O Chem Lab Midterm
2. Know what retention time is and how it is affected by the following: column (oven) temperature, carrier gas flow rate, column length, and the boiling point of the components.
Retention Time -The time required for a particular compound to pass through the column is a function of: • Carrier gas flow rate. Increasing gas flow rate shortens retention time. • Column temperature. Increasing column temperature shortens retention time. • Interactions between compound and stationary phase. • Boiling points of compounds being analyzed. The most volatile compounds usually move through the GC the fastest.
When a pure compound has a high vapor pressure its boiling point _____ and we say it is a ____ compound
Low ; volatile
1. Know the purpose of recrystallization.
* Purpose of recrystallization:-Recrystallization/fractional crystallization is a process involving dissolving an impure compound in a solvent at high temperature to create a concentrated solution and then cooling it slowly. * The cooling decreases solubility of both the compound and impurities. The compound crystallizes leaving behind the impurities .The pure crystals are filtered out.For the recrystallization process to be successful there must be only small impurities in the chemical compound and they must be soluble. * Recrystallization purifies chemical compounds in solvents. It works best when there is only a small quantity of impurities in the compound.
In a distillation, it is essential to have your thermometer properly placed. Choose the statement that best explains the outcome if the thermometer is placed too low in the Hickman still.
....
In the procedure, you are reminded to make sure that your ground glass joint is sealed tightly using vacuum grease. Choose the statement that explains this requirement.
....
Upon addition of water, sodium benzoate (structure shown) readily dissolves. However, when HCl is added to this solution, a white precipitate forms. Choose the chemical equation that best explains the reaction and the identity of the white precipitate.
.... -> Benzoic Acid (R-COOH)
Choose the chemical equation that best explains why benzoic acid is soluble in dilute NaOH.
.... -> R-COO-
Choose the chemical equation that best explains why ethyl 4-aminobenzoate is soluble in dilute HCl.
.... -> R-NH+3
The structure for the chloride salt of ethyl 4-aminobenzoate is shown below. Upon addition of water, this salt readily dissolves. However, when NaOH is added to this solution, a white precipitate is observed. Choose the chemical equation that best explains the reaction that is occurring and gives the correct identity of the white solid.
.... -> R-NH2
Kp practice problems
.....
Infrared Spectroscopy (IR)
* ... allows for the determination of the functional groups present in a molecule
The Rf value for spot A is ____________. Be sure to report your answer to two decimal places (0.00).
* 0.77
You are given a sample of benzoin that has been contaminated with anthracene and you purify this sample by recrystallization. Which of the following is a possible melting point you may measure if you did not successfully purify benzoin and it is still contaminated with anthracene? (literature melting points: benzoin 134-138C; anthracene 210-215C).
* 120-130 C
You are given a sample of benzoin that has been contaminated with anthracene and you purify this sample by recrystallization. Which of the following is a possible melting point you may measure if you did successfully purify benzoin? (literature melting points: benzoin 134-138C; anthracene 210-215C).
* 134-136 C
2. Know the procedure and setup for measuring an ultra-micro boiling point.
* 4 microlitres of liquid is considered in a melting tube, and is inserted in melting point apparatus and the temperature is rised to certain range which is below the boiling point and then heated at an incremental rate. As the liquid approaches its B.P you will observe that there will be no bubbles.
1. Know what is meant by melting point range. (Zubrick)
* A MP is the temperature at which the first crystal just starts to melt until the temperature at which the last crystal just disappears. Thus the MP is actually a melting range.
What does tlc plate tell about purity of impure solid and purity of the recrystallised solid?
* A TLC'd sample of an impure solid will appear to have multiple (more than 1) spot(s) that you will see after developing the TLC (i.e. running an eluent up the plate). Versus... * A TLC'd sample a recrystallized (and thus, pure) solid will appear to have a single spot.
Developing the Plate
* Developing Tank: either a coplin jar (with lid) or a beaker (with watch glass as lid) • Place a couple of millimeters of eluant in bottom of tank • Put TLC plate in the tank, with origin on the bottom (eluant should not touch spot/origin) • Allow eluant to move up the plate by capillary action
Choose the statement that best reflects the three basic steps of any recrystallization.
* Dissolve the impure solid in a minimal amount of boiling solvent; cool the solution to form crystals; vacuum filter the solution to collect the pure crystals.
* You are trying to recrystallize compound X. You consider using ethyl acetate as your recrystallizing solvent and test a small amount of compound X with ethyl acetate. You find that compound X is soluble in ethyl acetate at room temperature and at boiling. Is ethyl acetate a good recrystallizing solvent?
* No, the sample needs to be insoluble or sparingly soluble at room temperature so that the maximum amount of purified crystals form at room temperature and in the ice bath.
You are trying to recrystallize compound X. You consider using hexanes as your recrystallizing solvent and test a small amount of compound X with hexanes. You find that compound X is insoluble in hexanes at room temperature and at boiling. Is hexanes a good recrystallizing solvent?
* No, you need to choose a solvent that fully dissolves the sample at boiling so the slow, selective crystallization that occurs as the solution cools selects for compound X.
Know how to determine the melting point of an unknown sample without having to raise the temperature slowly over a large temperature range.
