Organic Chemistry Lab final
What are the two reasons that a distillation pot should never be distilled to dryness?
Without the cooling effect of the boiling liquid, the flask can become very hot and break. ii. Nonvolatile impurities left behind may explode when heated to high temperatures.
If the peaks obtained in a gas chromatogram are not well separated, what four conditions can you change to improve resolution?
You can change the following: i. The stationary phase (liquid) ii. the column length iii. the column temperature iv. the flow rate of the carrier gas
Benzoic acid is extracted from an organic solution using 20 mL of 2 M NaOH. How many mL of 3 M HCl are required to neutralize the NaOH in order to recover the benzoic acid? Show your work for full credit.
(2 moles/L) x 0.02 L = 0.04 moles of NaOH 0.04 moles / (3 moles/L) = 0.0133 L = 13.3 mL
How many theoretical plates would you need to get a greater than 90% concentration of A, if you start with a 50:50 solution?
2 Theoretical Plates
If a 15:85 mixture of A and B were distilled in an apparatus having two theoretical plates, what would be the composition of the distillate? (
74 % A, 26% B
Why does the following combination of reactants fail to undergo a Diels-Alder reaction?
A Diels-Alder reaction involving a benzene ring would destroy the aromaticity of the ring. This would result in the loss of approximately 36 kcal/mol of stabilization energy
2. Consider the following table of compounds and their solubilities in hot and cold water, and in hot and cold ethanol: Solubility in grams per liter (g/L) Compound Hot Water Cold Water Hot Ethanol Cold Ethanol A 26.4 23.6 18.9 1.21 B 5.11 3.53 14.56 12.78 C 3.05 0.011 2.36 0.12 D 7.14 1.06 1.33 0.98 a) Which of the compound(s) above could be effectively recrystallized from ethanol? Explain your answer.
A and C. These two compounds show a large difference in solubility between hot and cold solvent
Under what conditions can a good separation be achieved by a simple distillation?
A boiling point difference of > 100 ºC.
A mixed melting point will be used help distinguish your unknown from other organic compounds with similar melting points. How is this evidence used to point toward one compound or another?
A mixed melting point is measure with possible unknowns with a melting point in the range of your unknown compound. When mixing with the unknown, the melting point range should not change. When mixing with another organic compound, the substance becomes impure, broadening and lowering the melting point range
The carboxylic acid derived from trimyristin is myristic acid, shown below. It has a melting point of 55-56 oC, which is very close to the melting point of trimyristin. Explain how you could show, using only the melting point technique, that the material you isolated was trimyristin and not myristic acid. Assume you have access to pure samples of trimyristin and myristic acid
A mixed melting point would verify if the isolated material was trimyristin. A complete answer will describe the mixed melting point technique. Mixing the trimyristin isolated with a pure sample will not change the melting point, whil mixing with myristic acid will decrease the melting point and broaden the range.
Discuss, briefly, how you separated trimyristin from nutmeg. This should not be written like a procedure. Instead, discuss the methods used to separate components and why they worked (or didn't). You should comment on the method used to separate trimyristin from nutmeg, as well as the solvent choice. Also discuss the method used for purification of trimyristin, commenting on the solubility of trimyristin vs. impurities. (
A solid-liquid extraction was used to separate trimyristin from nutmeg. Dichloromethane was used as the solvent, which removed trimyristin from the other components of nutmeg. Trimyristin is soluble in hot acetone, but relatively insoluble in cold acetone, making this a good choice for recrystallization solvent. Any impurities were soluble in acetone, while trimyristin was not.
What is the drop test, and how and why is it used?
A test to determine which layer is the aqueous layer. Drop a small amount of water into the neck of the separatory funnel or test tube containing the two immiscible liquids. Watch it carefully: if it remains in the upper layer, that layer is the aqueous layer. If it sinks to the bottom of the upper layer, the bottom layer is the water layer. The test can also be carried out using a drop of the organic solvent.
"Bumping" is a term often used to describe the behavior of liquids during a distillation. Define bumping
Bumping is the instantaneous violent expulsion of vapors from a superheated liquid.
