SYNTHESIS OF ETHYLBENZENE

Method A. Huang-Minion modification of the Wolff-Kishner reduction

• Place 36.0 g (0.3 mol) of redistilled acetophenone, b.p. 201 °C, 300 ml of diethylene glycol, 30 ml of 90 per cent hydrazine hydrate and 40g of potassium hydroxide pellets in a 500-ml two-necked round-bottomed flask fitted with a reflux condenser; insert a thermometer supported in a screw-capped adapter in the side-neck so that the bulb dips into the reaction mixture. 

• Warm the mixture on a boiling water bath until most of the potassium hydroxide has dissolved and then heat under reflux for 1 hour either by means of a free flame or by using a heating mantle. 

• Remove the reflux condenser and fit a still-head and condenser for downward distillation. 

• Distil until the temperature of the liquid rises to 175 °C (1). 

• Separate the upper hydrocarbon layer from the distillate and extract the aqueous layer twice with 20 ml portions of ether. Dry the combined upper layer and ethereal extracts with magnesium sulphate, remove the ether on a water bath and distil the residue. 

• Collect ethylbenzene at 135-136 °C; the yield is 20g (62.5%). The i.r. spectrum shows absorptions at c. 3050 and 2950 cm⁻¹ for the stretching of the aromatic and alkyl carbon-hydrogen bonds respectively. 

• Monosubstitution is confirmed from the absorption at c. 690 and 745 cm⁻¹ arising from five adjacent hydrogens. The p.m.r. spectrum (CDCl₃, TMS) shows signals at 𝛿 1.19 (q, 3H, Me), 2.53 (t, 2H, CH 2 ) and 7.07 (s, 5H, C— H). The ¹³C-n.m.r. spectrum (CDCl₃ , TMS) shows signals at 𝛿 15.8, 29.1, 125.8, 127.9, 128.4 and 144.1.

Notes to keep in mind:

1. The reduction takes place at a comparatively low temperature and is fairly rapid for acetophenone. With higher ketones, the upper layer of the distillate should be returned to the contents of the flask and the heating under reflux continued for 3-5 hours. The reaction mixture and the aqueous distillate are then combined, extracted with ether and the ether extract treated as described above.

Method B

• The toluene-p-sulphonylhydrazone of acetophenone (0.721g, 2.5mmol) (1), m.p. 140-141.5 °C, is placed in a flame-dried, nitrogen-filled flask containing 5 ml of chloroform. 

• Catecholborane (0.52 ml, 5.0mmol) is added and the reduction allowed to proceed for 2 hours at room temperature (2). 

• Methanol (1 ml) is added to destroy the excess of hydride followed by the addition of tetrabutylammonium acetate (0.7g, 2.5mmol). 

• The reaction mixture is stirred for 4 hours when g.l.c. analysis indicates a 94 per cent yield of ethylbenzene. The product is isolated by distillation, yield 0.21g (79%), b.p. 132-136 °C.

Notes to keep in mind:

1. The general procedure for the preparation of toluene-p-sulphonylhydrazones.

2. The reaction may be monitored by removing aliquot portions with a syringe, mixing with deuterochloroform in an n.m.r. tube, and recording the spectra.