CHEM 232 Midterm Exam

CHEM 232 Midterm Exam Good recrystallization solvent - ANS-Favorable temperature coefficient: Solute is essentially insoluble at room temperature or below, and highly soluble below the solute melting point Good recrystallization solvent - ANS-Impurities insoluble in the solvent at all temperatures or remain soluble at room temperature (both will allow physical separation from the desired solute) Good recrystallization solvent - ANS-Low enough boiling point so easily evaporated from the desired solute. Good recrystallization solvent - ANS-Boiling point lower than the melting point of the solute. Good recrystallization solvent - ANS-Solvent does not react with solute. Assessing general solubility of solute in a variety of solvents - ANS-In general, polar or ionic solutes are soluble in polar solvents. Assessing general solubility of solute in a variety of solvents - ANS-Non-polar solutes are soluble in non-polar solvents. Assessing general solubility of solute in a variety of solvents - ANS-Typically, there is a difference in polarity between the solvent and the solute, it is not an exact match. Usually, the solvent is slightly more polar than the solute. Assessing general solubility of solute in a variety of solvents - ANS-Polarity of solvents assessed in terms of their dialectric constant-values: 2-3, nonpolar, value 10-polar Assessing general solubility of solute in a variety of solvents - ANS-Limit the volume of solvent used to the bare minimum possible; too much will limit the recrystallization yield. Trouble-shooting experiment 1 - ANS-Goal is to keep the dissolved solute being recrystallized as hot as possible until impurities have been removed. Trouble-shooting experiment 1 - ANS-To that end, the solution should be exposed to cooler surfaces (the walls and stems of the funnel) as little as possible; the surface of air-cooled points of contact with the hot solution should also be limited. Trouble-shooting experiment 1 - ANS-Crystals don't form after chilling on ice? Scratch the bottom of the flask containing the solution, "seed" the solution with a small amount of pure species. Seeding - ANS-The point is that the crucial energy-requiring step in recrystallization is the formation of the first aggregated collection of molecules....a "seed". Once formed, or provided, it serves as a magnet for the rest to come out of solution. How do you tell if the recrystallized material is purer? - ANS-Melting point Pure - ANS-In ____ solids, the constituent units have definitive "places in the alignment of the crystalline structure, one that maximizes the contact between molecules and the stability of the solid. An impurity will not fit into the alignment in the same way. As a result, it weakens the contact between the individual elements... making it easier to break those contacts. Impure - ANS-The melting point of an ____ substance will be lower than the pure material and the temperature range over which it melts will be broader. Identity - ANS-Mixed melting point analysis can be used to confirm the ____ of a solid. Calculations for Experiment 1 - ANS-How to predict the maximum possible yield of recrystallization: Use solubility at the higher temperature (given in mg/mL) to determine the maximum amount you could dissolve. Then use the solubility of a substance at a lower temperature; that will be the amount of solute that you will not be expected to recrystallize. Percent yield - ANS-____ is (mass retrieved/initial mass) x100 % Extraction of trimyristin from nutmeg (reflux) - ANS-Principles are slightly different in the choice of solvent than recrystallization: deciding on what makes the best solvent to dissolve the desired molecule; determining the yield and purity of the material isolated. Extraction - ANS-For ___ we want the solute to be very soluble in the solvent of choice at all temperatures. Typically, we also use a relatively low boiling solvent. Trimyristin - ANS-____ is a tri-ester of 14 carbon saturated fatty acid (carboxylic acid groups), myristic acid and glycerol (alcohol). As a result, it has three long non-polar tails, but also a head portion composed of three polar carboxyl groups. Diethyl - ANS-Non-polar solvents (like ___ ether which was used) interact with the non-polar tails, but a solvent like dichloromethane which is slightly more polar can also interact with the polar head group. Powder - ANS-To maximize the extraction of trimyristin, nutmeg ___ was used because of the increased surface area which would be exposed to the solvent, and thus the extraction; had whole nut megs been used, it would have been necessary to grind them up to achieve the same end. Alternatively, the heating and stirring time would have been increased. Lastly, solvent was removed under reduced pressure. Base-base extraction - ANS-The first requirement for an acid/base or base/base extraction is that we use two immiscible liquids. One of the liquids is always water, COTINUES...