The Substitution Reactions Organic Chemistry Lab Report

Virtual Lab: Substitution Reactions, SN2

Part 1 NC State Virtual Lab

Part 2 Beyond Labz Experiment

Part 1 NC State Virtual Lab (12 points)

Watch the NC State virtual lab https://go.ncsu.edu/vrlab-sn2-reactions

You can also view the lab on YouTube with speed and caption options. https://go.ncsu.edu/vrlab-sn2-reactions-captions Answer the questions as you follow along.

Questions

  1. Predict the reactivity of the following alkyl bromides in a substitution reaction with NaI in acetone. Rank them from 1-3 with 1 being the fastest. You will not be graded on the correctness of this answer. (0.5 pt)
  2. Circle the molecule(s) that reacted in the substitution experiment? (0.5 pt)
  3. Explain how a positive reaction was observed in the experiment. What chemical was observed? (0.5)
  4. Rank the molecules from 1-3 based on their rate of reactivity with NaI with 1 being the fastest. Explain the theory behind your ranking of each compound. (1 pt)
  5. Predict the reactivity of the following alkyl bromides in a substitution reaction with NaI in acetone. Rank them from 1-2 with 1 being the fastest. You will not be graded on the correctness of this answer. (0.5 pt)
  6. Circle the molecule(s) that reacted in the substitution experiment? (0.5 pt)
  7. Rank the molecules from 1-2 based on their rate of reactivity with NaI with 1 being the fastest. Explain the theory behind your ranking of each compound. (1 pt)
  8. Predict the reactivity of the following alkyl bromides in a substitution reaction with NaI in acetone. Rank them from 1-2 with 1 being the fastest. You will not be graded on the correctness of this answer. (0.5 pt)
  9. Circle the molecule(s) that reacted in the substitution experiment? (0.5 pt)
  10. Rank the molecules from 1-2 based on their rate of reactivity with NaI with 1 being the fastest. Explain the theory behind your ranking of each compound. (1 pt)
  11. Draw the mechanism and products for the following reaction. (2 pts)
  12. Which of the following reactions is the fastest? Explain your answer. (1 point)

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  1. Rank the molecules from 1-3 based on their rate of reactivity with NaI with 1 being the fastest. Explain the theory behind your ranking of each compound. (1 point)
  2. Rank the molecules from 1-3 based on their rate of reactivity with NaI with 1 being the fastest. Explain the theory behind your ranking of each compound. (1.5 point)

Part 2 Beyond Labz Experiment (8 points)

Figure 1 The conversion of 1-chlorobutane to 1-butanol.

Introduction: In a substitution reaction, a good leaving group (a weak base such as a halide ion), can be replaced by a nucleophile. There are two substitution pathways: SN1 (tertiary and sometimes secondary substrates) and SN2 (methyl, primary, and secondary substrates). There is a summary of SN2 vs SN1 reactions and all the governing factors on last page of this experiment. The SN2 substitution pathway is a concerted substitution, where the nucleophile attacks the electrophilic carbon atom and displaces the leaving group. In this experiment, you will be studying the SN2 reaction of 1-chlorobutane to form 1-butanol. The mechanism for a similar SN2 reaction is shown in figure 2.

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Figure 2 The concerted (SN2) substitution reaction of bromomethane to form methanol. (From Organic Chemistry by Bruice, 8th Ed.)

The SN2 reaction is one-step, and thus the reaction profile diagram shows only one activation energy barrier. Steric crowding in the transition state raises its energy and is the reason why this reaction proceeds the fastest with methyl or primary alkyl halides, such as 1-chlorobuane.

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Figure 3 The reaction profile diagrams for SN2 reactions. (From Organic Chemistry by Bruice, 8th Ed.)

The SN1 substitution pathway is a stepwise substitution, where the substrate first ionizes to form a stable tertiary carbocation, and then the carbocation is attacked by a nucleophile (Figure 4). Only substrates that can form stable carbocations can go through the SN1 mechanism. The rate determining step is formation of the carbocation intermediate (Figure 5).

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Figure 4 The mechanism for the hydrolysis of 2-bromo-2-methylpropane. (From Organic Chemistry by Bruice, 8th Ed.)

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Figure 5 The reaction profile diagram for an SN1 reaction. (From Organic Chemistry by Bruice, 8th Ed.)

Several common nucleophiles/bases are shown in figure 6. Either water (H2O) or hydroxide ion (OH) could be used in a substitution reaction to form an alcohol product.

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Figure 6 Common nucleophiles/bases. (From Organic Chemistry by Bruice, 8th Ed.)

A weaker nucleophile, such as water, is necessary for an SN1 reaction to prevent the competing Elimination (E2) reaction from occurring. Primary alkyl halides, such as 1-chlorobutane do not undergo E2 reactions as readily as tertiary alkyl halides, and so a stronger nucleophile like hydroxide ion can be used.

