SN1+Solvents+and+Rates

Zach Chandler Megan Bernard


 * Lab #2: SN1 Solvents and Rates**


 * Objective:** To determine if the choice of a solvent has an effect, if any, on a two-step nucleophilic substitution (SN1) reaction by observing the reaction between 2-chloro-2-methyl propane and water.


 * Introduction:** The purpose for this lab was for students to experiment with a two-step nucleophilic substitution (SN1) reaction while collecting data to prove that an SN1 reaction occurred via the addition of an alkyl halide Careful how you say this! It's not an addition reaction. . The main focus was to figure out what would happen when a particular solvent was put into the solute of 2-chloro-2-methylpropane and water. The assigned solvent used in the lab was isopropanol. 2-chloro-2-methylpropane in isopropanol (tert-Butyl chloride) also known as an alkyl halide is a colorless liquid that is soluble in water and can undergo spontaneous solvolysis (where solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule) when dissolved in a given polar solvent. It is flammable and volatile. This alkyl halide is mainly used as a starting molecule in SN1 reactions to produce different products ranging from alcohols to alkoxide salts. The fact that two solvents are given in the lab and divided between the class is a way to prove via numerous sets of class data with the same assigned solvents that and SN1 reaction occurred and not an SN2 reaction. I don't understand the previous sentence. "an" instead of "and?" We assume it's SN1: just looking at the solvent effects. The mechanism for this reaction is listed below:



The apparatus you need consists of a set of (five) 25 mL Erlenmeyer flasks, a stir bar, magnetic stirrer, graduated glass pipets, and a stopwatch. Any leftover chemicals at the end of the lab period should be disposed of in the waste bottle for halogenated organics, in the hood. Your stir-plate should be set to STIR without HEAT. Arrange your materials as shown, and as described below:
 * Procedure:**



Use a pipet to transfer 2.0 mL of 0.1 M alkyl halide into each of four labeled test tubes (A-D). Stopper the test tubes. Measure as precisely as you are able. Similarly, transfer 2.00 mL of 0.1M HCl into a labeled test tube (E). Stopper the test tube. Label five 25 mL Erlenmeyer flasks in the following manner: flask #1 HCl; flasks 2-5 with the numbers 2, 3, 4, 5. Place the contents from the table below into the five Erlenmeyer flasks:

Start the stirrer in flask #1 to create good mixing. Pour 2.0 mL of the 0.1 M HCl from the test tube (E) into flask #1 and immediately begin timing IN SECONDS. Record the time for the color of the indicator to change from blue to yellow (for bromothymol blue). Start the stirrer in flask #2 to create good mixing. Pour 2.0 mL of the 0.1 M alkyl halide from one of the test tubes (A) into flask #2 and begin timing IN SECONDS. Record the time for the color of the mixture to change from blue to yellow (for the indicator bromothymol blue). Repeat the procedure for the remaining three flasks (#3, 4, 5 and test tubes B, C, D). Calculate the percent composition of water in each flask immediately after the addition of the alkyl halide and place these values in Table #2. Complete Table #2 to show the concentration of NaOH in M 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).
 * Table #1**
 * ** 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 ||
 * ** Acetone or isopropanol as assigned ** || 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**
 * ** Material ** || ** Flask #1 ** || ** Flask #2 ** || ** Flask #3 ** || ** Flask #4 ** || ** Flask #5 ** ||
 * ** % Water ** || 30% || 30% || 40% || 50% || 60% ||
 * ** [NaOH] ** || 0.002 M || 0.002 M || 0.002 M || 0.002 M || 0.002 M ||
 * ** [HCl] ** || .02 M || ** X ** || ** X ** || ** X ** || ** X ** ||
 * ** [RCl] ** || ** X ** || .02 M || .02 M || .02 M || .02 M ||
 * Observations/Data:** The table below shows the rates of reaction for each of the five flasks containing either iPA (isopropanol) or acetone. In each reaction below, the more water there was and the less acetone or iPA there was, the rate of reaction had increased. YES. The rate of reaction for the three iPA groups were slower than the rate of reaction for the two acetone groups. Yes. The acetone would be the most effective solvent for the reaction performed in this experiment since it appears to happen more rapidly than the same reactions with iPA as the solvent.
 * // ** Alkyl Halides ** // || // ** Flask #1 ** //// ** Rate of rxn (sec) ** // || // ** Flask #2 ** //// ** Rate of rxn (sec) ** // || // ** Flask #3 ** //// ** Rate of rxn (sec) ** // || // ** Flask #4 ** //// ** Rate of rxn (sec) ** // || // ** Flask #5 ** //// ** Rate of rxn (sec) ** // ||
 * ** Isopropanol (IPA) **** (The-Mach’s) ** || ** Instantaneous ** || ** 677 ** || ** 328 ** || ** 83 ** || ** 12 ** ||
 * __ Isopropanol (IPA) ____ 2nd group __ || __ Instantaneous __ || __ 920 __ || __ 348 __ || __ 85 __ || __ 14 __ ||
 * ** Isopropanol (IPA) **** 3rd group ** || ** Instantaneous ** || ** 697 ** || ** 306 ** || ** 93 ** || ** 17 ** ||
 * __ Acetone ____ 1st group __ || __ Instantaneous __ || __ 538 __ || __ 134 __ || __ 39 __ || __ 13 __ ||
 * ** Acetone **** 2nd group ** || ** Instantaneous ** || ** 534 ** || ** 152 ** || ** 44 ** || ** 15 ** ||


 * Sources of Error:** When the indicator changed from blue to yellow, it was difficult to stop the timer at the same point of change in color in all five reactions, which could have effected the rate of the reaction in each flask. The measurements of the alkyl halide with the buret may have been a little over or under the 2.0 mL because of the difficulty reading the measurements on the buret.


 * Conclusion:** It appears that the choice of solvent does effect an SN1 reaction. The initial hypothesis was that the isopropanol would cause the reaction to proceed faster than with acetone because with SN1 reactions, Polar Protic solvents are more efficient than Polar Aprotic solvents. The isopropanol is Polar Protic, and the acetone is Polar Aprotic. However the Polar Aprotic solvent acetone was more efficient at converting reactants to products, based on the reduced time as more water was added. This could be because the acetone is more polar than the isopropanol, as shown in the images below. The acetone has a double bond to an electronegative oxygen, which is more polar than than the isopropanol, that has a single bond to a hydroxide. AWESOME explanation of your data: clear, concise, and does not overstate your confidence. Did you look up the polarity of the solvents? You can find it on Wikipedia, I believe, listed as "Dipole Moment."

Isopropanol structure

Acetone structure

Doxsee, K.M.; Hutchison J.E. Green Organic Chemistry - Strategies, Tools, and Laboratory Experiments, Print 2004. POGIL project, Creegan F., professor emeritus at Washington College.
 * References:**