* Obtain an unknown compound from your instructor. Prepare two capillaries containing the unknown. Determine an approximate melting point for it using the first tube and a heating rate of 15-20 degrees per minute. Then let the thermometer and Mel-Temp apparatus cool to at least 20 degrees below this approximate melting point and use the second tube to obtain an accurate melting point with a heating rate of no more than 3 degrees per minute.
Solubility: Non-polar Species
* Organic-Soluble Species - Organic molecule - Uncharged or neutral - Non-polar * Common Organic Solvents - Dichloromethane (methylene chloride) - Diethyl ether (ether) - Benzene
3. Know how to calculate percent recovery and why it is called percent recovery and not percent yield
* Percent Recovery: - Percent recovery is used in cases where no chemical reaction is taking place, as in purification of a sample. It is calculated as follows: % recovery = amount pure product recovered (g)/ amount of crude material used (g) x 100 Percent Yield: - Percent yield is used is cases where a chemical transformation occurs. To calculate the % yield, you need the following information: 1. The molar ratio of product to starting material. 2. Molecular weights of product and starting material. 3. Limiting reagent. * Percent yield is calculated as follows: % yield = actual yield (g)/theoretical yield (g) x 100
What is polar compound? How does it differ from a non-polar compound?
* Polar compounds have a net dipole moment and non-polar compounds don't. * A dipole moment is a result of uneven electron distribution within a chemical bond (or multiple bonds) owing to the difference in electronegativity of different elements. * A popular example of a polar compound is water.
Purification: Crystallization Theory
* Purification of solid compounds is based on their preferential ability to form crystalline solids. * Organic compounds that are solid at room temperature can usually be purified by crystallization. * The general technique revolves around dissolving the material to be crystallized in a hot solvent, then cooling the solution slowly. * The dissolved material has a decreased solubility at lower temperatures and will separate from the solution as it is cooled.
A student was given a dye sample believed to contain several components. In an attempt to determine the identity of those components, she decided to analyze the mixture using TLC. The student used both 100% hexanes and 50% hexanes in ethyl acetate as eluting solvents and spotted known dyes for comparison. The following TLC chromatograms were obtained.
50% hexanes in ethyl acetate
8. Know how to prepare a sealed capillary and to prepare a sample for melting point.
Zubrick pg. 75
2. Know the procedure and setup for measuring an ultra-micro boiling point. (Zubrick)
pg. 197
Distillation Theory
• For a liquid containing only a single compound. • When the vapor pressure of a liquid equals atmospheric pressure, it will boil. • For mixtures of liquids. When the sum of the vapor pressures (partial pressures) of all components equals atmospheric pressure, the liquid will boil. • Proportions of components in vapors does not equal the proportion in liquid. The vapor phase is enriched in the lower boiling component.
3. Know the physical property in operation for TLC separation and how the dielectric constant for a solvent influences the Rf values.
• It is useful if the eluant is relatively volatile, so that the plate dries quickly after development. • The Rf's can be changed by varying the polarity of the eluant. • Increasing the polarity of the eluant (dielectric constant) will increase the Rf's of the compounds being analyzed. • Decreasing the polarity of the eluant (dielectric constant) will decrease the Rf's of the compounds being analyzed.
spotter (Cappilary Tubes - open at both ends)
• Lightly touch the capillary tube on the plate and allow the solution to be deposited on the plate. Before spotting the sample, draw a line (with pencil!) ~ 0.5 cm from the bottom of the plate (the origin). When you spot your sample on the plate, place it on that line.
Analysis of Mixture Composition
• Peak area is directly proportional to the amount of compound, assuming the detector is equally sensitive to all compounds. • Triangulation method for determining areas. * Area Peak B = 122 * 19 = 2320 mm^2 * Area Peak A = 40 * 17 = 680 mm^2 * % B = (2320/ 3000) * 100% = 77.3% * % A = (680/3000) * 100% = 22.7% * Ratio B/A = 2320/680 = 3.35/1
Fractional Distillation (Basically a series of simple distillations all at one time)
• Separation of two liquids with similar boiling points is difficult using simple distillation.
Visualizing the Plate
• The method you use to visualize the spots depends on what kind of compounds are in your sample • Sometimes the compounds can be seen by the naked eye (colored spots) • If compound contain a chromophore (conjugated π-system such as an aromatic ring) then the spot can be viewed under UV light.
Retention Factor (Rf)
• The retention factor is a measure of the mobility of a compound in a given solvent system. It is calculated illustrated below. • The distance traveled by the solvent (solvent front) and the distance traveled by the compound(s) is measured with a ruler (you can obtain one from the window) • Rf values are always between 0 and 1. They are reported only to 2 decimal places. * Rf values are very sensitive to the exact concentrations of the eluting solvent as well the batch of solid support used. It is sometimes difficult to get exactly the same Rf for a particular compound if you run it more than once. Assume a 10-15% error. * Rf value = distance traveled by substance/ distance traveled by solvent front
3. Know the advantages/benefits of measure the ultra-micro boiling point.
* You can measure sharp boiling points using this method
Be able to describe the extraction techniques used to isolate a substance from a solid plant sample.
