Nucleophilic+Substitution+Reactions+(7TH+INNING)

One of the most important features for the success of a planned SN1 reaction is the selection of a suitable solvent. In organic chemistry, nucleophilic substitution is a reaction in which an electron rich nucleophile selectively bonds with the positive or partially positive charge of an electrophile which was attached to a displaced leaving group. This can be performed in an SN1 two-step reaction or SN2 biomolecular reaction. In order to determine a suitable solvent a few factors must be considered. One factor when choosing a solvent includes solubility which allows reagents to be in the same phase as the molecules and allows them to collide and react. Another factor is that the solvent needs to be unreactive towards the reagents and last how the solvent will affect the rate of reaction. In the following experiment the effects, if any, of the solvents acetone and isopropanol in an SN1 (two-step nucleophilic substitution) reaction are analyzed. To do this an acid-base indicator will be added to a mixture to monitor the production of acid as the reaction proceeds, although initial knowledge of SN1 reactions assumes that polar protic solvents will increase the rate of reaction. Acid indication allows for measurement of the dependent variable. This allows for further analysis in deciding the relationship of the solvent and reaction progress.
 * Introduction:**

Procedure:
This lab exercise was devised by Carol Higginbotham, Ph.D. Professor of Chemistry at Central Oregon Community College and Oregon State University - Cascades Campus. Inspiration for this lab activity comes from the POGIL project, especially Frank Creegan, professor emeritus at Washington College, and from Ken Doxsee and Jim Hutchison’s laboratory text “Green Organic Chemistry, Strategies, Tools, and Laboratory Experiments.” please cite me (CH) as the source o this procedure to you. I worked hard on it! :)

- ( five ) 25mL Erlenmyer flasks
- magnetic stir rrer - stir bar - graduated glass pipettes - stop watch - five test tubes

** Reactants: **
- 0.1 M alkyl halide - 0.1 M HCL - 0.1 M NaOH

** Process: **
1.Used a pipette to to transfer 2.0 mL of 0.1 M alkyl halide into four labeled test tubes.

2. Also using a pipette 2.0 mL of 0.1 M of HCL was transferred into a single test tube . 3. In 25 mL Erlenmeyer flask #1 3.0 mL of water, 2.0 mL of 0.01 M NaOH, 3.0 mL Acetone and 3 drops of indicator were combined.

4. In 25 mL Erlenmeyer flask #2 3.0 mL of water, 2.0 mL of 0.01 M NaOH, 3.0 mL Acetone and 3 drops of indicator were combined.

5. In 25 mL Erlenmeyer flask #3 4.0 mL of water, 2.0 mL of 0.01 M NaOH, 2.0 mL Acetone and 3 drops of indicator were combined.

6. In 25 mL Erlenmeyer flask #4 5.0 mL of water, 2.0 mL of 0.01 M NaOH, 1.0 mL Acetone and 3 drops of indicator were combined.

7. In 25 mL Erlenmeyer flask #5 6.0 mL of water, 2.0 mL of 0.01 M NaOH, 0.0 mL Acetone and 3 drops of indicator were combined.

8. The stirrer in flask #1 was started to create good mixing. 2.0 mL of the 0.1 M HCl from the test tube was poured into flask #1. Once the HCl was poured into flask #1 a stop watch was started.

9. The time for the indicator to change from blue to yellow was recorded in seconds.

10. The stirrer in flask #2 was started to create good mixing. One of the test tubes containing 2.0 mL of the 0.1 M alkyl halide was poured into flas #2 and a stop watch was started.

11, The time for the indicator to change from blue to yellow was recorded in seconds.

12. Steps 10 and 11 were repeated for flasks #3- #5.

13. The percent composition of water, NaOH, HCl and RCl in each flask immediately after the addition of the alkyl halide were calculated and recorded in a table (see data below.)

14. A table with the recorded times of the indicator changing from blue to yellow for the entire lab was filled in at the front of the classroom. See below table of times in data.

Solvents compared:
1-propanol:








 * *****NEW TABLE: **

You obviously put a LOT of work into these graphs, and they are very effective in showing trends. That is GREAT. Now, which data was generated by your small group? I don't think you have pointed it out anywhere.

Discussion/conclusion:
This discussion/conclusion section is very well thought out. I am giving you a boost on your score for the section because of this. Unfortunately in your enthusiasm to dig into the depths of polarity vs. protic influences on rate, you missed some other stuff which should be in your conclusion section--such as a discussion of error.

The data obtained during the lab points to one thing: the more water in the mixture, the faster the rate of reaction. While this statement holds true as a general trend for all of the data, there are other’s which must be outlined to show the true correlation between solvent choice and rate of reaction in an SN1 nucleophilic substitution.

