CHM 205 Post Labs
The solubility of an unknown compound X in ethanol is found to be 0.21 g per 10 mL at 0°C and 1.4 g per 10 mL at 78°C. What is the minimum amount of ethanol needed to recrystalize a 2.0 gram sample of compound x? After recrystallization of the 2.0 gram sample, how much of compound X will remain dissolved in the cold solvent (i.e how much of compound X is not recovered after the recrystallization?)
1. 1.4g(X)= 2.0g(10mL) X=14.3 14.3mL (0.21g)= X (10mL) X=0.3g 14.3 mL of ethanol is needed to recrystallize a 2.0 g sample, and 0.3g of the chemical will be dissolved in solution.
Given the procedures described for using Pasteur pipets from Part A of Experiment 0, and literature densities if needed, explain how you would: a) add about 0.8 mL of chloroform to a vial b) add about 1.2 g of methanol to a vial c) determine the volume and mass of 38 drops of methylene chloride (Ch2Cl2)
1. A) Determine the number of mL per drop, then divide 0.8mL by the number of mL per drop and add that many drops to the vial. B. Use the density of methanol to determine how many mL you need to add to get 1.2 grams. Then, divide the number of mL by the number of mL per drop to know how many drops to add to the vial. C. Add drops to a graduated cylinder up to the 2mL mark and divide the number of mL by the number of drops to determine how many mL are in a drop. Next, either look up the density of methylene chloride or determine it. Calculate the number of drops per 1mL, zero the balance with a beaker on it, and add one mL of methylene chloride to the beaker. The mass read by the balance is the density of the solution in grams/mL. Next, calculate the number of mL in 38 drops by multiplying the number of mL per drop by 38. Finally, multiply the number of mL in 38 drops by the density in grams/mL to determine the mass.
Your TA may have told you to use acetone to clean your glassware after you finished your experiment. Acetone is a very popular solvent for cleaning glassware. Given the properties of acetone listed in MtC, why do you think this is? Do you think acetone would make a good recrystallization solvent? Why or why not? From a safety standpoint, what do you suppose would be a major concern when using acetone?
1. Acetone is very good at dissolving solutes at room temperature, but acetone would not be useful for recrystallization because solvents for recrystallization should only dissolve the solute when they are boiling. Acetone is flammable, so care must be taken when using it near a heat source.
Arrange the following solvents in order of increasing polarity: ethyl acetate - dichloromethane - isopropanol, ethanol, toulene, heptane
1. Ethyl acetate < heptane < toluene < dichloromethane < ethanol < iso-propanol
Why does the ferrocene elute from the column first? Why was the solvent changed in the middle of the column procedure?
1. Ferrocene eluted from the column first because it does not have high polarity so it was easily eluted with a non-polar solvent, petroleum ether. The solvent was changed during the procedure because the polar acetylferrocene needed be eluted using a polar solvent. Therefore, the switch occurred so the two compounds could be separated through their interactions with a solvent.
Ferrocene eluted from the column first, but it ended up as the higher of the two spots during TLC analysis. Explain.
1. Ferrocene is less polar, so it is eluted first. It had fewer interactions with the TLC plate so it traveled further.
In running TLC under the following conditions, you get results that are less than ideal. Consider the situation in each case and suggest a correction. a) you run TLC on a mixture of two unknown halogenated alkenes, but you see only one spot with an Rf of 0.91. The solvent used was ethyl acetate. b) you run TLC on a mixture of thiol and an amine, and again you get only one spot with an Rf of 0.15. The solvent was a mixture of petroleum ether and dichloromethane c) When you put your TLC plate in the developing chamber, the solvent in the chamber covered the spots on the TLC plate baseline
1. For the following, these are possible corrections a. Use a less polar solvent b. Use a more polar solvent c. Make the starting point for the solvent further up on the plate, so that they do not touch the solvent and dissolve.
What do you think would happen to your recrystallization solution if you did not use a preheated filter during the hot gravity filtration step?
1. If a pre-heated filter is not used, the cold temperature will cause crystals to form on the filter instead of filtering through and forming in the beaker below the funnel.
Suppose you are trying to determine the mp of a compound and the following happens: a) the compound turns from white to brown before melting b) the compound slowly disappears from the capillary tube before melting. Describe what happened in each case. How would you either report or correct this?
1. If the compound changed from white to brown, this could mean that the compound has been overheated. For this, I would lower the start temperature and ensure that the compound is heated very slowly. If the compound slowly disappears from the capillary tube before melting it may have sublimed into a gas. The cap should be tightly secured to the capillary to prevent this.
Why is it important to use only the minimal amount of hot solvent needed to achieve a recrystallization? what would be the effect of using too much solvent while dissolving a solid for recrystallization?
1. It is easier to form crystals when the concentration of the solute is high. If the concentration of the compound isn't high enough (e.g. too much solvent is used), the crystals will not be able to form.
Traumatic acid has a distribution coefficient of 5.8 between MtBE and water. If 75 mg of traumatic acid is added to a centrifuge tube containing 3 mL of water and 2 mL of MtBE, how much traumatic acid would be in each layer after thorough mixing?
1. K = C1/C2 = 5.8 = [(75 mg /2)/(x/3) x = 15.41 mg
Adipic acid and citric acid have very similar melting points. Given two unknown samples, one being citric acid and the other adipic acid, how could you determine which was which without using spectroscopy?