* A familiar example of the first case is making a cup of tea or coffee - the soluble flavor and odor chemicals and caffeine are extracted from the solid tea leaves or ground coffee beans into hot water (the solvent). Insoluble plant material is left behind in the tea bag or coffee filter. An example of the second case is an experiment that is often done in an introductory organic lab - an organic solvent is used to extract the caffeine from an aqueous tea or coffee solution, leaving the more water-soluble compounds behind in the aqueous solution. This in fact is how decaffeineated tea and coffee are sometimes made.
Solvent Extraction
* A popular separation technique in the organic laboratory * A mixture of compounds can be separated if the individual components have differing solubilities in organic solvent vs. aqueous solvent * Polar and/or charged compounds are soluble in water * Non-polar compounds are soluble in organic solvents
7. Know how to use TLC to determine purity.
* A pure substance when spotted onto to a silica-gel plate and properly developed and visualized, will yield a single spot on the TLC Plate.
Absorption Spectroscopy
* A sample is irradiated with light, and the behavior of the molecule is observed, by detecting the energy of the light absorbed by the sample * The energy absorbed by the sample is recorded as a function of the radiation wavelength, described by the equation below: * E = hc/λ * h = Planck's constant (6.626 x 10-27 erg/s) * C = speed of light (3 x 1010 cm/s) * λ = the wavelength of radiation (cm)
The TLC Plate (solid phase / stationary phase)
* A solid backing (usually plastic, glass, or aluminum) that is coated with an adsorbent (the stationary phase). A wide range of adsorbents are available, silica gel is the most common. This is an example of a polar adsorbent. * As the solvent moves up the plate, the more polar the compound, the more time it spends adsorbed to the solid phase, and hence it does not move up the plate as quickly as less polar compounds. * When using silica gel plates, polar compounds move more slowly than nonpolar ones
eluant (dielectric constant)
* A solvent, or mixture of solvents, called the eluant, is allowed to flow up the plate by capillary action. * The ideal eluant is one in which the compounds you are analyzing have Rf's between 0.3 and 0.6 * It is also known as the mobile phase.
During your extraction, you lose track of your layers. Choose the answer that best describes how you can determine which layer is aqueous and which is organic.
* Add a few drops of water to each layer; the layer that dissolves the water is the aqueous layer.
5. Know what happens to the boiling point of a liquid if a volatile solute/impurity is added.
* Adding impurities increases the boiling point.
What piece of glassware is used in the fractional distillation experiment that was NOT used in the simple distillation experiment
* Air Condenser (Fractional Column)
3. Be able to describe the procedure used to isolate and separate the components of solid/liquid and liquid/liquid mixtures.
* As the liquid boils, a condensation line of vapor can be observed as it moves up the distilling head. Once these vapors reach the thermometer bulb, a dramatic temperature increase is observed. The temperature of the vapors in the distilling head provides information regarding the progress of the distillation. Initially, the vapors are rich in the more volatile compound, and the observed temperature is close to the boiling point of that compound. In a distillation with an efficient separation, the initial temperature remains relatively constant until all of that compound is collected. After the compound with the lower boiling point is completely distilled, the temperature rises sharply as the vapors of thehigher-boiling compound reach the thermometer bulb.At this time, the boiling point of the higher boiling compound is observed as it distills into the receiver. The higher-boiling compound will vaporize only if one continually increases the heat source on the pot. If the temperature is not increased, it is possible that the higher-boiling compound will onlyreflux in the fractionating column and one will see the temperature drop at the thermometer bulb because it is no longer bathed in heated vapors. * Procedure:Assemble the fractional distillation apparatus.Be careful to position the fractionatingcolumn vertically to promote mixing of the liquid and vapor phases. Place two boiling chips into the pot. Start the water flow through the condenser. Heat the mixture in the pot to boiling. Observe the condensation line as it moves up the fractionating column. When the vapors reach the top of the column packing, reduce the heating rate so the vapor condensation line remains just above the column packing and below the side arm of the distilling head. Maintain the vapor condensation line in this position for ~2 min to allow the vapor and liquid in the column to reach equilibrium. adjust the heating rate to produce distillate at a rate no greater than 1 drop per second. Record the temperature when you collect the first drop of distillate and again after every 20 drops or 1 mL of distillate you collect.
Fingerprint Region
* Below 1400 cm-1. This area of the spectrum is often complicated and difficult to analyze (although it can be useful). Every organic compound has a unique fingerprint region.
Define boiling point how does this definition describe what is occurring in micro boiling point determination method? (i.e. that the boiling point is the temperature at which the liquid levels inside and outside of the capillary tube are the same?
* Boiling point: The temperature at which the vapor pressure of a liquid becomes equal to atmospheric pressure. * In micro boiling point determination method, when the apparatus is heated, the air present in the tube expands and liquid vapors enters into the tube and an equilibrium will forms between vapor and heating liquid. this gives rise to a stream of initial bubbles.When the temperature reaches the boiling point of liquid, the vapor pressure inside the liquid becomes equal to atmospheric pressure.As temperature rises just above the boiling point the vapors will start to escapes that gives second set of bubbles. Once heating is stopped, apparatus begins to cool down. At the exact temperature for which the bubbles stop forming, the bell is no longer pressurized. That is, the pressure inside the bell is equal to atmospheric pressure. The boiling point..
