SYNTHESIS OF HEX-1-YNE (Butylacetylene)

STEP 1: Equip a 5-litre three-necked flask (or a 5-litre flange flask fitted with a mul­tiple head adapter) with a Herschberg or other efficient stirrer, a soda-lime guard-tube and a liquid ammonia inlet tube. 

STEP 2: Observing the precautions noted in this section, run in liquid ammo­nia until the flask is about two-thirds full (c. 3.5 litres). 

STEP 3: Prepare a suspension of sodamide from 138 g (6 mol) of clean sodium using 0.5 g of finely-powdered crystallised iron(m) nitrate as the catalyst. 

STEP 4: It may be necessary to add further quantities of liquid ammonia to maintain the volume of 3.5 litres. 

STEP 5: When the conversion of sodium into sodamide is complete, replace the ammonia-addition tube by a wide tube reaching almost to the bottom of the flask to allow for the passage of acetylene through the suspension of sodamide. 

STEP 6: The acetylene gas from a cylinder should be freed from acetone by passing through two Drechsel bottles half-filled with concentrated sulphuric acid; when the acid in the second wash bottle becomes discoloured the wash bottles should be recharged with fresh acid. 

STEP 7: An empty Drechsel bottle (to act as a safety trap) and a mercury escape valve should be inter­posed between the reaction flask and the Drechsel bottles charged with sul­phuric acid. 

STEP 8: Surround the reaction flask with an acetone-Cardice cooling bath and pass acetylene rapidly (2-3 litres per minute) into the sodamide sus­pension until a uniformly black liquid is formed (usually 4-5 hours) (1). 

STEP 9: Care­fully watch the mercury escape valve in case the wide entry tube becomes blocked by deposition of solid. If this should happen, temporarily stop the acetylene flow and clear the tube by inserting a glass rod of appropriate dia­meter. 

STEP 10: It may also become necessary to introduce some more liquid ammo­nia. 

STEP 11: Replace the soda-lime guard-tube with a pressure-equalising separatory funnel charged with 685 g (538 ml, 5 mol) of butyl bromide which is intro­duced over a period of 1.5-2 hours with stirring and while a slow stream of acetylene (c. 500 ml per minute) continues to pass through the reaction mix­ture. 

STEP 12: The reaction is exothermic so that it will be necessary to maintain the cooling bath at -50 °C by continued addition of solid carbon dioxide. 

STEP 13: When all the alkyl bromide has been added, discontinue the supply of acetylene and allow the ammonia to evaporate. 

STEP 14: Before evaporation is complete add cau­tiously 60 g of ammonium chloride with stirring to decompose the excess of sodium acetylide (or sodamide) if present. 

STEP 15: Now introduce 500 g of crushed ice followed by about 1.5 litres of distilled water. Steam distil the reaction mix­ture, separate the hydrocarbon layer in the distillate, dry over magnesium sul­phate and fractionally distil through a Hempel column filled with Fenske rings. 

STEP 16: Collect the hex- 1-yne, b.p. 71-72 °C. The yield is 280g (68%). The i.r. absorptions occur at 3300 (^---=^-CH) and 2120 cm ^-1 (C=---^-C). The "C-n.m.r. (CDC1₃, TMS) has absorptions at 6 13.7 (C₆), 18.3 (C₅), 22.1 (C₄), 30.9 (C₃), 68.4 (C₂) and 84.5 (C₁).

Notes to keep in mind: 

1. Occasionally the reaction mixture does not become completely black nor free from suspended solid; here the acetylide is an insoluble (or sparingly soluble) form, but it gives satisfactory results in the preparation of hex-1-yne. The saturated solution of the soluble forms of sodium acetylide in liquid ammonia at —34 °C is about 4.1 M.