SYNTHESIS OF HEXAN-1-0L

CAUTION: This experiment should be carried out in an efficient fume cup­board.

• Equip a 500-ml three-necked round-bottomed flask (1) with a sealed stirrer unit, a 100-ml pressure-equalising funnel fitted with an inlet adapter to allow flushing with dry nitrogen, and a two-necked adapter carrying a con­denser fitted with a calcium chloride guard-tube, and a thermometer reaching to the bottom of the flask. 

• Arrange the apparatus so that it may be cooled occasionally in an ice-water bath supported on a laboratory jack.

• Maintain a slow stream of dry nitrogen through the apparatus and place in the flask 90 ml of dry diglyme, and 3.41 g (0.090 mol, 20% excess over the theoretical requirement of 0.075 mol) of powdered sodium borohydride. 

• Stir until most of the borohydride has dissolved then add a solution of 25.2 g (0.30 mol) of hex-1-ene in 50 ml of dry diglyme. 

• Place 17.0g (15.1 ml, 0.12 mol; 20% excess over the theoretical requirement of 0.10 mol) of purified boron trifluoride-etherate (48%, w/w) in the dropping funnel followed by 25 ml of dry diglyme. 

• Adjust the flow of dry nitrogen so that a slow stream is maintained throughout the ex­periment. 

• Add the boron trifluoride-etherate in portions to the rapidly stirred mixture during 30 minutes while maintaining the temperature at 20­25 °C by occasional cooling with the ice-water bath. 

• Continue stirring at room temperature for 1 hour to ensure completion of the hydroboration reaction. 

• Add 20 ml of water dropwise from the dropping funnel to the reac­tion mixture during about 25 minutes to decompose excess sodium boro­hydride; vigorous hydrogen evolution may cause foaming during this addition. 

• When hydrogen evolution has stopped, place 40 ml (0.35 mol) of 30 per cent hydrogen peroxide in the dropping funnel. 

• Add 40 ml (0.12 mol) of 3 M aqueous sodium hydroxide in one portion down the condenser to the reaction mixture and then add the hydrogen peroxide dropwise keeping the temperature at 30-50 °C by cooling in a cold-water bath; this addition should take about 25 minutes. 

• Stir the reaction mixture at room temperature for a further 1 hour to ensure oxidation is complete and then pour it on to 250 ml of ice-water in a separatory funnel. 

• Rinse the reaction vessel with 50 ml of water and add to the contents of the funnel. 

• Extract the aqueous mixture with two 200 ml portions of ether (some insoluble inorganic material may separate in the aqueous layer at this stage) and then wash the combined ethereal extracts with eight 50 ml portions of water to remove diglyme (2). 

• Dry the ether solution over magnesium sulphate, filter and remove the solvent by flash distillation or on the rotary evaporator. 

• Transfer the residual colourless liquid to a 100-m1 round-bottomed flask and fractionally distil through a well-lagged 14 cm column filled with glass helices, collecting the fraction, b.p. 154-157 °C. 

• The product is hexan-1-ol containing approximately 6 per cent of hexan-2-ol (3); the yield is 24.7 g (81%). 

• A complete separation of the isomers may be effected by using a more efficient fractionating column such as a spinning band column; hexan-1-ol and hexan-2-ol boil at 155-156 °C and 137-138 °C respectively at 760 mmHg. Assign the "C-n.m.r. absorptions which occur at 6 14.1, 22.9, 25.9, 32.0, 32.9, and 62.6.

Notes to keep in mind:

1. The apparatus should be thoroughly dried in an oven and assembled under a stream of dry nitrogen and allowed to cool.

2. Diglyme, which has b.p. 162 °C at 760 mmHg, must be completely removed from the ether extract - otherwise it will contaminate the product. Its presence in the extract may be conveniently checked by g.l.c., using a 1.5-m column of 10 per cent Sili­cone oil on Chromosorb W held at 100 °C, with a nitrogen flow rate of 40 ml/minute, t_R 3.2 minutes.

3. The g.l.c. analysis under the conditions specified in Note (2) gives hexan-2-ol and hexan-1-ol with t_R 1.4 minutes and t_R 2 minutes respectively.

Cognate preparation: Oetan-l-ol

• Use 1.70 g (0.045 mol, 20% excess over the theoretical requirement of 0.0375 mol) of sodium borohydride, 45 ml of dry diglyme and a solution of 16.8 g (0.15 mol) of oct-1-ene in 25 ml of dry diglyme in a 250-ml three-necked round-bottomed flask (1).

• In the pressure-equalising funnel place 8.5 g (7.55 ml, 0.06 mol, 20% excess over the theoretical requirement of 0.05 mol) of purified boron trifluoride-etherate.

• Proceed as for hexan-1-ol; use half quantities in the alkaline oxidation step.

• After work-up, distil and collect the fraction at 191-192 °C.

• A product, 15.8 g (81%), is obtained which consists of octan-1-ol contaminated with about 7 per cent of octan-2-ol (2).

• These may be separated, if required, by using a more efficient fractionating column. At 760mmHg octan-1-ol has b.p. 194—195 °C and octan-2-ol has b.p. 179 °C.

Notes to keep in mind:

1. Alternatively the apparatus may be assembled with the funnel in the centre neck of the flask and the thermometer and condenser in the side-necks and stirred using a magnetic stirrer. However, the follower bar must be substantial as the reaction mixture becomes viscous towards the end of the experiment.

2. The purity of the product may be determined by g.l.c. using a 1.5-m column of 10 per cent Silicone oil on Chromosorb W held at 128 °C with a nitrogen flow rate of 40 ml/minute. Octan-1-ol has t R 3.3 minutes and octan-2-ol has t R 2.4 minutes.