Solvent Selection
* Choose several different organic solvents to test * Dissolve a small amount of solid in solvent at room temperature - If the compound dissolves at room temperature, THIS IS NOT A GOOD CHOICE * Heat the remaining samples to boiling - If the compound does not dissolve in the boiling solvent, THIS IS NOT A GOOD CHOICE * Cool the remaining samples to room temperature and then place on ice - The sample that gives the best crystals from the lowest boiling solvent is the BEST solvent to use
Phase Diagrams
* Consider the separation of a liquid A (bp = 90 °C) from a liquid B (bp = 170 °C). * If you start with a solution that is a 70:30 B:A mixture (W), heating it to boiling (X) provid es vapor over the liquid th at is 30:70 B:A (Y). If thi s vapor is condensed a solution of 30:70 B:A results (Z). Some things to keep in mind: As the distillation proceeds: • The liquid that distills (the distillate) is enriched in the lower boiling component (in this case A). • The liquid left behind (in the pot) is enriched in the higher boiling component (in this case B). • The temperature of the liquid distilling increases over time.
Theoretical Plate (The length of the fractionating column and the material it is packed with impact the number of times the vapors will recondense before passing into the condenser; the number of times the column will support this is referred to as the number of theoretical plates of the column.)
* Each condensation and revaporization that occurs on a fractionating column is called a theoretical plate. A fractionating column with a large number of theoretical plates accomplishes many condensation-revaporization steps and very efficiently separates the compounds in a mixture. It is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other.
When a solid is impure, its melting point is higher and broader than the melting point for a pure sample.
* False
When choosing a solvent for recrystallization, the solute should dissolve in the solvent at room temperature.
* False
When recrystallizing an impure solid, it is acceptable to use a solvent whose boiling point is higher than the melting point of the pure solid as long as many crystals form upon cooling.
* False
You can use either pen or pencil when writing on a TLC silica gel plate.
* False
Melting Point Technique
* Heat the apparatus quickly to within 25°C of the mp (literature mp) and then reduce the rate of heating (to 2°C/min) as you get close to the expected mp. - Rapid heating near the mp leads to errors resulting from uneven heating through the sample and block.
a compound with a low boiling point has a ____ vapor pressure
* High
Recrystallization Solvent
* IDEAL properties of a solvent used for recrystallization : - solid should be sparingly soluble or insoluble at room temperature - solid should be soluble at high temperature (boiling point of solvent) - solvent should be non-toxic - solvent boiling point should be LOWER than that of the solid * Every solid has a solvent that is good for recrystallizing it. * There is no "universal" solvent that is good for recrystallizing all solids.
Infrared Region
* IR radiation is the part of the electromagnetic spectrum (14,000 cm-1 to 10 cm-1), which lies between the visible and microwave regions * The part of the IR region that is of use to us (i.e. chemists) is between 4000 and 400 cm-1. The energy of light in this region corresponds to the vibrational energy within molecules * The energy a molecule absorbs (from the IR region) arise from molecular vibrations, primarily the bending and stretching of bonds
After recrystallizing an impure sample with isopropanol, you isolate your product by filtration. What solvent do you use to wash your crystals?
* Ice cold isopropanol
Choose four requirements that must be met when choosing a solvent for recrystallization.
* Impurities must be insoluble at the boiling point or soluble at room temperature. * Solvent must be easily removed (i.e., have a low boiling point and high vapor pressure). * Solute must be insoluble at room temperature and soluble at the boiling point. * The solvent must not react with the solute. * A large amount of large crystals should form when the solution is cooled.
Step 3 suggests the use of a "minimal amount of solvent to dissolve your impure solid". Choose the answer that best explains the reason behind this warning.
* In a recrystallization, you need to have a saturated solution (chemical solution containing the maximum concentration of a solute dissolved in the solvent.) in order for crystals to form when the solution is cooled.
adsorbent
* In thin layer chromatography, a solid phase, the adsorbent, is coated onto a solid support as a thin layer (about 0.25 mm thick). In many cases, a small amount of a binder such as plaster of Paris is mixed with the absorbent to facilitate the coating. Many different solid supports are employed, including thin sheets of glass, plastic, and aluminum.
4. Know what happens to the boiling point of a liquid when atmospheric pressure is increase/decreased and why.
* Increasing the atmospheric will result in increase in boiling point and vice versa because it eventually increases the vapor pressure of the liquid.
Reflux (Boiling Vessel)
* It is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. * This is where the mixture will reside while the distillation is performed. Eventually, once the distillation is complete, this will contain the solid component of the mixture (sometimes call the pot residue)
8. Know how to use TLC to follow the progress of a reaction.
* It's quite simple actually. * Basically, as the reaction proceeds you take aliquots of the reaction (however often you want) and run a TLC on them. * On that same TLC plate, if it's possible, you have a spot for your starting material and a spot for your product. So you'll have a spot for your starting material, reaction mixture (RM) and the product. * When you run the very first TLC plate, the RM should have mainly a spot for the starting material. As the reaction proceeds, later aliquots will show less and less starting material, and more product. Eventually, if the reaction goes to completion, the starting material spot should be completely gone from the RM fraction, and it should only show your product (and possibly by-products).
a compound with a high boiling point has a ____ vapor pressure
* Low
2. Know what kind of information about a sample can be determined from its melting point.
* Melting point (mp) is an excellent indicator of - Purity - Identity * Pure crystalline solids melt over a narrow range (2-3°C) * Impure crystalline solids melt over a wide range (>5°C) and are lower (are "depressed") than the corresponding pure sample * Identity is confirmed by comparison to the literature value for the compound.The presence of impurity has two effects on a substance's melting range: * To summarize, melting points can provide information about the identity and the purity of a solid sample.
