SYNTHESIS OF BUT-2-YNOIC ACID

3-MethylpyrazoI-5-one:

• Place 65g (0.5 mol) of ethyl acetoacetate in a conical flask and stir magnetically during the slow dropwise addition of a solution of 25g (0.5 mol) of hydrazine hydrate (98-100%) in 40 ml of absolute ethanol. 

• The temperature rises during this addition which should be regulated so that a temperature of about 60 °C is maintained; a crystalline deposit separates. 

• After further stirring for 1 hour at room temperature, cool the reaction mixture in an ice bath to complete the crystallisation, and filter. 

• Wash the product with ice-cold ethanol; it is then pure enough for use in the next stage. The yield is 43g (90%), m.p. 222 °C (phase change at 195 °C; microscope m.p. apparatus).

4,4-Dibromo-3-methyIpyrazoI-5-one:

• Dissolve 20.0g (0.2 mol) of 3-methyl-pyrazol-5-one in 80 ml of glacial acetic acid and stir magnetically during the slow dropwise addition of a solution of 32g (0.2 mol) of bromine in 20 ml of glacial acetic acid (1). 

• On completion of this addition, add 50 ml of water and continue the dropwise addition of a further 32g (0.2 mol) of bromine dissolved in 20 ml of glacial acetic acid. 

• On completion of this second addition of bromine solution allow the mixture to stand at room temperature overnight.

• Add water to precipitate the dibromopyrazolone, filter and wash the solid product under suction with distilled water until the washings are neutral. 

• The air-dried product, sufficiently pure for use in the next statge, has m.p. 130—132 °C, the yield is 41g (79%).

But-2-ynoic acid:

• Prepare a solution of 20g of sodium hydroxide in 500 ml of water and stir magnetically in an ice bath until the temperature reaches 0-5°C. 

• Add portion wise over 10 minutes 34g (0.132 mol) of 4,4-dibromo-3-methylpyrazol-5-one. 

• The bromoketone dissolves to give an orange-red solution which evolves nitrogen gas; the temperature of the solution during the addition shows only a slight tendency to rise. 

• Stir the reaction mixture for 1 hour at 0-5 °C and then at room temperature for 1 hour. 

• Cool the solution again and acidify it with concentrated hydrochloric acid. 

• Continuously extract the acidified solution with ether overnight, dry the ethereal extract with magnesium sulphate and remove the solvent on a rotary evaporator. 

• Place the flask containing the orange oil in a vacuum desiccator and allow to stand until it solidifies. 

• Extract the orange crystalline deposit with successive portions of boiling light petroleum (b.p. 60-80 °C) and concentrate the combined extracts to about 50 ml. 

• Filter the slightly off-white product, m.p. 74-75 °C; recrystallise by dissolving in the minimum volume of light petroleum (b.p. 80-100 °C), adding an equal volume of light petroleum (b.p. 40-60 °C) and allowing to cool. 

• The pure but-2-ynoic acid has m.p. 75-76 °C, the yield is 5.9g (54%). The i.r. spectrum shows absorption at 2950 (broad, —OH), 2240(sharp, disubstituted—C=C—), 1690 cm⁻¹ (broad, — 0=0 in carboxylic acid).

Notes  to keep in mind:

1. Removal of a portion of the reaction mixture when 1 mol of bromine has been added and addition to it of water results in the precipitation of the monobromo compound, m.p. 180-182 °C.