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Alkynes


Alkynes are also known as acetylenes. The general formula of the alkyne series is CnH2n-2. Ethyne (commonly called acetylene, C2H2) is the most important member in the alkyne series.Alkynes are hydrocarbons having a triple bond between two carbon atoms.

Alkynes are named by replacing the "-ane" ending in the name of the corresponding alkane by an "-yne" ending. The longest chain selected must contain the triple bond. Table 16.4 shows the IUPAC names of some alkynes.

Table: Names of some alkynes

Molecular formula

Structural formula

IUPAC

Common name

C2H2

CH CH

ethyne

acetylene

C3H4

CH3C CH

propyne-1

methylacetylene

C4H6

CH3CH2-C CH

butyne-1

ethylacetylene

C4H6

CH3-C C-CH3

butyne-2

dimethylacetylene

Isomerism in Alkynes


Alkynes exhibit position isomerism because the triple bond may be present at different positions in the molecule of the alkyne, e.g. butyne-1 and butyne-2 are position isomers.

The carbon atoms linked through a triple bond are sp-hybridized. Thus a triple bond represents a sigma (σ ) bond and two pi (π ) bonds. The π -electrons in a triple bond are delocalised around the σ bond. A molecule of acetylene is linear and the bond angle is 180° (Fig. 16.15).

Nomenclature


In IUPAC system the suffix -ane of the corresponding alkane is replaced by yne. E.g.
HC CH                        H3C - C CH
Ethyne                             Propyne

H3C - C C - CH3         H3C - CH2 - C CH
But - 2-yne                     But - 1-yne

The remaining rules of nomenclature are same as in case of alkanes and alkenes.




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