You have a 100-mg sample of an impure unknown. What type of recrystallization would be best to use?
* Microscale
Purpose of Recrystallization
* Recrystallization, also called fractional crystallization, is a process that involves dissolving an impure compound in a solvent at a high temperature to create a concentrated solution and then cooling the solution slowly. The drop in temperature decreases the solubility of both the chemical compound and the impurities it contains. The pure portion of the compound crystallizes, leaving behind the impurities in the solution. Filtering out the pure crystals separates them from any impurities, and repeating the recrystallization process creates an even purer compound. * For the recrystallization process to be successful, there must be only a small quantity of impurities in the chemical compound, and they must be soluble. * Performed correctly, the recrystallization procedure is a highly effective way to obtain a pure sample of a compound. However, it can take a long time because the slower the rate of cooling, the larger the pure crystals, which makes filtering easier.
'I forget which layer is ether and which is H2O.' or 'I can't figure out which layer is aqueous.'
* Remember that the position of a layer is determined by density. If you are using ether and water as the extraction solvents, ether is always the top layer because it is less dense than water. If you forget that, or are using solvents unfamiliar to you, simply add a little more ether (or water, or whatever solvent you are using) to the separatory funnel and watch which layer becomes larger in volume.
Retention Time (Chegg)
* Retention time: Retention time is the amount of time a compound spends on the column after it has been injected. If a sample containing several compounds, each compound in the sample will spend a different amount of time on the column according to its chemical composition i.e. each will have a different retention time. * Factors affecting retention time: - column (oven) temperature: A high column temperature will give shorter retention times, as more components stay in the gas phase but this can result in poor separation. For better separation, the components have to interact with the stationary phase. If the compound does not interact with the stationary phase, the retention time will decrease. - carrier gas flow rate: A high flow rate lowers retention times but also yields a poor separation. - column length: A longer column will produce longer retention times but better separation. Unfortunately, if a component has too long a transit time in the column, there can be a diffusive effect that causes the peak width to broaden. - boiling point of the components: If a component has a low boiling point, then it is likely to spend more time in the gas phase. Therefore its retention time will be lower than a compound with a higher boiling point. A compound's boiling point can be related to its polarity.
The organic solvent used for extraction must meet a few criteria:
* Should readily dissolve substance to be extracted. * Should not react with the substance to be extracted. * Should not react with or be miscible with water (the usual second solvent). * Should have a low boiling point so it can be easily removed from the product. * Common extraction solvents are diethyl ether and methylene chloride.
Simple Distillation
* Simple distillation is a procedure by which two liquids with different boiling points can be separated. Simple distillation (the procedure outlined below) can be used effectively to separate liquids that have at least forty degrees difference in their boiling points. As the liquid being distilled is heated, the vapors that form will be richest in the component of the mixture that boils at the lowest temperature. Purified compounds will boil, and thus turn into vapors, over a relatively small temperature range (2 or 3°C); by carefully watching the temperature in the distillation flask, it is possible to affect a reasonably good separation. As distillation progresses, the concentration of the lowest boiling component will steadily decrease. Eventually the temperature within the apparatus will begin to change; a pure compound is no longer being distilled. The temperature will continue to increase until the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected. This fraction of distillate will be primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated.
4. Be able explain why the boiling points of the components of a mixture should differ by at least 40 oC in order to get an effective separation by simple distillation.
* Simple distillation is effective only when separating a volatile liquid from a nonvolatile substance or when separating two liquids that differ in boiling point by 40 degrees or more. If the liquids comprising the mixture that is being distilled have boiling points that are closer than 40 degrees to one another, the distillate collected will be richer in the more volatile compound but not to the degree necessary for complete separation of the individual compounds. * 4. In simple distillation, the vapor is immediately channeled into a condenser. Consequently, the distillate is not pure. As a result, simple distillation is effective only when the liquid boiling points differ greatly.
Simple Distillation cont...
* Simple distillation is effective only when separating a volatile liquid from a nonvolatile substance or when separating two liquids that differ in boiling point by 50 degrees or more. If the liquids comprising the mixture that is being distilled have boiling points that are closer than 50 degrees to one another, the distillate collected will be richer in the more volatile compound but not to the degree necessary for complete separation of the individual compounds.
4. Know why it is important to let the crystals form slowly.
* Slowly cooling the solution results in crystals being large compared to rapid cooling which results in smaller crystals. * Rapid cooling causes impurities to be trapped in the crystals. The crystals are formed and then collected through vacuum filtration and washed with ice-cold solvent.
After doing a correct and careful recrystallization, you isolate your product by filtration. Which of the following will be present in the mother liquor?
* Small amount of product * Impurities * Recrystallizing solvent
Solvent Selection Summary
* Solvent should not react with the compound * Solid should be insoluble at room temperature * Solid should be soluble at boiling point * Impurities should be soluble at room temperature - More soluble in the solvent than the solid you are trying to purify * Solvent should have a boiling point lower than that of the solid * Solvent should be non-toxic
You have a bottle of white solid and have recorded the melting point to be 157-159 degrees C. You suspect, based on this data, that the solid is actually salicylic acid. How can you confirm that the solid is salicylic acid?