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Figure 7 Relationship between charge and nucleophilicity/basicity. (From Organic Chemistry by Bruice, 8th Ed.)

In this experiment, you will be conducting three trials of this reaction: two using water as the solvent/nucleophile (room temperature and reflux), and one using hydroxide ion as the nucleophile (in water solvent at room temperature). You are investigating the effect of nucleophile strength on the SN2 reaction. Can a weaker nucleophile such as water be used?

The conversion of 1-chlorobutane to 1-butanol.

Part 1: Collecting the IR and 13C NMR spectrum of the starting material 1-chlorobutane.

  1. To load the synthesis lab, under “Organic Worksheets”, select “Substitution Elimination” and then “VCL 4-2: Nucleophilic Substitution-1”.
  2. On the chalkboard (top right) mouse over “Alkyl Halide Solvolysis”, and you will see the available chemicals. You will notice that there are 5 different alkyl chlorides available from the “stockroom”, so be sure to select the correct starting alkyl chloride to record the IR spectrum.
  3. Clicking and dragging adds reagents into the flask. Click on the 1-chlorobutane and drag it into the flask.
  4. Check to see that only 1-chlorobutane has been added to the flask by hovering your mouse over the flask. The contents of the flask will display on the chalkboard.
  5. Drag the flask over to the clamp above the stirring hot plate.
  6. Record the IR spectrum of the 1-chlorobutane by clicking on the IR spectrometer and dragging the salt plate icon to the flask. This will display the IR spectrum on the screen. You can type the name of the compound on the spectrum and click save to save it to your lab notebook. Click ok to close the spectrum.
  7. By default, the NMR is set to record 1H- change to 13C by clicking the window on the NMR spectrometer. Record the 13C NMR spectrum of the starting material by clicking on the NMR spectrometer and dragging the sample tube to the flask. This will display the 13C NMR spectrum on the screen. You can type the name of the compound on the spectrum and click save to save it to your lab notebook. To view the chemical shift values (in ppm) mouse over each peak- the first number is the chemical shift. Click ok to close the spectrum.
  8. Reset the synthesis lab by clicking on the waste container to “clear lab”.

Part 2: The conversion of 1-chlorobutane to 1-butanol.