1. Mixed melting points could be used to identify which compound is citiric acid and which compound is adipic acid. You would need is a pure sample of either citric acid or adipic acid. Then, you could mix the known sample with each of the compounds and see which melting point range is the narrowest. The narrowest melting point range for the mixed melting point means that these two compounds are the same, so you can identify which compound you have.
Your run a TLC plate spotted with three compounds, napthalene, o-toulic acid, and fluorenol. Predict the relative Rf values
1. Naphtlalene would have the highest Rf value. The lowest value would be o-toulic acid. Fluorenol would have an Rf value in between the others.
o-Hydroxyacetophenone has a melting point of 4-6 C. p-Hydroxyacetophenone has a melting point of 109-111 C. Explain the basis for the sizable difference in melting points for these two compounds.
1. P-hydroxyacetophenone has a higher boiling point than o-hydroxyacetophenone because it is highly polar. These polar bonds are harder to break, which results in an increased boiling point.
Explain how each of the following contaminants is removed during the recrystallization process: A) sand (inorganic contaminant) B) colored organic contaminants C) a very soluble contaminant
1. Sand will be removed by the filter during the gravity filtration. Colored organic contaminants are removed by activated charcoal when the solution is boiling. A soluble contaminant will not form crystals with the pure chemical so it can be removed when the crystals are separated out of solution.
Benzyl alcohol has a boiling point of 205 C. It has solubility characteristics that would seem to make a good choice for use as a recrystallization solvent for fluorenol (mp 153-155C) but in fact benzyl alcohol is a poor choice of solvent for this recrystallization. Why do you think this is? (Hint: you do not need any other info to answer this question)
1. The boiling point of the solvent should be below the melting point of the solute so that the crystals don't melt.
For each of the following pairs, predict which compound will have the larger Rf values if both are run on a SiO2 TLC plate in 10% acetone/hexane: 4-decanone or 4-decanol; xylene or benzoic acid; cycloheptane or cycloheptanone
1. The following have the larger Rf value by comparison in 10% acetone/hexane i. 4-decanone ii. Xylene iii. Cycloheptane
What is the proper heating rate used to determine a compound's melting point range? what would happen if you exceeded this heating rate?
1. The rate at which a solid should be heated to determine its melting point is 1-2 °C/minute. If you exceeded the heating rate, it would be difficult to measure the melting point range because it would happen too quickly to be measured accurately.
Why is it important to limit the sand bath temperature to about 60 C when evaporating the solvent from your collected fractions?
1. The sand bath temperature had to be at 60°C because acetylferrocene has a low melting point and you do not want to decrease the percent yield by evaporating it.
define melting point range
1. The temperature range at which a solid melts and forms a liquid is the melting point range. This is found by measuring the temperature at which the solid begins to melt and the temperature at which the solid is completely melted.
name three important things to keep in mind while spotting TLC plates
1. Three important things while spotting TLC plates a. The spots used to spot the plate must be small b. The solvent cannot touch the starting line of the sports c. Do not touch the plate as oils on hands can affect results
Divise a general extraction scheme for separating the following pairs: a) an organic base mixed with an organic neutral compound b) an organic acid (HA) and a phenol (ArOH) careful, see the lab manual!
2. A) to separate an organic base mixed with a neutral compound, add HCl to the mixture so that the base is a different layer. Then, back extract using MtBE and finally treat with NaOH to form a precipitate. The neutral compound can be extracted by washing with DI H2O and sodium sulfate. Finally, remove the organic layer and evaporate. B) To separate an organic acid and a phenol, NaHCO3 should be added. The organic acid will be the bottom layer and the phenol will be the top layer. They can then be separated, HCl is added, and then a solid is extracted using vacuum filtration.
Suppose that just before adding a drying agent to an organic solvent that was used to extract an aqueous solution, you notice that there are still tiny water droplets in the organic layer. Can you proceed with adding drying agent? what should you do?
2. If there are small water droplets, you cannot add the drying agent. The visible water droplets should be removed with a pipet. The drying agent can only be used to remove trace amounts.
Potassium carbonate is an excellent drying agent, but it should not be used with some classes of organic compounds. Would it be a better choice to use in drying an ether solution containing an acid or a base? Why?
2. It would be best to do it with a base because potassium carbonate is basic and an acid would allow potassium carbonate to deprotonate.
When two immiscible solids are mixed, two layers form. Liquid liquid extractions commonly use organic solvents that are less dense than aqueous solutions, and therefore form the top layer. An important exception to this rule is chlorinated solvents, which are often more dense than aqueous solvents, and form the bottom layer. Suppose you were unsure which layer was which during your extraction. What simple method do you think you could use to determine which of the two layers was the aqueous layer?
2. To see which layer is aqueous, it would be useful to add a drop of water to the solution to see whether that goes to the top or bottom layer.
Find literature melting points for the following pairs of compounds: ferrocene and acetylferrocene, adipic acid and citric acid
The melting point of ferrocene: 173-174 °C and the melting point of acetylferrocene is 81-83 °C. The melting point of adipic acid is 151-154 °C and the melting point of citric acid 153-155 °C.
Given a solution of 10 mg of traumatic acid dissolved in 100 mL of water, and using the partition coefficient given in question one, show that extracting the 100 mL of aqueous solution with two 10mL portions of MtBE would recover more of the traumatic acid from the aqueous solution than one extraction using 20mL of MtBE.
see lab 2 postlab 2
Give an equation for each of the organic salts below, showing how you would recover the parent compounds:
see lab 2 postlab 3
