Alkynes

Alkynes:

Alkynes are unsaturated hydrocarbons that contain carbon-carbon triple bonds in their molecules. Their general formula is C_nH_2n-2. The first member of alkyne series is Ethyne popularly known as acetylene. Oxyacetylene torch is used in welding.

Nomencluture of alkynes:

Let us write the IUPAC name for the below mentioned alkynes by applying the general rules of nomenclature that we already discussed in unit No.11

General Methods Of Preparation Of Alkynes

1. Preparation of alkynes from alkenes:

This process involves two steps:

(i) Halogenation of alkenes to form vicinal dihalides

(ii) Dehalogenation of vicinal dihalides to form alkynes.

2. Preparation of alkene from gem dihalides:

A compound containing two halogen atoms on the same carbon atom is called gem dihalide (Latin word 'Gemini' means twins). On heating with alcoholic KOH, gem dihalides give alkynes.

3. Preparation Of Alkenes From Elec-trolysis Of Salts Of Unsaturated Di-carboxylic Acids. (Kolbe’s Electrolytic Method)

Electrolysis of sodium or potassium salt of maleic or fumaric acid yields alkynes.

4. Industrial prefaration of ethyne:

Ethyne can be manufactured in large scale by action of calcium carbide with water.

Calcium carbide required for this re-action is prepared by heating quick lime and coke in an electric furance at 3273 K

Physical properties of alkynes:

1. The first three members are gases, next eight are liquids and the higher alkynes are solids. They are all colour-less and odourless except acetylene which h as a garlic odour.

2. They are slightly soluble in water but dissolve readily in organic solvents like benzene, acetone and ethyl alcohol

Chemical properities of alkynes

Terminal Alkynes are acidic in nature. It un-dergoes polymerization and addition reaction.

1. Acidic nature of alkynes:

An alkyne shows acidic nature only if it contains terminal hydrogen. This can be explained by considering sp hybrid orbitals of carbon atom in alkynes. The percentage of S-character of sp hybrid orbital (50%) is more than sp² hybrid orbital of alkene (33%) and sp³ hybrid orbital of alkane (25%). Because of this, Carbon becomes more electronegative facilitating donation of H⁺ ions to bases. So hydrogen attached to triply bonded carbon atoms is acidic.

2. Addition reactions of alkynes

i) addition of hydrogen

ii) Addition Of Halogens:

When Br₂ in CCl₄ (Reddishbrown) is added to an alkyne, the bromine solution is decolourised. This is the test for unsaturation.

iii) Addition Of Hydrogen Halides:

Reaction of hydrogen halides to symmetrical alkynes is electrophilic addition reaction.

This reaction also follows Markovnikoff’s rule.

Addition of HBr to unsymmetrical alkene follows Markownikoff’s rule.

iv) Addition Of Water:

Alkynes undergo hydration on warming with mercuric sulphate and dilute H₂SO₄ at 333K to form carbonyl compounds.

3. Ozonolysis:

Ozone adds to carbon-carbon triple bond of alkynes to form ozonides. The ozonides are hydrolyzed by water to form carbonyl compounds. The hydrogen peroxide (H₂O₂) formed in the reaction may oxidise the carbonyl compound to carboxylic acid.

4. Polymerisation:

Alkyne undergoes two types of po-lymerisation reaction

(i) Linear Polymerisation:

Ethyne forms linear polymer, when passed into a solution of cuprous chloride and ammonium chloride.

(ii) Cyclic Polymerisation:

Ethyne undergoes cyclic polymeriza-tion on passing through red hot iron tube. Three molecules of ethynepolymerises to benzene.

Uses of Alkynes

1. Acetylene is used in oxy acetylene torch used for welding and cutting met-als.

2. It is used for manufacture of PVC, polyvinyl acetate, polyvinyl ether, orlon and neoprene rubbers.