CHEM LECTURE

MODULE 3A In order to study chemistry they have to be in PURE FORM (recrystallization for solid) Reflux to create new product (not usually pure form) need to separate from other components not interested in. For 2 reactant to meet to react (needs to be in common venue: which is the solvent) Property of reactant must be soluble in it or else 2 reactants will not react. Then, purify SOLID: recrystallization (choose proper recrystallizing solvent: has to dissolve solute at high temp and not dissolve solute at low temp; if double solvent: solute with dissolve in one but not the other, those 2 should be miscible with each other) Main objective: Separate physically and chemically the organic molecule Extraction Removal of a desired compound (solute) or undesired from one vehicle (solid or liquid mixture) by the use of an extracting solvent; transferring a solute from one vehicle to another using an extracting solvent Solid-liquid extraction Solvent (non-toxic, easily removed, solute is soluble, nonreactive) Liquid-liquid extraction Solvent (non-toxic, easily removed, solute is soluble, nonreactive, immiscible with vehicle) Extracting solvent: should DISSOLVE desired compound (not everything only desired); immiscible to liquid mixture; desired material dissolve in liquid mixture at certain solubility; desired material must be soluble in the extracting solven; separation depend based on relative solubilities in liquid mixture and extracting solvent. *desired material soluble in extracting solvent Developing solvent: Chromatography Extraction To remove desired compound/s or undesired compound/s (impurities) using an extracting solvent Washing Using extraction to remove impurities; it is the impurities that are soluble in the extracting solvent Salting-out To decrease solubility of organic compound in the aqueous layer Extracting solvent Should dissolve the desired compound/s or the impurities Drying Removal of traces of water in an organic solvent using a drying agent Solid-liquid extraction Percolation Passing a liquid (extracting solvent) through a solid material (pulverized to increase extraction of desired material/s); either at room temperature or elevated temperature Ex. Coffee filter/ percolator: hot water (elevated pressure) then what comes out coffee extract At room temp: stainless steel/glass where you put solid mixture/matrix inside and let extracting solvent drip on it then comes out at bottom with stopcock it drains *pulverize leave grind first (more surface area) where solvent would mix with pulverized sample for maximum contact = get more desired material *MAXIMUM CONTACT OF EXTRACTING SOLVENT WITH SAMPLE= sample better off in powder form (maximum extraction of desired material) Leachate Liquid that comes out of the percolator Continuous Solid Extraction Soxhlet Extraction – the extracting solvent in the collection flask is heated and evaporated, and condenses down the chamber where the solid material is placed; the extracting solvent with the extracted material drips down the collection vessel. [stop heating when colorless] heated->condensed->then reused as extracting solvent (only once then repeatedly) while percolator keeps on adding extracting solvent until colorless Liquid-Liquid Extraction Uses separatory funnel (common) Also known as partitioning Separate/extract the desired material from a liquid mixture based in its “relative” solubilities in the liquid mixture and the extracting Choice of solvent for liquid-liquid extraction: Solvent must be able to dissolve compound of interest. Solvent must not be very soluble in water. Solvent must be low boiling so that it can be easily removed by distillation or evaporation. Solvent must have a density that is different from water. Must not react with the compound being extracted. Relative solubilities Distribution coefficient Ratio of the concentration of the solute in each solvent at a particular temperature It is independent of the total concentration and the actual amounts of the two solvents used K= concentration in solvent 2/ conc. in solvent 1 (solute is soluble in both liquid mixture and extracting solvent) Take note: after extraction Example: Compound A is soluble in diethyl ether to the extent of 20g/100mL at 20C and soluble in water to the extent of 5.0g/100mL at the same temperature. (Solubility of compounds in a particular solvent is found in handbook of physical constants) What is the distribution coefficient of compound A in diethyl ether and water? (Distribution coefficient can also be based on experiment if solubility is not found in handbook) Calculate how much compound will be extracted by single extraction. Example: A solution containing 5.0g of compound A in 100 mL water. If we shake this with 100mL diethylether, how much a will be extracted by the ether? Original sample = 5.0 gms of A dissolve in 100 mL water After extraction: Concentration of A in diethyl ether = X g/100mL Concentration of A in water = (5.0 -X) g / 100 mL 4 grams extracted by diethyl ether; 1 grams left on water Because a solute distributes itself between two solvent, a single extraction may not be very efficient. The 100 mL of ether maybe divided into two portions of approximately 50 mL each. [refer to 2nd pic] Double solvent= more but not able to remove all of the compound A in the water mixture because of relative solubility, compound a has some degree of solubility in water it will still be a little bit that will be left behind in water. More and more extraction = less and less of compound A that will be left behind in liquid mixture. Continuous Liquid Extraction Objective: extract more of the solute in extracting solvent i. Continuous Liquid - 1. If heavier than water: no funnel; heat extracting solvent = evaporates go into