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INTRODUCTION TO ALKYNES AND THEIR PHYSICAL PROPERTIES

Introducing Alkynes 

In the previous two units, you have studied the chemistry of alkanes and alkenes. We shall now study another kind of hydrocarbons known as alkynes which contain carbon-carbon triple bond. Ethyne, CH≡≡CH, the simplest alkyne, was burnt in the miners’ lamps before electric lamps were developed. It is used in oxyacetylene torches for cutting and welding metals.It is extensively used as a fuel gas. In industry, it is the starting material for the preparation of many important chemicals, e.g., ethanoic acid, chloroethene (vinylchloride), propanone, butanol, ethanol, etc.
A large number of naturally occurring compounds containing triple bonds have been isolated from plant kingdom. For example, a triyne from safflower has considerable activity against nematodes and evidently forms part of the plant’s chemical defence against infestation.You will see in this unit that the carbon-carbon triple bond reacts with many of the reagents that react with alkenes. You will also study the most unique aspect of the chemistry of alkynes, the acidity of terminal alkynes.

Types of Alkynes

Alkynes, also known as acetylenes, constitute the homologous series of open chain unsaturated hydrocarbons that contain one or more carbon-carbon triple bond. Alkynes may be of two types: terminal and internal. In the terminal alkynes, the triple bond lies at the end of the carbon chain and in the internal alkynes, the triple bond lies any where except at the terminal position.
CH3C≡CH1-propyne (terminal alkyne)
CH3CH2C≡CCH2CH33-hexyne (internal alkyne)
CH3C≡CC≡CCH32,4-hexadiyne (internal alkyne)

Physical properties of Alkynes 

1. Physical State

The first three alkynes are gases while those containing five to thirteen carbon atoms are liquids and higher alkynes are solids.

2. Melting and Boiling Points

The melting and boiling points of alkynes are quite low and increase regularly with increase in molecular mass. Alkynes are less volatile than alkanes and alkenes.

3. Solubility

Alkynes are insoluble in water but are soluble in organic solvents such as benzene, hexane, ether, carbon tetrachloride, etc.

4. Density

All alkynes are lighter than water. Their densities increase regularly with increase in molecular mass.

 Table showing Physical Properties of Some Alkynes

ASSIGNMENT : INTRODUCTION TO ALKYNES AND THEIR PHYSICAL PROPERTIES ASSIGNMENT MARKS : 10  DURATION : 1 week, 3 days

 

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