Week+3-+Nucleophilic+Substitution+Reactions

=Week 3- Nucleophilic Substitution Reactions= = =

Introduction:
Nucleophilic substitution is a “class of reactions in which an electron nucleophile selectively bonds with or attacks the positive or partially positive charge of an atom or a group of atoms called the leaving group; the positive or partially positive atom is referred to as an electrophile” (Wikipedia).Two-step nucleophilic substitution (SN1) reactions occur at secondary (2°) and tertiary (3°) electrophilic carbons and are unimolecular. It is unimolecular because only one molecule is involved in the first step, the rate determining step, which is the electrophile. Since the nucleophile is not involved until after the slow step (the leaving group leaves creating a carbocation intermediate), its concentration has no effect on the rate. The ideal conditions for SN1 reactions to occur are with good nucleophiles, weak bases and polar protic (polar solvents with an H on O, N or X) solvents (such as water or acetone). Acetone is NOT protic, though it is polar (it does not have H attached to O, N, or X).  They can also occur with poor nucleophiles, weak bases and polar protic solvents under cool conditions. The polar protic solvent stabilizes the intermediate carbocation, allowing for a lower energy state increasing the rate. This lab demonstrates the effects the solvent has in an SN1 reaction. Below is an example of the pre-lab assignment that shows the mechanism of tert-butyl chloride (2-chloro-2-methylpropane) and water. While the inclusion of the prelab sheet below does add some valuable information to your Intro, it's not a real polished way to present that info. Better than not having it here, of course.



Procedure:
The set up for this lab requires five 25 mL Erlenmeyer flasks, five test tubes, a magnetic stirrer, a stir bar, graduated glass pipettes, a stirplate without heat, and a stopwatch. Number the flasks from (1 to 5) and label the test tubes from (A to E). Picture #1 shows how the tubes and flasks should be set up.

Next, place 2.0 mL of 0.1 M alkyl halide into the test tubes labeled (A thru D) using a pipette and place a stopper on them. Then in test tube (E) place 2.00 mL of 0.1 HCl and place a stopper on it.

In the 5 flasks, place the contents displayed in Table #1. (Once the indicator is added, the liquid in all flasks should be a light blue).


 * Acetone was used

Begin stirring the contents in flask #1 and allow to stir for a minute (get the stopwatch ready). Pour the contents in test tube (E) (2.00 mL of 0.1 HCl) into flask #1 and immediately start timing (in seconds). Record the time it takes for the color of the indicator to change from blue to yellow.

Begin stirring the contents in flask #3. Pour the contents from test tube (A) (2.0 mL of 0.1 M alkyl halide) into flask #2 and immediately start timing (in seconds). Record the time it takes for the color of the indicator to change from blue to yellow.

Repeat this procedure for flasks #3, 4 and 5, and test tubes B, C, and D.

Calculate the percent composition of water from each flask after the addition of the alkyl halide. Place the values in Table #2 as shown. Complete the rest of Table #2 to show the concentration of NaOH in M in each flask immediately after the addition of HCl as well as the concentration of HCl (Flask #1) and RCl (Flasks #2-5) (which for this lab has already been completed). Also report these results on the board for the other lab groups record to help support each lab groups conclusion.

Dispose of leftover chemicals in the waste bottle for halogenated organics in the hood.

Data/Analysis:


Including a graph of these results would help the reader see the trends in the data quickly and clearly. Consider the use of graphs and charts if they add clarity to your presentation--you may see scores improve as a result!
 * **Flask # ** || **1 ** || **2 ** || **3 ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">4 ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">5 ** ||
 * **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">Tube ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">E ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">A ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">B ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">C ** || **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">D ** ||
 * <span style="font-family: Arial,sans-serif; font-size: 10pt;">Time (sec) Acetone || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">2 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">825 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">160 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">41 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">20 ||
 * **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">Other Lab Groups Time ** ||
 * <span style="font-family: Arial,sans-serif; font-size: 10pt;">Time (sec) Acetone || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">2 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">700 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">213 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">56 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">18 ||
 * Time (sec.) IPA || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">0.41 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">693 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">464 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">106 || <span style="display: block; font-family: Arial,sans-serif; font-size: 10pt; text-align: center;">16.53 ||
 * **<span style="color: #632423; font-family: Arial,sans-serif; font-size: 10pt;">Table #3. Time for color of mixture to change. ** ||


 * Conclusion**: The reaction happened, and finished, almost instantaneously when the 2mL of HCl was added to the solution in flask #1. This result is expected, since a strong acid was added to the mixture, so the pH changed quickly. The rest of the data shows that the more water in the mixture, the faster the reaction completes (the color has completely changed). All available data, though slightly varied in detail, support this observation. Even the data from the experiment using isopropanol show the same trend. The difference, however, is that the isopropanol mixture did not have as severe a time change as the acetone mixture when the percent water increased. Perhaps this is due to the fact that isopropanol is a polar protic solvent and acetone is a polar aprotic solvent so the addition of a polar protic solvent would cause a greater change in acetone than in isopropanol. Overall, the initial hypothesis that adding a polar protic solvent (H2O) to the mixture would increase the rate of the reaction was supported.

What about the comparison of one concentration of IPA to the same concentration of acetone? For instance, under Trial B conditions IPA as a solvent resulted in a much slower reaction than acetone, even though IPA is polar protic and acetone is not. I'd be interested to hear a bit more from you about that situation (which also holds true for Trials C and maybe even D). Error: Some potential sources of error in this experiment would be inaccurate measurement of reactants, observer reaction time when using a stopwatch or timer, unclean glassware, incorrect information given by other groups, or incorrect calculations. Another issue could be due to the lack of data provided by groups using the isopropanol mixtures. There was only one presented. This could cause the entire conclusion to be incorrect. I assume you mean that you can't be really confident with only one data set.

Works Cited:
<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10.5pt;">Higginbotham, Carol. Ph.D. “Nucleophilic Substitution Reactions” CH-335: Organic Chemistry Course. COCC/UO: Blackboard, Winter 2012. Web. 1 Feb 2012.

<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">"Nucleophilic Substitution." //Wikipedia, the Free Encyclopedia//. Web. 03 Feb. 2012. <http://en.wikipedia.org/wiki/Nucleophilic_substitution>.

<span style="background-color: white; font-family: Arial,sans-serif; font-size: 10pt;">Straumanis, Andrei. //Organic Chemistry: A Guided Inquiry : A Process Oriented Guided Inquiry Learning Course//. Boston, MA: Houghton Mifflin, 2009. Print.