Chromatography Compounds separated by differential adsorption Liquid-solid Chromatography or Column Chromatography Stationary Phase Solid adsorbents typically polar alumina, silica gel, starch Polar compounds strongly adsorbed to stationary phase...separated from non-polar compounds Polar mobile phases selectively elute mixtures of polar compounds Mobile Phases Hydrocarbons - hexane, toulene, Increasing | petroleum ether Eluting | Halocarbons - chloroform, Ability | methylene chloride | Ethers - diethyl ether \/ Ketones - acetone Alcohols - methanol Organic Acids & Water Chromatography of Azobenzene. Syn-Azobenzene Anti-Azobenzene Stationary phase is alumina More polar syn isomer strongly adsorbed Non-polar anti isomer eluted with petroleum ether (Petroleum Ether - A mixture of low-molecular weight alkanes Thin-Layer Chromatography Stationary Phase - silica gel or alumina coated on glass or plastic Mobile Phase (hexane, ether, methanol, etc.) Farthest point of solvent migration (line drawn) Rf = c/s Known compounds & unknown mixtures applied Recrystallization Temp Boiling Point of Solvent Step 1 - Dissolve all visible material in minimum amount of hot solvent. (Except "mechanical" impurities - dirt, lint, glass) Step 2 - Add decolorizing charcoal if required. Step 3 - Filter hot solution by gravity. Step 4 - Cool solution and allow compound to crystallize. Step 5 - Collect crystals by vacuum filtration using water aspirator (use trap). Recrystallization Trial and error selection of solvent to dissolve impurities Necessarily must lose a certain amount of compound of interest Example of Recrystallization Target compound A contaminated with impurity B, both with identical solubilities: 5 g/100 mL at 20oC 20 a/100 mL at 80oC (boiling point) Have 10 g of mixture that is 80% A & 20% B At 80oC 40 mL solvent dissolves 8g A 100 mL 8g x ------ = 40 mL 20 g After cooling to 20oC 2g A remains in solution 5g ------ x 40 mL = 2g 100 mL And all of B remains in solution. Six grams A precipitates. 6g recovery is -- = 75% 8g Phase Diagram (Eutectic Diagram) Simplest case of two components (Actually N impurities) Pure compound has sharp high M.P.range ( <2 oC) All melting points of mixtures lower than pure compounds. Impure materials have broad melting point ranges. Mixed melting points of compound of interest with known compound do not change if compounds are identical. Melting point depression is proportional to mass of impurity. Notes on Procedure Recrystallize benzoic acid from water at bench. Recrystallize naphthalene from ethanol in hood. Determine melting points: impure benzoic acid impure naphthalene recrystallized benzoic acid recrystallized naphthalene 8:2 benzoic acid/acetanilide 9:1 benzoic acid/acetanilide Disposal: Water & ethanol down drain Capillary tubes in glass box Filter paper in trash Hand in recrystallized products Quotes of the Day "Hot glassware looks just like cold glassware." "Hot thermometers break on a cold surface." "If you're not part of the solution you're part of the precipitate." Boiling Point & Distillation Vapor pressure of ideal mixtures is determined by Raoult's Law: P = Pa x Xa P - vapor pressure of A in mixture Xa - mole fraction of A Pa - vapor pressure of pure A Vapor pressure of A in mixtures is proportional to its mole fraction. Also P = Pb x Xb P - vapor pressure of B in mixture Xb - mole fraction of B Pb - vapor pressure of pure B Notes on Procedure Simple distillation of 50:50 ethyl acetate/butyl acetate Return distillate from simple distillation to distilling flask to do fractional distillation Use low water flow rate through condenser. Check glassware for chips and cracks. Use boiling stones. Collect distillate in graduated cylinder. Record temp & volume data. Transfer contents to A, B, C. Distillate in "recovered ethyl acetate/butyl acetate." Isolation of a Natural Product Caffeine - Tea Trymiristin - Nutmeg A naturally occurring component of plants or animals useful as medicines, dyes, flavors, and poisons, etc. Natural source is a complex mixture of organic and inorganic compounds. 