CHEM 341 Western Kentuky University Organic Chemistry Lab Report

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CHEM 341

Western Kentuky University

CHEM

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ACID / BASE EXTRACTIONS Student learning objectives – What you will learn from this experiment • Use acid-base chemistry to increase water-solubility of organic compounds • Perform liquid-liquid extractions • Safely handle diethyl ether (a volatile, explosive solvent) Experiment • Strategy o In this experiment, you will separate a mixture of three organic compounds using acid/base chemistry and liquid-liquid extractions. The three compounds are naphthalene, benzoic acid and 3-nitroaniline; these compounds are neutral, acidic, and basic, respectively. All of the compounds are initially soluble in diethyl ether, and all of these compounds initially prefer ether over water. You will change the solubility of the reactive compounds in the mixture one at a time by adding acid or base to create salts that can be selectively taken into the water layer during a liquid-liquid extraction. You will then neutralize the salts of your reactive compounds in water to encourage precipitation of the acidic/basic compounds. Yields are calculated and compared with your starting masses of materials. • Procedure o Part I – Dissolve the mixture in diethyl ether 1. Dissolve 180 mg of the acid/base/neutral mix (1:1:1 by mass) into 4 mL of diethyl ether in a test tube (test tube A). Gently swirl to dissolve the solids. 2. If the mixture does not completely dissolve into a clear solution, add 0.5 mL of diethyl ether and swirl. Repeat this step until the mixture dissolves completely. There should be no cloudiness or floating particles. o Part II – Extract the reactive components 1. Using a Pasteur pipet, add 1 mL of 3 M HCl solution into test tube A. You will then use the pipet to draw the solution in and out of the pipet to facilitate mixing. Do this for 2-3 minutes. 2. After the solution has been completely mixed, allow the solution to separate into 2 layers. You will then draw the aqueous layer into a Pasteur pipet and transfer into a new test tube (test tube B). (Check your table of physical constants to help identify layers.) 3. To maximize your yield of the 3-nitroaniline, repeat steps 1 & 2 above two more times and add the aqueous layers into test tube B. 4. Test tube A now contains only the acidic and neutral component. Add 1 mL of sodium bicarbonate solution into test tube A. You will then use the pipet to draw the solution in and out of the pipet to facilitate mixing. Do this for 2-3 minutes. 5. After the solution has been completely mixed, allow the solution to separate into 2 layers. You will then draw the aqueous layer into a Pasteur pipet and transfer into a new test tube (test tube C). 6. To maximize your yield of the benzoic acid, repeat steps 4 & 5 above two more times and add the aqueous layers into Test tube C. 7. Test tube A now only contains the neutral component and diethyl ether. o Part III – Isolate the neutral component 1. Take test tube A to the hood designated for diethyl ether evaporation. 2. Evaporate the diethyl ether by gently using the air nozzle of the fume hood to completely dry the sample in test tube A. Your TA will demonstrate how to do this. 3. Once completely dry, scrape the solid onto a tared piece of weighing paper and record the mass. You will use this mass to calculate a percent yield for the neutral portion. Save the solid for characterization in the final step of the experiment. 4. Note, if the solid is not completely dried using the air stream, try using vacuum filtration to draw air over the sample and dry the naphthalene. o Part IV – Isolate the acidic component 1. Acidify the solution in test tube C. Begin by slowly adding 3M HCl dropwise, draw the solution in and out of a second pipet to facilitate mixing every few drops, and then test the acidity with a pH paper strip. Add more HCl dropwise and mix until the pH paper ACID / BASE EXTRACTIONS o o • • • gives a strongly acidic reading. Note, that you should place a very small drop of the solution onto the pH paper – do not “dip” the pH paper into your solution. You may need to repeat this step several times. 2. Benzoic acid will form a white precipitate at the bottom of the test tube at low pH (acidic). 3. Collect the benzoic acid by vacuum filtration and continue to vacuum until completely dried. 