SYNTHESIS OF 2-METHYL-6-PHENYLHEXA-3,5-DIYN-2-OL

w-Bromophenylacetylene

STEP 1: Prepare a solution of sodium hypobromite as follows. 

STEP 2: To a 250-m1 conical flask fitted with a ground glass joint add 60 g of ice, 30 ml of 10 m aqueous sodium hydroxide (0.30 mol) and 21.8 g (7.0 ml, 0.136 mol) of bromine. 

STEP 3: Swirl for a few seconds to dissolve the bromine, then add 13.0 g (14.0 ml, 0.127 mol) of phenylacetylene (1). 

STEP 4: Securely stopper the flask, and shake vigorously for 5 hours on a mechanical shaker. 

STEP 5: Extract the product from the reaction mixture with three 50 ml portions of ether, wash the combined extracts with water and dry the ethereal layer with anhydrous sodium sulphate. 

STEP 6: Filter the solution and remove the ether on a rotary evaporator. w-Bromophenylacetylene (2) sufficiently pure for use in the following reaction is obtained as a light yellow oil (3); the yield is 20.4 g (89%).

2-Methyl-6-phenylhexa-3,5-diyn-2-ol

STEP 1:  In a 100-m1 conical flask place 40 ml of 25-30 per cent aqueous dimethylamine solution (0.22-0.27 mol), 200 mg (0.002 mol) of copper(i) chloride and 650 mg (0.009 mol) of hydroxylamine hydrochloride. 

STEP 2: Add 12 ml (10.42 g, 0.124 mol) of 2-methylbut-3-yn-2-ol, add a magnetic follower bar and chill in an ice-water bath. 

STEP 3: Then, whilst stirring magnetically, and with continued cooling, add from a dropping pipette 20.4 g (0.113 mol) of w-bromophenylacetylene and ensure complete transfer of the bromo compound by rinsing several times the pipette and container with reaction mixture. 

STEP 4: When the addition has been completed, continue stirring and cooling for a further 10 minutes and then add 25 ml of 5 per cent aqueous potassium cyanide to complex the copper salts. 

STEP 5: Extract the solution with four 40 m1 portions of ether and dry the combined extracts with anhydrous sodium sulphate. Filter and evaporate the ether solution using a rotary evaporator. 

STEP 6: Purify the solid product by recrystallisation from light petroleum (b.p. 80-100 °C), and wash the filtered product with light petroleum (b.p. 40-60 °C). 

STEP 7: The yield of pure diynol (4), m.p. 55-58 °C, is 15.3 g (73%).

Notes to keep in mind:

1. Phenylacetylene has a penetrating and persistent odour and is mildly lachry­matory. It is recommended that it be handled in the fume cupboard and that protective gloves he worn.

2. co-Bromophenylacetylene has a very penetrating and persistent odour and is mildly lachrymatory. It should be stored in the fume cupboard and care taken during subsequent handling.

3. Gas-liquid chromatography analysis using a 1.5 m S.E. 52 chromatographic column, at 120 °C with a nitrogen flow rate of 45 ml/minute, gives retention times for phenylacetylene and w-bromophenylacetylene of 2.16 and 0.76 minutes respectively.

4. The purity of this alkynol may be checked by g.l.c. analysis using an S.E. 52 chro­matographic column. With a 1.5-m column, at 170 °C and with a nitrogen gas flow rate of 45 ml/minute, the retention time is 3.84 minutes. Similar conditions using an S.E. 30 column give a retention time of 3.16 minutes. The two reactants have very much shorter retention times [see Note (3) above].