Nucleophilic Substitution Lab Report PDF

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Lab report for nucleophilic substitution lab...

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Nucleophilic Substitution Experiment #2

Madison Bradley Section 3 TA: Maddie Parker October 2, 2020 1

Purpose: To determine if the nucleophile 2,6-dimethylphenol prefers an SN1 or SN2 mechanism, it was reacted with 1-bromopentane and 2-bromopentane, which prefer SN2 and SN1 respectively. Proton NMR then detected which substitution product was more abundant, which indicated the preferred mechanism.

Experimental: Experimental notes can be found on pages 10 and 11 of lab notebook, which are attached in Appendix D.

Results and Discussion: The SN2 product of 1-bromopropane would show a triplet in the 3.5-4.5 ppm region of proton NMR. The SN1 product of 2-bromopropane would show a multiplet in the same region. Both of these peaks correspond to the proton(s) located on the first carbon off of the oxygen in the alkoxy substituent. There is 1 proton there for the SN2 product, and there are 2 protons there for the SN1 product. These protons are highlighted in Appendix B. The proton NMR spectrum of the collected product can be found in Appendix B. It revealed a triplet at 4.02 ppm with an integration of 8.86, and a multiplet at 4.47 ppm with an integration of 2.04. These two peaks correspond well to the two peaks discussed above. According to the calculation located in Appendix A, the mole ratio of the products of SN2 to SN1 is 2.17:1. These results indicate that 2,6-dimethylphenol prefers to do nucleophilic substitution by an SN2 mechanism.

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According to the known proton NMR spectra of 1-bromopropane and 2-bromopropane, they both contain peaks around 3.5 ppm, corresponding to the proton(s) located on the same carbon as bromine. There was a peak around this point in the spectra of the collected product, indicating that this product contained some bromopropane, and therefore was impure. Given the relatively low boiling points obtained in the pre-lab assignment (71C for 1-bromopropane, and 59C for 2-bromopropane) and the high temperature of the sand bath used, it is probable that these compounds were evaporated and condensed into the collecting flask during reflux. 2,6-dimethylphenol and sodium hydroxide’s boiling points are much higher (203C and 1390C), so it’s not likely that they ended up in the product. Although ethanol has a relatively low boiling point (78.5C), and therefore could have made it into the collecting flask, it would have been located in the basic aqueous phase of the separatory funnel, so it’s unlikely that it ended up in the product. Drying of the organic layer over sodium sulfate also ensured that all three of these compounds would be removed. The rotary evaporator was used to remove tert-butyl methyl ether (boiling point: 54C) from the organic layer to isolate the substitution products. Known proton NMR spectra of this compound identifies 2 distinct singlets at 3.2 ppm and 1.2 ppm. Neither of these are found on the spectrum of the collected product, indicating that rotary evaporation was successful in removing all of the ether. We began the reaction with an excess of alkyl halide, so the nucleophile, 2,6dimethylphenol, was the limiting reagent. We began with 1.63*10^-3 moles of it. If all of our nucleophile reacted with the bromopropane, we would have obtained 0.17 grams of substitution products. The final yield of product was 0.05 grams. This is a yield of 29%, which is low. All of these calculations can be found in Appendix A. If the product also contained bromopropane, as

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the data suggested, the yield of substitution products is even lower than that. There are several potential reasons for the low yield: the reaction not being performed under reflux long enough for all of the nucleophile to react, some of the organic layer being lost during draining, some of the solutions being lost with each transfer, etc.

