SYNTHESIS OF ETHYL CYCLOHEXYLIDENEACETATE

CAUTION: Hydrogen gas is evolved in the first part of this experiment, hence the apparatus should be sited in an efficient fume-cupboard.

• Triethyl phosphonoacetate (11.2g, 0.05 mol) is added dropwise at 20 °C to a slurry of 50 per cent sodium hydride (2.4g, 0.05 mol) in 100 ml of dry 1,2-dimethoxyethane. 

• After the addition, the reaction mixture is stirred for 1 hour at room temperature until gas evolution has ceased. 

• Cyclohexanone (4.9g, 0.05 mol) is added dropwise at such a rate that the temperature is maintained below 30 °C. 

• After the addition, the solution is stirred for 15 minutes at room temperature during which time a viscous semi-solid appears. 

• The mixture is taken up in a large excess of water, and the aqueous solution extracted with ether. 

• The ether layer, after being dried over magnesium sulphate and evaporated, gives a liquid residue, b.p. 88-90 °C/10mmHg, 5.8g (70%), nᴅ²⁵ 1.4704. The i.r. spectrum shows a strong absorption band at 1660 cm⁻¹.

Cognate preparation: Ethyl (E)-but-2-enoate 2SS (PTC procedure)

• A solution of triethyl phosphonoacetate (35 mmol) and acetaldehyde (35 mmol) in dichloromethane (5 ml) is added dropwise to a stirred two-phase system consisting of dichloromethane (35 ml), aqueous sodium hydroxide (20 ml, 50%) and tetrabutylammonium iodide (0.7g) (1). 

• The strongly exothermic reaction is complete in 15 minutes. 

• The organic layer is separated, washed with water (5 ml), and dried with magnesium sulphate. 

• Evaporation of the solvent and distillation of the residue affords the product, b.p. 51-52 °C/25mmHg, in 54 per cent yield.

Notes to keep in mind:

1. To avoid undesirable reactions of the aldol-type, both substrates should be added simultaneously to the reaction mixture.