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.

SYNTHESIS OF DIMETHYL ADIPATE

• In a 500-ml single-necked flask containing a magnetic stirrer bar, place 58.5g (0.4 mol) of adipic acid, 16 g (20 ml, -0.5 mol) of methanol, 83.2 g (0.8 mol) of 2,2-dimethoxy propane and 0.5 g of toluene-p-sulphonic acid. 

• Fit a reflux condenser to the flask and stir the mixture magnetically for 4 hours in a water bath kept at 45 °C. 

• Rearrange the condenser for distillation and distil off acetone (b.p. 56 °C) and methanol (b.p. 64 °C) on the water bath. 

• Distil the residue under reduced pressure (water pump) and collect the dimethyl adipate, b.p. 130°C/25mmHg. The yield is 54.9 g (79%).

SYNTHESIS OF ISOPROPYL LACTATE

• Place a mixture of 53 g (0.5 mol) of 'AnalaR' lactic acid (85-88% acid), 75 g (95.5 ml, 1 .25 mol) of commercial anhydrous propan-2-ol (isopropyl alcohol), 300 ml of benzene and 20g of Zerolit 225 (acid form) (1) in a 1-litre flask, equipped with an automatic water separator (e.g., a large modified Dean and Stark apparatus with a stopcock at the lower end) carrying an efficient reflux condenser at its upper end. 

• Reflux the mixture using a magnetic stirrer/hotplate unit for 5 hours or until water no longer collects in appreciable amount in the water separator; run off the water from time to time. 

• Filter off the resin at the pump and wash it with two 25 ml portions of benzene. 

• Shake the combined filtrate and washings with about 5 g of precipitated calcium carbonate, filter, and wash with a little benzene. 

• Distil the benzene solution under reduced pressure (water pump) through a short fractionating column; the isopropyl alcohol-benzene azeotrope (2) passes over first, followed by benzene. 

• Collect the isopropyl lactate at 76°C/24mmHg; it is a colourless liquid and weighs 40 g (61%). The ester boils, with slight decomposition, at 157°C/771 mmHg.

Notes to keep in mind:

1. This resin is available as the sodium form. It may be converted into the hydrogen form by treating it with about twice its volume of 1 m sulphuric acid and stirring frequently: the resin is thoroughly washed by decantation with distilled water until the washings have a pH of 6-7, filtered and dried in the air.

2. The b.p. of the propanol-benzene azeotrope at atmospheric pressure is 71-72 °C.

Cognate preparations: Butyl oleate

• Proceed as for isopropyl lactate using 28 g (0.1 mol) of redistilled oleic acid, 37.0 g (46 ml, 0.5 mol) of butan-1-ol (the excess of the latter acts as the water carrier) and 8.0 g of Zerolit 225/H® in a 250-ml flask. 

• Reflux the mixture with magnetic stirring for 4 hours, allow to cool, separate the resin by suction filtration and wash it with three 5 ml portions of butan-1-ol. 

• Remove the butanol from the combined filtrate and washings by distillation under reduced pressure (water pump); the residue consists of crude ester. 

• Distil the residue under diminished pressure (oil pump) and collect the butyl oleate at 232 °C/9 mmHg. The yield is 27 g (85%).

Furfuryl acetate:

• Reflux a mixture of 39.2 g (34.8 ml, 0.4 mol) of redistilled furfuryl alcohol, 48 g (0.67 mol) of glacial acetic acid, 150 ml of benzene and 20 g of Zerolit 225/H® in a 500-ml flask, using the apparatus described for isopropyl lactate. 

• After 3 hours, when the rate of collection of water in the water separator is extremely slow, allow to cool, separate the resin by suction filtration and wash it with three 15 ml portions of benzene. 

• Remove the benzene, etc., from the combined filtrate and washings under reduced pressure (water pump) and then collect the crude ester at 74-90 °C/ 10 mmHg; a small solid residue remains in the flask. 

• Redistil the crude ester through a short fractionating column; pure furfuryl acetate passes over at 79-80°C/17mmHg. The yield is 14.5 g (26%).

SYNTHESIS OF DIETHYL ADIPATE

• Place 146 g (1 mol) of adipic acid, 360 ml (285 g, 6.2 mol) of absolute ethanol, 180 ml of toluene and 1.5 g of concentrated sulphuric acid in a 1 -litre round-bottomed flask, attach a short fractionating column connected to a down-ward condenser and heat in an oil bath at 115°C. 

• When the acid has dissolved, an azeotropic mixture of alcohol, toluene and water commences to distil at 75 °C; the temperature of the oil bath may then be lowered to 100-110°C. 

• Collect the distillate in a flask containing 150g of anhydrous potassium carbonate. 

• Continue the distillation until the temperature at the top of the column rises to 78 °C. 

• Shake the distillate thoroughly with the potassium carbonate, filter through a Buchner funnel or fluted filter paper and return the filtrate to the flask. 

• Heat the flask again until the temperature rises to 78-80 °C (1). 

• Transfer the warm residue to a flask of suitable size and distil under reduced pressure. 

• Alcohol and toluene pass over first, the temperature rises abruptly and the diethyl adipate distils at 138°C/20mmHg (2). The yield is 195 g (96%).

Notes to keep in mind:

1. The distillate contains ethanol, toluene and water, and may be dried with anhydrous potassium carbonate and used again for esterification after the addition of the necessary quantity of alcohol.

2. The b.p. may rise several degrees towards the end of the distillation owing to super-heating.