Chemistry 2633 Techniques of Organic Chemistry James S
Chemistry 2633 Techniques of Organic Chemistry James S Chickos Department of Chemistry and Biochemistry University of Missouri-St. Louis Louis MO 63121 E-mail: [email protected] Teaching Assistants: Wed 12:00-4:30: Ryan cantwell Thurs 12:00-4:30: Andrew Kamadulske John Zhou Thurs 5:00-9:30: Matt Queensen Fall 2016
CH3 C H2SO4 OH O Acetic anhydride: irritating liquid Acetic acid: also irritating (vinegar is a ~5% solution of acetic acid in water)
Sulfuric acid: concentrated sulfuric acid is a strong dehydrating liquid All work is done in a Hood! CO2H (CH3CO)2O CO2H + CH3CO2H OH H2SO4
OCCH3 O 1. Mix the appropriate reagents and heat on a water bath (90+ C); 2. Cool in ice and scratch until solid forms; 3. Add cold water and break up solid; 4. Filter and wash with cold water; 5. Dissolve the wet solid aspirin in NaHCO3; O O OCCH3
NaHCO3 OCCH3 + CO2 CO2H solid CO2 -1
in aqueous solution Filter solution then add HCl to the filtrate to a pH 1 and then filter again O OCCH3 CO2H solid
+ CO2 A likely structure of the tacky polymer O O O O O O
O O O O O CO2H (CH3CO)2O CO2H
+ CH3CO2H OH H2SO4 OCCH3 O 1. Mix the appropriate reagents and heat on a water bath (> 90 C). 2. Cool in ice and scratch until solid forms; 3. Add cold water and break up solid; 4. Vacuum filter and wash with cold water; 5. Dissolve the wet aspirin in NaHCO3;
6. Filter to remove polymer (tacky insoluble material); (save liquid) 7. Acidify liquid to pH ~ 1, filter (vacuum filtration again) 8. Allow to dry until next week. End of first lab Typical Calculations in Organic Chemistry Laboratory CO2H (CH3CO)2O CO2H + CH3CO2H OH OCCH3
salicylic acid: MW = 138 material used: 3.0 g acetic anhydride: MW =102 material used: 5.0 mL density: 1.10 g/mL aspirin isolated: 3.0 g What is the limiting reagent? What is the yield? O CO2H (CH3CO)2O
102 g/mol CO2H + CH3CO2H OH OCCH3 3 g used 138 g/mol 3g/138g/mol = 0.0217 mol SA 3 g isolatedO
180 g/mol 5 mL*1.10 g/mL = 5.5 g/102 = 0.0539 mol AA The limiting reagent is SA; Why is SA made the limiting reagent and not AA? The theoretical yield is? 0.0217 mol *180 g/mol = 3.906 g Aspirin % Yield: 3.0/3.906 = 0.768*100 = 76.8 % Why is the yield less than 100%? 1. Loss due to transfers 2. assume 100 % reaction 3. assume limiting reagent is 100 % pure
4. solubility of aspirin in water 5. Competing side reactions How do we know that what we isolated is aspirin? 1. One way is by comparing the physical properties of the material we isolated to that of pure aspirin What physical properties? Infrared Spectroscopy OH C=O ester
C=O acid conjugated acid: 1693 cm-1; non-conjugated ester: 1754 cm-1 Common Organic Functional Groups in IR C-H O-H broad band for carboxylic acids N-H C=O ester: 1754 cm-1; carboxylic acid: 1693 cm-1
C=C 1600 cm-1 CC C N Other Physical Properties Melting properties of a pure material melting temperature T / C
first crystal forms, supercooling Added heat remove heat Melting properties of an impure material solid disappears T / C beginning and ending of melting liquid appears
Added heat 1. Melting point of a known material should melt within 1-2 C of the value reported previously. 2. Melting point of a pure material melts sharply; If a compound melts sharply, is it necessarily a pure material? mp (pure A) mp (pure B) A
B C C 100 % A Temperature 100 % B The melting point behavior of a simple binary mixture
liquid + solid liquid region A B Temperature C solid region (60-40)%
100 % A 100 % B time The melting point behavior of a simple binary mixture liquid + solid liquid region A
B Temperature C solid region (40 A-60 B)% 100 % A 0%B 100 % B 0% A
time The melting point behavior of a simple binary mixture A B C C 100 % A
100 % B The melting point behavior of a simple binary mixture A B C eutectic 100 % A 100 % B
time The melting point behavior of a simple binary mixture at the eutectic composition The eutectic point is the lowest temperature solid and liquid exists in equilibrium A B C C
100 % A AB 100 % B The melting point behavior of a more complex mixture A B C
C dl 100 % d-menthol 100 % l-menthol The melting point of a chiral system Many chiral systems exhibit this type of behavior (for example: dl menthol) How do we purify an impure material?
A. solid B. liquid C. gas 1. Solids: recrystallization impure liquid material Solvent molecules not shown Slow crystallization Fast
crystallization Why doesnt the blue material crystallize? When does recrystallization not work? What are the factors affecting the quality of the crystal obtained? 1. rate of crystal growth 2. nature of the solvent and of the impurity Factors affecting the amount of the crystal obtained: 1. solubility of materials in the solvent 2. temperature How to choose a solvent for recrystallization? 1. Choose a solvent similar in polarity to the compounds being
recrystallized -like dissolves like- polar solvents dissolve polar substrates; non-polar substrates dissolve non polar substrates 2. Choose a solvent with a reasonable boiling point Why? Solubility g/mL 25 C Temperature When will crystallization work? 1. If the material is reasonably pure to begin with 90+%.
2. The impurities have different properties including polarity, solubility, structure If solid A has a solubility of 1 g /100mL of solvent at 25 C and 15 g /100mL at the solvents boiling temperature, how much can you recover by recrystallization of 167 g of this material if it is 90% pure? Lets assume the impurity has a similar solubility. If we use a minimum amount of boiling solvent to dissolve this material, how much solvent must we use? 167g *0.9 = 150.3 g of A requires ~1000 mL of boiling solvent At 25 C, 10 g of A will remain in solution along with the impurity. Thus ~140 g will be recovered. In order to achieve good separation, the solid and liquid should be
separated as quantitatively as possible. Any solvent that evaporates on the solid will deposit dissolved materials. Note: 16.7 g of impurity/1000mL or 1.67g/100mL Typical Solvents in Organic Laboratory (in decreasing polarity) Green Solvents water, methanol, ethanol, 1, and 2-propanol, 1-butanol, t-butanol acetone, 2-butanone, ethyl acetate Usable solvents acetic acid, dimethyl sulfoxide, acetonitrile, tetrahydrofuran, toluene, xylenes, cyclohexane, heptane, isooctane Undesirable dimethylformamide, pyridine, dioxane, diethyl ether, diisopropyl ether, chloroform, dichloromethane, carbon tetrachloride benzene, pentane, hexane
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