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35 Cards in this Set
- Front
- Back
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Difference between crystallization and precipitation?
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Crystallizaton - slow selective formation of the crystal framework resulting in a pure compound.
Percipitation - rapid formation of a solid from a solution that produces amorphous solid containing many trappe impurities. |
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What property does recrystallizaton rely on?
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As the temperature of a solvent increases, solutibility of the compound in that solvent also increases.
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5 steps in recrystallization
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find a suitable solvent
dissolve impure solid in a minimum volume of hot solvent remove any insoluble impurities by filtration slowly cool the hot solution to crystallize thedesired compound from the solution. Filter the solution to isolate the purified solid compound. slowly cool the hot solution to crystallize filter the solution to isolate purified soli compound |
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4 things in considering a solvent
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1. compound should be very soluble at the BP of the solvent ando nly sparingly soluble in the solvent at room temp.
2. UNwanted impurities should be either very soluble in the solvent at RT, or insoluble in the hot solvent. 3. the solvent should not react wiht the compound being purified. 4. The solvent should be volatile enough to be easily removed from the solvent after the compound has crystalized. |
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Why are erlenmeyer flasks preferred for dissolving solids?
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decrease amount of solvent lost to evaporation. Prevents formation of a crust around the glass, makes it easier to swirl the hot solution while dissolving the solid without splashing it out of the flask.
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When do you add decolorizing carbon
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After the ipure solid dsample is dissolved in hot solvent, add small amounts of decolorizing carbon.
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How do you make crystallization happen faster?
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Scratch wall of the flask to release small particles of glass to act as nuclei for growth.
Add small crystal - seed crystal. Boil off excess solvent. |
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Effect of impurities on MP?
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Lower or depress melting temp.
Broaden the melting range. |
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Eutectic point
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The melting point of a specific combination of compounds A and B at which both components in the mixture. Unique for two combinations.
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When do you use simple, and when do you use fractional distillation?
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When the temperature differences between BP is >100 = simple, otherwise, use fractional
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Where should the thermometer be placed?
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the tip of the thermometer must be correctly positioned slightly below the center of the condenser to accurately reflect the temperature of the vapors
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Azeotrope
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a mixture that cannot be separated by distillation due to the fact that the vapor emitted from boiling has the same relative makeup as that of the fluid
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Theoretical plate
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A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases are in equilbirium. or a "simple distillation" - efficiency of a fractionating column.
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What is the limiting factor in fractional distillation?
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Speed. The distillate can only be collected so fast otherwise fractionation cannot occur.
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Organic solvent extraction
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an organic solvent with a high affinity for the desired compound is used to extract the compound from another solution.
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Why is caffeine good candidate for sublimation?
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few intermolecular forces and possesses a lower boiling point than melting point
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Four advantages of sublimation
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no solvent is involved, can be performed at low temperatures, loss of material is kept at a minimum, and readily separates nonvolatile impurities
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How to compute Rf value?
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Distance traveled by the compound/distance traveled by the solvent front.
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How to handle an extraction mechanism?
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Invert and open stopcock to release pressure after a few swirls.
Never point the stopcock towards anyone. Make sure stopcock is closed before pouring DO NOT SHAKE VIGOROUSLY |
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Conditions favoring SN1
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tertiary alkyl halide
weak base polar protic solvent good leaving group. |
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What causes acid decrease in t-butyl experiment? What neutralizes the change?
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The formation of hydronium ion during the deprotonation step of the reaction.
NaOH |
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What are some factors affecting separation in GC?
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Nature of the stationary liquid phase.
Column length Column temperature Flow rate of the carrier gas |
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What determines the nature of the liquid phase?
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Dipole
Vander waals Polarity |
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What are good features of Solid support in GC?
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Porous, inert material,
High SA Efficacy of separation increases with decreasing particle size. |
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Retention time
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distance from inital ejection to peak maximum is retention time
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Two factors affecting separation in GC?
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Volatility and Polarity
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BP and Retention time
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Higher BP = longer retention time.
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What is oxidation
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The term is applied more generally to signify the loss of hydrogen atoms or the addition of oxygen or other electronegative atoms. Even more generally, oxidation can be defined as loss of electron density on a carbon atom.
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What does a primary alcohol oxidze to?
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Aldehyde
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What does a tertiary alcohol oxidize to?
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cannot be oxidized
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What can IR tell you?
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functional groups
purity structural information confirmation of a molecule's identity compared to a known spectrum. NOT Exact arrangement and structure. |
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What does the area under a GC curve represent
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Directly proportional to the number of moles that the compound present in the mixture injected onto the column.
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Bumping
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instantaneous, violent expulsion of vapor from the liquid
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Where to place the thermometer?
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Adjust the thermometer bulb so that the entire bulb is just below the level of sidearm of the distillation head.
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Forerun
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Volatile impurities that gets distilled first.
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