The following list of frequency ranges (in wavenumbers) are those that correspond to the four most important regions of the IR spectrum. For each region indicate the most important structural information that it indicates. 1750-1630 cm-1
C=O stretch of aldehydes, ketones, and carboxylic acid derivatives. C=C stretch is at the low end of the range; these are typically distinguished from carbonyl stretch by being weaker and narrower
Which compound below will have the highest Rf on a TLC plate? Explain your reasoning
Compound B will have the highest Rf on a TLC plate. Compound A is more polar and will therefore adsorb to the plate more strongly and travel less. The low polarity of B is a result of a lack of polar functional groups. Compound A, however has a ketone functional group with polar C-O bonds. These polar bonds interact strongly with the stationary phase, which slows the movement of A on the TLC plate.
4. Describe the cooling process during a recrystallization. Why is a slow cooling process important?
Cool to room temperature then place in an ice bath. Cooling too quickly can cause impurities to be trapped in the crystals.
True or False. The presence of a halogen atom will increase the polarity of an organic molecule to a greater degree than the presence of an oxygen atom.
False
A solution of 4 g of organic compound X in 300 mL of water was extracted with 300 mL of ethyl acetate. At equilibrium the concentration of X in the two phases was: 4.6 g / 100 mL of ethyl acetate; 0.8 g / 100 mL of water. a) Calculate the partition coefficient, Kd, for compound X in this system. Show your calculations for full credit
Kd = 4.6 g / 0.8 g = 5.75
Each scenario below describes an operational error that can lead to inaccurate or irreproducible TLC results. In each case describe the effect on the TLC results and briefly explain why. Samples were applied to a TLC plate in spots that were 6 mm in diameter
Large sample spots lead to large spots after development and the spots may overlap, leading to inaccurate interpretation of the plate.
Why is it important to keep all glassware hot during a "hot filtration step of a recrystallization? (
Letting the solution cool during a hot filtration will cause crystals to form prematurely in the filter funnel.
Describe the difference between the terms "soluble" and "miscible"
Miscible is only used for two liquids and means that the liquids are infinitely soluble in each other (i.e., soluble in all proportions). The term soluble differs from miscible in two ways: 1) it can be use when describing both liquids and solids; 2) there can be different degrees of solubility.
Describe two of additional sources of lost product. These sources should be based on your observations as well as logical reasoning.
Potential answers include: Premature crystallization on fluted filter paper Mechanical losses on filter paper, in flask, etc. Mechanical losses due to premature crystallization Losses due to incomplete dissolution in the hot solvent Losses due to spilling solution, dropping solid, etc.
What is an Rf value and how is it calculated?
Retention factor, ratio of the distance that a compound travels from to origin to the distance that the solvent front travels up the plate
Each scenario below describes an operational error that can lead to inaccurate or irreproducible TLC results. In each case describe the effect on the TLC results and briefly explain why. A lab worker frequently removed the developing chamber cover in order to better see the position of the solvent front during development of the plate.
TLC results are more accurate and consistent, and plates develop faster, if the atmosphere inside the developing chamber is saturated with solvent vapors. Opening the developing chamber will release solvent vapors and lower the level of saturation.
What was the purpose of stirring the nutmeg-dichloromethane mixture while it is being heated?
The reaction is biphasic so stirring keeps reactants mixed well throughout the reaction time. Stirring also promotes smooth boiling which prevents bumping.
Instructions for washing your crystallized product in the vacuum filter funnel call for keeping the wash solvent cold. Why is this important? How would the success of the experiment be affected if the wash solvent were warm?
The solubility's of organic compounds increase with temperature. More of the product will dissolve and be lost if the wash step is carried out with warm solvent.
Using the fractional distillation plot, estimate the percent composition of the starting MeOH:Water solution. Explain in words how you arrived at your answer, and draw lines on the plot to illustrate.
They should drop a line to the baseline from the spot on the fractional plot that has the highest slope. This spot should be approximately in the middle of the S-shaped curve. They also need to briefly explain that this is the correct position to estimate composition because it represents the middle of the transition between the lower- and higher-boiling components. The starting composition was 45:55 MeOH:Water.
"Bumping" is a term often used to describe the behavior of liquids during a distillation. What two techniques are used to prevent bumping?
Use of boiling stones (also called boiling chips) or boiling sticks ii. Use of magnetic stirring.
What two things affect the retention time of a compound on a gas chromatograph?
Volatility (or boiling point) and polarity
Polarity of compounds
determined by how far they move up the solvent front, the more polar the less it moves
In order to conduct a diels-alder reaction
dienophile must be conjugated (1,3) and cis
Benzophenone and piperidine consist mostly of carbon atoms, with only one heteroatom each. Yet, they show very different solubility in water. Provide two reasons why there is a difference in solubility.
