CHEM Lab Final Tuesday December 9 2014 12 54 PM Experiment 1 Who has my compound Purpose use physical properties melting point thin layer chromatography comparison of Rf values solubility in sodium bicarbonate solution and mixture melting point to identify an unknown compound Procedure 1 Determine melting point Use Mel Temp apparatus with high setting with a 6 10 C min and low setting with 1 2 C min i Temperature range from the point at which the first crystal is observed beginning to melt to the temperature at which the last crystal melts ii Melting points taken for determination of purity iii Melting points taken for identifying unknown 1 Taking mixture melting point taking small sample of unknown and mixing it with a known suspected to be the same sharp melting point 2 Solubility in sodium bicarbonate add small amount to 2 mL of 10 aqueous sodium bicarbonate to see if it dissolves i Weak base that reacts with protic acids to form sodium salt of acid and carbonic acid ii Any acids will react with sodium bicarbonate iii The results of this test allows to reduce number of compounds that you need to consider as identical to unknown 2 Thin Layer Chromatography use 6 mL of solvent 50 50 ethyl acetate hexanes and compound dissolved in 4 mL 95 ethanol Calculate Rf value distance spot traveled divided by distance solvent traveled i In which the adsorbent is stuck to a solid material ii Distance a compound elutes up the plate may be used for identification purposes 2 Mixture melting point mix both compound together and determine melting point If range is broader not the same compound if range is narrow compounds most likely the same Then melt standards to determine identity of compound 3 Mixture melting point with inorganic solids melt organic inorganic solid mixture Sapling 1 Tiny shard of broken glass inside melting point capillary melting range not affected 2 Composition of mixture of two components generate lowest possible melting point 3 Good to perform mixed melting point determination at two different ratios of unknown known materials in melting point capillary because it is possible for one particular combination of materials to generate a eutectic low temp 4 True lower limit of m t r occur when first crystal melts upper limit of m t r occur when last crystal melts and mp is physical measure of forces holding material into its crystal lattice 5 Rapid temperature increase will give a m p r that is too high 6 M t r of an impure sample is broader and lower than pure sample 7 TLC use thin glass spotter or capillary tube to apply dilute solution to TLC plate 8 Filter paper in TLC chamber prevent evaporation and air is saturated with solvent vapor 9 Polar compounds have higher Rf values Experiment 2 Recrystallization Favored for high melting solids thermally unstable solids Process of dissolving a solid material into a suitable solvent or mixture of solvents and allowing material to crystalize from the solution o Removing impurities present in compound o Getting maximum amount of compound back as a purified solid Choice of recrystallization solvent o Should not chemically react with the compound o Should dissolve compound while hot o Should not dissolve while cold o Should dissolve impurities at all temperatures or not at all Purpose Use recrystallization to purify solid impure compound Procedure o Heat 75 mL water in 250 mL beaker and invert Buchner funnel on top o Dissolving compound add 1 5 g acetanilide into 125 mL Erlenmeyer flask with 25 mL hot water Heat until all solid has dissolve o Hot vacuum filtration filtrate using flask and Buchner funnel with vacuum o Reheat filtrate again to dissolve acetanilide Cool solution again in ice water bath o Isolate and dry pure acetanilide vacuum filter cold solution to collect crystals of acetanilide o Determine weight of recovered acetanilide and melting point Sapling o Techniques included in four common methods for separation and purification extraction recrystallization and distillation o Recrystallization not for organic materials that are liquid o For suitable solvent for recrystallization solvent either dissolve impurities at all temps or not dissolve at all or solvent should not dissolve compound when cold o Initial hot vacuum filtration done to remove insoluble solid impurities o If filtrate solution is cooled too rapidly small impure crystals will form o To increase percent recover of acetanide solute after the first filtration place solution in ice bath for about 10 minutes o When oil form while solution cools reheat solution allow solution to cool once oil is dissolved and add a small amount of water Experiment 3 Molecular Modeling Purpose used computer to visualize 3D properties of molecules to gain understanding about steric interactions bond lengths bond angles dipole moments and bond rotation Sapling o True about chair conformation of cyclohexane contain six axial hydrogens that are parallel to ring axis and contain six equatorial hydrogens that radiate around ring equator o True about equatorial and axial positions in cyclohexane each carbon is attached to one axial and one equatorial hydrogen and each face of chair conformation ring has three axial and three equatorial hydrogen in an alternating pattern o True about cyclohexane after ring flip axial substituent become equatorial different geometries a molecule can attain by bond rotations and bends are called conformations substituent larger than H prefer equatorial due to smaller steric strain o If bond angle increase steric strain decrease because groups are moving further apart o o Increase dihedral angle of gauche butane will increase torsional strain Dihedral angle increase will decrease steric strain because groups are moving apart Experiment 4 Infrared spectroscopy Purpose use computer to learn about infrared spectroscopy identify different organic unknowns by their spectra and become familiar with common functional groups Sapling o IR measure vibrational frequency of a bond o IR recorded using ATR detector Attenuated Total Reflectance o IR over beam through sample spectrophotometer ATR crystal not degraded by water and sample preparation and cleanup of an ATR is simple o Region useful for distinguishing between isomers with same functional group fingerprint region 1400 400 o IR can detect impurities but cannot analyze almost any kind of sample Experiment 5 Simple distillation and gas chromatography Purpose Use method of purification of volatile compounds compounds