12 mg estradiol/3600 mg ovaries 10 mg testosterone/100 mg testes 20 mg progesterone/625 mg ovaries (50,000 sonis) Two common methods afford primary separation Steam distillation - water-involuble oils co-distill with water at less than 100oC. Extraction - natural product selectively dissolved in water or organic solvent Chromatography follows Liquid-Liquid Extraction Distribution coefficient, partition coefficient concentration of A in solvent 1(Org). K = ------------------------------------- concentration of A in solvent 2 (H2O). Let K = 2 and volumes of solvent 1 and 2 equal After one extraction 2/3 of A is in S1 A concentration in S1 = 2 x A concentration in S2 After second extraction of S2 with S1 Total A extracted = 2/3 + 2/3 x 1/3 = 2/9 After third extraction of S2 with S1 Total A extracted = 8/9 + 2/3 x 1/9 = 26/27 Isolation of Caffeine from Tea Alkaloid xanthine isolated from coffee in 1821 by P.J. Robiquet. Strong stimulant China 350 A.D. medical use Aminophylline - (Theophylline) Myocardial stimulant & dilates coronary artery - for congestive heart failure & bronchial asthmas Extracted into water. Filter cellulose and calcium salts of tannins. Extracted from water into methylene chloride. Decaf - whole beans extracted with trichloroethylene at 71oC. Isolation of Trymitristin from Nutmeg O þ CH2-O-C-(CH2)12CH3 A triglyceride (a fat) þ O þ þ Component of Cell membrane CH--O-C-(CH2)12CH3 þ O þ þ CH2-O-C-(CH2)12CH3 Glycerol myristic acid Triester tetradecanoic acid 20-25% by weight of dry nutmeg Extracted into refluxing methylene chloride or ether. Diethyl ether has very low flash pt Can be ignited by hot surface Distilled ether goes in bottle designated recovered ether. Filter paper in solid waste jar Acetone down the sink Distribution Coefficient [o]organic K = ---------- usually [o]aqueous defined > 1 (K could be defined as reciprocal ratio.) K usually proportional to maximum solubility of substance in two phases. Maximum solubility = 1/K (maximum solubility of o in organic (CH2CH2) of in aqueous [A]1 As1/V1 As1 V2 ---- = Ko = ------ = --- þ -- [A]2 As2/V2 As2 V1 Separation by Reactive Extraction Benzoic Acid p-Nitroaniline Anthracene All three compounds soluble in methylene chloride CH2Cl2. All three compounds insoluble in water. But benzoic acid reacts with base to produce Water-soluble benzoate anion. And p-Nitroaniline reacts with acid to produce water-soluble m-nitroanilinium ion. Caution with Separatory Funnel Aqueous layer contain: Organic layer carboxyl anion if basic contains neutral or aminium ion if acidic. anthracene Dry with anhydrous Na2SO4 Extract twice for both HCl and NaOH CaSO4 or HgSO4 Filter or decant PhCOO-Na+ + NaOH in water Distill all but a few O2N-Ph-NH3+Cl- + HCl in water Air-dry in hood milliliters of solvent Neutralize with acid or base PhCOO- Na+ + NaOH + HCl ----> CO2H + NaCl pKa<0 precipitate pKa=4.2 O2N-Ph-NH3+Cl- + HCl + NaOH -----> NH2-Ph + NaCl pKa~2.5 no precipitate + H2O pKa~16 Filter Precipitate & Dry Extract precipitate and aqueous layer twice with methylene chloride. Dichloromethane is toxic - keep in closed vessels. Vent separatory funnel. Pre-lab question #2 important. (Also #5) Dry solids in oven about 20 min. Do not determine m.p. of anthracene. Filter paper in solid waste. Distilled dichloromethane in dedicated bottle. Addition of Water to Alkenes: Acid-Catalyzed Hydration Mechanism is reverse of acid-catalyzed dehydration of alcohols Dehydration-hydration are in equilibrium Le Chatelier's Principle þG=þH-(TþS) Rearrangement of carbocation is observed. Hydration of Norbornene Sublimation pp. 56-58 For purification of solids with unusually high vapor pressures Dehydrohalogenation Elimination reactions of alkyl halides þ eliminations. 