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Competetive Nucleophiles Essay

Introduction:

The purpose of this experiment was to compare the relative nucleophilicities of chloride ions and bromide ions in two different reactions. One reaction involved n-butyl alcohol and the other involved t-pentyl alcohol. We performed the reactions and compared the percentages of alkyl chloride and alkyl bromide in the product. To perform this experiment, we used methods including heating reaction mixture under reflux, extraction using a separatory funnel, drying with anhydrous sodium sulfate, and refractometry.

Experiment Scheme:

First, we prepared the solvent-nucleophile medium. We combined 44mL 7.7M sulfuric acid with 4.75g ammonium chloride and 8.75g ammonium bromide, and we heated the mixture with stirring to dissolve the salts. We put 17mL into a separatory funnel for use in the second reaction, and placed the rest into a reflux apparatus for our first reaction. We performed the first reaction of the competitive nucleophiles with 1-Butanol. We added 2.5mL of 1-butanol to the solvent-nucleophile medium in the reflux apparatus with a boiling stone and heated the mixture under reflux for 75 minutes.

After reflux was completed, we allowed it to cool in an ice bath, and transferred the solution to a clean separatory funnel. The different phases separated, and we drained the lower aqueous layer. We added 5mL water to organic layer, mixed and collected the lower organic layer. We extracted the organic layer with 5mL sodium bicarbonate, drained organic layer and dried with anhydrous sodium sulfate. We decanted the alkyl halide solution and set it aside to be analyzed by refractometry.

We performed the second reaction of competitive nucleophiles with 2-methyl-2-butanol. We measured 2.5mL of 2-methyl-2-butanol into the separatory funnel containing 17mL of the solvent-nucleophile medium. We swirled the mixture, venting occasionally until pressure equalized, then shook it vigorously with occasional venting for 2 minutes. We then allowed phases to separate. We drained the lower aqueous layer and poured the top organic layer into a small beaker containing 0.5g sodium bicarbonate.

When bubbling stopped and clear liquid was obtained, we decanted the alkyl halide and began analysis by refractometry of the products from both reactions. By refractometry, we measured the refractive index of the products and used Equation 1 and 2 to calculate the percentages of each product (Pavia, Lampman, Kriz, and Engel, Organic Chemistry Laboratory Manual p. 47-50). Reaction 1 (Pavia, Lampman, Kriz, and Engel, Organic Chemistry Laboratory Manual p. 47-50)

Discussion:

In this experiment we used 2 nucleophiles, the bromide ion and the chloride ion. We performed 2 reactions with these nucleophiles in equimolar concentrations, and they competed with each other for the substrate. Normally, alcohols do not react well in nucleophilic substitution reactions because the hydroxide ion (a strong base) would need to be displaced, and it is not a good leaving group. For that reason, the substitution reaction must take place in acidic solution; the alcohol must first be protonated because water is a stable molecule and a better leaving group. After protonation, the substrate reacts by either the SN1 or SN2 mechanism. 1-Butanol will react by the SN2 mechanism because it is a primary alcohol while 2-methyl-2-butanol will react by the SN1 mechanism because it is a tertiary alcohol.

In Reaction 1, the major product was the alkyl bromide and the minor product was the alkyl chloride because bromine is more reactive; bromide is a stronger nucleophile. In Reaction 2, there essentially was no major product because the alkyl bromide and alkyl chloride formed in nearly equal amounts; the product was racemic. This is because it followed the SN1 reaction mechanism. The rate determining step is the loss of the H2O molecule forming a carbocation, and this step does not require a nucleophile (Pavia, Lampman, Kriz, and Engel, Organic Chemistry Laboratory Manual p. 47-50).

Important methods used in this experiment included heating reaction mixture under reflux, extraction using a separatory funnel, drying with anhydrous sodium sulfate, and refractometry. When heating under reflux, we used a condenser attached to the flask that we were heating. Under reflux, our system could heat the mixture to boiling and vapors condensed and drop back into the flask. We can heat our mixture for a long time at boiling point without losing any product. In refractometry, we measured the refractive index of our product mixture, which is the ratio of the speed of light in a vacuum to the speed of light in the media of interest. We can use our measured values with the standard values to calculate the percentages of alkyl chloride and alkyl bromide.

Sources of error could include the extraction process and the reading of the refractive index values. During extraction, it is possible that the phases may not have completely separated or the reactants did not fully react to form products, so some of the reactants could possibly enter into our final mixture. When finding refractive index values, it is possible to have some error when reading the value. Overall, error did not have a huge effect on our results. We could improve the procedure by being very careful to remove the entire aqueous layer during extraction so none gets into our final product for refractometry testing.

In conclusion, the experiment turned out well. Our results align with the expected results. We expected that the alkyl bromide will be the major product of Reaction 1 because it followed the SN2 mechanism, and we expected that the product from Reaction 2 would be a racemic mixture because it followed the SN1 mechanism.

Questions:

Bromide is a stronger nucleophile because the chlorine is more electronegative than bromine, so it holds electrons in closer. Bromine is less electronegative and has more electrons, and it is able to share unpaired electrons much more easily than chlorine.

References
ChemSpider Chemical Database. Royal Society of Chemistry, 4 July 2012. Web. Accessed 11 Sept. 2012. Sigma-Aldrich. Sigma-Aldrich. 2012. Web. Accessed 11 Sept. 2012. Pavia, Lampman, Kriz, and Engel. Organic Chemistry Laboratory Manual. Cengage Learning: Mason, OH, 2009. Print.


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