4. Scrape the solid obtained from the filter paper onto a tared weighing paper and record the mass. You will use this mass to calculate percent yield. Save the solid for characterization in the final step of the experiment. Part V – Isolate the basic component 1. Basify the solution in test tube B. Begin by adding 1 mL of 3M NaOH dropwise, draw the solution in and out of a pipet to facilitate mixing, and then test the acidity with pH paper. Add more NaOH dropwise until the pH paper gives a strongly basic reading. You may need to repeat this step several times. 2. 3-nitroaniline will form a yellow precipitate at high (basic) pH. 3. Collect the 3-nitroanaline by vacuum filtration and continue to vacuum until completely dried. 5. Scrape the solid obtained from the filter paper onto a tared weighing paper and record the mass. You will use this mass to calculate percent yield. Save the solid for characterization in the final step of the experiment. Part VI – Characterize your products 1. Spot each isolated product versus the starting mixture on a TLC plate and elute using hexane/isopropanol (90:10) as the mobile phase. Use the eluent to spot your plates. Special safety concerns o Always use slow addition of acid or base to avoid violent boiling of the solution. o Keep all ether solutions in the designated hood. Keep that sash as low as possible. Absolutely no heat sources are to be used in the ether hood. Remember – ether is explosive. o Never point the open end of a test tube at anyone, including yourself. Avoid rapid boiling. Student notes o Think about what chemical reactions are taking place when you add acid or base at each step. Make sure that you can write a mechanism for each of these reactions. Prep / TA notes o TAs – demonstrate evaporation of diethyl ether in a hood without any heat sources. o Please prepare the mobile phase for students (90:10 hexane:isopropanol). Questions to test your understanding that will help you with your report and exams: • Explain your observations in terms of what is happening at the molecular level. I.e., Why is base added to the solution, and what reaction(s) occurred? Why does benzoic acid precipitate after acidifying the solution? What happened when acid was added to 3-nitroanaline? • Data analysis – report mass yield and percent yield for each component. Report TLC results. Name: _____________________________________________________________ ACID-BASE EXTRACTIONS CHEM 341 Table of physical constants (5 points): (tabulate name, structure, molar mass, density (liquids only), melting point, boiling point, and safety hazards for all chemicals mentioned in the procedure) Observations: (Include pictures of TLC plate) Tabulated data (10 points): (construct a table including name of compound, distance spot traveled, distance solvent traveled, calculated Rf values for each spot visualized, amount (g) material recovered, and percent recovered for each material) Answer the questions below (25 points): 1. Which compound was primarily recovered in each test tube (A, B, and C)? How much of each material did you recover? Where is the rest of the material? How could you recover more material from each tube? 2. Explain any anomalies that you observed in your TLC plate (e.g., streaking or large spots). 3. Discuss the purity of each compound isolated and use your data to support your claims. 4. How would you change the procedure to increase the yield of any of the three materials? 5. How would you change the procedure to increase the purity of any of the three materials?
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Name: _____________________________________________________________

ACID-BASE EXTRACTIONS
CHEM 341
Table of physical constants (5 points): (tabulate name, structure, molar mass, density
(liquids only), melting point, boiling point, and safety hazards for all chemicals mentioned
in the procedure)
Name

Structure

Naphthalene

Molar mass

Melting
point
128.17Kj/mol 80.2 °C

Boiling
point
217.9
°C

Density
-----------

Safety hazards



Benzoic acid

122.12

122.4 °C

249.2
°C

------------




3Nitroaniline

114.0 °C
138.12

306.0
°C

--------





Hydrochloric HCl
acid

36.46

114.22°C

-85.05
°C

1.639 g/L




NaHCO3

84.007

50 °C

------

Sodium
hydroxide

NaOH

39.997

323 °C

1388 °C -------




68.7 °C




Hexane

86.18

-95.3 °C

---------



Sodium
bicarbonate

0.66g/ml

Harmful if
swallowed
Suspected of
causing cancer
Causes skin
irritation
Causes serious
eye damage
Toxic if
swallowed, in
contact with
skin or if
inhaled
Causes
damage to
organs
through
prolonged or
repeated
exposure...


Anonymous
Very useful material for studying!

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