S_N2: Synthesis of 1-Bromobutane

Alkyl halides can be prepared from their corresponding alcohols via an acid catalyzed substitution reaction. The mechanism of these acid catalyzed substitution reactions are labeled as S_N1 (substitution, nucleophilic, unimolecular) and S_N2 (substitution, nucleophilic, bimolecular). Tertiary alcohols follow the S_N1 route, primary alcohols follow the S_N2, route and secondary alcohols can follow either path.

Figure 1 Conversion of 1-butanol into 1-bromobutane by an S_N2 mechanism

The S_N2 reaction for converting 1-butanol to 1-bromobutane involves rapid protonation of the alcohol along with a concerted step where the nucleophile attacks the carbon, displacing water. The backside attack by the nucleophile is favored for primary substrates and less ideal for secondary and tertiary substrates.

The product will be verified by gas chromatography by comparing the chromatograms of the starting material, 1-bromobutane standard, and your product. Confirmation of your product,
1-bromobutane can also be performed by reacting the product with a solution of sodium iodide in acetone. The primary alkyl halide will react by means of an S_N2 mechanism with the sodium iodide to form an insoluble precipitate. The appearance of a precipitate indicates a positive result suggesting the presence of a primary alkyl halide.

OBJECTIVES

In this experiment, you will

• Synthesize 1-bromobutane via an S_N2 reaction.
• Confirm the presence of a primary alkyl halide using the sodium iodide test.
• Verify the product by gas chromatography.

MATERIALS

Part I Synthesis of 1-bromobutane

Part II Analyze by Gas Chromatography

Part III Sodium Iodide Test for Primary Alkyl Halides

PROCEDURE

Part I Synthesizing 1-bromobutane

1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood.

2. Set up the reaction.

Obtain an 18 150 mm test tube and place a boiling stone inside.

Weigh out 2.0 g of sodium bromide and transfer it to the test tube. Record the mass to the nearest 0.001 g.

Add 1 mL of water to the test tube.

Add 2.0 mL of 1-butanol to the test tube.

3. Prepare an ice water bath using the 250 mL beaker and cool the test tube.

4. Slowly add 1.5 mL of concentrated sulfuric acid to the solution in the test tube. CAUTION: Handle concentrated sulfuric acid with care. Can cause painful burns if it comes in contact with the skin.

5. Clamp the test tube to a ring stand and lower into the sand bath in the heating mantle.

6. Gently heat the mixture to boiling. Allow the mixture to reflux for one hour. Note: When refluxing the mixture, do not let the condensation ring rise higher than 2 cm to avoid the loss of reactants and product.

7. After the reflux is complete, carefully remove the test tube from the sand bath and allow it to cool.

8. Remove the lower acid layer using a Pasteur pipet. Transfer this layer to your 250 mL waste beaker.

9. Wash your synthesized 1-bromobutane with 2 mL of distilled water and remove the water with a Pasteur pipet. Discard the wash in the waste beaker. Note: 1-Bromobutane is denser than water.

10. Repeat the wash with 2 mL of a saturated sodium bicarbonate solution. Discard the aqueous layer in the waste beaker.

11. Repeat the wash with 2 mL of a saturated sodium chloride solution. Discard the aqueous layer in the waste beaker.

12. Add a pea-sized amount of anhydrous sodium sulfate to the test tube to dry your synthesized
1-bromobutane. Allow the product to dry for five minutes.

13. Carefully pipet your product into a tared 5 mL vial. Record the mass in your data table.

Part II Analyze the Product by Gas Chromatography

14. Obtain 1-butanol and 1-bromobutane standards for analysis. Be sure to label the vials.

15. Prepare the Vernier Mini GC for data collection:

1. Turn on the Mini GC and connect it to the USB port on your computer or LabQuest.
2. Choose New from the File menu of the data collection program.
3. To bring up the Temperature-Pressure profile, tap ► in LabQuest or click Collect in Logger Pro.
4. Set the Temperature-Pressure values to:

1. Select Done to initiate the Mini GC warm up. Continue with Step 16 during warm up.

16. Clean and flush the syringe with acetone. Important: The glass syringe and needle are fragile. Handle with care. Never pull the plunger back more than 50% of its total volume.

