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Alkynes: Nomenclature And Isomerism

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Alkynes: Nomenclature And Isomerism - Lesson Summary

The group of unsaturated hydrocarbons with the general formula C nH 2n-2 is known as alkynes. Alkynes contain at least one triple bond between two carbon atoms.

The first of the alkyne series is ethyne with the chemical formula, C 2H 2. Ethyne, also known as acetylene, is used for arc welding in combination with oxygen gas, which is known as oxy-acetylene flame.

Alkynes have less number of hydrogen atoms than that of alkanes and alkenes. In a structure, the carbon atom with the triple bond is assigned the lowest number. This lowest possible number of the carbon atom also denotes the position of the triple bond.

In some alkynes, there may be more than one possible structure because the position of the triple bond may be different in each structure, which are known as position isomers.
Ex:  Pentyne has two structures

In one structure, the position of the triple bond is 1 and in the other it is at 2. They are named as pent-1-yne and pent-2-yne.
 

 
The structures with similar formula but differ in the length of the chain are called chain isomers.
Ex:


 
An ethyne molecule has two hydrogen atoms bonded to two carbon atoms. Each carbon atom has two sp hybridized orbitals and two unhybridized p- orbitals. The head-on overlapping of the two sp- hybridized orbitals atoms along the inter-nuclear axis of the two carbon atoms forms a carbon-carbon sigma bond. The hydrogen-carbon-carbon bond angle is 180 °.


The un-hybridized 2p- orbitals in each carbon atom lie perpendicular to each other as well as to the plane of the C-C sigma bond. Thus 2p -orbitals undergo lateral or sideways overlapping. Thus a pi-bond is formed between two carbon atoms.



Hence, an ethyne molecule consists of one carbon-carbon sigma bond, two carbon-hydrogen sigma bonds and two carbon-carbon pi bonds. The electron cloud between two carbon atoms is cylindrically symmetrical about the inter-nuclear axis. Therefore, the shape of ethyne molecule is linear. The carbon-carbon triple bond is shorter than carbon-carbon double or single bond and is greater in terms of strength as compared to single and double bonds.

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