1) The amine group of piperidine has greater polarity than the ketone functional group of benzophenone. This is because the amine can serve as a hydrogen bond donor (it has a hydrogen atom on the amine nitrogen), as well as a hydrogen bond acceptor. The ketone function group can only accept hydrogen bonds. 2) Benzophenone is a larger molecule than piperidine and the hydrocarbon portion of the molecule dilutes the polarity of the functional group to a greater degree than in piperidine.
List the seven steps of recrystallization in proper order
1. Select the proper recrystallization solvent 2. Dissolve sample in the minimum amount (+10-20%) of solvent at or near the boiling point 3. Decolorize the hot solution with charcoal 4. Filter the hot solution through gravity filtration with a fluted filter paper to remove insoluble impurities 5. Cool the hot filtrate slowly to room temp. then in an ice bath 6. Collect solid by vacuum filtration and wash with cold solvent 7. Dry solid
Predict the relative C=O stretching frequencies of the following, listing them in order from higher to lower frequency. Explain your reasoning.
2 > 1 > 4 > 3 Their answers should be more than just "esters are at higher frequencies than aldehydes." That is too simplistic and is just copying from the textbook. They need to say WHY the order is what they have given; their reasoning should include justification based on resonance effects. This will be essentially the same reasoning as in their discussion section.
Write a balanced equation for the acid-base reaction that occurs when the carboxylic acid component of your mixture was extracted from the ether solution. Write a balanced equation for the acid-base reaction required to recover the carboxylic acid from the aqueous layer.
A complete answer will discuss how the acid-base extractions separated components. A weak base (NaHCO3) was used initially to separate benzoic acid (pKa = 4.17) from the mixture. A stronger base would have also deprotonated 4-tert-butylphenol (pKa = 10.17). Benzoic acid was isolated by adding HCl to protonate the salt. 4-tert-Butylphenol was then separated from biphenyl using NaOH and isolated by protonating the salt with HCl. Biphenyl was isolated by rotary evaporation.
Imaging that you are performing a fractional distillation of two miscible liquids, A and B, where A is the lower boiling component. The procedure calls for collecting distillate fractions over specific temperature ranges. How would the separation of the two components be affected if the thermometer in the distillation apparatus were positioned 1 cm higher than it should have been?
A thermometer that is positioned too high will display a low, incorrect temperature. Fractions collected at pre-determined temperatures would have an incorrect composition, with the first fraction contaminated with the lower-boiling component (B).
Consider the boiling point composition curve for a mixture of two liquids A and B (below). Draw lines on the graph to show how you arrived at your answers. a) A sample of vapor has the composition of 30% A and 70% B. What is the composition of the boiling liquid that produced this vapor?
About 9% A, 91% B.
When a fractional distillation of two miscible liquids (with very different boiling points) is being performed, the thermometer will register a boiling point for the first compound, and then record a decrease in temperature before rising to a higher temperature. What is the reason for decrease in temperature?
After the first component has distilled there is no vapor passing by the thermometer until the next component vaporizes. During this time a decrease in temperature is observed.
b. Based upon your answer to part a., when this 95 °C solution is cooled to 25 °C, how much benzoic acid will remain in solution and how much will precipitate/crystallize out of the solution? Show all calculations for full credit
Amount of benzoic acid in 14.7 mL of water at 25 °C is 14.7 mL × (3.4 g/1 L) x (1L/1000 mL) = 0.05 remaining in aqueous solution Amount of recovered benzoic acid is 1.0 g - 0.05 g = 0.95 g
You should have seen a difference between the solubilities of biphenyl and benzophenone in methanol. Explain this difference. Note: if both dissolved completely, use the rate of dissolution as the basis for your answer
Biphenyl is a pure hydrocarbon of low polarity; it is soluble in low polarity solvents, but only slightly soluble or insoluble in very polar solvents. It should have been partially soluble in methyl alcohol (or if it dissolved completely, it should have dissolved very slowly). Benzophenone is capable of hydrogen bonding to methyl alcohol and is therefore more polar than biphenyl and more soluble in methyl alcohol.
The following list of frequency ranges (in wavenumbers) are those that correspond to the four most important regions of the IR spectrum. For each region indicate the most important structural information that it indicates. 3100-2500 cm-1 .
