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# Alkyne Structure

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 Sub Topics Alkynes are hydrocarbons that have a triple bond between two carbon atoms, with formula CnH2n-2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. Like other hydrocarbons, alkynes are generally hydrophobic but tend to be more reactive. An alkyne is a hydrocarbon that contains a carbon-carbon triple bond. Acetylene the simplest alkyne was once widely used in industry as the starting material for the preparation of acetaldehyde, acetic acid, vinyl chloride and otherhigh volume chemical but more efficient routes to these substances using ethylene as starting material are now available.

## What is a Triple Bond?

A triple bond in chemistry bond between two chemical elements involving six bonding electrons instead of the usual two in a covalent single bond. The most common triple bond, that between two carbon atoms, can be found in alkynes. A triple bond in an alkyne consists of one sigma bond and two pi bonds.  Other functional groups containing a triple bond are cyanides and isocyanides.

Alkynes are highly unsaturated than ever the alkenes. Alkynes contain four hydrogen atoms less than the corresponding alkanes and are characterized by the presence of triple bond in the molecule. The chemistry of alkynes closely resembles that of alkenes. Both classes of compounds have $\pi$ bonds that dominate their chemical reactivity, which is largely addition of electrophiles.

## Physical Properties of Alkynes

The physical properties of alkynes are very similar to those of alkenes and alkanes, with the same number of carbons. For exaple, the boiling points of hexane, 1-hexene and 1-hexyne are 69oC, 63oC and 71oC. Because of their $\pi$ bonds, the chemical properties of alkynes are very similar to those of alkenes. They ubdergo many of the same reactions as alkenes and often react as both $\pi$ bonds.

Alkynes are less common in nature than that of alkenes. Alkynes are also less important in industry. The largest use of acetylene is as a fuel for the oxyacetylene welding torch which burns at a very high temperature. Alkynes are all colourless and odourless but acetylene has a slight garlic smell due to impurities. They are insoluble in water but soluble in organic solvents.

## Chemical Properties of Alkynes

In the presence of Ni or Pt as catalyst
R-C≡ C-R $\rightarrow$ R-CH=CH-R $\rightarrow$ R-CH2-CH2-R

For example, chlorine is added. Acetylene dichloride followed by acetylene tetrachloride is formed.
CH≡CH + HBr $\rightarrow$ CHCl=CHCl $\rightarrow$ CHCl2-CHCl2

CH≡CH + HBr $\rightarrow$ CH2=CHBr $\rightarrow$ CH3-CHBr2