Prelab 7- Preparation of Alkenes by Elimination PDF

Preview

Summary

Preparation of Alkenes by Elimination...

Description

Pre-Lab Grading Rubric NAME: Rachel Totos

COURSE-SECTION: CHEM 233- MW 12pm

Grading Rubric Fair

Poor

Pre-Lab & Reaction Table

3 pts: The pre-lab is well written, organized, and neat. It contains all required elements: title, introduction, equations/reactions, calculations and pre-lab questions. Every element is thorough and correct.

2 pts: All elements are present, but there are minor errors, misinformation, or slight omissions in one or two of these elements. Some mistakes are present in calculation of rxn table.

1 pts: One or two elements are missing, contain several errors, or significant omission of required detail is noted.

0 pts: More than two elements are missing or have not been completed and very little original thought is shown.

2 pts: The procedure is thorough and contains details that are specific to the experiment. Procedure is original and authentic, and a diagram of the setup is included.

1 pt: Generally, the procedure is well written and thorough, but may be missing some key steps that should have been performed in the laboratory.

Pre-Lab Questions

Good

Procedure

Performance Element

Excellent

0 pts: Significant omissions of required steps. Steps are incorrectly listed.

Points are assigned (5 pts)

T otal Score (10 pts) à

Earned Points

Pre-Lab Report 7 Preparation of Alkenes by Elimination Rachel Totos TA: Xuan Duong 17 July 2021

Preparation of Alkenes by Elimination Introduction The purpose of this experiment is to synthesize alkenes using E1 and E2 mechanisms, isolate the formed products by fractional distillation, and analyze the products using the bromine and Baeyer test. Finally, the ratio is analyzed using gas chromatography (GC). For this experiment, 2-methyl-1-butene and 2-methyl-2-butene are synthesized via E1 mechanism from 2-methyl-2-butanol and sulfuric acid (H2SO4), and from 2-chloro-2-methylbutane and potassium hydroxide (KOH) via E2 mechanism (see figure one). Elimination reactions are categorized as E1 and E2 and can only occur if a beta hydrogen atom is present. Elimination reactions proceed by dehydration of alcohols (E1) and dehydrohalogenation of alkyl halides (E2). See figure E2 reactions, also known as betaelimination reactions, are bimolecular in which the beta hydrogen atom bond and the leaving group bond are broken simultaneously. and a double bond is formed. In E2 reactions, there is no intermediate and only one transition state. E1 reactions are unimolecular and differ from E2 reactions because it forms a cationic intermediate. For E2 reactions, two products are formed the regioselectivity determines the major versus minor product. Regioselectivity refers to the preference of one reaction at a site over another. In other words, if a reaction is regioselective, it can proceed in more than one direction; However, one direction dominates the other in reactivity. The major product, or Zaitsev product, is the product that is most stable in which also is the more substituted alkene, whereas the minor product, or the least substituted product, is the Hofmann product. This is known as Zaitsev’s rule. In this case, the major product is the product formed by the loss of a hydrogen atom from the beta (B) carbon with the lowest number of hydrogens.

Exceptions to this rule where a Zaitsev product is either not formed or not the major product include steric hinderance and stereoelectronic factors. The Baeyer test, as completed in lab 3, is carried out in this experiment to determine the presence of an alkene. In this case, potassium permanganate (KMnO4) in water is used to create a reaction that will cause a color change to indicate a positive result that confirms the presence of an alkene. See figure three for the reaction of this process. The bromine test was also carried out in order to determine the presence of an alkene. This is done by adding Br2 in dichloromethane (DCM) to the product solution and observing color change. For a positive result for unsaturation, the orange-red color from the bromine will disappear and the solution will become colorless. See figure four for an illustration of this reaction. Gas chromatography is also used in this experiment to determine the distribution of products. Equations & Reactions Percent Yield = (actual yield / theoretical yield) x 100%

Mole % composition =

moles of compound A x 100 % (moles of compound A +moles of compound B)

Figure 1. This figure illustrates the elimination reactions that are carried out in this lab. This image is from the UIC Organic Chemistry Lab Manual, pg.119.

Figure 2. This figure illustrates elimination reactions by dehydration of alcohols (E1) and the dehydrohalogenation of alkyl halides (E2). This image is from the UIC Organic Chemistry Lab Manual, pg.222.

Figure 3. This figure illustrates the reaction taken place during the Baeyer test. This image is from the UIC Organic Chemistry Lab Manual, pg.61.

Figure 4. This figure illustrates the reaction taken place during the bromine test. This image is from the UIC Organic Chemistry Lab Manual, pg.120.

