Nucleophilic+Sn1

Nucleophilic Substitution Reactions (SN1) Lab

=** Introduction: **= This lab was an exploration into the process of performing a two-step nucleophilic substitution reaction (SN1). The collecting of time data for these reactions will help further our understanding of the process as well as the effects of different solvents (polar protic vs polar aprotic) on the reaction taking place. Polar protic solvents favoring E1 reactions. The focus for this experiment was in determining the effects of different concentrations of water and Acetone on the progress a synthetic reaction with a 0.1M alkyl halide (2-chloro-2-methylpropane). The rate of this reaction will be increased with higher concentrations of a polar protic solvent. A solution of 0.1M HCl (hydrochloric acid) is used as the control reaction for the purpose of verifying a pH shift through color change. Beginning with the alkyl halide the rate determining step is the leaving group chloride ion creating the tertiary carbocation. Water is then acting as the nucleophile and attracted to the electrophilic cation. Once the bond has formed the oxygen of the water molecule becomes positively charged and becomes the electrophile. Any adjacent chloride atom or water molecule then acts as the nucleophile and pulls the proton away from the H2O and forms a hydronium ion or HCl. The original alkyl halide molecule has transformed into tert-Butyl alcohol.

Mechanism for (SN1) It's good that you included this. Well done.



=** Procedure: **= __Procedure source handout provided by instructor Carol Higginbotham,CH-242/388 Organic Chemistry II Laboratory Nucleophilic Substitution Reactions__

At your workstation assemble five 25mL Erlenmeyer flasks, five test tubes, one magnetic stir bar, a stopwatch, two graduated glass pipets, a 10 mL graduated cylinder, a bottle of pH indicator (methelyne blue), and five stoppers for the test tubes. Label the Erlenmeyer flasks #1-5. Label the test tubes A-E. Use a pipet to add 2.0 mL of 0.1 M alkyl halide to test tubes A-D then stopper the tube. Add 2.0 mL of 0.1 M HCl to test tube E using a pipet then stopper the tube. Place the following contents into the Erlenmeyer flasks as follows…


 * Table #1 Contents used in procedure**
 * Material || Flask #1 || Flask#2 || Flask#3 || Fask#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 ||
 * Acetone || 3.0 mL || 3.0 mL || 2.0 mL || 1.0 mL ||= - ||
 * Indicator || 3 drops || 3 drops || 3 drops || 3 drops || 3 drops ||

Begin by placing the flask #1 on the stir plate and place the magnetic stir bar into the flask. When ready to time the reaction pour the contents of test tube E into the flask and time the reaction. The color will change to yellow when the reaction is complete. Record the time in seconds on the data sheet. Remove stir bar and rinse with distilled water and place into the flask #2. Place on stir plate and begin stirring. Empty the contents of test tube A into the flask and time the reaction. The same color change will occur when the reaction is finished. Record the time. Repeat this same procedure for flask #3 with test tube B, flask #4 with test tube C, and flask #5 with test tube D. Calculate the composition of the contents of the flasks and record the data in the table provided.

percent composition= Mass due to specific component / Total molar mass of compound X 100 =**Data:**= Collected data recorded with stop watch and converted to seconds. Once completed and data recorded transfer your results by copying your table #2 on the board, or by photocopying the table and taping it up at the front of the room.
 * Table # 2 Each Reactions Reagent Composition**
 * Calculations:**
 * Material ||= Flask #1 || Flask #2 ||= Flask #3 ||= Flask #4 ||= Flask #5 ||
 * % Water ||= 30% || 30% ||= 40% ||= 50% ||= 60% ||
 * [NaOH] ||= 0.002M || 0.002M ||= 0.002M ||= 0.002M ||= 0.002M ||
 * [HCl] ||= 0.02M ||= - ||= - ||= - ||= - ||
 * [RCl] ||= - || 0.02M ||= 0.02M ||= 0.02M ||= 0.02M ||

How did you get data to the 0.01 sec for the isopropanol #1? This is beyond our measuring capability, I think, so hold your sig figs to the level of precision I called for in the procedure: to the nearest second.
 * Table #3 Includes data collected from experiment and data from the rest of the class recorded in (seconds)**.
 * = data (seconds) ||= Group Data ||||||= Class Data ||
 * =  ||= Acetone #1 ||= Acetone #2 ||= Isopropanol #1 ||= Isopropanol #2 ||
 * = Flask #1 ||= 3.28 ||= INSTANT ||= INSTANT ||= INSTANT ||
 * = Flask #2 ||= 534 ||= 538 ||= 677.38 ||= 677 ||
 * = Flask #3 ||= 152 ||= 135.4 ||= 305.56 ||= 328 ||
 * = Flask #4 ||= 44.4 ||= 39 ||= 93.19 ||= 83 ||
 * = Flask #5 ||= 14.7 ||= 13 ||= 17.15 ||= 12 ||

A graphical representation of this data somewhere in your report would be very helpful (and would boost you toward the max score in the "data" category!).

=Conclusion/Analysis:= The purpose of this procedure is to investigate the reaction between 2-Chloro-2-methylpropane, acetone and water. In order to Identify the SN1 rate reaction depending on which solvent is used (Acetone and Isopropanol). According to Table #3 from the data section the Acetone appeared to react faster than the Isopropanol each trial. This occurrence points to the fact that the mixture of the assigned solvent IPA or Acetone is the factor that dictates the rate. What is known, Acetone is a polar aproic solvent. Isopropanol and water are polar protic solvents. The reaction time was significantly faster in direct proportion to the percentage of water in the reaction flask. Yes: a clear trend and just what we would expect. The problem with the data is the fact that the reactions taking place using the isopropanol were considerably longer in duration then those using Acetone. The results should have been just the opposite. If you investigate the polarity of acetone vs. isopropanol, this is less surprising! But it's not an "error" in the result; just not what you expected to see. The problem could have been due to the switching of bottles labeled for the two solvents. Also there could be some stabilization effect of the protic and non protic solvents (H2O and Acetone) that facilitate an increase in reaction rates. Another answer could involve the solvent effect of solvolysis reaction rates. Obtaining the constant k values for Acetone and isopropanol and running the reaction again with a fixed rate mixture of the solvents in water could prove to answer the rate question. Could the polar aprotic Acetone have shifted the reaction towards an E2 synthesis creating a faster reaction pathway for a portion of the reactants and in conjunction with the polar protic H2O facilitating the E1 portion of the reaction reduced the overall duration of the experiment. Or could the two polar protic solvents H2O and isopropanol somehow been in competition with one another thus hindering the progress of the E1 pathway.

Common errors during procedure
 * Difficulty measuring solvents for the procedure with the pipet and graduated pre-measured device.
 * transferring the magnetic stir rod could have been contaminated with solvent from other flasks when not washed good enough after each trial.
 * When timing the trials there was fluctuation of error when to start the timer and when exactly the color was completely changed to indicate reaction was complete
 * When using the five flasks there was excess water unable to dry from previous class, created error of measurements and composition of water minor but could be a possibility. These are all reasonable errors to suspect. Good thinking.

=Post-lab Questions:= 1) Possibly using other solvents which are similar to Acetone and Isopropanol and are capable of conducting a SN1 reaction.

2) With procedure tasking out groups to conduct trials with alternate solvents and possibly a third alkyl halide to help with gathering more data to prove this SN1 reaction is possible. =Reference:= Green Organic Chemistry: Strategies, Tools, and Laboratory Experiments, Doxsee and Hutchison, Thompson- Brooks/Cole. modified version provided by Carol Higginbotham at Central Oregon Community College Winter Term 2012 Organic Chemistry II CH-242/388.