SYNTHESIS OF 2-METHYLPENTAN-2-OL (tertiary alcohol from an ester)

CAUTION: Owing to the carcinogenic properties of methyl iodide and the tox­icity of benzene all operations should be conducted with great care and in an effi­cient fume cupboard.

• Prepare an ether solution of methylmagnesium iodide from 49 g (2 mol) of dry magnesium turnings, 284 g (124.5 ml, 2 moles) of dry methyl iodide and 400 ml of sodium-dried ether; use the apparatus and the procedure described in cognate preparation nonan-l-ol. 

• Allow to cool, and slowly add a solution of 116 g (132 ml, 1 mol) of ethyl butanoate (1) in 100 ml of anhydrous ether into the vigorously stirred solution of the Grignard reagent. 

• Reflux the mixture on a water bath for 1 hour to complete the reaction. 

• Pour the ethereal solution into a mixture of 200 ml of approximately 2 M sulphuric acid and 750 g of crushed ice. 

• Separate the upper ethereal layer and extract the aqueous solution with two 150 ml portions of ether. 

• Wash the combined ethereal extracts with dilute sodium hydrogen carbonate solution, followed by a little water, then dry with anhydrous potassium carbonate or anhydrous calcium sulphate, distil off the ether on a water bath and distil the residue through a short fractionating column. 

• Collect the 2-methylpentan-2-ol at 117-120 °C. A further small quantity of the tertiary alcohol may be obtained by redrying the low-boiling distillate, filtering and redistilling. The yield is 90 g (88%).

Cognate preparations. 2-Methylbutan-2-ol (tertiary alcohol from an ester)

• From ethyl propanoate and methylmagnesium iodide. 

• Collect the tertiary alcohol at 100-102 "C.

Triphenylmethanol (tertiary alcohol from an ester)

• Prepare a solution of phenylmagnesium bromide from 14 g (0.57 mol) of magnesium turnings, 90.5 g (60.5 ml, 0.57 mol) of dried redistilled bromobenzene and 250 ml of anhydrous ether. 

• Treat this Grignard reagent with a solution of 37.5 g (36 ml, 0.25 mol) of dry ethyl benzoate in 100 ml of sodium-dried ben­zene (CAUTION) following the procedure described above. 

• Decompose the reaction product by pouring it slowly, with constant stirring, into a mixture of 750 g of crushed ice and 25 ml of concentrated sulphuric acid. 

• Continue the stirring until all the solid dissolves; it may be necessary to add 25 g of solid ammonium chloride to facilitate the decomposition of the magnesium com­plex, and also a little more benzene to dissolve all the product. 

• When all the solids have passed into solution, separate the benzene layer and wash it suc­cessively with 100 ml of water, 100 ml of 5 per cent sodium bicarbonate solu­tion and 100 ml of water. 

• Remove the benzene as completely as possible from a 1-litre round-bottomed flask using a rotary evaporator; steam distil the residue in order to separate unchanged bromobenzene and biphenyl (by-product). 

• Filter the cold residue in the flask at the pump, wash it with water and dry. 

• The resulting crude triphenylmethanol weighs 62 g. Recrystallise it from carbon tetrachloride (4 ml per gram of solid): the first crop of crystals, after drying in air to remove the solvent of crystallisation, weighs 56 g (86%) and melts at 162 °C. 

• Treat the mother-liquid with 1 g of decolourising charcoal, concentrate to one-quarter of the original volume and cool in ice: a further 3 g of pure triphenylmethanol is obtained.

3-Ethylpentan-3-ol (tertiary alcohol from diethyl carbonate)

                   

• In a 1-litre three-necked flask, equipped as in Expt 5.39, prepare a solution of ethylmagnesium bromide from 36.5 g (1.50 mol) of magnesium turnings, 163 g (112 ml, 1.50 mol) of ethyl bromide and 600 ml of anhydrous ether. 

• Run into the resulting ethereal Grignard reagent a solution of 52 g (53.5 ml, 0.44 mol) of pure diethyl car­bonate in 70 ml of anhydrous ether, with rapid stirring, over a period of about 1 hour. 

• A vigorous reaction sets in and the ether refluxes continually. 

• When the diethyl carbonate has been added, heat the flask on a water bath with stirring for another hour. 

• Pour the reaction mix­ture, with frequent shaking, into a 2-litre round-bottomed flask containing 500 g of crushed ice and a solution of 100 g of ammonium chloride in 200 ml of water. 

• Transfer to a separatory funnel, remove the ether layer and extract the aqueous solution with two 175 ml portions of ether. 

• Dry the combined ethereal extracts with anhydrous potassium carbonate or with anhydrous calcium sulphate, and remove the ether on a water bath. 

• Distil the alcohol through a short fractionating column and collect the fraction boiling at 139­142 °C as pure 3-ethylpentan-3-ol. 

• A further small quantity may be obtained by drying the low boiling fraction with 2 g of anhydrous potassium carbonate or anhydrous calcium sulphate, filtering and redistilling. 

• The total yield is 44 g (86%).

• The following tertiary alcohols may be prepared similarly from the appro­priate Grignard reagent and diethyl carbonate in yields of 75-80 per cent; 4- propylheptan-4-ol, b.p. 89-92 °C/20 mmHg; 5-butylnonan-5-ol, b.p. 129­131 °C/20 mmHg; 6-pentylundecan-6-ol, b.p. 160-163 °C/19 mmHg.