Alkynes are unsaturated hydrocarbons that contain a carbon-carbon triple bond -C≡C-. These are more unsaturated than alkenes. The first member of this series of hydrocarbons is acetylene (CH≡CH) and these are called acetylenes. The triple bond is often referred to as the acetylenic linkage. Acetylenic linkage is regarded as the functional group of alkynes.
Formula of alkyne
The alkynes are represented by the general formula CnH2n-2, where n is any positive integer. Alkynes can be described by the condensed formula R-C≡C-R, where R may be an alkyl group or H.
Structure of Alkynes
The structure of alkyne can be best understood by taking the example of acetylene.
In acetylene, the carbon atoms are sp-hybridized. Carbons are bonded to each other by a σ-bond and two π-bonds and with hydrogen by a σ-bond. The σ-bond is formed by the head-to-head overlap of the sp-hybridized orbital of carbon, and the π-bond is formed by the side-by-side overlap of the unhybridized p-orbital of the two adjacent carbon atoms. The sp-hybridized orbital of each carbon atom overlaps with the s-orbital of hydrogen in acetylene.
The acetylene molecule thus formed is linear in shape and each bond angle is 180o, the C≡C bond length is 1.21 Ao and C-H bond length is 1.08Ao. Alkynes are more reactive and unsaturated than allkene because of the presence of two π-bond.
Nomenclature of Alkyne
For the alkyne, there are three systems of naming.
- Common system,
- Acetylene derive system, and
- IUPAC system
The common name of the first member of the series is acetylene (CH≡CH). This name has been adopted even by the IUPAC system. The common name of the alkynes do not bear any relationship and are based on the names of related compounds. For examples:
However, except acetylene other names are no longer used.
Acetylene derive system
In this system, acetylene is considered to be the parent members of the alkyne series and all other alkyls are named alkyl or aryl derivatives of acetylene. Thus,
In this system of nomenclature the ending ‘ane’ of the corresponding alkane is replaced by ‘yne’ as a suffix which indicates the presence of a triple bonds other numbering systems is similar to an alkene. Some IUPAC names are given below:
Uses of alkyne
- Acetylene is commonly used to make organic compounds such as ethanoic acid, ethanol, and acrylic acid.
- Alkyne is also used in the production of polymers and polymer raw materials.
- Acetylene is used as a fuel, with many kilograms produced each year through fractional oxidation of natural gases.
- Alkynes are commonly used in the chemical ripening of fruits and vegetables.
Physical properties of alkyne
The first three numbers are gases, next eight members are liquids, and the higher alkynes are solids. they all are colorless and odourless except acetylene which has garlic odour.
They are slightly soluble in water but highly insoluble in organic solvent such as benzene, acetone, chloroform etc.
The boiling point, melting point and specific gravity show a regular increase with the increase in molecular weights. The branched chain alkyne have relatively lower boiling point.
Dipole moment of some alkynes, through small, are grater than those of the corresponding alkenes. The reason for the difference is that the elctron density in a sp-orbital lies closer to the nucleus than it does sp2 and sp3. The sp hybridized carbon in alkyne is some what more electronegative than sp2 carbon which in turn is more elctronegative than sp3 carbon. Hence, the dipole moment of alkyne is slightly greater than alkene and alkane.