lab 1 in Chemistry at University of Houston.
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Date Created: 10/12/16
Orgo Lab 1 Midterm Study Guide Experiment 0704 - Separating Cyclohexane and Toluene by Distillation (Macroscale technique) - Purpose: separate two miscible liquids by macroscale technique using simple and fractional distillation and compare the efficiencies - Important reagents: Cyclohexane is flammable and irritating. Toluene is flammable and toxic. Both must be used under fume hood - Concepts: - Distillation is a technique used for separating compounds based on differences in their boiling points - Volatile compounds are heated to boiling in one container, vapors are produced and then cooled and reliquefied by passing them through a water-cooled condenser and collected in a receiver - The composition of the liquid mixture is different than the composition of the vapor mixture; more of the volatile molecules will be present in the vapor mixture and when vapors move into the water-cooled condenser, these vapors condense to a distillate which has the same composition as the vapor from which it was formed - This method is used to separate volatile product from nonvolatile impurities, remove a volatile solvent from a nonvolatile product, or to separate two or more volatile products that have significant different boiling points - Simple distillation- if one compound is much more volatile than the other, the compounds can be separated in one vaporization step - Apparatus: pot, distilling head, condenser, adapter, receiver - Fractional distillation- when the bp of two compounds differ by less than 40 C, the compounds have to go through the effect of many simple distillations - Apparatus: same as simple distillation except fractional column which is packed with glass beads is placed between the pot and the distilling head - vapors generated in the pot rise up the fractional column and encounter cooler surfaces and they condense. The condensed liquid is revaporized by the rising hot vapors from the pot. This process is repeated until the vapors rise up to the column. Then the vapors are condensed in the condenser and are collected in the receiver in pure compound - each condensation-revaporization process results in an increase in the more volatile compound and the bp decreases as the mixture moves up the fractionating column (b/c volatile compounds have a lower bp) - Theoretical plate- each condensation and revaporization that occurs on a fractionating column - Temp of the volatile vapors remains constant until all of the compound with the lower bp is collected. Temp spikes as the vapors of the higher bp rise to the thermometer - Evaporation - transition from liquid to vapor of only those molecules at the liquid surface, occurs at temperatures below the boiling point of a compound - Equilibrium vapor pressure- pressure exerted by gaseous molecules on the wall of the container, eq. vp increases as temperature increases - Boiling Point- temp at which the vapor pressure of the liquid is equal to the external pressure applied to the surface of the liquid - When a mixture of two or more volatile compounds is heated, the vapor pressure of the mixture equals the sum of the vapor pressures of each compound in the mixture - P= X1P1 + X2P2 - Homogenous mixture will boil at a temp. between the boiling points of the pure compounds. - You should never add boiling chips to a boiling liquid because the large surface area of the chips will cause the liquid to bubble, foam and rise to the top. This may cause overflow and burns - Thermometer must be positioned properly in order to accurate data. The right way is to insert it into get the thermo. adapter so the top of the thermometer bulb is at the the level or slightly below the bottom side of the distilling head. This makes sure the bulb is immersed in the vaporous properly thus giving us an accurate reading of the thermometer. - The outer jacket of the fractioning column ensures that heat is not lost from the inner jacket during distillation. This ensures that the data collected is not erroneous. The outer jacket is not for passing water. Fractionating column must be placed vertical for the condensed liquid to fall down through the rising hot vapors, this ensures equilibrium of the vapor and liquid phases - An increase in heating rate does not change the bp during distillation. Only the rate at which the rate changes from liquid to vapor increase. The energy that is supplied by the increased heating rate is absorbed as more liquid molecules overcome intermolecular interactions and enter the vapor phase. - When the vapor re-vaporizes, after condensation and move up the fractional distillation column the concentration of the more volatile liquid increases and the bp decreases, so each condensation-vaporization increases the concentration of the more volatile liquid Experiment 0701 - Measuring the melting points of compounds and mixtures - Purpose: measure melting points of pure benzoic acid and determine eutectic composition and eutectic temp. of benzoic acid and identify unknown using mixture melting points - Important reagents: benzoic acid is irritant, unknown maybe flammable, toxic, and irritating - Concepts: - Melting point- temp. at which the solid is in equilibrium with its liquid - Melting points can be used to identify unknown compounds and assess compound purity - mix a known and unknown compound and if the mp is same then both compounds are most likely the same but if there is a decrease in mp and