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Alkyne Structure


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?

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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

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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

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1. Addition of H2 (Reduction)

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

2. Addition of halogens

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

3. Addition of hydrogen halides 

In the addition of hydrogen halides, the order of reactivity is HI>HBr>HCl. Markownikoff's rule is obeyed 
CH≡CH + HBr $\rightarrow$ CH2=CHBr $\rightarrow$ CH3-CHBr2

Nomenclature of Alkynes

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Alkynes are named by selecting the longest continuous carbon chain that contains the triple bond. The chain is numbered to assign the lowest number to the first carbon of the triple bond. Aklyl groups and halogens are disregarded in selecting the direction of numbering unless the same number for the triple bond is obtained from either end of the chain. 

For compounds containing both double and triple bonds, the chain is numbered from the end nearer the first multiple bond. However, in equivalently placed multiple bonds, the double bond takes precedence over triple bonds in the direction on numbering. Compounds with both double and triple bonds are called enynes, not ynenes.