Intro-to-Nucleophilic-Substitution

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Iowa State University *

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331L

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Chemistry

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Dec 6, 2023

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TEMPLATE: 331L Intro to Nucleophilic Substitution Name: Kim Huynh Section: TA First Name: TA email: Combination: Pre-lab Questions Reminders: • All work must be legible or it will not be graded. • Do not copy and paste structures or mechanisms unless instructed to do so. 1. Read the Safety and Procedure for this experiment. a. (10 pts) Write a bullet list outline for the procedure. • Weigh a 50mL Erlenmeyer flask. • Add 2,5-dimethyl-2,5-hexaniediol into flask • Add HCl to a flask, stir 5-10 min by a stir bar • Add 10mL hexane and put in centrifuge tube • Invert and vent, then layers to separate • Remove the organic layer and transfer to 125 mL Erlenmeyer flask • Transfer the organic layer to rotovap flask and repeat extraction once more • Give flask to TA for rotovating • Use TLC plate with 10 mL of 95% hexanes/5% ethyl acetate • Dip TLC paper in PMA stain • Take to TA to heat by hot gun • Take photo and calculate Rf • Determine the melting point of the product • Obtain the IR spectrum of the product b. (5 pts) Write a brief summ ary of the key safety concerns specific for this experiment.  The chemicals used in this lab are very toxic, flammable, irritating, and highly corrosive ; so they must be handled carefully, carefully change new gloves when these chemicals come to contact your gloves. 2,5- Dimethyl-2,5-hexanediol is causes serious eye damage, must wear safety goggles, if it comes to contact eyes, must call for help and wash eyes with cool water. And all containers containing these substances must be closed to avoid causing danger. 2. (15 pts) Label the substrate, leaving group and nucleophile in the reaction scheme below. 3. (10 pts) Carbocation stability is important to examine when determining whether a substitution is S N 2 or S N 1. Look at the reaction scheme above. Is the substrate a 1 o , 2 o , or 3 o carbon? Does this type of carbon form a stable carbocation? The substrate is a 3 o carbon. Yes, it forms a stable carbocation. 4. (10 pts) Based on what you learned from the tutorials on C anvas for today’s experiment, which substitution mechanism do you predict for the substitution reaction involving 2,5-dimethyl-2,5- hexanediol and HCl? Explain briefly why you chose this mechanism.

The substrate carbon is 3 o carbon, forms a stable carbocation; the reaction takes over one step, so it is not SN2. It is SN1 reaction. 5. (35 pts) Draw todays synthesis using the reaction mechanism you chose in question 4. 6. (15 pts) calculate how much substrate and reagent to use based on moles. The following table guides you in this calculation. Amounts to Use for Reaction Reactant Reactant 2,5-dimethyl-2,5- hexanediol Conc HCl (aq) Moles 7 mmol 15x excess = 15 * 7 mmol = 105 mmol MW 146.23 g/mol NA Concentration NA 11.65 M (or mmol/mL) Amount Calculate and write in pre- lab notes. T Be sure and record exact amount Calculate and write in pre-lab notes. In-lab Notes 2,5-dimethyl-2,5-hexanediol: mass= moles x MW= 7/1000 mol x 146.23g/mol= 1.024 (g) Volume of HCl V = 105 mmol ÷ 11.65 mmol/mL = 9.012 (ml) In-lab Notes Erlenmeyer Flask 50mL: 41.223g Erlenmeyer Flask 50mL + 2,5 Dimethyl-2,5 hexanediol: 42.244g 2,5 Dimethyl-2,5 hexanediol: 1.021g HCL: ~9 mL Mass of 100ml rotovap flask: 75.581g Mass of 100ml rotovap flask+ product: 76.531g 18mg for NMR sample. Fumehood 25 for proton NMR Melting point 65 o C -69 o C.

TLC plate: - Solvent front: 6.3cm. - Crude product distance 5.4 cm. - Reactant distance: 0 cm Analysis and Application Questions • Do not copy and paste structures or mechanisms from any source. • Answers must be legible or they will not be graded. Data and Observations 1. (7 pts) Include a photo of your TLC plate showing product purity. Calculate the R f value for each spot (compound) on your TLC plate. 2. (8 pts) Calculate the percent yield. Show your calculations in order to get credit. From the reaction, 2,5-dimethyl-2,5-hexanediol is limited reactant, so the product 2,5-dichloro- 2,5- dimethylhexane is 7 mmol. Theorical mass of product: 7/1000 mol x 183.12 g/mol = 1.282 (g)

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Actual mass of product is 76.531g - 75.581g = 0.95 (g) %yields: (Actual mass of product/ Theorical mass of product) x 100%= (0.95/1.282)x 100%= 74.1% 3. (2 pt) Record the melting point range. Melting point 65 o C -69 o C. 4. Include copies of the FTIR spectra. a. (4 pts) Draw the structure of the product. b. (6 pts) Label peaks according to functional groups present in product. 5. Include a copy of the 1 H-NMR spectra for the product. Annotate with:

a. (4 pts) the product structure, b. (10 pts) pts) prediction table showing shift, integrations and splitting Proton Chemical Shift (ppm) Intergration Splitting HA 1-2 4 1s HB 1-2 12 1s c. (8 pts) relevant peaks clearly labeled according to corresponding protons on the product. Analysis 6. (6 pts) Explain how your TLC plate demonstrated that your product was pure. On TLC plate, a pure substance will only create one point. From my TLC plate, I observed was that the substrate was pure, but crude product is not pure. 7. Explain how your data give evidence that you synthesized the product that you expected and that it is pure. If your synthesis did not go well, explain what you could have done differently for better results. a. (4 pts) Compare observed melting points with expected melting points. Explain deviation if relevant. My observed melting points from 65 o C -69 o C, which is higher expected melting points (63 o C-64 o C). So it may not 100% pure, the deviation may be due to impurities in the sample, or due to a slower heating rate during the melting point determination. b. (8 pts) Compare FTIR spectra of products with what was expected. Comment on possible

contaminants if relevant. FTIR spectra was similar with the expected product. If relevant, it's likely due to water or impurities from the aqueous layer in the separation process. c. (8 pts) Compare 1 H-NMR spectra with what was expected. Comment on possible contaminants if relevant. 1 H-NMR spectra is similar what I was expected. It has 2 peaks, with intergation is 4 and 12.32 ( near 12) , with chemical shift in range 1-2 ppm. If relevant, maybe impurities appeared during practice, leading to the product not being as expected. Application 8. Pyrazole rings are a part of the structure of a vast array of pharmaceuticals and agrochemicals. As such, reactions resulting in substituted pyrazoles are widely used in industry. The following is an example of a substitution reaction which adds complexity to the pyrazole ring. a. (9 pts) Label the substrate, leaving group and nucleophile in the reaction scheme above. b. (8 pts) Can the substituent form a stable cation? Explain briefly. No. The substituent can not form a stable cation because the substituent is primary alkyl halide, it has only one alkyl group, so it is relatively unstable, then it forms a primary carboncation, which is unstable. c. (8 pts) Is this reaction more likely to an S N 2 or S N 1 mechanism? Explain briefly. This reaction is more likely S N 2 mechamism. It has only one step mechanism and appearance of primary alkyl bromide.

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