* Take a mixed melting point. If the melting point remains the same, it is salicylic acid.
3. If given a GC of a mixture, be able to calculate the relative amounts of components (as percentage) using the triangulation method (rulers provided). (Area = h * w1/2)
* The area under a gas-chromatograph peak is proportional to the amount ( moles ) of compound eluted. Hence, the molar percentage composition of a mixture can be approximated by comparing relative peak areas. * h -- the height of the peak above the baseline w1/2--the width of the peak at half of it height This method works only when the peak is symmetrical. To obtain a percentage composition for the mixture, first add all the peak areas. Then, to calculate the percentage of any component in the mixture, divide its individual area by the total area and multiply the result by 100.
Forerun
* The first fraction, (or the first mixture between fractions) usually just a few drops, is called the forerun. It contains any highly volatile substances that were present in the sample. * If you observe condensation in the still head, remove the first few drops that collect and put in a vial labeled "forerun."
What is the relationship between wavelength, frequency and energy?
* The greater the energy, the larger the frequency and the shorter (smaller) the wavelength. * Given the relationship between wavelength and frequency — the higher the frequency, the shorter the wavelength — it follows that short wavelengths are more energetic than long wavelengths.
4. Know how to determine if two samples that have the same melting point are identical or not.
* The identity of two samples that have the same melting point can be determined by taking a mixed melting point. Grind roughly equal portions of the two samples together to mix well and take a melting point of the mixture. If the two substances are identical the melting point should be the same as that of either sample. If the two substances are not identical, then the melting point will be depressed.than the lowest of the melting points of the individual samples * Also we need to notice that when the two susbatnces are mixed and the if the mixture is supplied with heat to melt, and if the mixture melts at wide range of temperature with a range of more than 3-4 degrees then the substances are NOT identical. While if the mixture of two substances melt at a temperature range of 2-3 degrees then the substances are identical.
Know how to determine which layer is the aqueous layer or the organic layer when separating two phases during an extraction.
* The location (either the top or bottom layer) of an extraction solvent is determined by density. The density of ether is 0.713 g/cm3 and the density of H2O is 1.0 g/cm3; therefore, ether is always the top phase when the extraction solvent pair is ether and water.
In today's experiment, you will be recrystallizing a "dirty" sample of benzoin. Choose the statement that best explains how you will determine the purity of your sample.
* The melting point range should be less than 3 oC.
The Partition Coefficient
* The partition coefficient is a number that describes the solubility of a compound between two solvents * The compound is partitioned between an organic solvent and an aqueous solvent * The coefficient (Kp) is determined by dividing the concentration of the solute in the organic phase by the concentration of the solute in the aqueous phase. Kp = [ conc. of X in organic layer ] / [ conc. of X in aqueous layer ]
Retention Times Injection Mixture of Hexane, Cyclohexane, Toluene and Ethyl Benzene (1:1:1:1)
* The peak on the left is the first eluting compound (most volatile) (Lowest BP) * The peak on the right is the last eluting compound (least volatile) (Highest BP)
Microscale Extraction Technique
* The sample is dissolved in an organic solvent (A) * The solute is partitioned between two immiscible solvents (B) * The solvents are mixed and allowed to separate * The layers separate due to the difference in density * The lower phase is removed with a pipet
6. Know when/why you should measure the evacuated melting point of a compound.
* The sample is evaluated by means of melting point , should analisarse at the moment it starts to melt and the reading is correct at the time the sample is completely melted .
A student measures the melting point of a sample of salicylic acid and records 140-150 degrees C. The known value of salicylic acid is 158-161 degrees C. What does this information tell you about the sample of salicylic acid?
* The sample is impure and may or may not be salicylic acid.
A student analyzes the product of his fractional distillation and is dismayed by his results. The GC showed a nearly equal concentration of the two components he was trying to separate (the GC should show a more pure product than what he started with). Choose the statement that best describes the human error responsible for these results.
* The student was eager to finish and cranked the heat.
1. Know what is meant by melting point range.
* The temperature at which a solid melts is known as the melting point (MP) of that substance. The melting point is a physical property of a solid and can be used to help identify a substance. In practice, a solid usually melts over a range of temperatures rather than at one specific temperature. * For this reason it is more useful to speak of a melting point range. Although the term "melting point" is usually used, what is meant is "melting point range". If the compound melts over a very narrow range, it can usually be assumed that the compound is relatively pure. Conversely, compounds that melt over a wide range are assumed to be relatively impure. * To summarize, an impure solid melts over a wide range and at a temperature lower than that of the pure solid.
A student conducted a simple distillation and recorded a bp of 120 oC. The literature bp for this compound was recorded to be 130-132 oC. GC analysis showed the sample to be pure, and the thermometer was calibrated correctly. Suggest a reason for the discrepancy between the two values.
* The thermometer was placed too high in the apparatus.
What is the purpose of placing boiling stones in the distillation flask before starting a distillation procedure?
* They promote smooth boiling of the solution.
Distillation, How is it useful?