17. Follow the process in Step 16 to clean and flush the syringe with 2-butanone.

18. Prepare for injection and the start of data collection.

1. Collect 0.2 L of 1-butanol for injection.
2. The Mini GC should now have reached the correct start temperature and pressure and the LED turned to green.
3. Insert the needle of the syringe into the injection port as shown in Figure 2.
4. Simultaneously, depress the syringe plunger and select Collect to begin data collection. Pull the needle out of the injection port immediately.

19. While the data collection proceeds, repeat Step 16 to thoroughly clean the syringe and needle.

Figure 2

20. Analyze your chromatogram to determine the retention time and record your result.

21. Save the file, if desired.

22. Repeat Steps 1721 for the 1-bromobutane standard and your synthesized 1-bromobutane.

23. When you have completed your final data-collection run, turn off the Mini GC.

Part III Sodium Iodide Test for Primary Alkyl Halides

24. Set up a 50°C hot water bath in a 250 mL beaker. Monitor the temperature using a
Wide-Range Temperature Probe or thermometer.

25. Set up the reaction.

1. Obtain three 13 100 test tubes and label 13.
2. Pipet 10 drops of distilled water into Test Tube 1.
3. Pipet 5 drops of 1-bromobutane into Test Tube 2
4. Pipet 10 drop of your product into Test Tube 3.

26. Add approximately 1 mL of the 15% sodium iodide in acetone solution to each test tube. Swirl each test tube to mix the contents. The presence of a precipitate indicates the presence of a 1° halide. Record your observations in the data table.

27. If no precipitate is observed after five minutes, place the test tubes in the hot water bath. After five minutes, remove the test tubes and cool to room temperature. Record your observations. Note: Do not boil the solutions.

28. At the end of the experiment, discard the solutions as directed by your instructor.

DATA TABLE

Part I Synthesizing 1-bromobutane

Part II Analyze the Product by Gas Chromatography

Part III Sodium Iodide Test for Alkyl Halides

DATA ANALYSIS

1. What is the theoretical yield of 1-bromobutane in your synthesis? What is the actual yield?

2. Why is the S_N2 mechanism favored for primary substrates and less likely for a tertiary substrate?

3. What experimental evidence proves your product is 1-bromobutane?

S_N2: Synthesis of 1-Bromobutane

Alkyl halides can be prepared from their corresponding alcohols via an acid catalyzed substitution reaction. The mechanism of these acid catalyzed substitution reactions are labeled as S_N1 (substitution, nucleophilic, unimolecular) and S_N2 (substitution, nucleophilic, bimolecular). Tertiary alcohols follow the S_N1 route, primary alcohols follow the S_N2, route and secondary alcohols can follow either path.

Figure 1 Conversion of 1-butanol into 1-bromobutane by an S_N2 mechanism

The S_N2 reaction for converting 1-butanol to 1-bromobutane involves rapid protonation of the alcohol along with a concerted step where the nucleophile attacks the carbon, displacing water. The backside attack by the nucleophile is favored for primary substrates and less ideal for secondary and tertiary substrates.

The product will be verified by gas chromatography by comparing the chromatograms of the starting material, 1-bromobutane standard, and your product. Confirmation of your product,
1-bromobutane can also be performed by reacting the product with a solution of sodium iodide in acetone. The primary alkyl halide will react by means of an S_N2 mechanism with the sodium iodide to form an insoluble precipitate. The appearance of a precipitate indicates a positive result suggesting the presence of a primary alkyl halide.

OBJECTIVES

In this experiment, you will

• Synthesize 1-bromobutane via an S_N2 reaction.
• Confirm the presence of a primary alkyl halide using the sodium iodide test.
• Verify the product by gas chromatography.

MATERIALS

Part I Synthesis of 1-bromobutane

Part II Analyze by Gas Chromatography

Part III Sodium Iodide Test for Primary Alkyl Halides

PROCEDURE

Part I Synthesizing 1-bromobutane

1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood.

2. Set up the reaction.

Obtain an 18 150 mm test tube and place a boiling stone inside.

Weigh out 2.0 g of sodium bromide and transfer it to the test tube. Record the mass to the nearest 0.001 g.

Add 1 mL of water to the test tube.

Add 2.0 mL of 1-butanol to the test tube.

3. Prepare an ice water bath using the 250 mL beaker and cool the test tube.

4. Slowly add 1.5 mL of concentrated sulfuric acid to the solution in the test tube. CAUTION: Handle concentrated sulfuric acid with care. Can cause painful burns if it comes in contact with the skin.