  1. On the chalkboard (top right) mouse over “Alkyl Halide Solvolysis”, and you will see the available chemicals. You will notice that there are 5 different alkyl chlorides available from the “stockroom”, so be sure to select the correct starting alkyl chloride.
  2. Clicking and dragging adds reagents into the flask. Click on the 1-chlorobutane and drag it into the flask.
  3. Click on the water and add it to the flask.
  4. Check to see that the reagent and the solvent have been added to the flask by hovering your mouse over the flask. The contents of the flask will display on the chalkboard.
  5. Drag the flask over to the clamp above the stirring hot plate.
  6. For this first trial, the reaction will be attempted at room temperature using water as both the solvent and the nucleophile.
  7. Start the reaction by clicking the right knob on the stirring hotplate.
  8. Advance the clock 5 hours, note the contents of the flask that are displayed on the chalkboard.
  9. Reset the synthesis lab by clicking on the waste container to “clear lab”.
  10. Repeat steps 9-13 to place the 1-chlorobutane and the water in the flask.
  11. This second trial will be conducted at high temperature using water as both the solvent and the nucleophile.
  12. Click and drag the heating mantle below the flask.
  13. To prevent the solvent from evaporating, add a reflux condenser to the flask.
  14. Notice that the top of the condenser is sealed with a rubber septum. If we heat a closed system, it will explode! Click and drag the nitrogen line to the top of the condenser.
  15. Start the reaction by clicking the right knob on the stirring hotplate.
  16. Advance the clock 5 hours, note the contents of the flask that are displayed on the chalkboard.
  17. Reset the synthesis lab by clicking on the waste container to “clear lab”.
  18. Repeat steps 9-13 to place the 1-chlorobutane and the water in the flask.
  19. Add potassium hydroxide to the flask by clicking and dragging from the reagent bottle labeled “KOH”.
  20. This third trial will be conducted at room temperature using hydroxide as the nucleophile and water as the solvent.
  21. Check the chalkboard and verify that your flask contains the correct alkyl halide, the water solvent, and the potassium hydroxide.
  22. Start the reaction by clicking the right knob on the stirring hotplate.
  23. Advance the clock until the chalkboard shows that the starting material has been consumed and note the amount of time (in minutes) that it took for the reaction to go to completion.
  24. Stop the reaction by dragging the separatory funnel over to the flask. The chalkboard will display what is in the flask, which should be the product alcohol, the by-product potassium chloride, and water.
  25. Add water by clicking and dragging the water to the separatory funnel. After you have added the additional water to your aqueous reaction solution, you will see two layers of liquid in the funnel. There is clearly an assumption that diethyl ether has been added in this step.
  26. Recall that the less dense ether layer (“organic layer”) will float on top of the aqueous layer. If you hover your mouse over the layers, it will display if the layer is organic or aqueous and the chalkboard will display the chemicals contained within the layer.
  27. Remove the lower aqueous layer by clicking and dragging to the cork ring support. Do not throw this layer away- it contains your product!
  28. Remove the ether layer and discard by clicking and dragging the flask to the waste container and the separatory funnel to the rack.
  29. The aqueous layer contains the product alcohol and the by-product potassium chloride. To separate the product (a liquid) from the water and the potassium chloride you will need to perform a distillation.
  30. Drag the flask containing the aqueous layer back to the clamp above the stirring hot plate.
  31. Click and drag the distillation apparatus over to the flask.
  32. Click and drag the nitrogen line to the distillation apparatus.
  33. Start the distillation by clicking the right knob on the stirring hotplate. Mouse over the thermometer and record the temperature (this is the starting temperature).
  34. Advance the clock for 10 minutes, mouse over the thermometer again and record the temperature (this is the collection temperature).
  35. Distilling for 30 more minutes (advance the clock 30 minutes) should remove the lower boiling water, leaving the product alcohol and by-product potassium chloride in the flask.
  36. Empty the collection flask by clicking and dragging it to the waste container.
  37. Continue the distillation by advancing the clock for 10 more minutes, mouse over the thermometer again and record the temperature (this is the collection temperature).
  38. Distilling for 30 more minutes (advance the clock 30 minutes) should remove the product alcohol, leaving the by-product potassium chloride in the flask. Note: In the simulation it appears as though all of the liquid has been removed from the flask, which is known as “distilling to dryness”. This is actually really dangerous, and we should never distill to dryness!
  39. Stop the distillation by clicking and dragging the collection flask (containing the product) to the cork ring support.
  40. Drag the distillation apparatus back to the support and click and drag the flask containing the potassium chloride to the waste container.
  41. Record the IR spectrum of the product by clicking on the IR spectrometer and dragging the salt plate icon to the flask. This will display the IR spectrum on the screen. You can type the name of the compound on the spectrum and click save to save it to your lab notebook. Click ok to close the spectrum.
  42. By default, the NMR is set to record 1H- change to 13C by clicking the window on the NMR spectrometer. Record the 13C NMR spectrum of the product by clicking on the NMR spectrometer and dragging the sample tube to the flask. This will display the 13C NMR spectrum on the screen. You can type the name of the compound on the spectrum and click save to save it to your lab notebook. To view the chemical shift values (in ppm) mouse over each peak- the first number is the chemical shift. Click ok to close the spectrum.
  43. Your virtual experiment is now complete!

Lab Data/Questions

Table 1: Reaction Trials (1 pt) Indicate the time to reaction completion. Write amount of time lapsed + “no reaction” if a reaction did not occur. (ex. 2 hours- no reaction)

 

Time to completion

Room Temp + H2O

 

Reflux + H2O

 

Room Temp + KOH/H2O

 

Q1 Which trials were successful at forming the desired alcohol product? Can a weaker nucleophile, like water, be used for an SN2 reaction? Explain your response. (2 pt)

Table 2: IR Results (1 pt)

If the absorption peak(s) was present in the IR spectrum, fill in the approximate wavenumber(s). If the absorption peak(s) was absent in the IR spectrum, write “absent”.

 

O-H stretch, alcohol

sp3 C-H stretches

1-chlorobutane

  

1-butanol

  

IR spectrum 1-chlorobutane IR spectrum 1-butanol

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Q2 How could the results of the IR spectroscopy be used to indicate that the reaction was successful? (1)

Tables 3 & 4: 13C NMR results

Report the 13C NMR signals from highest chemical shift (signal 1) to lowest chemical shift (signal 4) for the starting alkyl halide and the product alcohol.

13C NMR spectrum 1-chlorobutane: (0.5 pt)

Signal

~ Chemical shift in ppm

1

 

2

 

3

 

4

 

Which signal # corresponds to the carbon attached to chlorine in the starting material? (in ppm) (0.5 pts

13C NMR spectrum 1-butanol: (0.5)

Signal

~ Chemical shift in ppm

1

 

2

 

3

 

4

 

Which signal # corresponds to the carbon attached to oxygen in the product? (in ppm) (0.5 pts)

How could the results of the 13C NMR spectroscopy be used to indicate that the reaction was successful? (1 pt)

A summary of SN2 vs SN1 reactions is on the last page of the experiment.

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