In each case the data from test tube one can largely be disregarded. It served as a control to show experimenters the color change that occurs when the SN1 reaction goes to completion, and as a means of testing the composition of each solution. The following analysis will focus on the trends observed in test tubes two through five.

In any collaborative situation, there is the potential for error. While this can be a problem, any detrimental effect is typically outweighed by the positive consequences associated with collaboration. As the experimentally derived data came from each individual team in the lab-section and was then combined, the entire group was able to make more precise predictions about the processes involved during the reactions.

The main drawback of this method is the degradation of accuracy that occurs when there are so many moving parts. One such example is in how the different teams reported the raw data: some reported to the hundredths of a second, while others only used whole numbers, with some even reporting in minutes.

The second cause of error comes from the method with which the ratio of solvent to solute was calculated. For the sake of time and focusing on the goal of the lab, many figures were rounded. This was an effective tool, simplifying the web of information cluttering each experimenters head, but it is a strong source of potential error.

Starting with a 50/50 mixture of acetone and water and moving stepwise to a mixture containing 80% water, 20% acetone, reaction rates dropped significantly. Put simply, more water and less acetone creates faster reaction rates. It can be extrapolated that this occurs due to the interactions between water, a polar protic solvent, and acetone which is polar aprotic since the alkyl halide remained constant throughout the experiment. This data suggests that alkyl halides react more readily with polar protic solvents than with polar aprotic solvents. It can be further deduced that the presence of acetone slowed the reaction by consuming some of the energy from water that otherwise would go to the alkyl halide.
 * Acetone: **

The process for this reaction followed the same parameters as listed above, but 1-propanol was used instead of acetone. The data for 1-propanol follows the general trend of acetone, but at slower rates. 1-propanol is a polar protic solvent so one would assume that it would increase the reaction rate. This is not the case and here’s why: dipole interactions. When 1-propanol is mixed with water, each molecule has an effect on the rate of reaction. Because water is so much more reactive, it “hogs” the reaction with the alkyl halide. As more water and less 1-propanol are included in the mix, the reaction rate increases.
 * 1-propanol: **

As shown by the charts, there is an inversely proportional relationship between both reagents (1-propanol and water) and the rate of reaction: as reagents decrease, reaction rate increases. In each case, however, acetone proved to be a “faster” reagent than 1-propanol. This contradicts the theory that SN1 reactions favor polar protic solvents. Because the only thing changed was the choice of reagent (all other conditions held constant), it must be a result of each one’s characteristics. Although both species have dipole interactions, acetone is significantly more EN than 1-propanol. This explanation accounts for why acetone shows faster reaction rates. Taken as a whole, none of the data disputes water’s potency as a solvent in SN1 reactions.
 * Acetone vs. 1-propanol: **

Post Lab Questions:
1) Suggest 3 other solvents that might be effective for completing the SN1 reaction performed in this lab. a. DMSO – Dimethyl sulfoxide and water mixture. The dipole on this compound is 3.96D, which is significantly more than that of acetone. This is a relatively large polar aprotic molecule, but the significant dipole interactions should facilitate SN1 nucleophilic substitution reactions.  b. Methanol – methanol is a small polar protic solvent with a slightly smaller dipole than water. This compound was chosen because it is similar to water in both size and dipole.  c. MeCN – Acetonitrile and water mixture. This polar aprotic compound has a dipole of 3.92 D. As shown by this experiment, electronegativity can be more important in determining the rate of a reaction than solely polar protic vs. polar aprotic. This molecule should have the ability to facilitate SN1 reactions with such strong dipole interactions. Very nice.

2) Suggest a different alkyl halide that might be used if we want to get evidence that the reaction actually happened by SN1 rather than SN2. (S) 2-bromobutane: This is a chiral molecule: in a SN1 reaction with water, there would be a racemic mixture of (R)-2-butanol and (S)-2-butanol enantiomers as products . An SN2 reaction would create only products of the opposite absolute configuration to the reactant. Polarimetry tests will confirm product configuration and verify or discredit hypotheses.

(R)-1-bromo-1,2-methylpropane is not a correct name. Whoops. But you've got the idea right. You need to pick a chiral starting molecule (that is also secondary) and watch for the formation of chiral products.

Statistics for the post-lab question were obtained from: Loudon, G. Mark. Organic Chemistry 4th ed. New York: Oxford University Press. 2002. pg 317.

This report earned the following scores for: format (2/2) style (1.5/2) data (2.5/3) quality of result (1/1) quality of reported data (1/1) conclusion (2/2) error analysis (0/1) post-lab Q (1.5/2) for a total of 11.5/14. You worked hard! It shows in a good way.