mp&recryst theory & concepts 8-19-07

mp&recryst theory & concepts 8-19-07 - Purification...

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Unformatted text preview: Purification of Organic Solids Identification using Melting Point Determination Experimental Objectives become familiar with common heating & cooling methods purify organic unknown using recrystallization identify unknown by melting point & solubility data Recrystallization crystalline material (solute) is dissolved in a hot solvent then returns to solid when cooled important to understand "dissolving" solvent solid choice & amount of solvent is critical Solubility solvent temperature - solubility increases with temperature solvent volume - amount of solute vs. volume of solvent solvent properties - solute/solvent interactions "like dissolves like" - polar H2O dissolves most (polar) inorganic compounds - nonpolar organic solvents dissolve most (nonpolar) organics Recrystallization Solvents Polarity Index 0.0 2.3 2.9 3.4 4.3 4.3 5.2 5.4 6.6 9.0 Common Name of Solvent hexane toluene diethyl ether (ether) methylene chloride ethyl acetate chloroform ethanol acetone methanol water Structure CH3(CH2)4CH3 C6H5CH3 (CH3CH2)2O CH2Cl2 CH3CO2CH2CH3 CHCl3 CH3CH2OH (CH3)2C=O CH3OH H2O (least polar) (most polar) 5 Steps to Recrystallization 1. 2. 3. 4. 5. find "best" suitable solvent dissolve bulk sample in minimum of hot solvent remove insoluble impurities by hot gravity filtration cool solute/solvent mixture. scratch glass/liquid interface or use seed crystal to induce crystallization if SUPERSATURATED collect crystals by suction filtration & air dry. Step 1 1. find "best" suitable solvent dissolves crystals when hot not when cold some typical solvents water ethanol ethyl acetate methylene chloride (dichloromethane) toluene hexane Steps 2 - 3 1. dissolve bulk sample in solvent minimum amount of solvent to dissolve solute with heat this will be unique to solute & solvent choice use boiling chips or sticks (prevents "BUMPING" : a sudden, explosive release of a large volume of vapor) add a few mL of excess hot solvent 1. remove insoluble impurities hot gravity filtration (hot NOT BOILING) rinse residue in filter and in flash with about 1 mL hot solvent Hot (not boiling) Gravity Filtration using a fluted (fan shaped) filter paper Hot Plate Step 4 4. cool solute/solvent mixture cool slowly to R.T. (10-15 min) to crystallize (not precipitate) and eliminate soluble impurities from getting trapped inside the crystals If crystals do not form try... scratching air/liquid interface with a glass stir rod add a seed crystal evaporate some excess solvent (remove about of total) and repeat Step 5 5. collect crystals vacuum filtration using Buchner Funnel allow air flow through for <5min to help dry to vacuum Percent Recovery evaluates recrystallization quantitatively not the same as a percent yield do not forget to record weight of unknown amount of solid recovered %= X 100 amount of initial solid Melting Points temperature where solid & liquid phases are in equilibrium characteristic physical property of solid the identity of an organic solid the purity of an organic solid most organic solids melt 25 - 250 oC A. Sublimation B. Melting C. Evaporation Melting Points Organic molecules within the solid are held together by intermolecular forces Hydrogen bonding Dipole-dipole interactions Van Der Waals forces (temporary dipole) 2 10 kcal/mol 0.5 - 2 kcal/mol < 0.5 kcal/mol compare to an intramolecular covalent bond ~100 kcal/mol Melting Points Consider the effects of intermolecular forces on the melting point of three organic compounds of similar mass: O H O O O benzoic acid ethyl benzoate ethyl benzene Hydrogen Bonding Benzoic acid melting point 122-3 oC O H O O H O O H O O O H O O H Dipole-Dipole Ethyl benzoate, melting point -23 oC - C + O O O + C O - Dipole-dipole interactions are weaker. Ethyl benzoate is heavier than benzoic acid, yet its melting point is below room temperature, almost 150o below benzoic acid. Van Der Waals Ethyl benzene, melting Point -95 oC Temporary dipoles are the weakest form of intermolecular force. Solids held together by these forces have low melting points. Melting Point Determination The energy (heat) required to break these intermolecular interactions is the same for any two molecules within the solid pure organic compounds have a distinct melting point that can be used for identification melting point should also occur over a very narrow range of temperatures impurity in the solid disrupts intermolecular forces takes less energy to melt solid therefore lowering and broadening melting point range Pure Solid The Van Der Waals forces that hold the solid together are regular and are the same from molecule to molecule Pure naphthalene melts at 82-83oC Impure Solid cyclohexane contaminated naphthalene, mp 62-69oC Melting Point Determination Problem: Suppose you are cleaning the chemical stockroom and you encounter a reagent bottle whose label has decomposed. You suspect that the compound could be either benzoic acid (mp 122-3 o C) or succinimide (mp 123-5 oC). You take a melting point, and sure enough the unknown melts at 123 oC. How would you use what you have learned to determine the true identity of the contents of the bottle? Theory & Background a. Melting Point Temperature at which a transition occurs between solid and liquid phases Temperature at which an equilibrium exists between the well-ordered crystalline state and the more random liquid state. b. Melting Point Range The first point (lower temperature) is the temperature at which the first drop of liquid forms amongst the crystals. The second point (higher temperature) is the temperature at which the entire mass of solid turns to a clear liquid. c. Uses Identify Compounds Establish Purity of Compounds Melting Point of mixtures Melting Point Indicates Purity in Two Ways 1. The Purer the Compound, the Higher the Melting Point 2. The Purer the Compound, the Narrower the Melting Point Range mpB > mpA mpB mpA Liquid A + B So li d +L So l id + Range First Drop of Liquid id Solid A + B 0 mol %B Eutectic 0 mol % A 1. Melting point of A decreases as impurity B is added 2. Eutectic Point is the Solubility Limit of B in A; Thus, it is the Lowest Melting Point of an A/B mixture (Note: Sharp melting point no range at eutectic point) Temperature { Li ui q d Complete Melting iq u Mel-Temp uses a heated aluminum block three samples simultaneously aluminum is quick to respond to temperature changes higher temps can be achieved more quickly advantage in determining high melting points good choice for organics that melt over 200 C MP Sample Size Use the minimum amount of sample & slowest heating rate to obtain the sharpest, most accurate melting point. Sample size: samples placed in a mp capillary tube. Use the minimum amount seen through the magnifier (1-2 mm) Heating Rate: quickly go to 20 C below the expected MP, then slow to 1-2 C per minute to observe the correct MP Mixed M.P. to determine unknown substance Solution: obtain known sample (most likely to be your unknown) thoroughly mix the known sample with a portion of the unknown (50/50 homogeneous mix) determine the melting point for unknown, mix, and known (3 simultaneously). MP Pure A MP Pure B Temp (C) 100% A 100% B ...
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This note was uploaded on 09/05/2008 for the course CHEM 3201 taught by Professor Haney during the Fall '07 term at University of Houston - Downtown.

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