the range is more than 5 C, then the impure (meaning the unknown is different from the known compound) - Melting point range (mpr)- span of temp. from the point at which the crystals first begin to liquefy to the point at which the entire sample is liquid - Pure organic compounds have a mpr of 1-2C - Presence of an impurity decreases expected mp of the pure compound and broadens the mpr - Eutectic composition- both compounds are in equilibrium with the liquid - Eutectic temperature- lowest possible melting point for a mixture, anything below the eutectic temp., both compounds are present in solid form only - Adding more of the pure compound increases the overall mp of the pure compound - Afinely powdered sample should be used in a mp measurement to ensure good heat transfer. If the sample is not powdered, then it may cause air pockets that can slow the process of heat transfer - Heating a sample slowly is the correct way to obtain an accurate melting point because heating it too fast can alter the thermometer reading from the actual heat source. - Detecting the temp at which crystals first began to liquefy can be difficult because small samples produce small amount of liquid which is hard to detect.Also because when the solvent is present, it leads to sweating which is mistaken for melting.Achange in shape from the heat is mistaken for melting - When crystals began to disappear from the bottom of the capillary tube it means that the compound went through sublimation (solid to vapor). When the capillary tube is closed, crystals will vaporize and recrystallize at the top of the tube Experiment 0703 - PurifyingAcetanilide by Recrystallization - Purpose: select an appropriate recrystallizing solvent, separate and purify acetanilide from a mixture by recrystallization, compare melting points of impure and recrystallized acetanilide - Important reagents: acetanilide is toxic and irritating, acetone and ethanol are flammable and irritating, petroleum ether is flammable and toxic, activated carbon is irritant. These compounds must be used under fume hood - Concepts: - Recrystallization- a purification process used to remove impurities from organic compounds that are solid at room temperature - As a heated solution cools down, a seed crystal forms in the solution, molecules of the seed crystal attracts the same molecules in the solution due to greater affinity thus the crystal grows layer by layer and is removed from the solution - Selecting a recrystallizing solvent: 1. Compound has to be insoluble in solvent in room temperature and soluble near the solvent’s bp 2. Compound must be soluble in boiling solvent (most organic compounds are non polar (like dissolves like)) 3. Boiling point of the recrystallizing solvent should be lower than the mp of the compound to be recrystallized (otherwise compound will oil out: when compound is a solid in a solution at a temp. above the compound’s mp) 4. An abundant amount of crystals must be produced as the solution cools to room temp. or below - Solvent pair- two solvents are selected that are miscible with each other but have opposite abilities to dissolve the compound - Compound is added to solventAnear the solvent’s bp, then solvent B is added until the mixture becomes cloudy (indicating that with solvent B, the compound is soluble), then a few drops of solventAis added to redissolve the precipitate and produce a clear solution (indicating that with solventA, compound is soluble). This combination of solvent is seen as a single recrys. solvent.Add five % of excess solvent to prevent premature recrystallization - Asample may contain a soluble impurity that produces a colored solution and that solution colors crystals that would otherwise be colorless, activated carbon or decolorizing carbons’ large surface area is used to absorb dissolved impurities from the solution. Excess carbon can absorb the recrystallized compound which reduces the percent recovery. Do not add activated carbon to heated solution, the solution could boil over. - Hot solution is filtered through gravity filtration through funnel with fluted filter paper (increases surface area inside the funnel and speeds filtering process). To prevent premature recrystallization in the funnel stem, the mixture should be hot and the gravity filtration apparatus should be heated on an hot plate - Filtered solution is cooled in ice bath to maximize crystal formation. If crystals form too rapidly, it means that there are impurities trapped in crystals - Scratch the side of flask with glass stirring rod to initiate crystal formation or place a seed crystal to promote crystal growth - Vacuum filtration is used to separate crystals form the remaining solvent - Bernoulli effect- partial vacuum is created because of the reduced pressure at the point where the rapidly moving water passes the hole and at that point, air is pulled into the aspirator sidearm. The trap can be used in tandem with water aspirator to prevent contamination of the solution in the filter flask with water - % recovery =[(mass of recrystallized compound grams)/(mass of crude compound grams)] x 100% - Purity of recrys. compound can be assessed by observing its color and by measuring its melting point range. If the recrystallized compound’s color matches with the actual compound’s color, if not, the compound should be recrystallized using activated carbon.A pure compound melts over a range of 1-2 C, if the dry compound has a mpr higher than 4 C, then recrystallize again - Putting the hot solution to the ice