SYNTHESIS OF PENTYLBENZENE (1-Phenylpentane)

• In a 1500-ml three-necked flask equipped with a dropping funnel, a sealed stirrer unit and a double surface condenser to which is attached a guard-tube filled with a mixture of calcium chloride and soda-lime, prepare an ethereal solution of benzylmagnesium chloride from 24.3 g (1 mol) of clean, dry magnesium turnings (under 100 ml of anhydrous ether) and a solution of 126.5g (115 ml, 1 mol) of freshly distilled benzyl chloride in 500 ml of anhydrous ether. 

• Use a crystal of iodine as a catalyst. 

• Finally, cool the flask by immersion in a bath of ice-water. 

• Place a solution of 456g (2 mol) of butyl toluene-p-sulphonate in about twice the volume of anhydrous ether in the dropping funnel, and add it slowly to the vigorously stirred benzylmagnesium chloride solution, at such a rate that the ether just boils; a white solid soon forms. 

• The addition is complete after about 2 hours. 

• Pour the reaction product slowly into a mechanically stirred mixture of 1 kg of finely crushed ice, 1 litre of water and 125 ml of concentrated hydrochloric acid contained in a 4- or 5-litre beaker; the precipitated magnesium toluene-p-sulphonate will ultimately pass into the solution. 

• Separate the ether layer, extract the aqueous layer with 250 ml of ether and wash the combined ether solutions with about 100 ml of water. 

• Dry the ether solution with about 10g of anhydrous potassium carbonate. 

• Distil off the ether on a rotary evaporator, add to the mixture 5-7g of sodium cut into small pieces and heat under reflux for about 2 hours in order to remove any benzyl alcohol which may have formed by atmospheric oxidation of benzylmagnesium chloride. 

• Decant the solution and distil it from an air bath through a well-lagged and efficient fractionating column; collect the fraction, b.p. 190-210 °C. 

• Redistil and collect the pentylbenzene at 198-203 °C. The yield is 90 g (61%). 

• Record the i.r. spectrum and the p.m.r. spectrum and assign the absorptions using the spectra quoted above for butulbenzene a guide. Interpret the m.s. which shows principal fragment ions at m/z 148, 105, 91, 77 and 65.