super quiz 2 Flashcards
• Reason for carrying out
o Process of removing a compound of interest from a solution or solid mixture
• How acid base reactions work (what is used to extract what species and what occurs after extraction)
o Use base to react with an organic acid to form an ionic compound (water soluble)
o Use acid to react with an organic base from an ionic compound (water soluble)
• Flowchart on how to separate strong acids, weak acids, bases and neutral compounds – know what substances are used for what
something
• Why extractions are carried out in the sequence they are (weak base then strong base)
to ensure that the strong acid doesnt react and neutralize the weak base
- Weak base only reacts w/ strong acid
- Strong base only reacts w/ weak acid
- If strong base were added first, would react w/ weak acid & strong acid, so both would move to aqueous layer
- The base being used to extract an acid must have a conjugate acid that is weaker than the acid that it is trying to extract
• pKa requirements for conjugate acids of bases used to extract organic acids
o The base being used to extract an acid must have a conjugate acid that is weaker than the acid it us trying to extract
o Example
• Benzoic acid (pKa 4.17) is reacted with sodium bicarbonate to form sodium benzoate ion (water soluble)
• Sodium bicarbonate’s conjugate acid is carbonic acid (H2CO3, pKa = 6.52)
• Carbonic acid is weaker than benzoic acid
• So sodium carbonate can deprotonate benzoic acid to form sodium benzoate ion
Be able to recognize and draw species in aqueous and organic layers before and after acid base extractions and after acid or base regeration.
mixture of phenol (weak acid), aniline (a base) and naphthalene (neutral compound) in ether. extract the mixture with NaOH
- org. layer: naphthalene & aniline - aqu. layer: sodium phenoxide (ionic comp) and water
- mixture of aniline (a base) and naphthalene (a neutral compound) in ether. extract the mixture with HCl
- Org: naphthalene - Aqu: anilinium chloride
Reactions of acid-base extractions
- Benzoic acid + Na+ HCO- —–> Sodium Benzoate +water + CO2
- Sodium benzoate + HCl —-> benzoic acid + NaCl
- phenol + NaOH —–> sodium phenoxide + water
- sodium phenoxide + HCl ——–> phenol + NaCl
• Requirements/Criteria for extraction solvents
o Should readily dissolve substance to be extracted
o Should not be miscible with water (like oil and water)
• Organic solvents such as methanol and ethanol are not good extraction solvents because they are soluble in water.
o Should not react with solute
o It should not be highly flammable or toxic.
o It should be relatively inexpensive.
o It should have a low boiling point so that it can be removed readily
solvents should share as much surface area as possible to ensure sufficient transfer of solute from one layer to another, so all solid mixture should be dissolved in ether before beginning extraction process
How to mix, how to separate layers (must know why the action depicted in Figure 7.6 is advisable)
a. 3 methods for mixing layers
1. Flicking bottom of tube
2. Extracting and releasing with pipette action
3. Dilute reaction mixture w/ water & extract w/ organic solvent in separatory funnel
b. Separating Layers: Use pipette to remove lower layer
c. Advisable Action: Grasp full reaction tube and clean, empty tube in one hand when transferring material from one tube to another with a Pasteur pipette
i. Done in order to avoid losing any solution
How to mix, how to separate layers (must know why Fig. 7.6 is advisable)
1) Insert pipette, forcefully suck and expel → what we used
2) flick tube with finger holding top of reaction tube
a. good: violent motion effectively mixes layers, nothing lost through top
3) inset cork into tube and shake
a. bad: high vapor pressure of solvent will force solution out around cork
4) Macroscale experiments: dilute rxn mixture with water & extract it with organic solvent in a separatory funnel
Why large partition coefficient for organic solvent is desirable?
more solute will transfer over to the ether layer with each level of extraction and with that there would be lower number of ether that would be essential to remove solute completely
• Purpose of each step you carried out in the extraction experiment
o Mixing the layers
o Separating the layers
o Drying the organic layer
o Removing the solvent
purpose of backwashing.
• Serves to remove any organic material that might contaminate aqueous layer
o Add 0.2 mL of ether to tube 2, mix it thoroughly, remove the ether layer, and discard it.
• Serves to remove any organic material that might contaminate the contents of tube 2.
A backwash is used when a washing step has resulted in the undesired extraction of
the compound being isolated. To recover the lost compound, one extracts the wash
layer with fresh solvent identical to the solvent originally holding the compound.
