Question

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Determine the limiting reagent for the preparation of 2-bromo-2-methylbutane and calculate the theoretical yield of product.

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CHEM-2223 Organic Chemistry Laboratory I
Exp. 7: Synthesis of 2-chloro-2-methylbutane
In this reaction a tertiary alcohol, 2-methyl-2-butanol, is converted to a tertiary halide, 2-chloro-
2-methylbutane through a SN1 reaction.
1) Chemicals
 2-Methyl-2-butanol
 Hydrochloric acid, HCl (12 M)
 Saturated sodium bicarbonate
 Saturated sodium chloride (380 g/L)
 Anhydrous sodium sulfate
2) Required materials
1. 25-mL round-bottom flask
2. 125-mL separatory funnel
3. Simple distillation apparatus
3) Procedure for the experiment
1. Put 10 mL 2-methyl-2-butanol in a separatory funnel
2. Add 25 mL concentrated (12 M) HCl (wear gloves and do this in the fume
hood)
3. Swirl gently for about 1 minute without stopper to mix the reactants
4. Stopper and carefully invert
5. Release pressure by carefully opening the stopcock
6. When all pressure is released, close stopcock and shake for several minutes
7. Vent periodically to release pressure
8. Let mixture separate in two layers
9. Separate aqueous and organic layer
a) What is the organic layer?
b) What is the aqueous layer? (DO NOT DISCARD! SAVE AS AQUEOUS
LAYER!)
10. Wash organic layer by adding 10-mL portions of cold saturated sodium bicarbonate
CH3CH2COH + HCl
CH3
CH3
CH3CH2CCl +H2O
CH3
CH3
2
11. Vigorous gas evolution will happen. Let the gas escape from the unstoppered separatory funnel. Swirl gently until gassing stops
12. When gas evolution stops, stopper, and carefully invert. Vent immediately
13. Shake gently with frequent venting, then shake vigorously with frequent venting
14. Separate organic layer and wash again with 10-mL portions of water and saturated sodium chloride (DO NOT DISCARD AQUEOUS LAYER!)
15. Remove aqueous layers (DO NOT DISCARD!)
16. Put organic layer in Erlenmeyer flask and dry with anhydrous sodium sulfate
17. Swirl Erlenmeyer flask for 10-15 minutes until the product is dry. Add additional sodium sulfate if the liquid is cloudy
18. Carefully decant in a weighed container
19. Determine the weight of the product and calculate reaction yield
20. Analyze with the sodium iodide/acetone and ethanolic silver nitrate tests (see below).
When a more purified product is required for IR or GC analysis, the product obtained in the previous steps needs to be further purified by distillation.
Distillation
1. Carefully transfer the crude product with a Pasteur pipet to a round-bottom flask
2. Add stirbar and set-up for “shortpath” distillation (see Fig. 1)
3. Distill the product and collect in the product in a pre-weighed container in an ice bath
4. Collect the fraction that has a boiling point higher than 75oC (760 torr)
5. Boiling point for 2-chloro-2-methyl-butane: 85-86oC
6. Determine mass of distillate
7. Calculate reaction yield
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Figure 1: Shortpath Distilllation
Analysis
Silver nitrate, ethanolic solution
To prepare a 0.1 M ethanolic silver nitrate solution, dissolve 0.34 g of silver nitrate in 20 mL of 95% ethanol.
a) Add 1 drop alkyhalide to 2 mL of the ethanolic silver nitrate solution
b) Let stand for 5 min. If no precipitate, warm in a beaker of boiling water
c) Note color of precipitate
a. Chloride: white, turning purplish
b. Bromide: pale yellow
c. Iodide: dark yellow
Sodium iodide in acetone solution
To prepare the test solution, dissolve 3 g of sodium iodide in 20 mL of acetone. Keep the solution in a dark bottle and discard it when a red-brown color appears.
a) Add 2 drops of alkyl halide to 1 mL of sodium iodide-acetone solution
b) Shake tube and let stand for 3 min
c) Observe formation of precipitate
d) If no precipitate, warm in beaker with water @ 50oC
a. Primary bromides: precipitate
b. Secondary and tertiary bromides: precipitate after heating
c. Primary and secondary chlorides: precipitate after heating
d. Tertiary chlorides: no reaction
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Cleaning up
1) Dilute aqueous layers form 1st separation with H2O; neutralize with Na2CO3
2) Combine with other aqueous layers and discard in “Aqueous Waste”
3) Put sodium sulfate used for drying in “Organic Waste”
4) Dilute silver nitrate test solution with water and put in “Aqueous Waste”
5) Discard sodium iodide/acetone test solution in “Organic Waste”
Physical Properties
COMPOUND Molar mass Density (g/mL) b.p. (oC) m.p. (oC) 2-methyl-2-butanol 88.15 0.805 g/mL 101-103oC -9oC 2-chloro-2-methylbutane 106.59 0.866 g/mL 85-86oC -73oC Hydrochloric acid (37%) 36.46 1.188 g/mL 51oC -25oC Water 1.00 100 0
Practical and Safety Aspects of Synthesis of 2-Chloro-2-Methylbutane
 Concentrated hydrochloric acid is very corrosive and can cause severe chemical burns. Wear gloves, eye protection and a lab coat.
 Do not add concentrated acid too fast. It will heat up and may splatter.
 The aqueous phase needs to be neutralized before putting in the aqueous waste.
 DO NOT throw any layer away before you re ABSOLUTELY sure that you do not need them again.
 Check all layers before discarding them. It is not possible to retrieve them after discarding!
 Do NOT use excess drying agent, as this may reduce the yield of the reaction.
 Do not distill until dryness. It may result in cracked glassware.

Answer 1

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