addition product takes place according to the following steps. The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. Majority of the reactions of alkynes are the examples of electrophilic addition reactions.
A few addition reactions are given below: (i) Addition of dihydrogen HC ≡ CH+H Pt/Pd/Ni [H C = CH ] H CH –CH ( . ) CH –C ≡ CH + H Pt/Pd/Ni [CH –CH=CH ] Propyne Propene ↓ H CH –CH –CH Propane ( . ) (ii) Addition of halogens ( . ) Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised.
This is used as a test for unsaturation. (iii) Addition of hydrogen halides Two molecules of hydrogen halides (HCl, HBr, HI) add to alkynes to form gem dihalides (in which two halogens are attached to the same carbon atom) H–C ≡ C–H+H–Br [CH = CH–Br]→ CHBr Bromoethene , -Dibromoethane ( . ) ( . ) (iv) Addition of water Like alkanes and alkenes, alkynes are also immiscible and do not react with water.
However, one molecule of water adds to alkynes on warming with mercuric sulphate and dilute sulphuric acid at K to form carbonyl compounds. ( . ) ( . ) (v) Polymerisation (a) Linear polymerisation : Under suitable conditions, linear polymerisation of ethyne takes place to produce polyacetylene or polyethyne which is a high molecular weight polyene containing repeating units of (CH = CH – CH = CH ) and can be represented as —(CH = CH – CH = CH) n — Under special conditions, this polymer conducts electricity.
Thin film of polyacetylene can be used as electrodes in batteries. These films are good conductors, lighter and cheaper than the metal conductors. (b) Cyclic polymerisation: Ethyne on passing through red hot iron tube at 873K undergoes cyclic polymerization. Three molecules polymerise to form benzene, which is the starting molecule for the preparation of derivatives of benzene, dyes, drugs and large number of other organic compounds.
This is the best route for entering from aliphatic