EK B1 Ch4 Separations, purifications, blotting etc. Flashcards
distillation
condenser needs to be cool, series of vaporatizations and condensations, condenser so cold condesnes down distilling flask to land in other receiving flask as liquid, ice bath to keep it cold* want one side of set up to be cold so liquid can readily condense back into pure form*
Aluminum oxide is commonly used in thin-layer chromatography, most likely because it possesses which property?
A. Polarity
Thin-layer chromatography (TLC) separates compounds based on polarity. Typically, a highly polar stationary phase is used, along with a nonpolar solvent.
For which pair of compounds would fractional distillation (P = 1 atm) be an ideal separation method? The boiling point of each compound is given.
For which pair of compounds would fractional distillation (P = 1 atm) be an ideal separation method? The boiling point of each compound is given.
A. Ammonia (-35.5°C) and bromobenzene (156°C)
B. Chloroform (62°C) and bromobenzene (156°C)
green check
C. Ethanol (72.4 °C) and chloroform (62 °C)
Fractional distillation is used instead of simple distillation when the boiling points of the compounds to be separated are within 25°C of each other.
D. Formic acid (101°C) and camphor (204°C)
chromatography
Chromatography is used to separate, identify, and purify the components of a mixture. Four common MCAT-tested techniques are ion exchange, surface adsorption, partition, and size exclusion.
Although there are many chromatography techniques, they share the principle that the molecules in a mixture are applied onto a stationary phase (usually a solid), while a fluid known as the mobile phase (generally a solvent chosen to match the target molecules, e.g. polar or nonpolar) containing the molecules of interest travels through the stationary phase. Molecules of interest in the mobile phase will interact with the stationary phase with different levels of intensity. Molecules that interact more strongly with the stationary phase will take longer to pass through it, whereas molecules that interact more weakly with the stationary phase will pass through it more quickly. Common factors that shape these interactions include molecular characteristics related to adsorption, polarity- or charge-based affinity for the stationary/mobile phase, and differences in molecular weight. A chiral stationary phase can also be used to separate stereoisomers based on the principle that the various enantiomers of a compound may interact differently with such a stationary phase.
Which of the following molecules will have the highest Rf value when studied with thin-layer chromatography using a silica plate?
CH3CH2CH2CH2CH3
Silica is a highly polar molecule; thus, when chromatography is done on a silica plate, a polar substance will move slower on the plate due to polar-polar interactions between the substance and the plate. This means that the Rf value of a polar substance will be smaller than that of a nonpolar substance since the substance will move less on the plate compared to the solvent front than a nonpolar substance would. Of the options given, the compound in answer choice A is the least polar.
Remember, Rf is equal to the distance the compound of interest traveled along the plate divided by the distance traveled by the solvent front. A sample TLC plate, with those distances labeled, is shown below. Note that A, B, and C represent arbitrary compounds in this image.
solubility
Solubility describes the degree to which particles, called solutes, dissolve in another substance (usually a fluid), called a solvent. Solute-solvent interactions can be used to predict solubility. For example, hydrophilic substances are molecules that contain polar or charged groups and are capable of dissolving in water and other hydrophilic solvents, whereas hydrophobic substances are nonpolar molecules that are capable of dissolving in nonpolar, hydrophobic solvents. An easy way to remember this phenomenon is recognizing that “like dissolves like”—solutes will dissolve in solvents that they share similar properties with. This also applies if two equal proportions of liquids are mixed. (Be careful not to take “like dissolves like” too far; on the MCAT, it virtually always refers to polarity. For example, this principle breaks down in the context of acids and bases, since acids are most soluble in basic solution and vice versa.)
crystallization
As solute is added to a solvent, the solution is considered saturated when the maximum amount of solute that can be dissolved has been added.
Upon heating, more solute can be dissolved in the solution, and then upon slowly cooling the solution, the same concentration of solute will remain dissolved in what is now considered a supersaturated solution.