bath to recrystallize without letting it cooling down first could lead to rapid crystal formation and impurities will be trapped into the crystals. - If the decolorizing step is forgotten, impurities could still remain in the solution.After properly cooling it down to room temp. and then placing in ice bath, recrystallizing will not be rapid and impurities or colored compound will not be trapped in crystals Experiment 0707 - Separating a mixture of Biphenyl, Benzhydrol, and Benzophenone by Thin-Layer Chromatography - Purpose: Select a solvent to separate a mixture of biphenyl, benzhydrol, and benzophenone by thin later chromatography, identify mixture compounds by Rf values with reference compounds, use TLC to investigate solvent polarity and mobility, and identify unknown - Important reagents: benzhydrol, benzophenone and biphenyl in acetate, ethyl acetate, hexane, toluene, methanol are flammable and irritant,dichloromethane is toxic and irritant, iodine is corrosive and toxic - Concepts: - TLC- simple and inexpensive analytical technique that can efficiently separate quantities of less than 10 micrograms of material, can be used for analysis of reagent and product purity, progress of a reaction, select a suitable solvent before attempting a large TLC separation, determine the number of compounds in a mixture, and identification of unknowns - Mobile phase- a sample of mixture is placed into a liquid or gas (sample is placed near the bottom of TLC plate) - Ashallow layer of mobile phase liquid (eluent) is placed in the developing chamber - Stationary phase- as the mixture compounds (adsorbent) dissolve in the mobile phase ascends the plate, the mixture compounds dissolve in the mobile phase to different extents due to differences in their relative attractions for the mobile and stationary phases.After completion, the TLC plate is called a chromatogram - compounds with less attraction for the adsorbent move rapidly with the eluent - Retention factor (Rf)- ratio of the distance that a compound moves to the distance that the eluent front moves - Rf= (distance traveled by compound, mm)/(distance traveled by eluent front, mm) - Rf value is small when the adsorbent is strongly attracted to the eluent and doesn’t move far from the origin - Rf value is big when the eluent polarity increases, greater eluting power, it would increase the attraction of eluent and compound - Chromatographic behavior of individual compounds is reproducible as long as the stationary and mobile phase and the temperatures are kept constant - Adsorbents-Alumina and silica gel - Two compounds can have the same Rf values in the same eluent but different Rf values in different eluents if both of the compounds are not identical - TLC experiment: - Spotting a plate- origin is marked using a pencil, a line is drawn 1cm from the bottom and samples are placed on line - Developing a plate- place a filter paper on the walls of the beaker and pour the eluent over it to wet it. Lean the TLC plate on the filter paper and close developing chamber with aluminum foil, remove after eluent reaches 1 cm below the top - Visualizing a plate-After drying, place under UV light to see the spots. Compounds that absorb the UV light are dark.Another way is to place TLC plate in a Iodine chamber containing iodine crystals and vapor Experiment 0708- Separating Ferrocene andAcetylferrocene byAdsorption Column Chromatography - Purpose: prepare an adsorption chromatography column and separate two- compound mixture using column chromatography and calculate % recovery for each compound - Important reagents: acetylferrocene is toxic, tbme is flammable and irritant, hexane is flammable and irritant - Concepts: - Chromatography refers to analyzing, identifying, and separating mixtures of compounds; all have a mobile and stationary phase (due to different attractions between the two phases, the sample mixture moves in different rates through the stationary phase) - Adsorption column chromatography- a technique that uses a solid stationary phase, the adsorbent, packed in a glass column, and a solvent the mobile phase, that moves slowly through the packed column - Amixture of compounds added to the eluent.As the eluent moves through the column, the stationary phase and the mobile phase interact with the compounds in the mixture. The differences in attraction of the compounds to the stationary and mobile phases result in the compounds moving at different rates through the packed column, separating from one another. - Amixture of low polarity compounds, the more polar compounds of the mixture will bind to the adsorbent tightly and the low polar compounds will bind more loosely - Less polar compounds of the mixture dissolve in the eluent and move through the column - More polar compounds have a stronger attraction to the adsorbent than to the moving eluent - Column bed- packed adsorbent - In a Pasteur pipet from the bottom, cotton plug, sand, alumina, sample, alumina, sand - Cotton plug provides a level base for the adsorbent - Eluent is added from the top of the pipet an air pipe is used to force the eluent to flow down the pipet - Column has to be kept straight to prevent the bands from eluting unevenly and all the layers must be even and packed properly to prevent air pockets in the stationary phase - Hexane was used for ferrocene and TBME was used for acetylferrocene - Less polar ferrocene moved down the column as a yellow band, leaving behind the orange band of the more polar acetylferrocene - Alumina separates non polar