Procedure E2 Experiment The first part of the experiment conducted is the E2 reaction. To start, KOH pellets are added to a round bottom flask, or “stillpot”, followed by the addition of 1-propanol using a graduated cylinder. Next, a boiling stone is added, and the flask is gently swirled. The flask is then clamped to ring stand and heat is applied. The solution is gently heated to allow the hydroxide to dissolve. Once most of the hydroxide has dissolved, the flask is removed from the heat and put on ice. The SN1 product from the last experiment, 2-chloro-2-methylbutane, is added to the flask via pipette and the flask is then clamped back onto ring stand. Next, a Hempel column is filled with Raschig rings, and similar to the setup of the West Condenser, the tubing used as a water source and another as a drain is attached to column. The joints of the column are greased before placing on top of the stillpot to prevent the glassware from sticking together. See figure five for an illustration of this reflux setup. In this particular case, the Hempel column is acting as the reflux condenser. The water and heat are then turned on and the reaction is refluxed for one hour. During this hour, the procedure for the E1 experiment should be carried out—the steps for this process are explained in the next section. Once the hour is up, the heat is removed and the flask is allowed to cool, while the water remained turned on to prevent the product from evaporating. Once cooled, the water is turned off and the remaining of the fractional distillation apparatus is assembled. See figure six for an illustration of the fractional distillation setup. The stillhead is attached to the top of the Hempel column, and the West condenser with the water and drain tubing is attached to the arm of the stillhead and secured using Keck clips. The vacuum adapter and a pre-weighed stillpot, used as a receiving flask, are then attached to the condenser and held with Keck clips. The receiving flask is put on ice for the duration of the distillation

process. A thermometer with an adapter are placed on top of the stillhead and the heat is turned on and the solution is allowed to distill until the temperature reaches 45°C. Once the temperature has reached 45°C, the flask is removed from the heat, put on ice to cool and then weighed to calculate the percent yield. E1 Experiment The second part of this experiment is to synthesize an alkene using E1. To start, t-amyl alcohol is measured in a graduated cylinder and added to a stillpot using a pipette. Next, 6M H2SO4 is measured and added to the stillpot and gently swirled. A boiling stone is added to the flask and is then clamped to a ring stand. Next, the fractional distillation apparatus is assembled. See figure six for the setup of the fractional distillation apparatus. The receiving flask is again pre-weighed and attached to the vacuum adapter using Keck clips, and put on ice. The thermometer with the adapter is attached to the top of the still head and the water and heat are turned on. Same as before, the distillate is collected until the temperature reached 45°C. Once the temperature reaches 45°C, the flask is removed from the heat and allowed to cool, and finally weighed to calculate the percent yield. Baeyer & Bromine Tests The analysis for this experiment consists of conducting a Baeyer test and a Bromine test. For the Baeyer test, using three small test tubes, 1mL of water is added to each. Next, one test tube will remain as the control, the second test tube containing the E1 product and the third test tube containing the E2 product. Next, a small amount of potassium permanganate (KMnO 4) solution is added to the three test tubes and the results are observed and recorded. For a positive result, the color of the solution will turn brown instead of purple.

Next, another three test tubes are used to conduct the Bromine test. To start, 1mL of dichloromethane (DCM) is added to each of the three test tubes and a small amount of the E1 product is added to one test tube, a small amount of the E2 product is added to the second test tube and the third and final test tube is used as the control for this test. Finally, 0.1M Br2 in DCM is added to each test tube and the results are observed and recorded. For a positive result, there should be a rapid disappearance of the orange color and produce a colorless solution. Gas Chromatography (GC) Finally, the analysis of the product distribution is carried out via gas chromatography. This allows us to determine the ratio of Hofmann to Zaitsev product that was formed. To do this, about two or three microliters of the product solution is injected into the GC port. The peaks are then recorded and integrated. These steps were repeated for the other product formed. Next, the molar percentage of 2-methyl-1-butene and 2-methyl-2-butene are calculated in each fraction. No correction factors are necessary as the products have identical molar masses and polarities. The molar percentages can be used to calculate the percent yield.

Figure 5. This image is the reflux setup with the stillpot and Hempel column with Raschig rings. This image is from the UIC Organic Chemistry Lab Manual, pg.121.

Figure 6. This image is the setup of the fractional distillation apparatus with the receiving flask on ice. This image is from the UIC Organic Chemistry Lab Manual, pg.122.

References 1. Gilbert, John C. Experimental Organic Chemistry: a Miniscale & Microscale Approach. 5th ed., Cengage Learning, 2015.

2. Landrie, Chad L., et al. Organic Chemistry: Laboratory I Lab Manual and Course Materials. 11th ed., Macmillan Learning Curriculum Solutions, 2021.

Pre-Lab Questions 1. (1 pt) Write equations for the chemical reactions that you will use to demonstrate the presence of alkenes in your distilled products.

2. (1 pt) What is the function of the acid catalyst in promoting the E1 dehydration of 2methyl-2-butanol? The acid catalyst adds H+ to alcohol to form water that can then be easily removed to form a carbocation, in which increases the rate of the reaction. 3. (1.5 pts) Predict the b-elimination product(s) for the following reactions. If two or more products are formed, circle the major product.

4. (1.5 pts) Answer the following questions: a) Nucleophilic substitution and b-elimination often compete with each other. What are the factors that favor elimination? Factors that favor elimination include a strong bulky base interacting with a tertiary alkyl halide (E2) or a bulky base interacting with a primary, secondary or tertiary alkyl halide (E2). b) Using curved arrows to symbolize the flow of electrons write the complete reaction mechanism for the formation of compound C in this reaction.

c) Predict whether the following reaction proceeds predominately by substitution (SN2 or SN1) or elimination (E1 or E2) or whether the two compete. Write structural formulas for all products. Circle the major product(s). For a secondary alkyl halide, a strong base (OH-) favors E2. SN2 would occur if the compound is not sterically hindered....