Crystals can form in supersaturated solutions with the addition of a small amount of solute, which creates a nucleation site for solute to precipitate and form a crystal. This process is called crystallization.
solubility 2
Solubility is an equilibrium process between the non-dissolved form of a substance (usually a solid in most MCAT contexts, but gases and liquids are also possible) and the dissolved form. For an ionic species that is dissolved in solution, the proportion that is in the dissociated state reaches equilibrium with the proportion that is in the undissociated state. In other words, for a dissociation reaction such as that given by MgCl2 (s) ⇌ Mg2+ (aq) + 2Cl−(aq), we can define an equilibrium constant that is known as the solubility product constant (Ksp): Ksp = [Mg2+][Cl−]2. Ksp is similar to, but distinct from, molar solubility, which refers to the moles of a substance that will dissolve in a solvent, because Ksp takes into account the relevant ions from any source.
In most cases, the solubility of ionic substances in water increases with temperature, while the opposite pattern is observed for gases. This is because higher temperatures provide gases with more kinetic energy that they can use to escape the solution. Additionally, pressure favors the solubility of gases.
methanol versus water
Methanol is similar in many ways to water as a solvent - both are polar protic solvents with similar pKa
values. You can think about them as H-OH (water) and H-OCH3 (methanol). It is likely that it will facilitate a similar reaction as a solvent, while perhaps exchanging the -OH group when water is the solvent with -OCH_3 when methanol is the solvent.
Hint #44 / 4
***Fatty acids (and other carboxylic acids) are known to react with alcohols to form esters (RCOOH + R’OH → RCOOR’ + HOH). Therefore, it is likely that the bio-oil will contain esters when it is produced in the presence of methanol. There is no evidence in the passage to suggest that ethers will be formed.
Which procedures could the researchers have used to separate and identify hexadecanoic acid and oleic acid as the two largest constituents of bio-oil?
Bio-oil is a mixture of products. To figure out what is in it, you would need a separation method and an identification method.
Gas chromatography and thin-layer chromotography are both methods of separating compounds, so it would be difficult to identify compounds with just these two methods.
Although 13^C-NMR can provide information on the electronic environment of carbon atoms and can help figure out a molecule’s structure, it cannot usually identify specific, complex molecules such as oleic acid.
Mass spectrometry can provide the molecular weight of a compound, which can be combined with the retention time of each molecule in the gas chromatography to identify the molecule. (It is extremely unlikely that two different molecules will have the same retention time in gas chromatography AND the same molecular weight).
Therefore, the correct answer is gas chromatography and mass spectrometry.
mass spec
Mass spectrometry can provide the molecular weight of a compound, which can be combined with the retention time of each molecule in the gas chromatography to identify the molecule. (It is extremely unlikely that two different molecules will have the same retention time in gas chromatography AND the same molecular weight).
How vacuum assists in the sublimation
The vacuum assists in the sublimation by keeping the external pressure on the caffeine as low (close to zero) as possible.
When a higher external pressure is present inside the apparatus while a solid is undergoing a phase change, the solid will more likely change into a liquid before changing into a gas, moving the arrow up to a higher pressure.
This vacuum is key to the process of sublimation. For a substance to change phase into a gas, its molecules must reach a vapor pressure equal to that of its external air pressure. Because a vacuum lowers the air pressure, the caffeine can reach this vapor pressure at a lower temperature and bypass the liquid phase.
dichloromethane used to extract caffeine
The dichloromethane was used as the organic solvent in the liquid/liquid extraction procedure.
In extractions, solvents are liquids used to separate compounds of interest using their chemical and physical characteristics.
Compared to dichloromethane, water has a strong dipole through hydrogen bonding and would tend to segregate with more polar and charged molecules
Miscibility describes the mixing of two substances while each retaining their chemical structure.
Dichloromethane is immiscible with water and is miscible, but not chemically reactive, with caffeine, separating it from the aqueous layer.
Despite heating the apparatus, the scientist detects no temperature change for a short period during the sublimation step. Which of the following explains this observation
Chemical sublimation is the second purification step.
Sublimation is a phase transition.
Compounds like caffeine undergoing phase transition do not undergo temperature change as energy is used to break intermolecular bonds instead of increasing the kinetic energy of molecules.
