CHEM 262L Final
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1. What is chromatography good for?
2. What are the two main phases of chromatography?
3. Why is it better than distillation?
4. What are the two most combos of phases? what is column? What is TLC? what is gas?
5. How does polarity compare b/t SP and MP? Unless?
1. For separating a mixture of two or more compounds/ions by distribution between two phases: stationary vs. mobile
- 2. stationary phase and mobile phase
- 3. B/c it can be done on a very small scale
- 4. Solid/liquid; liquid/gas. TLC & column are solid/liquid while GC is liquid/gas
- 5. SP is usually more polar than MP unless we're doing REVERSE PHASE CHROMATOGRAPHY
1. What is chromatography based on? (4)
2. What does elution rate of a molecule depend on?
3. What size can column be?
4. What can stationary phase be?
1. Chromatography is based on partitioning of molecules between SP and MP. Can be based on polarity, solubility in the solvent, hydrogen bonding, and in GC - volatility.
2. Elution rate depends on attraction of molecule to the SP vs. the MP.
3. Any size
4. Any material as long as it doesn't dissolve in associated mobile liquid phase
What are steps of column chromatography? 7
- 1. Selecting column
- 2. Selecting stationary phase (absorbent) and filling column
- 3. Selecting eluent
- 4. Loading column
- 5. Running column
- 6. Analyzing eluted fractions
- 7. Isolating different components
SELECTING A COLUMN:
1. When should you select a larger column?
2. What does the diameter depend on?
3. What does length of column depend on?
4. What type of column should you generally try to use? Why?
5. How much absorbent should be used?
6. What does a longer column allow?
7. How do you keep absorbent in?
- 1. When you have larger samples with mixtures that are difficult to separate
- 2. Amount of sample
- 3. Difficulty to separate
- 4. Shortest, smallest one to save time
- 5. 20x the amount of sample
- 6. better separation
- 7. Use a cotton plug at the end.
1. Why are the size of the particles important?
2. What are the two types of packing methods?
3. Explain each type of packing method.
4. Which type of packing method should be used for small columns?
1. Too small - separation will be very good (b/c of larger surface area interaction b/t stationary phase and molecules), but will take FOREVER. Too large - column will drain effectively but poor separation
2. Dry and wet
3. Dry - column is first filled with dry absorbent then organic solvent (eluent) is added to column to saturate the absorbent
Wet - more common - absorbent is mixed w/ organic solvent in beaker to form slurry. Slurry is poured into column using funnel w/ stopcock at bottom open to allow excess solvent run through.
4. For small columns, should use dry packing method.
1. Why should the column never run dry?
2. What is the purpose of the sand?
1. B/c cracks can form within the absorbent or within the absorbent and the glass column, adversely affecting separation.
2. Sand - to protect shape of the orgnaic layer from the velocity of the newly added eluent.
SELECTING AN ELUENT:
1. How important is this?
2. How can you test which eluent to use?
3. If you have a polar analyte, what type of eluent should you use?
4. What difference in Rf values is needed to effectively separate different components of a mixture?
- 1. VERY
- 2. Using TLC chromatography - the SP of the TLC plate should match the absorbent of the column as closely as possible.
3. Polar eluent.
SELECTING AN ELUENT (PART II)
1. Describe the characteristics of a good solvent? (4)
2. What should the BP be like?
3. What are examples of very non-polar solvents? (3)
4. What are examples of slightly polar solvents? (3)
5. Polar? (2)
6. Very polar? (2)
- 1. Nontoxic, readily available, BP b/t 50-120 degrees celsius, inexpensive
- 2. BP should be high enough so rapid evaporation isn't a problem, but low enough so that the product can be isolated by rotary evaporation after the fractions are collected.
3. Very non-polar: hydrocarbons, toluene, carbon tetrachloride
4. Slightly polar - diethyl ether, dichloromethane, chloroform (most common)
5. Polar - acetone, ethyl acetate
6. Very polar - ethanol, methanol, water.
1. When dissolving mixture, what should it be dissolved in?
2. What controls elution rate?
3. As different fractions elute off the column, what can be increased?
4. What is the most common method used to analyze different fractions?
5. What can be done based on the method above?
6. What happens after evaporation of the solvent?
1. In eluent or something less polar.
3. Polarity of the eluent
4. TLC analysis
5. Fractions can be combined if they contain same components of the mixture
6. After evaporation of the solvent, different components are isolated and can be further purified.
What are other options instead of column chromatography? When?
Flash chromatography - applies air pressure to column to increase elution rate.
HPLC - using extremely fine absorbent --> smaller particle size means larger surface area and better separation, but this takes longer. So use high pressure.