condenser drip down in liquid mixture then goes down (liquid mixture carry with it the desired solute) recycling solvent; desired solute left behind in solvent vessel so that when evaporate solvent drips down to liquid together desired solute and so on. 2. If lighter than water: w/ funnel; extracting solvent goes down to liquid mixture then floats up carrying with it the desired compound. TWO TYPES: Solid-Liquid| Percolation and Soxhlet Liquid-Liquid| Separatory funnel, Continuously -> Liquid Extractor MODULE 3B Distillation (Purification/Separation of Liquids) Objectives: 1. Explain the difference in the boiling points of different liquids 2. Separate components of liquid mixture 3. Construct distillation curve 4. Calculate amount of liquid component in a mixture 5. Compare the efficiency of separating liquid mixture using simple and fractional distillation Organic synthesis: need solvent as common venue for reactants to react to form product and solvent not evaporate (reflux to elevate the temp. when rxion will react preventing reactants and solvent from evaporating Recrystallization (purifying solid samples to remove impurities; can be single or double recrystallizing solvents), Filtration, Melting Point Determination Extracting (purifying & separating mixtures using extracting solvent) Boiling Point: Temperature at which a compound changes from LIQUID TO VAPOR at standard atmospheric pressure. Used to TEST PURITY of liquid impure liquids have wide boiling point range pure liquids have sharp boiling point Used to PURIFY LIQUID in distillation as an impure liquid is heated the low boiling component is transformed to vapour leaving the high boiling component in the distilling flask (residue), the vapour can be condensed as pure liquid (distillate) basis of separation of liquid sample in distillation is the DIFFERENCE IN BOILING POINT of components of a mixture. LOW BOILING COMPONENT -> transformed to vapor (evaporates) -> condensed back to liquid then distillate HIGH BOILING COMPONENT-> left in flask as “residue” separate it from a distillate DISTILLATION Process of heating a liquid mixture until its more volatile component evaporates leaving the less volatile component in the liquid form, then cooling the vapor until it condenses into a liquid (distillate) in a pure form. Separation of the components of the liquid mixtures is based on the difference in the boiling points of their individual components. Boiling Point and Structure * Intermolecular forces of attraction - H-bonding (2 butanol therefore break the bonding to evaporate butanol is 117; boiling point affected) - Dipole-dipole interaction - Van der Waals forces/London forces (non-polar, weak interaction) *to break attraction between two hexane molecule needs low temperature to break bp of hexane=69 * Molecular mass example: boiling point of methanol= 65 while ethanol = 78 (because methanol only has one carbon while ethanol has 2 carbons) Boiling Point and Vapor Pressure For a pure liquid/liquid mixture to boil, the total pressure (Ttotal) of the liquid must be equal to the atmospheric pressure (Patm) As the temperature of the liquid increases, the vapor pressure also increases. Dalton’s Law of Partial Pressure The sum of the partial pressures of all the gases equals the total pressure in a system Ptotal = P1+P2+P3 determine vapor pressure of components of liquid mixture Immiscible Liquids The total pressure of the liquid mixture is equal to the sum of the vapor pressure of each component of the mixture. no interaction between A and B Miscible Liquids Raoult’s Law – the partial pressure of the vapor of liquid A (PA) is equal to the vapor pressure of the pure liquid A (PA) multiplied by its mole fraction (XA) in the mixture PA=XAPA For mixture of liquid A and B *interaction between A and B, the amount of A or B contributes to how much vapor pressure is contributed by those compounds. *HOW MUCH OF A/B is PRESENT in the mixture 3 moles of A, vapor pressure of A = 0.3 atm 4 moles of B, vapor pressure of B = 0.4 atm What is the the total pressure of the liquid mixture? When a miscible liquid mixture is heated, it will boil and eventually evaporate (produce vapour) The vapour that will be produced will be have more of the more volatile component than the liquid. When the vapour is condensed, it will have more of the volatile component that the original liquid mixture AZEOTROPE MIXTURE (not separated by distillation=strong interaction) Constant boiling mixture Further distillation will not change its composition Ex. 95% ethanol, 5% water BP=78.4 TYPES OF DISTILLATION Simple Use to separate mixtures of liquids with large difference in boiling point, separate volatile from the less volatile component *volatile to distillate and non-volatile residue Fractional Used when components of a mixture have close boiling points by doing a series of or repeated distillation steps in the fractionating column Steam HIGH BOILING POINT The process of separating or purifying/separating a liquid which is immiscible with water by passing steam (H2O) through it, used to isolate water-immiscible organic compounds from complex mixture by separating it from the nonvolatile impurities Vacuum MISCIBLE Distilling liquid at low pressure so that it boils at a lower boiling temperature than the boiling point of the liquid, used of heat sensitive mixtures * Like simple but attached to a vacuum line| LOWER boiling point normal 0.78 can boil up to 65 1. SIMPLE DISTILLATION To separate liquid mixture To separate liquids with large boiling point difference (good but not a total separation) To separate liquids with small boiling point difference (distillate will contain more of the low-boiling component) To separate liquid from a solid that is dissolved in it (solution)