SYNTHESIS OF PENTA-1,4-DIENE (Divinylmethane)

STEP 1:    1,5-Diacetoxypentane. 

• Equip a 5-litre three-necked flask with an efficient sealed stirrer unit of the Hirschberg type, a double-surface reflux condenser and screw-capped adapter carrying a thermometer. 
• Add 516 g (582 ml, 6.0 mol) of tetrahydropyran, 480 g (434 ml, 6.0 mol) of acetyl chloride and 4g of powdered fused zinc chloride. 
• Heat the mixture with a heating mantle with vigorous stirring under reflux, and continue to heat until the temperature of the reaction mixture reaches 150 °C (about 3-5 hours). 
• Allow the reaction mixture to cool to near room temperature and add 980 g (10 mol) of solid potassium acetate and 20 g of sodium iodide. 
• Heat the mixture again with stirring to 160 °C and maintain the temperature for 12 hours. 
• Cool to room temperature, add 1 litre of light petroleum (b.p. 40-60 °C), filter under suction and wash the residue thoroughly with more light petroleum. 
• Combine the filtrate and washings and remove the solvent on a rotary evaporator. 
• Fractionally distil the residue under reduced pressure and collect the 1,5- diacetoxypentane at 108-114 °C/3.5-4 mmHg. 
• The yield is about 790 g (70%); the product is sufficiently pure for use in the next stage, but a sample may be purified by redistillation and then has b.p. 102-104 °C/3 mmHg (or 92-94 °C/ 1.0 mmHg).

Figure 1

STEP 2:      Penta-1,4-diene.

• Assemble the pyrolysis apparatus shown in with the pyrolysis tube packed with glass beads but with the acetone-Cardice cooling trap arrangement attached to the water condenser. 
• Pass a gentle stream of nitrogen gas through the apparatus and heat the com­bustion tube to 580 °C when 900 g of 1,5-diacetoxypentane is added dropwise, during about 12 hours (1). 
• Fractionally distil the total pyrolysate carefully and slowly at atmospheric pressure and collect the fraction boiling below 50 °C which should be refractionated to give penta-1,4-diene of b.p. 24-27 °C.
• The yield is 170 g (52%). Assign the i.r. absorptions which occur at 3100, 1840, 1650, 995 and 910 cm ^-1. The 13C-n.m.r. (CDC1₃, TMS) reveals signals at 6 38.1 (C₃), 115.5 (C₁ and C₅), 136.6 (C₂ and C4).

Notes to keep in mind: 

1. Solidified acetic acid often blocks the flow of nitrogen in the cooling trap.It can be dislodged by careful warming with a warm-air blower.