Organic Chemistry Lab Exam I Essay

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What is the purpose of recrystallization
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primary method for purifying solid organic compounds
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Explain why the solvent is heated and cooled in ice bath
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The desired compound along with the soluble impurities are dissolved in a minimum of near boiling solvent, the solution is then allowed to cool slowly without interruption. As it cools, the solubility of the compound decreases and the solution becomes saturated with the desired compound and the compound begins to crystallize.
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Why do we use a minimum amount of solvent to dissolve the solid that is to be recrystallized?
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It cleans off any residual soluble impurities clinging to the surface of the desired crystals
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State the factors that limit the amount of solid that forms in the recrystallization process?
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-solubility -temperature cooling -temperature heating
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The purpose of Rotoevaportation
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a device used for efficient and gentle removal of solvents in large amounts
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Why is rotation required in Rotoevaporation
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to reduce the possibility of bumping and greatly increase the surface area of the solution
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Why is a vacuum required for the rotoevaporation
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to reduce pressure
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When is rotoevaporation utilized
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to evaporate dilute solutions obtained from extraction procedures or from chromatographic separations
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State the requirement of the solvent for effective use of the rotovap
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The boiling point has to be high
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State the purpose of the vacuum filtration
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is a technique for separating a solid product from a solvent or liquid reaction mixture
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Why do we wash the solvent from vacuum filtration
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because too much material can stick to the dry agent and reduce the yield.
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How can you lose product with vacuum filtration
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Washing with too much solvent
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State the purpose of drying agent
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When we need to remove water, we use chemicals to remove excess water
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How can using too little or too much drying agent
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You can lose your product, use too much agent can cause a loss of product by absorption from the drying agent. -use too little and water will remain and the product will be wet
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What affect does impurities have on the melting point
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impurities lower the melting point
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Whats the purpose of melting point
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tells us how pure the compound is
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What effect does the sample size of the melting point have on the melting point
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Too little can cause a mp range of too little.
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What effect does the rate of heating have on the melting point of a solid
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broad melting range, melting range will be high
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Purpose of acid/base extraction
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a powerful method for purifying mixtures in the work up procedure of a reaction
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State the purpose that the addition of an acid or base serves to promote the extraction process
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It helps us bring out the wanted product or convert the original solvent into what we want or need.
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Ka and Kb
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The strongest acids have large Ka values and small pKa values [pKa = -log(Ka)]. As acid strength increases, pKa decreases. The strongest bases have large Kb values and small pKb values [pKb = -log(Kb)]. As base strength increases, pKb decreases. The conjugate base of a strong acid has a very small Kb value. In general, as acid strength (and Ka) increases, the strength of the acid’s conjugate base decreases. The conjugate acid of a strong base has a very small Ka value. In general, as base strength (and Kb) increases, the strength of the base’s conjugate acid decreases.
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How are the organic layer and aqueous layer identified in the separatory funnel
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It goes by density, the heavier compound will be on the bottom, however the less dense will be on top.
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State factors that affect the amount of compound extracted from the organic layer into the aqueous layer
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Certain compounds such as salt can disrupt the aqueous layer, by making it more polar. This will decrease the solubility of the compound in the aqueous layer.
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The purpose of spectroscopy
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used to confirm identity of a compound
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What information can we get from the spectroscopy?
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We can identify unknown compounds and the functional groups
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Why are salt plates used in IR rather than glass
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-glass can absorb the compound, whereas salt is less likely too. They are also more transparent.
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The purpose of distillation
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-to purify a compound that is pretty much pure, completely
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The advantage of using distillation to carry out a chemical reaction over simply heating a reaction mixture.
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Compounds would decompose at high temperatures necessary to vaporize. However, because the boiling point is dependent on pressure, the distillation at reduced pressure, under a vacuum can make this possible.
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Purpose of reflux
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process of boiling reactants while continually cooling the vapor returning back to the flask, as a liquid Used to heat a mixture for long period of time at certain temperatures
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Advantage of using reflux rather than simply heating a reaction mixture
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-it can be left for long periods of time without need to add more solvent or fear of the reaction vessel boiling dry, as any vapors is returned from the condenser
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Thin layer chromatography purpose
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is the separation of a mixture of two or more different compounds or ions by distribution between two phases, stationary and mobile
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Stationary phase
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-a solid or a liquid supported on a solid -is the silica gel on the plate
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Mobile phase
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a liquid or a gas -is the solvent or mixture of solvents
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Process of stationary and mobile phase
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-as a solution is placed on the column, each molecule in the solution has to decide if it will stay in the solution (mobile phase) or adhere to the solid (stationary). The two compounds dissolved in the original solution will have different partitionary factors, meaning that one of them will adhere more to the stationary phase than the other. This partitioning can be governed by a multitude of factors, polarity, solubility, hydrogen boding, volatility. A more polar molecule will adhere more strongly to the more polar stationary phase than a less polar molecule.
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Rf values of TLC
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-value for each dye is then worked out by distance traveled by component/distance traveled by solvent -the less polar compound moves higher up the plate resulting in a higher Rf value -a more polar won’t move far
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Salt plates
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-polished sodium chloride -transparent to infrared radiation -salt dissolves on plates when placed on the plates -moisture in the air causes salt plates to become cloudy, this is why we store them. -if damaged it will through off the IR spectrum

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