C-H stretch. The position of the band allows differentiation between aliphatic and aromatic/alkene. Bands at frequencies just higher than 3000 are due to hydrogens on aromatic rings or alkene sp2 carbons; bands at frequencies just below 3000 are due to hydrogens on sp3 carbons. Aldehydic C-H stretch is at the low end of this range. The O-H of carboxylic acids is in the range, but can be distinguished from the others by the fact that it is a very broad band.
The following list of frequency ranges (in wavenumbers) are those that correspond to the four most important regions of the IR spectrum. For each region indicate the most important structural information that it indicates. 1350-1000 cm-1
C-O stretch; the position and number of the bands allow one to distinguish between alcohols, phenols, carboxylic acids, and esters.
Which compound below will have the highest Rf on a TLC plate? Explain your reasoning
Compound D will have the highest Rf value (move fastest). It has lower polarity than compound C and will therefore be less strongly adsorbed onto the silica gel. The lower polarity of D is the result of: 1) more carbon atoms than structure C; 2) a functional group of lower polarity - the oxygen atom of B can accept hydrogen bonds, but has no hydrogen atom to act as a hydrogen bond donor.
The result of a TLC experiment showed that Compound X has an Rf values of 0.75 on a silica plate using ethyl acetate (CH3CO2Et) as the mobile phase. Indicate if the Rf values of Compound X would be higher or lower in each of the following solvents. Briefly explain your reasoning for full credit, simply stating "higher" or "lower" will receive no credit. The structures of the solvents have been provided in the box to the right.
Dichloromethane c. 50:50 Hexane:Ethyl Acetate Lower Lower b. Toluene d. 95:5 Ethyl Acetate:Methanol Lower Higher They must rationalize their answers by comparing the polarity of the solvent to ethyl acetate. Increasing polarity of the solvent will increase the Rf values, decreasing polarity will decrease the Rf value.
1. Why would ethanol (EtOH) be a poor choice of solvent for extraction of an aqueous solution?
EtOH is totally miscible with water and there would only be a single layer. Therefore extraction would be impossible.
The mixture you separated consisted of a neutral compound and two acidic compounds. What additional steps would you have to add to the procedure if your mixture also contained a water-insoluble, basic amine that you wished to isolate?
Extract the amine into two portions of an aqueous acid, such as 1.0 M HCl. Combine the aqueous extracts and make strongly basic (pH 11), for example by use of 3 M NaOH. Cool and collected the precipitated amine by vacuum filtration, wash with cold water, and dry.
describe the mistakes and the effect on the product: While attempting to extract the phenolic component, you mistakenly extracted the ether solution with 1.5 M HCl rather than 1.5 M NaOH.
HCl is an acid, not a base. This will result in very little phenol extracted into the aqueous layer. The phenol will therefore have a low yield, and the neutral component will have a high yield. In addition the neutral component will have low purity due to the contamination by the phenolic component.
Which distillation technique did a better job of separating MeOH and water? Explain your answer. (
Hopefully their plots will suggest that the fractional technique is best. They should refer to the answer above about the more rapid transition from low boiling component to high boiling component.
A hydrogen bond is a special type of dipole-dipole interaction, and only occurs with a small group of elements. List the specific grouping of elements that must be present in the hydrogen bond donor, and the specific elements that must be present in the hydrogen bond acceptor.
Hydrogen Bond Donor: N-H, O-H, F-H (0.5 points each) Hydrogen Bond Acceptor: N, O, F
During distillation why should the heating mantle and stirrer be clamped above the bench top instead of placed directly on the bench top?
In an emergency situation, the heat can be quickly removed from the distillation flask by dropping the heating mantle.
Arrange the following four compounds in order of increasing Rf when using TLC plates similar to those you used. Place lowest Rf on the left and highest Rf on the right. Explain how you arrived at your choices for full credit.
In order of increasing Rf: C < D < B < A The most polar compound will have the lowest Rf and the least polar compound will have the highest Rf value.
Percent recoveries for this type of experiment are usually slightly less than 100% due to miscellaneous mechanical losses. Comment on your percent recoveries and what they say about the success of the experiment. If the yields are significantly below 100% or above 100%, you should try and think of logical reasons why based on the operations you carried out; don't just make a laundry list of excuses.