5. Clamp the test tube to a ring stand and lower into the sand bath in the heating mantle.

6. Gently heat the mixture to boiling. Allow the mixture to reflux for one hour. Note: When refluxing the mixture, do not let the condensation ring rise higher than 2 cm to avoid the loss of reactants and product.

7. After the reflux is complete, carefully remove the test tube from the sand bath and allow it to cool.

8. Remove the lower acid layer using a Pasteur pipet. Transfer this layer to your 250 mL waste beaker.

9. Wash your synthesized 1-bromobutane with 2 mL of distilled water and remove the water with a Pasteur pipet. Discard the wash in the waste beaker. Note: 1-Bromobutane is denser than water.

10. Repeat the wash with 2 mL of a saturated sodium bicarbonate solution. Discard the aqueous layer in the waste beaker.

11. Repeat the wash with 2 mL of a saturated sodium chloride solution. Discard the aqueous layer in the waste beaker.

12. Add a pea-sized amount of anhydrous sodium sulfate to the test tube to dry your synthesized
1-bromobutane. Allow the product to dry for five minutes.

13. Carefully pipet your product into a tared 5 mL vial. Record the mass in your data table.

Part II Analyze the Product by Gas Chromatography

14. Obtain 1-butanol and 1-bromobutane standards for analysis. Be sure to label the vials.

15. Prepare the Vernier Mini GC for data collection:

1. Turn on the Mini GC and connect it to the USB port on your computer or LabQuest.
2. Choose New from the File menu of the data collection program.
3. To bring up the Temperature-Pressure profile, tap ► in LabQuest or click Collect in Logger Pro.
4. Set the Temperature-Pressure values to:

1. Select Done to initiate the Mini GC warm up. Continue with Step 16 during warm up.

16. Clean and flush the syringe with acetone. Important: The glass syringe and needle are fragile. Handle with care. Never pull the plunger back more than 50% of its total volume.

17. Follow the process in Step 16 to clean and flush the syringe with 2-butanone.

18. Prepare for injection and the start of data collection.

1. Collect 0.2 L of 1-butanol for injection.
2. The Mini GC should now have reached the correct start temperature and pressure and the LED turned to green.
3. Insert the needle of the syringe into the injection port as shown in Figure 2.
4. Simultaneously, depress the syringe plunger and select Collect to begin data collection. Pull the needle out of the injection port immediately.

19. While the data collection proceeds, repeat Step 16 to thoroughly clean the syringe and needle.

Figure 2

20. Analyze your chromatogram to determine the retention time and record your result.

21. Save the file, if desired.

22. Repeat Steps 1721 for the 1-bromobutane standard and your synthesized 1-bromobutane.

23. When you have completed your final data-collection run, turn off the Mini GC.

Part III Sodium Iodide Test for Primary Alkyl Halides

24. Set up a 50°C hot water bath in a 250 mL beaker. Monitor the temperature using a
Wide-Range Temperature Probe or thermometer.

25. Set up the reaction.

1. Obtain three 13 100 test tubes and label 13.
2. Pipet 10 drops of distilled water into Test Tube 1.
3. Pipet 5 drops of 1-bromobutane into Test Tube 2
4. Pipet 10 drop of your product into Test Tube 3.

26. Add approximately 1 mL of the 15% sodium iodide in acetone solution to each test tube. Swirl each test tube to mix the contents. The presence of a precipitate indicates the presence of a 1° halide. Record your observations in the data table.

27. If no precipitate is observed after five minutes, place the test tubes in the hot water bath. After five minutes, remove the test tubes and cool to room temperature. Record your observations. Note: Do not boil the solutions.

28. At the end of the experiment, discard the solutions as directed by your instructor.

DATA TABLE

Part I Synthesizing 1-bromobutane

Part II Analyze the Product by Gas Chromatography

Part III Sodium Iodide Test for Alkyl Halides

DATA ANALYSIS

1. What is the theoretical yield of 1-bromobutane in your synthesis? What is the actual yield?

2. Why is the S_N2 mechanism favored for primary substrates and less likely for a tertiary substrate?

3. What experimental evidence proves your product is 1-bromobutane?

SN2- Synthesis of 1-bromobutane

VIDEO LINK

https://www.youtube.com/watch?v=Y4bC_buRAmk

DATA

Questions/Analysis: As given in the Vernier handout.

SN2- Synthesis of 1-bromobutane

VIDEO LINK

https://www.youtube.com/watch?v=Y4bC_buRAmk

DATA

Questions/Analysis: As given in the Vernier handout.