1.2 eliminations E2 Reaction - Elimination, Bimolecular E1 Reaction - Elimination, Unimolecular R H- CH3- + ---------------------- ~40% ~60% ~80% ~60% ~40% ~20% Synthesis of Alkenes by Elimination Reactions. Dehydrohalogenation Qualitative Tests for Alkenes Bromine in Carbon Tetrachloride Potassium Permanganate Gas-Liquid Chromatography (GLC) Gas Chromatography (GC) Separates Cases Stationary Phase - Non-volatile Liquid Film Coating a Solid Support Mobile Phase - Carrier Gas He or N2 Compounds Separated in Gas Phase on Heated Stationary Phase (up to 200o-300oC) Detector Response Usually Different for Each Compound Peak Area Response Factor = ------------- must be determined by calibration Unit Quantity of Compound Determined by Calibration Dehydration of Alcohols Cyclohexene from Cyclohexanol Cyclohexene is distilled from reaction mixture as it is formed. Collect in cooled round-bottom flask not open test tube. Dehydration of Alcohols Equilibrium Favored by þH Favored by þS þG-þH-TþS Alkene production enhanced by concentration shift of equilibrium. Exploit boiling point difference of reactant & product Dean-Stark water separator Rearrangement Accompanying Dehydration Catalytic Hydrogenation of 4-Cyclohexene-cis-1,2-Dicarboxylic Acid EGE - pp. 312-318 Pt or Ni R-CH=CH-R' + H2 ---------> R-CH-CH-R' solvent þ þ H H Remember syn-addition on surface of catalyst Sodium borohydride, NaBH4, is source of hydrogen in situ H- hydride H- + H+ -----> H2 NaBH4 + HCl + 3H2O -----> 4H2 + B(OH)3 + NaCl (1% sodium hydroxide added as stabilizer) Catalyst, platinum on carbon, generated by Brown & Brown Method. Pt/C Chloroplatinic acid, H2PtCl6, costs $25/mL Pto generated in situ deposits on carbon powder to give very high surface area. H2PtCl6 + 2H2 -----> Pt + 6HCl from NaBH4 After formation of catalyst add excess hydrochloric acid and hot aqueous solution of alkene substrate. Inflated balloon indicates slight excess hydrogen pressure over atmospheric pressure. Blow it up first to check for leaks. Filter the reaction mixture. Immediately return catalyst to "recovered catalyst" container. NaBH4/OH- Solution Prepared Test starting material & product w/KMnO4. Catalyst in "recovered catalyst" jar Ether in "recovered ether" bottle Sodium sulfate in trash pp. 204-211 Pre-lab p. 220 7,8,10 Sample Preparation For Report Questions EGE Chap. 10 p. 221 17-0.15 224 19 Will get unknown liquid & molecular formula Identify liquid from IR spectrum & molecular formula. (or come close with possible structures) Identify another unknown from elemental composition & IR, NMR, UV & Mass Spectrum. IR Operation 1. Make sure that both beams are unblocked before turning instrument on. Switch the power toggle switch to on and wait one minute for the system to initialize. The wavenumber readout will display 4000 cm-1. 2. Set the chart paper to 4000 cm-1 using the CHART and PARAMETER ADJUST controls. 3. Place the sample in the sample (front) beam. 4. Use the baseline control (in the sample compartment) to bring the pen to about 80% T on the chart paper. If necessary, also swing the reference (rear) beam attenuator into the lightpath to cause the pen to rise to 80% T. 5. Press GAIN CHECK. Use PARAMETER ADJUST controls to obtain a 10-division downward deflection of the pen. Press GAIN CHECK again to return to the scanning mode of the instrument. 6. Choose the SCAN TIME. A 12 minute scan provides a higher-resolution spectrum than a 3 minute scan. 7. Press the SCAN control. 8. When the scan is finished, remove spectrum, sample, and swing reference beam attenuator completely out of the lightpath. 9. If you are the last user of the lab period, turn the power toggle switch to off and cover the instrument with the dust cover. Otherwise, leave the instrument on for the next user. 10. Clean salt plates for nest user. Sign-up sheet outside 303. Arrive 15 min. early. Monday & Tuesday sections: Students work in pairs Oxidation of Alcohols Alcohol -----> Aldehyde or Ketone or Carboxylic Acids First example of functional group transformation. Important in synthesis. General Equation oxidizer 1o Alcohol Aldehyde oxidizer 2o Alcohol Ketone oxidizer N.R. 3o Alcohol (no useful reaction) (4H+ + 3e- + MnO4- -----> MnO2 + 2H2O) 2 8H+ + 2MnO4- -----> 2MnO2 + H2O Common Oxidizers 1. Chromic acid H2CrO4 Not stable for long periods so it's produced when required by sodium dichromates & sulfuric acid. H2O Na2Cr2O7 + 2H2SO4 -----> [H2Cr2O7] -----> 2H2CrO4 + 2HSO4- or chromium trioxide in water. CrO3 + H2O -----> H2CrO4 Typically used with water or acetone as solvent. 2. Sodium Hypochlorite NaOCl NaOCl + H3O+ -----> HOCl + Na+ + H2O 3. Potassium permanganate KMnO4 used under basic conditions. Reaction proceeds best with regular agitation. Test for Hypochlorite HOCl + I- -----> I2