Dehydration of 3,3-dimethyl-2-butanol PDF

Preview

Summary

Some background to prepare you for ochem lab work,...

Description

Dehydration of 3,3-dimethyl-2-butanol Introduction: Alkenes are found as starting materials in the industrial production of a variety of important products including plastics, fuels and cleaning agents. While most of the commercially important alkenes are made from petroleum, they can also be synthesized in ways that are a bit more conducive to an educational lab setting. (Ethylene is made by steam cracking of aliphatic hydrocarbons which requires temperatures on the order of 800 °C.) Alcohols can be used as starting materials in the preparation of a variety of different classes of compounds including alkenes. Alkenes can be formed under fairly mild conditions by heating an alcohol in the presence of an acid, typically concentrated sulfuric or phosphoric acid, to yield the alkene and water, thus this is referred to as a dehydration reaction (See Figure 1). Figure 1: Dehydration of 2-propanol to form propene and water OH

conc. H2SO4

CH3CHCH3

CH3CH=CH2

Δ

+ H 2O

The mechanism for this reaction is fairly straightforward. A hydroxide ion is not a very good leaving group so we have to do something to make it a better leaving group. By protonating the hydroxyl group (recall that the reaction is performed with sulfuric or phosphoric acid) we create a much better leaving group, i.e. water, and the reaction proceeds via an E1 mechanism as shown in Figure 2. Figure 2: Mechanism for dehydration O HO S O

O

O H

H

O

H

OH

O

CH3CHCH3

CH3CHCH3

H

H C

CH3

H

C

S

OH

H

O

H C

CH3

H

C H

One complication can arise if the alcohol that you are using can give different alkene products depending upon which carbon is deprotonated in the final step. For instance, dehydration of 2-butanol will yield the more substituted alkenes (i.e. B and C) and the less substituted alkene A. If either the more substituted or the less substituted alkenes predominate, then we would say that this reaction exhibits regioselectivity. This example also highlights an additional complication that when possible, you will observe cis/trans isomers as well. Again, if either the cis or trans product predominates, we would say that there is stereoselectivity. Figure 3: Dehydration of 2-butanol conc. H3PO4 OH

Δ

+

+ A

B

C

Secondly, when the elimination proceeds via an E1 mechanism, requiring the formation of a carbocation intermediate, we can observe the formation of alternative products which result from rearrangement 1

reactions. For instance, if we submit 3-methyl-2-butanol to dehydration conditions (see Figure 4), we will observe the anticipated less (D) and more (E) substituted products. However, we also obtain product F. Figure 4: Dehydration of 3-methyl-2-butanol conc. H3PO4

+

+

Δ

OH

F

E

D

How did this happen? In this case, upon formation of the carbocation intermediate a rearrangement can occur, referred to as a 1,2-hydride shift, resulting in the formation of the more substituted and more stable 3° carbocation. This carbocation can then go on to deprotonate and form compound F. As a sidenote, not only can hydrogens shift but alkyl groups (methyl, ethyl, etc.) and phenyl groups can also shift, as long as the rearrangement results in the formation of a more stable carbocation intermediate. Figure 5: 1,2-hydride shift mechansim O H H

1,2-hydride shift

O

S

OH

O

F Description of the Experiment: Today you will perform a dehydration experiment using 3,3-dimethyl-2-butanol as your starting material and 85% phosphoric acid as your acid catalyst. You will isolate the product(s) of this reaction and use gas chromatography (GC) to determine the alkene product(s) that were obtained. Reagents: 3,3-dimethyl-2-butanol 85% phosphoric acid 5% sodium bicarbonate Pre-Lab 1. Create a table of important physical properties of reagents and possible products in your lab notebook. 2. Write out the reaction that you are performing and the potential product(s) that you expect to obtain. 3. Prepare a flow chart of the steps that you will take in today’s lab. Check-Out Items Aluminum Heat Transfer Block Hickmann Still with Side-Arm

2

Safety - 3,3-dimethyl-2-butanol is a flammable liquid. - 85% phosphoric acid is a corrosive material. Appropriate PPE (Personal Protective Equipment) must be worn at all times. Waste Disposal - Aqueous solutions can be poured down the drain with plenty of water. - Organic liquids should be disposed of in the non-halogenated waste container. - Solids should be disposed of in the solids waste container. Experimental Procedure Set Up the Reaction Place an aluminum heat transfer block on your stirring hot plate. Add 1 mL of 3,3-dimethyl-2-butanol to a 3 mL reaction vial equipped with a magnetic stir bar. Cool the solution in an ice bath and then add 0.3 mL of 85% phosphoric acid. Place the reaction vial in the appropriately sized hole in the aluminum plate and clamp the reaction vial to the ring stand. Equip the reaction vial with a Hickmann still w/side arm attachment, a Claisen adapter (another clamp should be placed here to keep your apparatus from falling over!), thermometer adapter, thermometer and condenser (the thermometer will go in the side of the Claisen adapter that allows the thermometer to extend into the Hickmann still, the condenser can go in the other side). Maintain a slow trickle of water thru the condenser and begin heating the reaction. Develop a gentle distillation of material into the Hickman still, from which you should remove materials frequently (as it fills up), combining the distilled materials into a 15 mL centrifuge tube. Keep an eye on the temperature of the distillation and make a note of the temperature range over which you collect materials. Once the temperature drops (after it reaches approximately 70 °C) you should immediately stop heating the reaction, allow the apparatus to cool to room temperature (let it cool while you go on to work up the reaction, etc.), rinse the residual material into the non-halogenated carboy with WATER, wash your glassware and return all check-out items to the stockroom. Work Up Wash the organic layer with 2 x 1 mL of 5% aqueous sodium bicarbonate solution, being sure to retain the organic layer! Dry the organics over anhydrous magnesium sulfate. Isolate your product from the drying agent by using a filter tip pipette, filtering into a pre-weighed 3-mL reaction vial. Analysis Obtain a weight of your product mixture so that you can determine a percent yield (this is a yield of the alkene mixture). Determine the product(s) formed in this reaction by the use of GC. Since all of the products are alkenes with identical molecular weights and similar structures, we can assume that their thermal conductivities are not appreciably different. Thus, the areas measured for each compound can be assumed to be proportional to the amount of the total isolated product for each compound. Using this data, you can calculate an approximate yield of each component. Lab Report Be sure to include all relevant data including: - Crude weight - Crude percent yield - GC Data (copy of chromatogram) - Final weight and percent yield of each product 3

-

Discuss which product was the major product. Write a mechanism for the formation of this product. Why did it predominate?

4...