Nucleophilic+Substitution+Reactions



=__ Nucleophilic Substitution Reactions __= Heather Browning and Merissa Honey

The purpose of this lab is determine if a nucleophilic substitution reaction is observed. A nucleophilic substitution is the reaction of an electron pair donor (nucleophile, Nu) with an electron pair acceptor (electrophile). In the lab an alkyl halide, 2-chloro-2-methyl propane and water were allowed to react in a solvent of isopropanol. Our initial hypothesis was that the reaction would create a acidic product due to the polar protic solvent, isopropanol and the 3 degree carbocation that would result from 2-chloro-2-methyl propane when the two substances were allowed to react. The rate of the reaction is measured by monitoring the production of acid as it proceeds. An acid-base indicator (bromothymol blue) will be added to the mixture and when enough acid has been produced the color indicator will change. Recording and measuring the time required to reach the point of the color change will allow us to estimate the rate of the reaction. We are also, importantly, investigating the effect of the solvent on the rate of reaction. Your report could really benefit from some discussion of this phenomenon here.
 * Introduction:**

1. Tools needed include: five 25 mL Erlenmeyer flasks, five labeled test tubes, test tube rack, five test tube stoppers, stir bar, magnetic stirrer, graduated glass pipets and a stopwatch. 2. Use a pipet to transfer 2.0 mL of 0.1M alkyl halide (2-chloro-2-methyl propane, C4H9Cl) into each of the four labeled test tubes. Stopper the test tubes. Measure as precisely as possible. 3. Similarly, transfer 2.0 mL of 0.1M HCl into a labeled test tube. Stopper the test tube. 4. Label five 25 mL Erlenmeyer flaks in he following manner: flask #1 HCl; flask #2-5 iwth the name of the alkyl halide and the numbers 2, 3, 4, and 5. 5. Place the contents from the table below into the five Erlenmeyer flasks.
 * Procedure:** please cite me as the source of this procedure.

//Table #1 Non-alkyl halide reagents for the reaction// 6. Start the stirrer in flask #1 to create mixing. Pour 2.0 mL of the 0.1M HCl from the test tube into flask #1 and begin timing IN SECONDS. Record the time for the color of the indicator to change from blue to yellow (for C4H9Cl). 7. Wash off the stir bar and transfer it to flask #2. 8. Start the stirrer in flask #2 to create good mixing. Pour 2.0 mL of the 0.1M alkyl halide from one of the test tubes into flask #2 and begin timining in SECONDS. Record the time for the color of the mixture to change from blue to yellow. 9. Repeat the about two steps for the remaining three flasks. 10. Calculate the precent composition of water in each flask at the point of the addition of the alkyl halide and place these values in a table (table #2 below). Complete the table to show the concentration of NaOH in each flask immediately after the addition of HCl or RCl as well as the concentration of HCl (flask #1) and RCl (flasks#2-5).
 * **Material** || **Flask #1** || **Flask #2** || **Flask #3** || **Flask #4** || **Flask #5** ||
 * water || 3.0 mL || 3.0 mL || 4.0 mL || 5.0 mL || 6.0 mL ||
 * 0.01M NaOH || 2.0 mL || 2.0 mL || 2.0 mL || 2.0 mL || 2.0 mL ||
 * Isopropanol || 3.0 mL || 3.0 mL || 2.0 mL || 1.0 mL || none ||
 * indicator || 3 drops || 3 drops || 3 drops || 3 drops || 3 drops ||

//Table #2 Reagent Composition in each reaction// || ||  ||  || || 50% || 40% || 30% || 20% || //Table #3 Recorded times of experimental groups from lab//
 * Data:**
 * Material || Flask #1 || Flask #2 || Flask #3 || Flask #4 || Flask #5 ||
 * % water || 52% || 50% || 60% || 70% || 80% ||
 * [NaOH] || miniscule || miniscule || miniscule || miniscule || miniscule ||
 * [HCl] || miniscule ||
 * [RCl] ||
 * Flask # || Group #1 Acetone solvent || Group #2 Acetone solvent || Group #1 Isopropanol solvent || Group # 2 Isopropanol + ||
 * 1 || 2.09 seconds || 0 seconds || 1.01 seconds || 1.02 seconds ||
 * 2 || 5.58 seconds || 5.15 seconds || 286.15 seconds || 400.38 seconds ||
 * 3 || 1.38 seconds || 1.15 seconds || 81.10 seconds || 209.22 seconds ||
 * 4 || 33.25 seconds || 23.00 seconds || 16.80 seconds || 42.01 seconds ||
 * 5 || 11.00 seconds || 13.00 seconds || 8.60 seconds || 19.05 seconds ||