This “backwash” is recombined with the original solution.
what is a remedy for using low part. coefficient?
find another solvent biut if not possible, then conduct multiple extractions
• How to mix (why is it important to mix thoroughly), how to separate layers (must know why the action depicted in Figure 7.6 is advisable), advantages and disadvantages of other methods of mixing and what causes the disadvantages
o Layers must be thoroughly mixed – the solvents must share as much SA as possible in order for the efficient transfer of solute from one layer to another
o Method used in our experiment
• By pulling layers into pipette and forcefully expelling it back into the test tube
o Other Methods
• In macroscale experiments, a frequently used method of working up a reaction mixture is to dilute the mixture with water and extract it with an organic solvent, such as ether, in a separatory funnel
• When the stoppered funnel is shaken to distribute the components between the immiscible solvents t-butyl methyl ether and water, pressure always develops through volatilization of ether from the heat of the hands, and liberation of a gas (CO2) (in acid/base extractions) can increase the pressure.
• The organic reaction product is distributed wholly or largely into the upper ether layer, whereas inorganic salts, acids or bases pass into the water layer, which can be drawn off
o • Separation of layers – how? How do you know which layer is which – what could change the order of the layers and how to test
o The best method to avoid a misidentification is to perform a drop test.
o Add a few drops of water to the layer in question and watch the drop carefully.
o If the layer is water, then the drop will mix with the solution. If the solvent is the organic layer, then the water drop will create a second layer.
• Besides drying agents, what other method can be used to remove water from an ether layer?
o If ether that contains dissolved water is shaken with a saturated aqueous solution of NaCl, water will be transferred from the t-butyl methyl ether to the aqueous layer.
• NaCl dissolves in water, reduces the solubility of the organic compound in aqueous layer → ‘salting out’
• Why drying agent was used? What observations indicate that an organic layer is dry? You must know what substance(s) you have used as a drying agent in the experiments you have carried out
o Usually anhydrous salts; used to remove traces of water
• Salts bond to water molecules to form hydrated salts that can be then easily separated from the dried layer
o Used CaCl2 pellets as drying agent
• Criteria for choosing drying agent
o (1) The possibility of reaction with the substance being extracted
o (2) The speed with which it removes water from the solvent
o (3) The efficiency of the process
o (4) The ease of recovery from the drying agent.
o MUST not react with solute water quickly – swirling drying agent with solution speeds up drying.
o Why? More surface area
• Advantages of using calcium chloride as drying agent when compared to other drying agents in this experiment
o A very fast and effective drying agent
o It has the advantage that it clumps together when excess water is present, which makes it possible to know how much to add by observing its behavior.
observation when wet layer becomes
when solution becomes clear
• How do you ensure all product is transferred from drying agent
o Wash the drying agent left in the reaction tube with several small quantities of pure solvent to transfer all the extract
) You add small amounts of ether to the drying agent two more times after letting them dry for five minutes and use a pipette to transfer the ether to the holding vial
• Removing solvent – purpose, methods and their advantages/disadvantages, what tends to occur during the different methods and remedies
o Used leave behind solute
• In our experiment, evaporating the solvent by gentle heating
o If the quantity of extract is relatively small, say 3 mL or less, then the easiest way to remove the solvent is to blow a stream of air
• The easiest way to add heat is to hold the tube in your hand.
o To attach the Pasteur pipette to an aspirator and pull air over the surface of the liquid.
• Why don’t you get 100% recovery (liquid-liquid AND acid-base extractions)? What can you do to minimize loss of recovery? What aspect of ions helps minimize loss of recovery in acid-base extractions?
o From the partition coefficient equation you should see that you CANNOT get 100% → RECOVERY OF SOLUTE
• Since the solute is never completely transferred to organic layer
o The ionic substances will be in the aqueous layer; the nonionic ones will be in the organic layer.
• Why don’t you get 100% recovery (liquid-liquid AND acid-base extractions)? What can you do to minimize loss of recovery? What aspect of ions helps minimize loss of recovery in acid-base extractions?
) You add small amounts of ether to the drying agent two more times after letting them dry for five minutes and use a pipette to transfer the ether to the holding vial
• Sources of Error for extraction
- incomplete acid base rxn
- not all ions go into aquous layer
• Precautions to take when spotting, developing and visualizing – impact on observations and results if precautions not followed and reasons for these flawed observations/results.