compounds best - Silica separates polar compounds best Experiment 0705 - SeparationAcids and Neutral compounds by solvent extraction - Purpose: use solvent extraction techniques to separate a mixture consisting of a carboxylic acid, a phenol and neutral compound - Important reagents: acetanilide is toxic and irritating, sodium hydroxide is toxic and corrosive, HCl is toxic, foaming between HCl and NaHCO3, NaHCO3 and acids produce CO2 which result in foaming, Na2SO4 is irritating and hygroscopic - Concepts: - Extraction- a process that selectively dissolves one or more of the mixture compounds into an appropriate solvent; removal of a soluble compounds from a solid matrix. The reverse process is called washing in which the impurities are removed to the second solvent - Salting out- adding inorganic salts in water to reduce solubility of organic compound in the aq. layer - Separating organic compounds if one compound in the mixture is converted to an ionic form. The ionic form is soluble in an aq. layer and non-ionized organic compounds in the mixture will remain dissolved in the non polar solvent layer. Separation of the two layers results in the separation of the dissolved compounds - Strong acids, those with smaller pKa will react with the conjugate bases of weaker acids with bigger pKa - pKa=-logKa - Adding p-toluate instead of p-toulic acid to the two layers will dissolve in water more than TBME because it is polar enough - To remove any remaining traces of TBME that might inhibit the crystallization of the phenol, heat the NaOH extract before acidification - When the NaHCO3 extract is acidified with HCl the mixture foam and then the p-toluene will precipitate Experiment 0722 - Isolating Clove Oil from Cloves Using steam Distillation - Purpose: Isolate clove oil from cloves by steam distillation and extraction and analyze the product purity by TLC - Important reagents: - Concepts: - Raoutl’s Law- total vapor pressure is equal to the sum of all the individual partial pressures and their mole fractions - P= P1xM1 + P2xM2 - Total vapor pressure resembles the partial vapor pressure of the more volatile compound in the mixture of greater abundance - Total vapor pressure is between the partial pressures of the compounds in the mixture - Codistillation- distillation on mixtures in which one of the two compounds are not miscible - Steam distillation- one of the two compounds is water - Dalton’s Law- when two immisicle liquids are distilled, the total vapor pressure above the liquid is equal to the sum of the vapor pressures of each compound - P= P1 + P2 - Mole fractions are not included in this equation because each lied vaporizes independently of the other - Essential oils are used as flavorings and perfumes - The purpose of steam distillation is to separate one of two immiscible compounds one of which is water. It is a special case of codistillation. The other compound involved may have a bp close to the temp at which it decomposes. So the water and compound of interest mixture decreases bp of the mixture to below 100 C making it less preferable for the other compound to decompose. This principle is clear from Dalton’s Law: P = P1 + P2 - Reasons for presence of impurities could be because the ether layer after adding NaOH and ether wasn’t completely removed or extraction from the aq layer wasn’t complete Experiment 0709- Gas Chromatography - Purpose: Separate, detect compounds of mixtures by gas chromatography - Compounds with more branching will have higher boiling points - Properly cleaning the syringe is important to prevent imperfect results - Achange in injection temp., the retention time if the temp. increases - Achange in column temp. shortens retention time if it is too high - Detector attenuation does not directly affect retention rate. It affects whether the detector gets clogged or not - The retention time increases as column length is increased - The retention time is lowered if the sample size is small - If the temp. is too low for the injector, the compound will not be instantly vaporized. The compound will move slowly onto the column in a broad band, thus making separation and quantification difficult - % composition = (corrected integral compound/total of all corrected integrals) x 100 Experiment 0705 - SeparationAcids and Neutral compounds by solvent extraction Compound Solubility in water Density compared Rxn with Br2 Rxn with KMnO4 to water Cyclohexene insol less dense loss of color no loss of color, ppt formed cyclohexane insol less dense no loss of color no loss of color Toluene insol less dense no loss of color no loss of color, ppt formed Heptane insol insol - To test if unknown is heptane or water, take a sample of each and place in separate test tubes and add water. The test tube that mixes is water and the test tube that forms a layer on top of water is heptane - To test if unknown is hexane or hexene, take a sample of each and place in separate test tubes and add a few drops of Br2. If there is a loss of color then it is hexene. - To test if unknown is hexane or toluene, take a sample of each and place in separate test tubes and add a few drops of KMnO4, if ppt forms then it is toluene otherwise it is hexane - Alkanes don’t have double bonds so they are unsaturated and they do not react; CnH2n; they are inert - Alkenes have double bonds and are reactive, they exhibit substitution reactions - Aromatic hydrocarbons combust when ignited, they are cyclic compounds related to benzene
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