 * +These results relate to the following lab report.**


 * Graph of experimental groups reaction times.**

The graph of the experimental groups differing reactions times demonstrates the differences in the speed of the reaction when Acetone, a polar aprotic solvent, and Ispropanol, a polar protic solvent are used. The speed of reactions with Acetone in flasks #2 and #3 were roughly 281 to 395 seconds faster than the reactions times of flask #2 and #3 with Isopropanol.
 * Observations:**

The purpose of this lab was to contemplate, through experimentation, the effect that the solvent has in a two step nucleophilic substitution reaction (SN1). The rate of expression for SN1 Nucleophilic Substitution reactions This is a misstatement. Say "the rate of a SN1 reaction..." will vary depending on the solvent chosen for the reaction as well as the amount of solvent used. The SN1 reaction observed in this lab, displayed a bond breaking between the leaving group and the electrophile creating a stable 3 degree better to call it a "tertiary" carbocation. The nucleophile, which may vary in each individual reaction, was added to the electrophile resulting in the introduction of a new functional group to the organic molecule. That initial step of the leaving group breaking its bond to the electrophile is the rate determining step. However, this step also relies heavily on the particular solvent that the reaction occurs in. The solvent that allowed for the initiation of the reaction was the 2-chloro-2-methyl propane. Whoa there! This is a reactant, but it is made available to you in your solvent, which was the alcohol. This particular solvent was polar protic. By having an understanding of the results that a reaction will yeild using a polar protic solvent, we were able to predict that the end product would be acidic. Because the solvent that was assigned was 2-chloro-2-methyl propane, it is reasonable to predict that the reaction time would be less than that of the lab groups using acetone (a polar aprotic) solvent.
 * Conclusion/Analysis:**

All reactions with a combination of water and isopropanol were faster than just water. This data suggests that the higher concentration of polar protic solvent (ignoring water) why ignore water? leads to a faster reaction time. The data collected in this laboratory exercise is a real-life example of the solvent effects on SN1 and SN2 reactions. The polar aprotic solvent did not benefit the reaction time of the SN1 reaction, with the reaction times slowing down with more solvent present. The polar protic solvent did not show a strong relationship between reaction time and percent composition. However, it suggests that this type of solvent would speed up the reaction time for SN1 reactions, rather than the opposite seen with polar aprotic solvents. The polar protic solvent helps to stabilize the carbocation intermediate that is created in the first step of the SN1 reaction; with this lower potential energy of the intermediate, the activation energy decreases, thereby decreasing the reaction time. While the polar aprotic solvent resulted in a higher potential energy, leading to a higher activation energy and slower reaction time.

Some possible sources of error may include the fact that measuring the solvents and substances with precision presented a challenge. Another source of error may include that it was difficult to indicate the exact time the color in each flask changed. The timing for some flasks was immediate while others took many minutes. Another source of error could also be traced back to the magnetic stir rods that were used in each flask. Because the stir rods were in each flask they may have contained particles of chemicals from the previous flask, thus altering the reaction time observed. I think you've thought through these sources of error well. I'm glad you took the time to do so.

This lab illustrated the importance of choosing an effective solvent for an SN1 reaction. By gaining an understanding of polar protic and aportic solvents, future lab students may be able to make wise decisions regarding what type of solvent they should chose for a reaction that is effective and does not require much time. Overall this lab connected the ‘paperwork’ practice of SN1 reactions practiced during lecture time to the actual lab time where SN1 reactions are able to be observed through color change, time and solvent choice.

Post Lab Questions: 1) Suggest 3 other solvents that might be effective for completing the SN1 reaction we performed. Formic Acid  n-Butanol  Isopropanol

2) Suggest a different alkyl halide that might be used if we wanted to get evidence from polarimetry that the reaction proceeded by SN1 not SN2. (R)C4H9Br You need to show me a line structure, or a structural formula rather than a molecular formula. Or give me a name.

Citations: http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/alhalrx1.htm

This lab earned the following scores for: format (2/2) style (1.5/2) data (3/3) quality of result (1/1) quality of reported data (1/1) conclusion (0.5/2) error (1/1) post-lab Q (1/2) for a total of 11/14. Deductions are pretty much connected to the substance of the report, rather than the form....which you did a nice job on. Oh, except you didn't cite me.