o Make sure lines are drawn lightly. Do NOT flake adsorbent
• Gap might form that stop flow of eluent
o When spotting, lightly touch it to TLC plate, be careful again do NO flake absorbent
o Avoid streaking spots → Streaking may lead to the overlap of two or more compounds, thus skewing results
• Try to make spots small and compact
• Sample should not be too concentrated
o Make sure solvent level is below spotting line → so that solvent does not dissolve samples at spotting line
o Make sure bottom of TC plate is level with bottom developing chamber → solvent could rise up unevenly
o Make sure top is placed back on chamber → so solvent does not evaporate
o Make sure solvent does not rise pass 5mm line → difficulty visualizing, Rf would be too high
o Avoid leaning paper against filter paper. Eluent on filter paper can absorbed by absorbent interfering with ascending eluent
• Extra eluent may prevent compounds travelling from moving up plate in straight line
• Why pencil needed to mark and why ink is never used?
o Note that a pencil is always used to mark TLC plates because the graphite (carbon) is inert. If ink is used to mark the plate, it will chromatograph just as any other organic compound, interfering with the samples and giving flawed results.
• Sample preparation specifics
o Dissolve material in volatile substance like diethyl ether (or in our case methylene chloride)
• What are typical characteristics for solvents used as eluents and why?
o In most cases, a combination of two solvents is the best choice.
• If the spots stay at the bottom of the plate, add more of the polar solvent.
• If they run with the solvent front (move to the top), increase the proportion of the nonpolar solvent.
• Characteristics of solvents used with respect to samples
o A polar solvent will carry along with it polar substrates; vice versa with nonpolar solvent/compounds
• ‘like dissolves like’
• Characteristics of solvents that allow them to migrate rapidly
o Low boiling points that allow them to be easily evaporated
o Low viscosities that allow them to migrate rapidly
• Adsorbent materials and specific uses (what kind of adsorbents would be better for what kind of substances)
o The extended covalent network of these adsorbents creates a very polar surface.
o The electropositive character of the aluminum or silicon and the electronegativity of oxygen create a very polar stationary phase
• More polar the molecules to be separated, the stronger the attraction to the stationary phase.
• Nonpolar molecules will tend to stay in the mobile phase.
o Aluminum is absorbent of choice for separation…. (more active when anhydrous → absorbs strong)
• Relatively nonpolar substrates, such as hydrocarbons, alkyl halides, ethers, aldehydes, and ketones
o Silica is absorbent of choice for the separation…(less active)
• More polar substrates, such as alcohols, carboxylic acids, and amines
o In extreme situations…
• Very polar substances chromatographed on alumina will not migrate very far from the starting point (low Rf values)
• Nonpolar compounds chromatographed on silica gel will travel with the solvent front (high Rf values)
• Uses of TLC and how it is used– ALL 6
- 1) To determine the number of components in a mixture
- Easy method for analyzing such things as a crude reaction mixture, an extract from a plant substance, or the ingredients in a pill
- 2) To determine the identity of two substances
- If two substances spotted on the same TLC plate give spots in identical locations, they may be identical. (If not same place, substances cannot be the same)
- It is possible for two or more closely related but not identical compounds to have the same positions on a TLC plate.
- 3) To monitor the progress of a reaction
- By sampling a reaction at regular intervals, it is possible to watch the reactants disappear and the products appear using TLC.
- The optimum time to halt the reaction can be determined, and the effect of changing such variables as temperature, concentrations, and solvents can be followed without having to isolate the product.
- 4) To determine the effectiveness of a purification
- The effectiveness of distillation, crystallization, extraction, and other separation and purification methods can be monitored using TLC, with the caveat that a single spot does not guarantee a single substance.
- 5) To determine the appropriate conditions for column chromatographic separation
- Column chromatography is used to separate and purify up to 1 g of a solid mixture. The correct adsorbent and solvent to use for column chromatography can be rapidly determined by TLC.
- The adsorbents most commonly used for TLC—silica gel and alumina—are also used for column chromatography
- 6) To monitor column chromatography
- Solvent is collected in a number of small flasks
- The various fractions must be analyzed in some way to determine which ones have the desired components of the mixture. TLC is a fast and effective method for doing this.
• Effects of using too polar or too nonpolar a solvent
o Using eluent with too high a polarity with respect to samples…all spots will move to the top of TLC plate (unable to tell difference b/w them)
• More polar eluent the faster a compound movies
o Using eluent with too low polarity with respect to samples…all spots will stay toward the bottom of TLC plate (no movement at all)
• Visualization techniques – why certain kind of visualization technique was used for certain types of compounds
o UV Light – used for conjugated compounds (forms dark purple/bluish spots)
o I2 Vapor – used for most compounds (alkanes, alcohols, and ethers); forms brown spots
• Why are tweezers for handling TLC plates suggested
o Use tweezers to place the plate in the development chamber; oils from your fingers can sometimes smear or ruin a TLC plate.