1. What is TLC used for? (2)
2. What happens in TLC?
3. What is the purpose of TLC (6)
1. Fast, qualitative analysis of a mixture OR for very rapid separation of small amounts of material
2. Thin coating of silica gel on a plate of glass is used as SP while moving liquid phase is allowed to climb up plate due to capillary action, separating compounds in a mixture.
3. (1) establish identicalness (2) determine # of components (3) det appropriate solvent for column chrom (4) monitor progress of rxn (5) monitor column chromatography separation (6) check effectiveness/purity of separation by column chromatography, recrystallization or extraction
1. How should one spot onto a TLC plate?
2. What should your developing solvent depend on?
3. What do more polar solvents cause? (2)
4. How do you know if a solvent is too polar? Too nonpolar?
5. What are the most common solvents? 2 What polarity are they? What are they a good choice for?
6. What's a good solvent for hydrocarbons? (2)
7. What's a good solvent for more polar materials? (3)
1. Several times to achieve high concentration of sample in a small spot.
2. Depends on materials to be separated
3. High Rf values and faster movement up the plate
4. Too polar if the spotted materials move with the solvent front; too nonpolar if the solvent does not create any movement in the material in the spot
5. Dichloromethane and toluene (int polarity) good for a variety of functional groups
6. Hexanes and petroleum ether
7. Ethanol, methanol, acetone
1. What is important about the developing jar?
2. What happens if the spotting line is below the solvent line?
3. How does one visualize the spots? (2)
4. What do plate conditions depend on? 5
1. Must be small enough and closed off so that the atmosphere is saturated with vapor of the solvent to facilitate the capillary action.
2. Then the material to be analyzed will be dissolved into the solvent.
3. Using UV light or with visualization reagent like iodine (but be careful b/c it will sublime quickly).
4. Absorbent used, thickness of absorbent layer, how much compound has been spotted, solvent used, and ambient temp.
1. What is the SP? What is the MP?
2. Define SP further
3. What is generally used for the MP? define further
4. Describe what happens
1. SP = nonvolatile liquid phase; MP = inert carrier gas
2. SP is a high boiling organic compound adsorbed ont he solid inert packing in the column
3. He. Pushes molecules through column but does not significantly interact with analyte on a molecular level
4. Mixture of compounds to be separated is introduced to carrier gas stream where its components are equilibrated between moving gas phase and stationary liquid phase.
What is GC used for? 4
1. Test purity of substance and separate components of a mixture
2. Determine relative amount of components in a mixture
3. Identify a compound
4. Preparative method to isolate pure compounds from a small amount of a mixture.
1. What determines effectiveness of GC column?
2. How can liquid component interact with compounds being analyzed?
3. What is temp range of the column?
4. What determines how fast sample moves through column?
5. What does a high flow rate do?
6. What does a low flow rate do?
1. The liquid stationary phase
2. Van der waals, dipole interactions, hydrogen bonding, etc.
3. 250-350 degrees celsius
4. The flow rate of the carrier gas
5. A high flow rate does not allow enough time for the analyte to properly solubilize in the stationary phase, adversely affecting separation
6. A low flow rate broadens peaks and decreass resolution, molecules can migrate backwards.
1. What qualitatively should the temp of the injection port be for GC? Why?
2. When must all molecules start separation?
3. What happens if there's too much sample?
4. What if there's too little sample?
5. What must the detector be? To keep the sample from what? What is the temp usually?
6. What is generally used?
1. The temp should be high enough so that the entire sample instantly vaporizes. It's essential for good separation
2. At the same time
3. Too much sample - not all of it vaporizes adversely affecting separatinon
4. Too little sample - will be difficult to detect at the end of the column
5. The detector must be hot enough to keep the sample from condensing. Temp is usually around 300.
6. Thermal conductivity detector.
1. What happens if oven temp is too high? What if it's too low?
2. What happens once mixture reaches column?
3. What determines how much time is required for elution? Relative to other samples
4. What are characteristics of a quickly eluting compound?
1. Too high - different components elute too quickly and no separation will be observed. peaks will be broadened giving poor resolution
2. Different components of the mixture will equilibriate between liquid and gas phases.
3. Length of time required for elution depends on how much time it spends in vapor phase vs. liquid phase.
4. The more time it spends in vapor phase (low BP, high vapor pressure) the faster it will elute. MORE VOLATILE
1. What affects separation? (3)
1. Boiling point of compounds - lower BPs travel faster
2. Flow rate of carrier gas - too fast - sample in vapor phase can't equilibrate with liquid phase. Too slow - poor resolution, difficulty in chromatograph analysis
3. Choice of liquid phase used in column - MWs, functional groups, polarities in the component molecules in the mixture to bes eparated will help determine choice of packing of the column.