- Separate a mixture of two or more liquids, as long as bp's differ by 40 °C or more (see reading as to why) - Separate a liquid from a solid. - Purify a liquid sample (remove impurities) • For mixtures of liquids... When the sum of the vapor pressures (partial pressures) of the components equals the atmospheric pressure, the liquid mixture will boil. • The proportions of the components in the vapor will not be the same as the proportions in the liquid. The vapor will always be enriched in the lower boiling compound!
Thin layer chromatography (TLC)
* Thin layer chromatography (TLC) is a chromatographic technique used to separate the components of a mixture using a thin stationary phase supported by an inert backing. It may be performed on the analytical scale as a means of monitoring the progress of a reaction, or on the preparative scale to purify small amounts of a compound. TLC is an analytical tool widely used because of its simplicity, relative low cost, high sensitivity, and speed of separation. * Predominantly an analytical technique that is used for determining the progress of reactions and the purity of compounds * Compounds are separated largely on the basis of polarity.
Still Head
* This is the part of the apparatus that is just above your fractional column and just before your condenser. It also has an opening for the thermometer, so that you may note the distillation temperature of the product you are purifying. * This is where the liquid component (the distillate) is collected.
Know what happens to the melting point of a pure compound if it is contaminated by impurities.
* To affect the MP the impurity must be soluble in the solid. * The effect of impurities on the MP can actually be used to help identify a compound. For example if an unknown solid is known to be one of two possible known compounds, both having the same MP, the unknown can be mixed with one of the known compounds and a MP taken of the mixture. If the MP range is lowered and widened, it means that the two are different compounds. If the MP stays the same it means that the two compounds are likely identical. This technique is known as a mixed melting point determination.
2. Know the uses for TLC in organic chemistry.
* To determine the number of components in a mixture. * To determine the identity of two substances. * To monitor the progress of a reaction. * To determine the effectiveness of a purification
6. Know how to use Rf values for identification of an unknown or the components of an unknown sample.
* To identify an unknown by TLC, one must run chromatograms of the standard compounds and the unknown at the same time. By comparing Rf's or the distance that the spots move, the identification can be made.*
When choosing a solvent for recrystallization, it is important that the solvent does not react with the solute.
* True
When spotting the TLC plate, it is best to keep the spots small and concentrated.
* True
You have purified compound X and measured that it has a melting point of 159-161 C. Salicylic acid has a melting point of 158-161 C. You run a mixed melting point with compound X and salicylic acid and measure it to be 157-159 C. Compound X is salicyclic acid.
* True
Solubility: Charged Species
* Water-soluble species are charged, ionic compounds * Inorganic salts and mineral acids - NaCl, MgCl2, HCl, H2SO4 * Salts of organic compounds - sodium benzoate, pyridinium hydrochloride * Water-soluble species can also be highly polar neutral organic compounds (contain numerous polar bonds) - Carbohydrates (D-glucose) - Peptides - Alkaloids
Why is the micro boiling point determined at the point when the last bubble has left the bottom of the bell and the liquid begins to rise in the bell?
* When bubbles first start to appear, the bell is pressurized at a point above atmospheric pressure. Thus gas bubbles are released. As gas bubbles are released the vapor pressure in the bell drops, until the last gas bubble leaves. at that point the bell is no longer pressurized and the vapor pressure in the bell is equal to atmospheric pressure (definition of boiling point).
1. Know what happens to the vapor pressure (and boiling point) of a liquid when it contains a non- volatile solute
* When solution contains non-volatile solute, the solvent becomes diluted P = Xsolvent Posolvent where P is the vapor pressure of solution Xsolvent decreases as number of moles of solven decreases, so there will be a decrease in the value of the vapor pressure
2. Know how to use a phase diagram to predict the vapor composition above a liquid that boils at a given temperature and be able to predict the boiling point of the liquid that condenses from the vapor.
* When we heat a liquid to its boiling point, the composition will change as the more volatile component (B in these examples) is selectively removed as vapor. The remaining liquid will be enriched in the less volatile component, and its boiling point will consequently rise. * In order to understand it, you need to know about the lever rule, which provides a simple way of determining the relative quantities (not just the compositions) of two phases in equilibrium. * The lever rule is easily derived from Raoult's and Dalton's laws, but we will simply illustrate it by example. * The plot at the right shows the boiling point diagram of a simple binary mixture of composition 1. At the temperature corresponding to the tie line, the composition of the liquid corresponds to 2 and that of the vapor to 4. So now for the lever rule: the relative quantities of the liquid and the vapor we identified above are given by the lengths of the tie-line segments labeled a and b. Thus in this particular example, in which b is about four times longer than a, we can say that the mole ratio of vapor (of composition 3) to liquid (composition 2) is 4.
Using the chromatogram shown below, choose the dyes that are present in the unknown sample (U).
* Y5 and B1
5. Know how to increase the efficiency of a fractional distillation.
* You can improve the purity of the fraction by increasing the surface area of the fractionation column. A fractionating column with a large number of theoretical plates accomplishes many condensation-revaporization steps and very efficiently separates the compounds in a mixture. * Remove the fractionation column from the distillation unit and pack the internal space with steel wool. The steel wool increases the surface area the vapors interact with and slows the movement of the vapors up the column. The longer the vapors take to reach the top of the column, the higher the purity of the fraction and the greater the efficiency of the column. You can use any highly porous, non-absorbent material for the column packing material. * Raise the temperature of the heat source slowly to give the fractions time to interact and complete their movement up the column and down the condenser arm. Many industrial manufacturers increase the surface area in the column to recover fractions that have a boiling point separated by less than a degree. * Wrap the distillation column with an insulator like aluminum foil. The insulation will allow the vapors to travel all the way to the top of the column and out the condenser. The increase in efficiency results from the repeated interaction with the column packing. The vapor condenses on the metal and drops back to the boiling pot to evaporate again.
