Alkynes and their preparation

Preparation Of Alkynes

From calcium carbide From vicinal dihalides

From calcium carbideAcetylene is prepared in the laboratory as well as an

industrial scale by the action of water on calcium carbide.

CaC2 + 2H2O HC≡CH + Ca(OH)2

Calcium carbide required for this purpose is obtained by heating calcium oxide (from limestone) and coke in an electric furnace at 2275 K.

CaCO3 Heat CaO + CO2

From vicinal dihalidesVicinal dihalides on treatment with alcoholic

potassium hydroxide undergo dehydrohalogenation. One molecule of hydrogen halides is eliminated to form alkenyl halide which on treatment with sodamide gives alkyne.

Addition of dihydrogen

Alkynes react readily with hydrogen in the presence of finely divided Ni, Pt or Pd as a catalyst. The reaction is called hydrogenation.

HC≡CH+H2 Pt/Pd/Ni [H2C=CH2] H2 CH3−CH3

Addition of halogensReddish orange colour of the solution of

bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation. This test is known as bromine water test.

Addition of hydrogen halidesTwo molecule of hydrogen halides(HCl, HBr

and HI) add to alkynes to form gem dihalides (in which two halogens are attached to the same carbon atom). For example:

Addition of water• Alkenes react with water in the presence of

mercuric sulphate (HgSO4) and sulphuric acid at 337K. The product are carbonyl compounds (aldehydes and ketones). For eg:

polymerisationa)Linear polymerisationLinear polymerisation of ethyne takes place

to produce polyacetylene of polythyne which is a high molecular weight polyene containing repeating units of (CH=CH−CH=CH).

Cyclic polymerisationAlkynes have larger tendency to polymerize then alkenes

and, therefore these give low molecular mass polymers alkynes when passed through a red hot iron tube at 873k polymerize to give aromatic hydrocarbons. For eg:

This is the best route for entering from aliphatic to aromatic compounds.

b