SYNTHESIS OF METHYLENECYCLOHEXANE

Formula of Triphenylmethylphosphonium

Triphenylmethylphosphonium bromide:

• Dissolve 68.1 g (0.26 mol) of tri-phenylphosphine in 60 ml of dry benzene (CAUTION). Place the solution in a stout-walled wide-necked glass bottle of about 1-litre capacity and cool in an ice—salt bath. 
• Add 33.4 g (20 ml, 0.35 mol) of methyl bromide (1) and seal the bottle with a rubber bung secured with wire. 
• Remove the bottle from the cooling bath and leave for one day at room temperature. Cool the bottle in an ice—salt bath before opening, filter off the white crystalline solid under suction and wash with hot benzene (fume cupboard). 
• Dry the product in the oven at 100 °C and store in a desiccator over phosphorus pentoxide. 
• The yield is 91.5 g (99%), m.p. 232-233 °C

Notes to keep in mind: 

1. Methyl bromide (b.p. 4.5 °C) is supplied in sealed vials. 

2. The vial must be thoroughly cooled in an ice-salt bath before any attempt is made to open it. 

3. It is advisable to wear a face mask and thick gloves during the operation. 

4. Wrap a cloth around the vial and score around the neck with a glass-knife. Place several layers of cloth around the scored neck and break of the end of the neck.

Process of Methylenecyclohexane

STEP 1: Fit a 250-m1 three-necked flask containing a mag­netic stirring bar with a condenser, nitrogen inlet and a thermometer; surround the flask with a water bath supported on a hot-plate/magnetic stirrer unit. 

STEP 2: Pass a steady stream of dry nitrogen through the flask and main­tain the flow throughout the reaction. 

STEP 3: Place 125 ml of dry dimethyl sulphox­ide in the flask and stir vigorously for 30 minutes to remove oxygen gas. 

STEP 4: Briefly remove the condenser from the flask and add 4.8 g (0.1 mol) of a 50 per cent suspension of sodium hydride in oil by means of a solids funnel. 

STEP 5: Warm the mixture to 65 °C on the water bath and stir until all the sodium hydride dissolves (about 1.5 hours). 

STEP 6: Cool the mixture to room temperature in a cold-water bath. 

STEP 7: Remove the condenser, place a solids funnel in the central neck of the flask and add 35.7 g (0.1 mol) of triphenyl­methylphosphonium bromide in portions over 15 minutes; the reaction is slightly exothermic. 

STEP 8: Replace the condenser and stir at room temperature for 45 minutes to complete the formation of the methylenephosphorane. 

STEP 9: Add 9.8 g (0.1 mol) of cyclohexanone in one portion; an exothermic reaction ensues and the temperature rises to about 70 °C. Stir the mixture at 50 °C for 1 hour and then transfer to a single-neck 250-ml flask, add a boiling stick and attach a still-head and condenser arranged for vacuum distillation. 

STEP 10: A calcium chloride drying tube should be placed in the line between the water pump and receiving flask to prevent water vapour condensing in the flask. 

STEP 11: Distil and collect the volatile products boiling below 75 °C at 50 mmHg in a receiver cooled in an acetone—Cardice bath; the distillate consists of methylenecyclo­hexane contaminated with a small amount of benzene. 

STEP 12: Redistil the crude product at atmospheric pressure through a 7-cm fractionating column packed with glass helices. 

STEP 13: Reject the material (mainly benzene) which boils below 99 °C, and collect the pure methylenecyclohexane as a fraction of b.p. 99-100 °C; the yield is 6.2 g (64%). Confirm its homogeneity by g.l.c. analysis on a Silicone oil column at 60 °C.