Incomplete extraction due to use of too little acid or base (not checking pH) Incomplete precipitation from aqueous layer due to slight solubility of cold in water Incomplete precipitation from aqueous layer due to too short a cooling period Mechanical loss of product on filter paper, funnels, flasks, etc.
describe the mistakes and the effect on the product: When recovering the benzoic acid component, instead of using pH paper you adjusted the pH to 7 using litmus paper.
Little if any benzoic acid will precipitate at pH 7, so the reported percentage of benzoic acid will be low.
At 25 °C, the vapor pressure of acetone is 230 mm Hg, and the vapor pressure of toluene is 28.4 mm Hg. A solution of these two miscible liquids having a 5:1 molar ratio of acetone:toluene is at a temperature of 25 °C. What is the total vapor pressure above this solution? Show your calculations for full credit.
Mole fraction (C) of acetone = 5 / 6 = 0.83 Mole fraction (C) of toluene = 1 / 6 = 0.17 PT = PAcetone + PToluene = (Po Acetone * CAcetone) + (Po Toluene * CToluene) PT = (230 mm Hg * 0.83) + (28.4 mm Hg * 0.17) PT = 190.9 mm Hg + 4.83 mm Hg = 195.7 mm Hg
Chromatograph of Unknown Xylenes Mixture. Calculate the percentage of each xylene isomer in the unknown mixture using the triangulation method. Show all calculations and measurements on the gas chromatograph. What is the percent composition of the xylenes mixture?
Mole percentage = (area under individual peak / total peak area) x 100 Area = height x width at ½ height Check the calculations of peak area of the both the unknown mix and standard. Peaks should elute para, then meta, then ortho. Each chromatogram: 2 points for each area calculation, 1 point for peak assignment.
Calculate the theoretical yield of 9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic anhydride.
Moles of Anthracene. 0.60 g ÷ 178 g/mole = 3.37 mmoles Moles of maleic anhydride. 0.30 g ÷ 98 g/mole = 3.06 mmoles Reactant stoichiometry is 1:1, therefore limiting reagent is maleic anhydride One mole of limiting reagent gives one mole of product, therefore theoretical yield of product in moles = 3.06 mmoles Theoretical yield of product in grams = 0.00306 moles x 276 g/mole = 0.845 g
The literature claims that trimyristin represents 25-30% of the weight of nutmeg. Your value is likely to be lower than this. Where are the most likely points in your procedure for losing trimyristin?
Most likely losses would include Incomplete extraction from nutmeg Loss during recrystallization due to low level of solubility in acetone Mechanical losses on fluted filter paper. This is an especially large contributor. DCM evaporates quickly and it is often possible to see crystals of product forming near the top edge of the filter paper Mechanical losses on glassware
A simple solubility test in an aqueous solution of NaOH could be used to distinguish between trimyristin and myristic acid. Which compound would dissolve? Explain, using words or structures, what would be observed. HINT: Think about the reactions of the acid-base extraction com
Myristic acid would dissolve in an aqueous NaOH because of the acid-base reaction with the carboxylic acid group. Trimyristin is a very nonpolar molecule and has no acid functional group; it would be insoluble in aqueous NaOH.
Write a balanced equation for the reaction of sodium bicarbonate with hydrochloric acid.
NaHCO3 + HCl NaCl + CO2 + H2O
What is the effect on the boiling point of an insoluble substance (such as sand or charcoal) suspended in the liquid?
No effect. Because the solid is not in solution, it does not reduce the mole fraction of the liquid.
You are trying to find a TLC solvent system that will separate the compounds X, Y, and Z. You developed the compounds on a TLC place using 95:5 hexanes:ethyl acetate as the eluting solvent and obtained the chromatogram shown to the right. a. Is this solvent system appropriate for separating X, Y, and Z? Why or why not?
No, the separation of the three compounds is poor. The all have a similar Rf value.
You are given a sample that is believed to be either cis- ot trans-stulbene. Would TLC be a good method to determine which compound you have? Why or why not
No, the two structure differ only in the stereochemistry around the C-C double bond. This will result in only a very small different in polarity, if any. As a result, they will only have a very small, if any, difference in Rf values.
Would water be a suitable solvent for extracting trimyristin from nutmeg? Explain who or why not. Use the structure of trimyristin as a "hint"
No, trimyristin is too nonpolar to have any significant solubility in water. The hard hydrocarbon chains dilute the effect of the polar C-O and C=O bonds.