1. Define. When does it occur?
2. When is it a useful method for purifying a compound?
3. What can facilitate sublimation and require less heat?
4. What must you prevent from happening? 2
1. Sublimation is the transition of a substance directly from solid phase to gaseous phase without passing through intermediate liquid phase. It occurs when vapor pressure of solid equals ambient pressure.
2. When the desired compound has a relatively high vapor pressure at a temp below its melting point. Or when the pure substance has a lower vapor pressure than its impurities.
3. Decreasing atmospheric pressure by vacuum.
4. (1) moisture from entering as it can disrupt the sublimation process and (2) prevent heat from surpassing melting point of solid
1. Define distillation
1. the process of purifying a liquid by boiling it and condensing its vapors.
1. What is it based on? 2
2. What is the technique?
3. What happens when crystals are formed?
4. What does solubility of a compound depend on?
5. What is a "nice solubility gradient" and why is this important?
6. Why can the impurity be more or less soluble than the desired compound?
1. Based on differing solubilities of solutes in different solvents as well as the fact that the impurity will be present in a smaller amount
2. Dissolve crystals in hot solvent then cooling slowly. Solution will become oversaturated and the solid will separate from the solution as it cools.
3. When crystals are being formed, the same individual molecules will fit in a specific crystal lattice. Equilibrium process that produces very pure material
4 .It depends on intermolecular forces b/t compound and solvent and the polarity of both.
5. Nice solubility gradient means that it's poorly soluble in the solvent at low temps and highly soluble at high temps. This is important for picking a solvent.
6. If it's less soluble, then it can be filtered out before solutionc ools. If it's more soluble, it will be left behind in solution.
1. liquid-liquid extraction - what is it based on? What is on top?
1. Based on varying solubilities of different solutes in immiscible solvents. Organic layer.
1. When should distillation be used vs. recrystallization?
2. When should gas chromatography be used?
3. What are the benefits of chromatography over distillation?
1. Distillation should be used to purify or separate liquids based on boiling point. Lower boiling points will distill first.
Recrystallization should be used in the separation of solids - prerequisites - the impurity is found in much smaller amounts and there are different solubilities.
3. Can be done on small scale
How to pick a solvent for recrystallization?
1. What should it be?
2. How can it be tested?
3. If everything is too soluble, what should you do?
4. If nothing is solubilizing, what should you do?
5. What is sometimes needed?
1. Should be poor solvent at low temperatures and great solvent at high temperatures to ensure that oversaturation of the compound can occur.
2. Dissolving ~50 mg of solid in 0.5 mL of solvent.
3. If everything dissolves, it's too polar and should switch too less polar solvent.
4. Too nonpolar and should switch to more polar solvent.
5. Mixed solvent system - ethanol and water for example. After solubilizing all solids (impure and pure) in one solvent system, can add another solvent in which one is soluble but the other one isn't then filter.
1. When will a solid sublime?
2. What is the melting point?
3. Why do impurities lower then melting point?
4. How would you confirm that two solids with the same melting point were really the same substance?
5. What leads to "sweating"?
1. When vapor pressure of a solid equals the ambient pressure.
2. The temperature at which liquid and solid coexist at a pressure of 1 atm.
3. Because the mixing of any two compounds leads to the phenomenon of "melting point depression."
4. By grinding the solids and mixing them together. The melting point should stay the same. If they're different, usually melting point depression is observed.
5. Traces of leftover solvent or other volatile materials.
1. How is solvent chosen in reflux?
2. What are you assured of if you heat the mixture to reflux?
3. What is the difference between melting and dissolving?
1. It's chosen so that the boiling temp of the solvent (is slightly higher) and coincides with the ideal temp for the reaction.
2. That the solution is staying at a constant appropriate temperature.
3. Melting is a phase transition a solid undergoes when heated - the regular crystal structure of the solid is lost but the IM distance b/t molecules doesn't change. Dissolving - solvent molecules surround each individual molecule or ion.
1. How to pick solvent in column chromatography?
2. How to pick a solvent for TLC?
1. The more polar the analyte, the more polar the solvent should be - sometimes as column chromatography progresses, can switch to more and more polar eluents.
BP should be high enough so that rotary evaporation during CC isn't a problem, but low enough so that solvent can be evaporated over gentle heat to collect solid.
- 2. More polar solvents result in higher Rf values and faster movement up the plate. A solvent causing all the spotted material to move w/ the solvent front is too polar, while a
- solvent that does not create movement in any of the material in the spot is not polar enough.
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