SYNTHESIS OF 3,3-DIMETHYLBUT-1-ENE

CAUTION: This preparation should be conducted in an efficient fume cup­board.

STEP 1: From 51 g (63 ml, 0.5 mol) of 3.3-dimethylbutan-2-ol prepare 041,2,2- trimethylpropyl]S-methyl dithiocarbonate (the xanthate ester), up to and including the removal of toluene and residual alcohols with a rotary evaporator. 

STEP 2: Transfer the red residual crude xanthate ester to a 250-m1 round-bottomed flask. Fit a vertical air con­denser (about 12 inches), and attach a still-head with thermometer leading to an efficient double surface condenser terminating in a receiver adapter and flask. 

STEP 3: Immerse the receiver flask in an ice-salt bath and arrange for the water supply to the condenser to be cooled in an ice-salt bath. Heat the xanthate ester to boiling using a Bunsen burner; smooth decomposition occurs forming the alkene which slowly distils. 

STEP 4: On completion of the pyrolysis wash the distillate with three 10 ml portions of ice-cold 20 per cent potassium hydroxide solution, then with ice-cold water and finally dry the organic layer with anhydrous calcium chloride. 

STEP 5: Distil and collect 3,3-dimethylbut-l-ene as a fraction having b.p. 40-42 °C (cool the receiver flask in ice). 

STEP 6: The yield is 23.5 g (53%) (1). Further purification may be effected by redistillation from sodium metal. 

STEP 7: The product shows the characteristic absorption bands in the i.r. at 3050 (=C—H), 1820 (=CH₂ overtone), 1640 (C=C), 1385 (m), 1365 (s) ((CH₃)₃C), 995 and 915 cm^-1 (CH and CH₂ deformation). 

STEP 8: Assign the "C-n.m.r. signals which occur at b 29.4, 33.7, 109.0 and 149.8.

Notes to keep in mind: 

1. Gas-liquid chromatography analysis using 10 per cent Silicone oil column (1.5 m) at 40 °C with a nitrogen flow rate of 40 ml per minute gives t_R 1.18 minutes