MCAT Organic Chemistry Review
Separations and Purifications
Don’t forget, the MCAT won’t ask you to get into your lab coat and extract the product of a reaction! As long as you understand the principles governing these techniques and when you should apply them, you’ll be in great shape. Remember that separation and purification techniques exploit physical properties of compounds, such as polarity, solubility, size and shape, and charge, to obtain a purified product. These properties can be traced back to intermolecular forces or properties of the molecules themselves. Having a variety of tools and methods to separate and collect a purified product is essential in practical Organic Chemistry, and choosing the proper techniques often requires knowledge and consideration of the desired product. When you look at the bigger picture, these methods may be easier to conceptualize than to actually apply in the lab!
Despite the subject’s compelling relevance to everyday life, college organic chemistry often terrifies and alienates its students. The MCAT, on the other hand, doesn’t ask you to memorize tables of reactants or regurgitate hundreds of named reactions from scratch. Instead, the MCAT asks you to look at the bigger picture, to know trends, and to participate in the logic of chemistry. We hope that studying for the MCAT has given you a chance to rediscover organic chemistry—to focus on the how and the why, instead of the what. Organic chemistry, like the MCAT as a whole, should be seen not as an obstacle but as an opportunity. So work hard, have some fun along the way, and keep thinking about where you’re heading—you can almost feel that white coat.
· Extraction combines two immiscible liquids, one of which easily dissolves the compound of interest.
o The polar (water) layer is called the aqueous phase and dissolves compounds with hydrogen bonding or polarity.
o The nonpolar layer is called the organic phase and dissolves nonpolar compounds.
o Extraction is carried out in a separatory funnel. One phase is collected, and the solvent is then evaporated.
o Acid–base properties can be used to increase solubility.
· A wash is the reverse of extraction, in which a small amount of solute that dissolves impurities is run over the compound of interest.
· Filtration isolates a solid (residue) from a liquid (filtrate).
o Gravity filtration is used when the product of interest is in the filtrate. Hot solvent is used to maintain solubility.
o Vacuum filtration is used when the product of interest is the solid. A vacuum is connected to the flask to pull the solvent through more quickly.
· In recrystallization, the product is dissolved in a minimum amount of hot solvent. If the impurities are more soluble, the crystals will reform while the flask cools, excluding the impurities.
· Distillation separates liquids according to differences in their boiling points; the liquid with the lowest boiling point vaporizes first and is collected as the distillate.
· Simple distillation can be used if the boiling points are under 150°C and are at least 25°C apart.
· Vacuum distillation should be used if the boiling points are over 150°C to prevent degradation of the product.
· Fractional distillation should be used if the boiling points are less than 25°C apart because it allows more refined separation of liquids by boiling point.
· All forms of chromatography use two phases to separate compounds based on physical or chemical properties.
o The stationary phase or adsorbent is usually a polar solid.
o The mobile phase runs through the stationary phase and is usually a liquid or gas. This elutes the sample through the stationary phase.
o Compounds with higher affinity for the stationary phase have larger retardation factors and take longer to pass through, if at all; compounds with higher affinity for the mobile phase elute through more quickly. Compounds therefore get separated from each other, calledpartitioning.
· Thin-layer and paper chromatography are used to identify a sample.
o The stationary phase is a polar material, either silica, alumina, or paper.
o The mobile phase is a nonpolar solvent, which climbs the card through capillary action.
o The card is spotted and developed; Rf values can be calculated and compared to reference values.
o Reverse-phase chromatography uses a nonpolar card with a polar solvent.
· Column chromatography utilizes polarity, size, or affinity to separate compounds based on their physical or chemical properties.
o The stationary phase is a column containing silica or alumina beads.
o The mobile phase is a nonpolar solvent, which travels through the column by gravity.
o In ion-exchange chromatography, the beads are coated with charged substances to bind compounds with opposite charge.
o In size-exclusion chromatography, the beads have small pores which trap smaller compounds and allow larger compounds to travel through faster.
o In affinity chromatography, the column is made to have high affinity for a compound by coating the beads with a receptor or antibody to the compound.
· Gas chromatography separates vaporizable compounds according to how well they adhere to the adsorbent in the column.
o The stationary phase is a coil of crushed metal or a polymer.
o The mobile phase is a nonreactive gas.
· High-performance liquid chromatography (HPLC) is similar to column chromatography but uses sophisticated computer-mediated solvent and temperature gradients. It is used if the sample size is small or if forces such as capillary action will affect results. It was formerly called high-pressure liquid chromatography.
Answers to Concept Checks
1. The two solvents must be immiscible and must have different polarity or acid–base properties that allow a compound of interest to dissolve more easily in one than the other.
2. It is better to do three washes with 10 mL than to do one with 30 mL; more of the compound of interest would be extracted with multiple sequential extractions than one large one.
3. Acid dissolves better in aqueous base because it will dissociate to form the conjugate base and, being more highly charged, will become more soluble. Note that like dissolves like applies to polarity; acids and bases dissolve more easily in solutions with theoppositeacid–base characteristics.
1. Distillation takes advantage of differences in boiling points in order to separate solutions of miscible liquids.
2. A solution of ether and methylene chloride, which have very close boiling points, can be separated by using fractional distillation.
3. Vacuum distillation would be the best technique to separate two chemicals with such high boiling points because the decreased ambient pressure will allow them to boil at a lower temperature.
1. Each of these methods separates compounds using charge and polarity.
Type of Column Chromatography
Method for Separating Sample
Column is given a charge, which attracts molecules with the opposite charge
Small pores are used; smaller molecules are trapped, while larger molecules pass through the column
Specific receptors or antibodies can trap the target in the column; the target must then be washed out using other solutions
3. As the name suggests, gas chromatography is simply the same technique of mobile and stationary phases performed with a gaseous eluent (instead of liquid). The stationary phase is usually a crushed metal or polymer.
4. Historically, HPLC was performed at high pressures, whereas column chromatography uses gravity to pull the solution through the column. Now, HPLC is performed with sophisticated and variable solvent and temperature gradients, allowing for much more specific separation of compounds than column chromatography; high pressures are no longer required.
Equations to Remember
(12.1) Retardation factor:
· Biochemistry Chapter 3
o Nonenzymatic Protein Function and Protein Analysis
· General Chemistry Chapter 7
· General Chemistry Chapter 8
o The Gas Phase
· General Chemistry Chapter 9
· General Chemistry Chapter 10
o Acids and Bases
· Organic Chemistry Chapter 11