0.3 E2 elimination  (Page 3/3)

Print Name: _________________________________________

1. Predict the products when the following compounds are treated with sodium hydroxide (base). (3 points)

2. Which is the more stable structure and why? (2 points)

3. Write the major product from the following reactions. (1+ 4= 5 points)

(b)

Write the mechanism for this reaction. (Hint: carbocationic rearrangement takes place)

Grading

• Completion of Pre Lab Questions.
• Write-up in your Lab Notebook.
• Completion of Report Questions.
• TA Points.

Materials required

Safety

Wear gloves all the time, especially when you use the metals. Keep safety glasses on all the time.

Experimental procedure

Preparation of the Alkoxide Base: Add to a 5.0 mL conical vial containing a magnetic spin vane 3-3.5 mL of the anhydrous alcohol (see Table 4.2). Add a 60-mg piece of potassium (or sodium) metal (or ~2 pellets) and immediately attach the vial to a reflux condenser closed by a lid. Place a needle above the reflux condenser to remove extra pressure generated during reflux. Place the whole arrangement in a water bath and, heat the mixture gently (~50°C) with stirring. (DO NOT HEAT THE CLOSED SYSTEM) When all the metal has reacted, remove the assembly from the water bath and cool to near room temperature (remove the needle).

Remove the lid from the condenser and use a calibrated Pasteur pipette to introduce 100 $\mu$ L of 2-bromobutane down through the condenser into the vial. Place the assembly in the preheated water bath (see Table 1.3). Attach the gas delivery tube to the top of the condenser so that the open end of the tube is beneath the water level of the reservoir. If the connection to the top of the condenser is not made with an O-ring cap seal connection, lightly grease the ground-glass joint to insure a gas-tight seal. After about 10-15 minutes, air bubbles emerge. Place the water-filled gas collection tube over the open end of the gas delivery tube.

Isolation of Product: Collect about 6-7 mL of gas in the collection reservoir and then use a hypodermic syringe to withdraw a 0.7 to 0.8 mL sample through the rubber septum for GC analysis.

NOTE. Remove the gas delivery tube from the collecting reservoir and then from the water before discontinuing the heat on the reaction vial. This order of events prevents water from being drawn back into the reaction flask.

Purification and Characterization: The collected gas is analyzed by gas chromatography without further purification.

CAUTION

Handle sodium and potassium with care. These metals react vigorously with moisture and are kept under paraffin oil or xylene. Remove a small piece of metal from the oil using a pair of forceps or tongs-never use your fingers! Dry the metal quickly by pressing it with filter paper (to soak up the oil), and immediately add it to the alcohol in the reaction vial. Any residual pieces of sodium/potassium should be stored in a bottle marked“sodium/potassium residues." Never throw small pieces of these metals in the sink or in water.

Waste disposal

There will be a large beaker of isopropanol for you to dispense your unused metal. To destroy the metal, add small amounts to methanol.

Approximate lab time: 2.5-3 hours

Report: E2 Elimination (Total 30 Points)

On my honor, in preparing this report, I know that I am free to use references and consult with others. However, I cannot copy from other students’work or misrepresent my own data.

…………………………………………………………….. (signature)

Print Name: _________________________________________

1. Predict the more stable alkene and supply with reasons for your choice: (6 points)

(a) 1-hexene or trans-3-hexene

(b) trans-3-hexene or cis-3-hexene

(c) 2-methyl-2-hexene or 2,3-dimethyl-2-pentene

2. Starting with the appropriate alkyl halide and base-solvent combination, outline a synthesis that would yield each of the following alkenes as the major or only product and include your reasoning: (8 points)

(a) 1-Butene

(b) 3-Methyl-1-butene

(c) 2,3-Dimethyl-1-butene

(d) 4-Methylcyclohexene

3. When cis-1-bromo-4-tert-butylcyclohexane reacts with sodium ethoxide in ethanol, it reacts rapidly to yield 4-tert-butylcyclohexene. Under similar conditions, trans-1-bromo-4-tert-butylcyclohexane reacts very slowly. Using conformational chair structures, explain the difference in reactivity between these stereo isomers. (4 points)

4. Write the major product of the following reactions with proper stereochemistry.

(4+ 2+ 2+ 4= 12 points)