Describe the mistakes and the effect on the product: During the NaHCO3 extractions you failed to thoroughly mix the aqueous and organic layers
Not all of the benzoic acid will be converted to the sodium salt, so it will remain in the organic phase. Thus the reported percentage of benzoic acid will be low. The benzoic acid in the organic phase will be extracted with the subsequent NaOH extraction, and will contaminate the phenolic component. Thus, the phenolic component yield will be high and the purity will be low.
The following list of frequency ranges (in wavenumbers) are those that correspond to the four most important regions of the IR spectrum. For each region indicate the most important structural information that it indicates. Region 1: 3600-3200 cm-1 .
O-H and N-H stretch of alcohols, phenols, and amines.
Two solvents, ether and petroleum ether, are often confused. Describe and differentiate these two solvents.
Petroleum ether does not have an ether functional group; it is a mixture of hydrocarbons having (mostly) 5 and 6 carbons. It is also referred to as "low boiling petroleum ether". Ether usually refers to diethyl ether, which does have the ether functional group
Plate A, below, represents the TLC chromatogram of a compound developed in hexanes. The same compound was then spotted on a longer TLC plate and again developed in hexanes. Which TLC place, B, C, or D, correctly represents the appearance of the developed longer plate? Justify your answer
Plate C is the equivalent of plate A. Calculation of Rf values shows that A and C have similar Rf values. The distances CANNOT be compared.
Why is it a mistake to carry out a distillation in a closed system (i.e., to plug all openings in the apparatus)?
Pressure can build up inside the apparatus if there is no vent to the atmosphere. This can cause the joints of the apparatus to come apart, releasing vapors to the air. In the worst case, the apparatus could explode from the internal pressure.
Define reflux. What is the main advantage of this technique?
Reflux is heating a liquid to boiling and condensing the vapors so they return to the original reaction flask. The benefit is it keeps the reaction at constant temperature (the boiling point of the solution).
Diels-Alder reactions of anthracene take place only at the center ring. Use the resonance energies provided in the introduction to the experiment to explain why reaction occurs only at the center ring. Your answer should include calculations of resonance energies for the Diels-Alder reactions at the different rings of anthracene, as well as a brief discussion of what these numbers mean.
Resonance energies: Benzene: 36 kcal/mole Naphthalene: 60 kcal/mole Anthracene: 84 kcal/mole If a Diels-Alder reaction were to occur at one of the end rings of anthracene, a naphthalene nucleus would remain, and the loss in resonance energy would be (84 - 60), or 24 kcal/mole. Diels-Alder addition to the center ring of anthracene leaves two intact benzene rings, so the loss of resonance energy is only (84 - [2 x 36]) = 12 kcal.
You calculated the minimum amount of water required to recrystallize 1.0 g of your unknown based on the assumption that it was adipic acid, but your sample was actually salicylic acid. How will the yield be affected and why? Use the solubility data in the reagents table to answer this question
Salicylic acid is less soluble in water than adipic acid. Therefore, too little water will be used and the sample will not dissolve entirely in the hot water. Undissolved product will be removed by the hot filtration, resulting in a lower yield.
Spots were applied to a TLC plate so that they were exactly even with the solvent level in the developing chamber.
Sample may be washed from the plate when first placed in the developing solvent
What is the structural difference between saturated, monounsaturated and polyunsaturated fats? Draw an example structure of each fatty acid. Of which kind is trimyristin?
Saturated fats have no carbon-carbon double bonds. Monounsaturated fats have a single carbon-carbon double bond in one or more of the carboxylic acid moieties. Polyunsaturated fats have two or more carbon-carbon double bond in the carboxylic acid moieties. Since trimyristin doesn't have any double bonds it is a saturated fat.
The solubility of benzoic acid in water at 95 oC is 68 g/L. At room temperature (25 oC), the solubility of benzoic acid in water is 3.4 g/L. a. What is the minimum amount of water in which one gram of benzoic acid can be dissolved at 95 oC? Show all calculations for full credit.
Set up a proportion 1 g/x mL = 68 g/1000mL x = 1000 mL ÷ 68 = 14.7 mL
One of the most common errors in measuring melting points is too rapid heating of the melting point apparatus. If you were to make this mistake, how would your observed melting point compare to the true melting point?
Since the thermal mass of the thermometer is greater than the sample, the observed MP would be lower than the actual MP of the sample. This is because the thermometer heats up slower than the sample.
