SYNTHESIS OF METHYL HYDROGEN ADIPATE

• Place 175g (1.2mol) of adipic acid, 50ml (1.25 mol) of absolute methanol, 15 ml of concentrated hydrochloric acid and a few fragments of porous pot ('boiling chips') in a 500ml round-bottomed flask provided with a reflux condenser (1). 

• Heat cautiously at first until the mixture becomes homogeneous and then reflux for 8 hours. 

• Transfer the mixture to a flask fitted with a fractionating column filled with glass helices and arrange to heat the column with a heating tape, the current to which is controlled by a Variac transformer. 

• Careful fractionation under reduced pressure yields dimethyl adipate, b.p. 113-114 °C/6 mmHg (21g), and methyl hydrogen adipate, b.p. 154-156 °C/6 mmHg (66g, 34%). 

• Unchanged adipic acid remains in the flask.

Cognate preparation: Methyl hydrogen sebacate

• Place 1 15 g (0.56 mol) of sebacic acid, 20ml -(0.5 mol) of absolute methanol, 6 ml of concentrated hydrochloric acid and a few fragments of porous pot in a 500ml round-bottomed flask fitted with a reflux condenser. 

• Warm the mixture on a water bath until it becomes homogeneous and then reflux gently for 8 hours. 

• Transfer the mixture to a flask fitted with a fractionating column as for methyl hydrogen adipate and fractionate under reduced pressure; due precautions must be taken so that the distillate does not solidify in the condenser or receiver. 

• Collect the dimethyl sebacate at 153-1 54 °C/6 mmHg (20g, m.p. 26 °C) and the methyl hydrogen sebacate at 185-186 °C/6mmHg (46g, 43%, m.p. 37 °C). 

• The residue in the flask consists of unchanged sebacic acid.

Notes to keep in mind:

1. The acid ester may also be prepared by either of two alternative procedures: (a) 1 mol of the diester is heated with 1 mol of the diacid for several hours; 164 (b) 1 mol of the diester is dissolved in 3 to 4 volumes of ethanol to which is added a solution of 1 mol of potassium hydroxide dissolved in the minimum amount of ethanol, and the solution allowed to stand at room temperature overnight. The ethanol is removed on a rotary evaporator, water is added to the residue and the solution extracted with ether to remove unreacted diester. The residual aqueous solution is then cautiously acidified at °C, the acid ester extracted with ether and the ether extract washed, dried and evaporated. The residue is fractionally distilled under reduced pressure.