Conclusion: The experiment was successful in performing a nucleophilic substitution reaction between the nucleophile (2,6-dimethylphenol) and the electrophiles (1-bromopropane and 2bromopropane), and in collecting the substitution products. Proton NMR was successful in determining the ratio of SN2 to SN1 products, which was 2.16:1. This indicated that 2,6dimethylphenol prefers SN2 over SN1 for its nucleophilic substitution mechanism. It was predicted that SN2 would be the preferred mechanism. 2,6-dimethylphenol is a bulky nucleophile. SN2 prefers to occur on a less substituted carbon, which is necessary for bulky nucleophiles to physically reach the carbon. 2,6-dimethylphenol was also in its deprotonated form during the reaction, because NaOH is a strong enough base to deprotonate it. Being present in phenoxide form made 2,6-dimethylphenol a strong nucleophile, which is also characteristic of SN2 reactions. The collected product was impure because, according to the proton NMR spectrum, it contained some of the reactant bromopropane. While carrying out the reaction at reflux and collecting the products via condensation was effective in collecting the product, it was ineffective in the fact that it allowed for these impurities to be collected as well. When reactants that have lower boiling points are used, this is perhaps not the best technique to use if a pure product was required. However, this impurity did not compromise the results of the experiment.

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This is because proton NMR was used to interpret the products, and the peaks of the protons from bromopropane did not interfere with the peaks of the relevant protons from the substitution products. Therefore, the technique was sufficient for the purpose of this experiment. There were no other impurities present in the collected product. This indicates that the acid-base extraction and sodium sulfate were effective in removing any 2,6-dimethylphenol, ethanol, and sodium hydroxide that could have ended up in the collecting flask or the aqueous layer. It also indicates that rotary evaporation was successful in removing all of the tert-butyl methyl ether from the organic layer. There was a low yield of product, at 29%. This was likely a result of the time constraints of the lab period, and of some material being lost in each of the many steps and transfers of the experiment. In the end, the purpose of the experiment was achieved, so the low yield did not have a negative effect on the experiment.

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Appendix A

Mole ratio of SN2:SN1 products:

8.86 / 2 4.43 = =2.17 2.04 2.04

Moles of 2,6-dimethylphenol: .2 g ×

1 mol =.00163 mol 122.167 g

Mass of substitution products (C6H16O):

( 12.01 × 6 ) +( 1.01 × 16 ) +16.00=104.22 g

Theoretical yield of substitution products: .00163 mol dimethylphenol×

Percent yield:

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104.22 g 1 mol product =.17 g product × 1 mol dimethylphenol 1 mol product

.05 g =.29=29 % .17 g

Appendix B

Relevant peaks: Proton A B

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Shift 4.02 ppm 4.47 ppm

Multiplicity triplet multiplet

Integration 8.86 2.04

Appendix C

Pre-lab questions: Observation:

Mechanism:

1. The ratio A:B = 1

Both

2. The ratio A:B > 1

SN2

3. The ratio A:B < 1

SN1

Post-lab questions: 1. Because 2,6-dimethylphenol is an aromatic alcohol, it is more acidic than the simple alcohol ethanol (pKA of ~10 versus ~16). Therefore, it is more likely to be deprotonated into its nucleophilic counterpart. The base used in the experiment was NaOH. Hydroxide is the conjugate base of water, whose pKA is also around 16. This means that it is not strong enough to deprotonate ethanol, but strong enough to deprotonate 2,6dimethylphenol. Therefore, in our reaction, only 2,6-dimethylphenol was present in deprotonated nucleophilic form, while ethanol was in its protonated form and could not act as a nucleophile. 2. NaOH in the work up functioned to make the aqueous layer basic. This ensured that any stray ethanol or 2,6-dimethylphenol was deprotonated and transferred to the aqueous layer, and therefore would not make our product (in the organic layer) impure. 3. 2-bromo-2-methylpropane is very sterically crowded, and therefore highly geared towards SN1. The bulky nucleophile 2,6-dimethylphenol prefers SN2, but did a small amount of SN1 on 2-bromopropane. However, I think the new alkyl halide is just too

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crowded for SN1 to occur. I predict that the reaction would not be able to proceed. If it did, I expect that it would produce only trace amounts of 2-tert-butyl-3-methyl toluene.

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