Indicate whether the following pairs were miscible or immiscible. For the three pairs of compounds in the second column, identify the principle intermolecular force acting between the two solvents.
Solubility Intermolecular Force Water and ethanol Miscible H-Bonding (1 point) Water and diethyl ether Immiscible H-Bonding (1 point) Water and dichloromethane Immiscible (1 point) Water and hexane Immiscible Hexane and dichloromethane Miscible Dipole-Dipole (1 point)
Briefly discuss the IR spectrum of your product and describe which absorption bands support the structure of the product.
The absorbances supporting the adduct are: i. C=O stretch at 1780 cm-1 ii. C-O stretch at 1228 cm-1 iii. C-O stretch at 1068 cm-1 They should try to identify peaks and relate them to the structure of the product
a) What is the effect on the boiling point of a liquid (such as water) when a soluble nonvolatile substance (e.g., NaCl) is dissolved in it? Hint: Use the equation derived by combining Dalton's Law and Raoult's Law. (
The boiling point is increased because the nonvolatile solid does not contribute to the total vapor pressure, but it decreases the mole fraction of the liquid. A higher temperature is for the vapor pressure to equal the surrounding pressure. PT = Po ACA + Po BCB If B is the nonvolatile component, then Po B = 0, and the second term contributes nothing to the total vapor pressure. But because the nonvolatile component is soluble, the mol fraction of A (CA) is less than 1 and the total vapor pressure of the system is reduced at any given temperature.
Development and visualization of a TLC plate showed a single spot at the origin. The lab worker concluded that the sample consisted of a single pure compound. Was this conclusion justified? Why or why not?
The conclusion was not justified. You do not know how many components may be present because nothing has moved. The only conclusion that can be made is that a more polar developing solvent should be tried
When using a drying agent (sodium sulfate for example) to dry an organic layer, how do you determine is your layer is dry?
The drying agent should not clump together upon swirling. (It should be granular and flow like sand when the flask is swirled.)
Why is a fractionating column packed with glass beads more efficient than an empty column of the same length and same diameter?
The glass beads provide a larger surface area on which evaporation-condensation cycles can occur. The more evaporation-condensation cycles the greater the efficiency.
How would a melting point affect the solubility of a compound?
The higher a compounds melting point, the more stable the crystal lattice. This will decrease the solubility compared to another compound with, all other things being equal, a lower melting point.
Each scenario below describes an operational error that can lead to inaccurate or irreproducible TLC results. A pen was used to label the origin of the TLC plate
The mobile phase will dissolve the dyes present in ink, and this will contaminate the plate.
The structures of two spinach pigments are shown below. Use adsorption theory to explain why they have the relative Rf values given in the procedure.
The only difference in the two structures is the presence of two hydroxyl groups in Zeaxanthin. These hydroxyl groups make the molecule more polar and cause Zeaxanthin to interact strongly with polar sites on the surface of silica gel by hydrogen bonding and dipole-dipole interactions. As a result Zeaxanthin is more strongly adsorbed onto the silica gel and moves more slowly than beta-Carotene.
Describe the ideal recrystallization solvent in terms of solubility of the solute and impurities
The solvent should afford high solubility of the desired product at the boiling point and very low solubility in ice-bath temperatures. Impurities are either completely insoluble at all temperatures, or infinitely soluble at all temperatures.
Explain the terms "stationary phase" and "mobile phase". What is the mobile phase used for the analgesic part of the experiment?
The stationary phase is the substance that is fixed in place for a chromatography procedure. Examples of this include silica and alumina. The mobile phase is the solvent (in TLC) that carries the sample being analyzed along the stationary phase. The mobile phase for the analgesic portion of the experiment is 99:1 ethyl acetate : acetic acid.
. After a rather lengthy organic chemistry synthesis procedure, a student analyzed the product of the reaction on a TLC plate and obtained the result at right. What might he/she have done wrong, if anything? If you conclude that a mistake was made, provide an explanation as to why it leads to the observed results.
The streaking indicates that too much sample was applied to the plate. When a plated is overloaded the solute saturates polar active sites on the stationary phase; a portion of the solute will therefore move faster, resulting in streaking.
An infrared spectrum shows energy absorbed by a molecule that corresponds to the energy needed for a bond in that molecule to vibrate. Explain why they carbonyl stretch (C=O) varied for each of these molecules. Why do certain compounds show a higher vibrational frequency, while others show a lower vibrational frequency? HINT: The introduction given before the procedure is a good source for this information.
