Solvent Effects in SN1

Introduction
In organic chemistry, most of the reactions chemists see occur between a nucleophile and an electrophile. One of the main reactions studied is nucleophilic substitution which can be through the 1-step SN2 or the 2-step SN1, which both generally accomplish the same goal yet with different rate determinations and different effects on chirality. In either substitution reaction, however, an important factor to consider is the solvent.

In this experiment the reaction between the solvent 1-propanol or acetone and an alkyl halide were studied. Special attention was paid to the rate of the reaction between the solvent and reagents and were measured by monitoring the production of acid throughout the experiment. The acidity was tested by way of an acid-base indicator being added to the mixture. Measuring the time for this color change to happen allows for a good estimation of rate of the SN1 reaction at different solvent levels, to accurately observe the relationship.


sn1rxnmechanism.png
The SN1 Mechanism for this Reaction



Procedure
The specific procedure was created and given as a physical handout by Dr. Carol Higginbotham:
  • Part 1
Lab_2_-_Proc_1.JPG
  • Table of Reagents
Lab_2_-_Proc_2.JPG
  • Part 2
Lab_2_-_Proc_3.JPG
  • Addendum: Isopropanol was not actually used; the alcohol used for a reagent was 1-propanol instead.
The chemicals used in this lab:
  • Name: 1-propanol
    • IUPAC Name: Propan-1-ol
    • CAS #: 71-23-8
    • Compound Formula: C3H8O
    • Structure:
      • MSP110419e4bi83i5egh7c90000518cbb0a55765e6g.gif
  • Name: 2-chloro-2-methylpropane
    • IUPAC Name: 2-chloro-2-methylpropane
    • CAS #: 507-20-0
    • Compound Formula: C4H9Cl
    • Structure:
      • MSP318219e4bb6a732c1ba1000018gc79f6iea40b04.gif
  • Name: Bromothymol (blue)
    • IUPAC Name: sodium 2-[(Z)-(3-bromo-4-hydroxy-2-methyl-5-propan-2-yl-phenyl)-(3-bromo-2-methyl-4-oxo-5-propan-2-yl-1-cyclohexa-2,5-dienylidene)methyl]benzenesulfonate
    • CAS#: 34722-90-2
    • Compound Formula: C27H27Br2NaO5S
    • Structure:
      • MSP400519e4badb80a2befc000025d4e20gihh4f1ie.gif
  • Name: Sodium Hydroxide
    • IUPAC Name: Sodium Hydroxide
    • CAS#: 1310-73-2
    • Compound Formula: HNaO
    • Structure:
      • MSP424919e4bb1d918i390g000040fg7f91de9ee7e4.gif
  • Name: Hydrochloric Acid
    • IUPAC Name: Hydrogen Chloride
    • CAS#: 7647-1-0
    • Compound Formula: ClH
    • Structure:
      • MSP513819e4be3e3731ehc100003680b583cb0ecc8e.gif

Data
  • Observations
    • In all flasks, mixture started out blue and smoothly transformed to a pale yellow, at varying rates listed in the tables below.

sn1rxntable_final.jpg
Individual group results.


sn1solventclass.png
Class Results




Analysis
The more water present in the system (a polar protic solvent itself) correlated with reduced reaction times as all of the reagents remained constant. The amount of actual given solvent (1-propanol) had no effect on increasing the reaction rate. Also, from looking at the group results, the type of given solvent also had no correlating effect.

sn1composition.png


Discussion/Conclusion
From the tables above, one can see that when water dominated the mixture the reaction process was noticeably faster. However, there are a few things to consider when looking at the results. Precision was a large factor in the procedure and while the measurements of the reagents and solvents were measured out as accurately as possible there was still a large amount of room for potential error. The timing of reaction progress was done through use of stopwatches, and possible error could result from inaccuracies and variability in timing methods. It would have also been beneficial to utilize more data points throughout the entire class, which would give a wider range of specifics and could possibly help identify trends at a quicker pace.

After the nucleophilic substitution reaction was complete it became apparent that the amount of water in the system affected the time it took for the reaction to take place. Water being a polar protic solvent is what enabled it to have an affect on the rate at which the reaction was completed. The more polar protic solvent in the system the faster an SN1 reaction will be carried out. Protic solvents involve H-Bonding which will push the SN1 because the formation of the carbocation is the rate-determining step and thus, water, for this procedure was the best solvent for the procedure to take place as it stabilizes ions in the mixture such as the carbocation intermediate.

With that being said, when one looks at the data further one can see that the type of solvent had a negligible effect in determining the rate of reaction the case of this procedure. Both 1-propanol and acetone had little effect on the reaction progress thus leading to the conclusion that the rates were entirely dependent on the concentration of water; being the most abundant solvent.

Post Lab Question #1: Suggest 3 other solvents that might be effective for completing the Sn1 reaction we performed.

There are many potential solvents that would complete the reaction that was performed in lab however, all must be polar protic in order to expedite the SN1 reaction. A few good polar protic solvents are:
  • Ammonia (NH₃)
  • Acetic Acid (C₂H₄O₂) or (AcOH)
  • Methanol (CH₄O)
  • Ethanol (C₂H₆O)

Post Lab Question #2: Suggest a different alkyl halide that might be used if we wanted to get evidence (from polarimetery) that our reaction actually proceeds by Sn1 rather than Sn2.

A good alkyl hallide that will turn the mixture racemic (50/50 chance of there being an (S) or (R) configuration) is 3-bromo-3-methylhexane. It is chiral in nature and therefore a polarimetry test would tell us the degrees rotated during the reaction process. Regardless of starting out with an (R) or an (S), SN1 reactions tend to proceed when the carbon atom is surrounded by large bulky groups. These large groups prevent SN2 reactions from happening and sterically hinder the nucleophile from coming in and attaching to the electrophile. Therefore since SN1 is allowed to proceed the nucleophile can choose to attack the electrophile from either side, once again, leading to a racemic mixture.

Notes
The chemical structures, formulas, and vital statistics of each compound was researched using Wolfram Alpha computational knowledge engine and confirmed using the CRC Handbook of Chemistry and Physics [90th Edition].