SYNTHESIS OF 3-METHYLPENTA-1,2-DIENE

STEP 1: 3-Chloro-3-methylpent-1-yne

• Place 73.6 g (0.75 mol) of 3-methylpent- 1 -yn-3-ol and 750 ml of concentrated hydrochloric acid (d 1.18) in a 2-litre separating  funnel and shake the mixture vigorously. 

• Periodically remove portions of the upper organic layer and record the infrared spectrum as a liquid film; con­tinue the shaking until the broad band centred at 3390 cm ^-1, arising from the 0—H stretching vibration of the alcohol, has completely disappeared (about 15 minutes).

• Discard the dark lower aqueous acid layer and dry the pale brown organic layer over anhydrous potassium carbonate for about 2 hours; a fresh portion of anhydrous potassium carbonate should be added after 30 minutes. 

• Remove the drying agent by filtration. The yield of deep straw-coloured liquid, which consists of 3-chloro-3-methylpent-l-yne (c. 73%) and 1-chloro-3-methylpenta-1,2-diene together with traces of other rearrange­ment products, is about 70 g; it can be used without purification for the next stage. 

• The chloroacetylene has t_R 2.2 minutes on a 1.5-m column 10 per cent Silicone oil on Chromosorb W column held at 70 °C with a nitrogen flow rate of 40 ml/minute.

STEP 2: 3-Methylpenta-1,2-diene

• Prepare a zinc-copper couple as follows. 

• Place 65.4 g (1 mol) of zinc dust in a 250-ml conical flask and wash successively with four 50 ml portions of 3 per cent hydrochloric acid, two 50 ml portions of dis­tilled water, two 100 ml portions of 2 per cent aqueous copper(ii) sulphate, two 50 ml portions of distilled water, 100 ml of 95 per cent ethanol and 100 ml of absolute ethanol. 

• Each washing operation should last for about 1 minute and the liquid should then be drained as thoroughly as possible by decanta­tion before continuing with the next liquid wash. 

• Finally transfer the zinc-copper couple by washing with 100 ml of absolute ethanol into a 250-m1 three-necked, round-bottomed flask fitted with a reflux condenser, separa­tory funnel and an efficient (e.g. paddle-type) sealed stirrer. 

• Heat the stirred zinc-ethanol suspension to boiling, remove the source of heat and then add the crude chloroacetylene (69.5 g) dropwise from the separatory funnel at such a rate so as to maintain gentle reflux; this should take about 50 minutes. 

• Then heat the stirred reaction mixture under gentle reflux for 45 minutes, allow to cool slightly and rearrange the condenser for downward distillation. 

• Distil the reaction mixture with stirring and collect the allene-ethanol azeo­trope which passes over between 64 and 78 °C. 

• Wash the colourless distillate with three portions of water (1 x 150 ml; 2 x 50 ml) and then separate the upper organic layer and dry over magnesium sulphate. 

• Distil the crude prod­uct through a fractionating column (15 cm x 2 cm) filled with glass helices. 

• Reject any material distilling below 70 °C, which contains appreciable quanti­ties of acetylenic material, and collect 3-methylpenta-1,2-diene (> 95% pur­ity) as a colourless liquid, b.p. 70-72 °C; the yield is about 18.9 g (approximately 31% overall yield from 3-methylpent-l-yn-3-o1). 

• Record the infrared spectrum as a liquid film. 

• The strong absorption bands at 1964 cm^-1 and 840 cm ^-1 are characteristic of the asymmetric C=C=C stretching and =CH₂ out-of-plane wagging vibrations respectively of a terminal allene. 

• The presence of a trace of acetylenic impurity may be observed as a weak absorp­tion at 3305 cm ^-1 (=--^-C—H stretch). 

• The purity of the 1,2-diene may be checked by g.l.c. on a 1.5-m column of 10 per cent Silicone oil on Chromosorb W held at 61 °C with a nitrogen flow rate of 40 ml/minute; the compound has t_R 1.7 minutes.