5. Understand why it is important to slowly raise the temperature of the melting point apparatus in order to obtain an accurate melting point.
* Your melting range will be too broad, but this time on the high end! If a sample should melt at 130-131 C, but you are heating fast, it will still probable begin to melt at about 130 C, but the full sample won't have time to absorb heat and finish melting by 131 C. Instead, the heating device may have warmed well above 131 C before the interior liquefies, so the observed range may appear to be 130- 136 C. Both the magnitude of the range and the high end of the range may be misleading.
Absorption
* absorption = Absorptions are measured in wavenumbers (cm-1), which is equal to 1/λ * Recall that λ is in units of cm, and represents the wavelength of the absorbed light * Absorptions occurring at low wavenumbers are lower in energy. * Absorptions occurring at high wavenumbers are higher in energy. * The measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample.
5. Know what is meant by the following terms: stretching, absorption, infrared region, fingerprint region, frequency, and wavelength.
* absorption = Absorptions are measured in wavenumbers (cm-1), which is equal to 1/λ Recall that λ is in units of cm, and represents the wavelength of the absorbed light Absorptions occurring at low wavenumbers are lower in energy. Absorptions occurring at high wavenumbers are higher in energy.
Using eluent with too high a polarity with respect to samples
* all spots will move to top of TLC plate ; more polar an eluent the faster a compound moves
Using eluent with too low a polarity with respect to samples
* all spots will stay towards the bottom of the TLC plate;
Boiling Points
* as the temperature of cyclohexane is increased the vapor pressure also increases. This is true for all liquids. At some point, as the temperature is increased, the liquid begins to boil. This happens when the vapor pressure of the liquid equals the applied pressure (for an apparatus that is open to the atmosphere the applied pressure equals atmospheric pressure (1 atm = 760 mm Hg)). For cyclohexane, this occurs at 81° C. * Fact: at any given temperature, the vapor pressure of the lower-boiling (lower BP) compound > the vapor pressure of the higher-boiling (higher BP) compound. Thus, the vapor above the liquid will be richer in the lower-boiling compound, compared to the relative amounts in the liquid phase.
The liquid or gas that carriers the sample across the solid support.
* mobile phase
1. Know the meaning of the following terms: mobile phase, stationary phase, adsorption.
* mobile phase: the mobile phase is a carrier gas, usually an inert gas such as helium or an unreactive gas such as nitrogen. * stationary phase: The stationary phase consists of a packed column in which the packing or solid support itself acts as stationary phase, or is coated with the liquid stationary phase (high boiling polymer). * adsorption: A lab technique in which molecules are separated according to their adsorptive properties, where a mobile-fluid phase is passed over an immobile-solid adsorptive stationary phase.
efficiency of fractional distillation: theoretical plates are the ______
* number of plates needed to achieve a standard separation
Which of the following binary mixtures can be separated using simple distillation? Choose all that apply.
* sand and methanol
Choose the answer that best describes what is meant by the phrase "resolve a mixture of compounds".
* separate the mixture into its components
Distillate
* the liquid that has been condensed from vapour during distillation (purified form). * The remaining liquid that is collected is the "distillate," and is what you want (about 0.5 mL total volume)
A student injects a 50:50 mixture of ethylbenzene and toluene on the GC. Predict the order of elution for this mixture
* toluene; ethylbenzene
Distillation
1. During distillation, compounds are heated to boiling in one container, called the pot. * A fractional distillation apparatus includes a fractionating column placed between the pot and the distilling head. The vapors generated in the pot rise up the fractionating column and encounter cooler surfaces, upon which they condense. To obtain good separations, a fractionation column is required.
The presence of impurity has two effects on a substance's melting range:
1. Melting range depression (lower end of the range drops) 2. Melting range broadening (the range simply increases. Often the low end drops a lot, the high end less so or sometimes not much at all.) A melting range of 5 or more indicates that a compound is impure.
when a pure compound has a low vapor pressure its boiling point (temperature) is ___ and we say that it is ____
High ; non - volatile
1. Know what happens to the vapor pressure (and boiling point) of a liquid when it contains a non- volatile solute (Yahoo)
a. The presence of a non-volatile solute causes a decrease in the vapor pressure of the solvent. The magnitude of the decrease is directly proportional to the concentration of solute, expressed as the mole fraction: ΔPvap = Xsolute P°vap where P°vap is the vapor pressure of the pure solvent. b. Similarly, a non-volatile solute causes the solvent to boil at a higher temperature. The magnitude of the so-called boiling point elevation is directly proportional to the concentration of solute, expressed as the molality, m: ΔTb = Kb*m Kb is called the boiling point elevation constant of the solvent.
Thin Layer Chromatography Objectives
• Analysis of Food Coloring • Separation of Anthracene and Fluorenone
Mass Spectrometry (MS)
• Determine molecular weight and molecular formula • Destructive
