Lab E Procedure DS

Lab E: Diels-Alder Reaction

Objective: Perform a Diels-Alder reaction between 1,3-Butadiene (generated in situ from 3-Sulfolene) and Maleic anhydride to synthesize 4-Cyclohexene-cis-1,2-dicarboxylic anhydride.

Techniques:

• Reflux

• Vacuum Filtration

• Melting Point Determination

• IR Analysis.

Diels-Alder Reaction Background

A cycloaddition reaction where a diene and a dienophile (alkene) react to form a cyclohexene ring.

It is concerted (bond breaking/forming happens simultaneously) and stereospecific. Discovered by Otto Diels and Kurt Alder (Nobel Prize 1950).

The dienophile reactivity increases with electron-withdrawing groups.

Reaction Scheme Overview

Step 1 (Cheletropic Elimination): 3-Sulfolene (Butadiene sulfone) + heat (in xylenes) → 1,3-Butadiene + SO₂(g). Step 2 (Diels-Alder): 1,3-Butadiene + Maleic anhydride → 4-Cyclohexene-cis-1,2-dicarboxylic Anhydride. 1,3-Butadiene is generated in situ to avoid handling a gas.

Starting Materials in RB Flask

The 50 mL Round Bottom Flask provided contains:

1.5 g Maleic anhydride (pre-weighed),

2.7 g 3-Sulfolene/Butadiene sulfone (pre-weighed),

4 mL Xylenes (solvent). Add a magnetic stir bar to the flask.

Safety: Corrosive Solids

Maleic anhydride and 3-Sulfolene are corrosive solids causing skin/eye burns and are respiratory irritants.

3-Sulfolene has a foul smell. ALWAYS wear gloves and work inside the fume hood with the sash down.

Safety: Solvents and Gas

Xylenes (bp 137-140°C) and Petroleum Ether (bp 36-50°C) are toxic, flammable. Avoid inhalation/skin contact. Sulfur Dioxide (SO₂) is a toxic, foul-smelling gas evolved during reaction. Keep fume hood closed.

Step 1: Initial Setup

Obtain your pre-loaded 50 mL Round Bottom Flask. Add a stir bar. Cover the flask and transport it to your fume hood.

Step 2: Reflux Setup

1. Clamp ONLY the neck of the RB flask to the ring stand.

2. Place the smaller cloth heating mantle (from bottom 'F' drawer) on the hot plate/stirrer. Ensure the RB flask sits flush inside the mantle.

3. Attach a water-cooled reflux condenser to the flask. Ensure water hoses are connected (CHWS = Chilled Water Supply, CHWR = Chilled Water Return).

4. Check that water is flowing and the condenser feels cool.

5. Ask TA to check setup. TA will plug the heating mantle into the Variac (set to 55%).

6. Turn ON STIRRING (green light).

7. Do NOT turn on the hot plate heat (NO orange light).

Step 3: Reflux Reaction

1. Insulate the RB flask with foil for the first 3 minutes.

2. Wait for the solution to reach a boil (this starts the reflux time).

3. Remove the foil and return it to the TA table.

4. Continue stirring.

5. Observe solvent vapors condensing in the condenser.

6. Ensure the condenser stays cold.

7. Reflux for 30 minutes with gentle stirring.

8. Keep the fume hood CLOSED to contain SO₂ gas.

9. Thinking Ahead: Obtain 9 mL of Xylenes from the Organic Prep Hood using a labeled 10 mL graduated cylinder (covered with a 30 mL beaker).

Step 4: End Reflux & Cool

After 30 minutes of reflux, unplug the heating mantle from the Variac. Carefully remove the condenser from the RB flask. DO NOT disconnect the condenser hoses. Allow the reaction mixture to cool for 3 minutes.

Step 5: Prepare Warm Water Bath

Heat a 250 mL beaker (3/4 full of water) on a medium heat setting until warm (takes 3-5 minutes).

Step 6: Add Xylenes & Warm

Using a Pasteur pipette, add the 9 mL of Xylenes (from Step 3) to the reaction RB flask. Securely clamp the RB flask in the warm water bath for 2 minutes.

Step 7: Decant Solution

Decant ONLY the supernatant solution into a 50 mL beaker. Leave any brown sludge impurities behind in the RB flask. (Note: Often no sludge forms; pour all solution if clear).

Step 8: Obtain Petroleum Ether

Bring a labeled 25 mL or 50 mL graduated cylinder (covered with a 100 mL beaker) to the Organic Dispensing Hood to obtain 10 mL of Petroleum Ether.

Step 9: Precipitate Product

1. Using a Pasteur pipette, add the 10 mL of Petroleum Ether to the solution in the 50 mL beaker. DO NOT clamp this beaker.

2. The Diels-Alder product will precipitate as crystals.

3. Note the physical appearance of the crystals.

4. Cool the beaker in an ice-water bath for 2 minutes to complete crystallization.

Step 10: Isolate Product

Set up a Vacuum Filtration. DO NOT wet the filter paper with water. Collect the crystals.

Step 11: IR Sample & Drying

Transfer a spatula-tip amount of product into a vial. Label it with your full name, TA name, and lab section. Give it to your TA for IR analysis. Leave the remaining solid on a watch glass to dry until next week.

Step 12: Week 2 Analysis

The following week, record the dry mass and the melting point range of the Diels-Alder product in your notebook.

Waste Disposal 1: Solvents

Dispose of the filtrate (contains Xylenes/Petroleum Ether) and any unused solvents into the specially labeled "Diels Alder Waste Container".

Waste Disposal 2: Sharps & Acetone

Dispose of all used pipettes into the Red Sharps container. Waste Acetone from cleanup must be disposed of into the "DIELS-ALDER WASTE CONTAINER".

Glassware Cleanup for Lab F

Collect 20 mL of Acetone from your TA into your 100 mL RB Flask. Rinse the 100 mL RB Flask with this acetone. Cycle the same acetone through the following glassware in order: 1) 25 mL RB Flask, 2) 10 mL Graduated Cylinder, 3) 25 mL Graduated Cylinder, 4) 50 mL Round Bottom Flask, 5) 50 mL Beaker, 6) Buchner Funnel and Vacuum Filter Flask. Dispose of the waste acetone into the "DIELS-ALDER WASTE CONTAINER". Give your cleaned 100 mL Round Bottom Flask to your TA.

Critical Note: Variac Use

The heating mantle is plugged into the Variac (on right side below hood), NOT the hot plate. The hot plate is only used for stirring. The Variac dial is set to 55% by the TA.

Critical Note: Water Condenser

Ensure water is flowing through the condenser and it feels cool. A warm condenser means solvent is being lost. If it warms up, notify your TA immediately.

Critical Note: Hood Sash

Keep the fume hood sash closed as much as possible during reflux to contain the toxic Sulfur Dioxide (SO₂) gas produced.

Critical Note: Filtration

Do NOT wet the filter paper with water for the vacuum filtration. The product must not contact water as the anhydride can hydrolyze. Use a dry filter paper.

Product Characteristics

Product: 4-Cyclohexene-cis-1,2-dicarboxylic anhydride. A white crystalline solid. The cis stereochemistry from maleic anhydride is retained in the product due to the stereospecific nature of the Diels-Alder reaction.

Notebook Requirements

Record observations (color changes, precipitation). Note crystal appearance. Next week: record dry mass, melting point range. Include mechanism understanding and stereochemistry.