SYNTHESIS OF (Z)-BUT-2-ENOIC ACID

STEP 1: In a 250-m1 hydrogenation flask, place 100 mg of Lind­lar catalyst (1) and a solution of 2 g (0.024 mol) of but-2-ynoic acid in 100 ml of ethanol. 

STEP 2: Attach the flask to the adapter of the atmospheric hydrogenation apparatus, and fill the flask and gas burettes with hydrogen; note the volumes in the gas burettes. 

STEP 3: Agitate the flask and note the burette readings when uptake of hydrogen practically ceases (the volume of hydrogen absorbed should be in the region of 540 m1). 

STEP 4: The hydrogen in the apparatus with air. Disconnect the hydrogenation flask, and filter the solution to remove the catalyst which should then be transferred to a residue bottle. 

STEP 5: Evaporate the ethanol on a rotary evaporator; a white solid is obtained if the temperature of the flask is below 15 °C. 

STEP 6: Gas—liquid chromatography analysis (using a 1.5 m column of 10 per cent diisodecyl phthalate containing 1 per cent phosphoric acid on Chromosorb W held at 90 °C with a nitrogen flow rate of 20 ml per minute) of a solution of the acid in ether shows the crude product to contain 95 per cent of (Z)-but-2-enoic acid (2). 

STEP 7: The pure acid, m.p. 15 °C, may be obtained by low temperature recrystallisation using light petroleum (b.p. 40-60 °C). 

STEP 8: The p.m.r. spectrum should be recorded and com­pared with that of crotonic acid.

Notes to keep in mind:

1.  A suitable preparation is available from, for example, Fluka-A.G. or Aldrich.

2. Under these conditions isomerisation to the more stable (E) isomer is not appar­ent. The retention times for (Z)-but-2-enoic acid, (E)-but-2-enoic acid and but-2-ynoic acid are 6.8, 10.0 and 19.8 minutes respectively.