The stronger the bond, the higher its vibrational frequency. Resonance theory can help predict vibrational frequencies. Less double bond character due to resonance results in vibration at a lower frequency. They should specifically talk about the compounds above and related the resonance structures to bond strength and observed frequency
Looking back at the procedure... We dissolved both sugar and benzoic acid in water at a high temperature. Upon cooling the flask benzoic acid precipitates from solution. What happened to the sugar and why doesn't it precipitate from solution?
The sugar has many hydroxyl groups (-OH), which are very soluble in water. The sugar stays dissolved in water while the benzoic acid precipitates out.
Explain the results for the tube in which 1.0 M HCl was added to ethyl 4-aminobenzoate. Write a balanced equation for the reaction giving complete structures for all reactants and products. Now explain what happened when 6.0 M NaOH was added to this same tube, and provide a balanced equation. Explain why the different forms of the molecules display such dramatically different solubility behavior
Their explanation for the difference in solubility should center on the change in polarity between the amine with the polar covalent N-H bonds and the conjugate acid (ammonium ion) with the much more polar ionic bond.
Explain the results for the tube in which 1.0 M NaOH was added to benzoic acid. Write a balanced equation for the reaction giving complete structures for all reactants and products. Now explain what happened when 6.0 M HCl was added to this same tube, and provide a balanced equation. Explain why the different forms of the molecules display such dramatically different solubility behavior
Their explanation for the difference in solubility should center on the change in polarity between the carboxylic acid with the polar covalent O-H bond and the conjugate base (carboxylate ion) with the much more polar ionic bond.
Graphing of your data should result in two distillation plots having distinctly different shapes. Explain what these differences tell us.
They need to briefly explain that the sudden increase in slope for the fractional plot represents a rapid transition from the lower boiling component to the higher boiling component. Also that the greater slope of the curve during this transition represent a more efficient separation of the two liquids.
What does the pH of the aqueous solutions of malonic acid and piperidine say about these two compounds?
This is a demonstration of the fact that piperidine (like all amines) is mildly basic, and malonic acid (like all carboxylic acids) is mildly acidic.
Imagine you repeated the fractional distillation of this experiment using 2 identical fractional columns stacked on top of each other. How would this affect the efficiency of the separation of methanol and water? Explain.
This would increase the efficiency of the separation. Adding an additional fractional column increases the number of theoretical plates (evaporation-condensation cycles).
Why is it important to use a minimal amount of boiling solvent to dissolve the solute?
To ensure that the solution is saturated, using too much solvent will reduce the % recovery and possibly prevent any solute from precipitating.
You are trying to find a TLC solvent system that will separate the compounds X, Y, and Z. You developed the compounds on a TLC place using 95:5 hexanes:ethyl acetate as the eluting solvent and obtained the chromatogram shown to the right How could you change the solvent system to give you a better separation of X, Y, and Z?
You need to increase the differences in Rf values to get a better separation. If the compounds travel further up the plate, this will allow the differences in the Rf values to be larger. Increasing the polarity of the solvent will result in a higher Rf. This can be done by increasing the amount of ethyl acetate in the developing solvent.
The following TLC plates have been spotted with two different compounds and are about to be eluted. For each plate, explain is it has been spotted correctly or incorrectly and why
a) Improper: The solvent level is too high and it will wash the spots off the plate. b) Improper: The Rf values can't be directly compared because the origins of the two spots are not at the same level. c) Improper: Origin spots are overlapping. This may make it difficult to identify which sample the developed spots came from. d) Proper: The spots are at the same height, above the solvent line, and spaced well.
At what temperature would a 10:90 mixture of A and B boil?
about 79 C
Assume you were given a mixture of A and C. Devise a plan to separate and purify them.
i. Extract A with cold water. A is very soluble in water at all temperatures, while C is very insoluble in cold water. Concentrate the water solution and dry the resulting solid to give slightly impure A. Dry remaining C to give slightly impure C. ii. Recrystallize A from EtOH. The high differential solubilities of A between hot and cold EtOH will result in a good recovery of pure A. iii. Recrystallize C from either water or EtOH. Both solvents show good differential solubility between hot and cold.
Which of the xylene isomer would be expected to move fastest through the GC column, and why?
para-Xylene. Because the isomers have similar polarities, they are separated based on their boiling points. para-Xylene has the lowest boiling point.
