Alkynes are hydrocarbons with atleast one triple bond between carbon atoms. They will be called as terminal alkynes if the triple bond is present in the end of the carbon chain. They have the general formula of CnH2n-2.
Alkynes are unsaturated because the valency of carbon is not satisfied and they can undergo addition reactions to become saturated and the triple bonds will become single bonds. Hence, they are highly reactive and their properties are different that of alkenes and alkanes.
The alkynes can be named in two ways.
1. General naming of alkynes
In this method the alkynes are named as acetylene derivatives. The lowest alkyne with formula C2H2 is called acetylene
and other compounds are named as substituents of acetylenes. The naming of some alkynes is given below.
- CH ≡ CH - Acetylene
- CH3-C ≡ C-CH3 - Dimethyl acetylene
- CH3-C ≡ C-CH2-CH3 - Ethyl methyl acetylene
- CH3-CH2-CH2-C ≡ CH - Propyl acetylene
- CH3-C ≡ CH - Methyl acetylene
The general naming of alkynes is restricted to some fewer alkynes and will give complex results as many alkynes will be possible for complex structures like methyl isopropyl acetylene.
2. IUPAC naming of alkynes
- In IUPAC system the alkynes are named as derivatives of alkanes. The suffix 'ane' is replaced with 'yne' in the alkane name.
- For example the following compound have three carbon atoms and hence the root name is propane.
- If the suffix 'ane' is replaced with 'yne' we will get the IUPAC name of the compound as propyne. CH3-C ≡ CH - Propyne.
- If there is a substitute in the chain, the longest chain is selected in such a way that the chain contains the triple bond.
- Similarly the chain is numbered in such a way that the carbon containing the triple bond gets the least number.
- For example the following compound contains five carbon atoms in the longest chain. Hence it is named as pentyne.
- It is named from right to left so that the carbon containing triple bond gets the number 1.
- There is one substitute methyl group at 3rd carbon atom. Hence the name of the compound is 3-methyl 1-pentyne. CH3-CH2-C(CH3)-C ≡ CH - 3-methyl 1-pentyne.
The following is the list of alkynes with their General name and IUPAC name.
||CH ≡ CH
||CH3-C ≡ CH
||CH3-CH2-C ≡ CH
||CH3-C ≡ C-CH3
||CH3-CH2-C ≡ C-CH2
||Ethyl methyl acetylene
||CH3-CH(CH3)-C ≡ CH
Alkynes are important starting materials in any synthesis. The acetylene gas which is used in oxy-acetylene torch in welding is one example of alkyne. When burnt with oxygen acetylene produces heat which can heat objects up to 3000C. Hence it is used for welding and joining metal parts.
In the structure of alkynes, the carbon atom undergoes sp hybridization. The outermost electronic configuration of carbon is 2s2, 2p2. In the excited state one electron from 2s orbital is excited to 2p.
Hence, the electronic configuration in the excited state is 2s1, 2px1, 2py1, 2pz1. Here the carbon undergoes sp hybridization to give 2 sp hybridized orbitals with linear shape and with bond angle as 180o
One of the sp hybridized orbitals is connected with other carbon and other is connected with hydrogen. Hence the structure of acetylene molecule is linear.
- The two orbitals that do not take part in hybridization overlap side wise to give 2π bonds. Hence there are two pi bonds and three sigma bonds in acetylene.
- In alkyne the carbon containing triple bond is sp hybridized with more s character. Hence it is more electronegative than carbon atoms in alkenes or alkanes where the hybridization is sp2 and sp3 respectively.
Compounds having same molecular formula with different structures are called as structural isomers. Alkynes have different structural isomers like chain isomerism, position isomerism, functional isomerism etc.
1. Chain isomers
Compounds having same molecular formula with difference in carbon chain pattern like linear or branch are called chain isomers. 1-Pentyne is chain isomer for 3-methyl Butyne.
CH3-CH2-CH2-C ≡ CH and CH3-C(CH3)-C ≡ CH
2. Position isomers
Compounds having same molecular formula with difference in position of the functional group are called as position isomers. 1-Butyne and 2-Butyne are position isomers.
CH3-CH2-C ≡ CH and CH3-C ≡ C-CH3
3. Functional group isomers
Compounds having same molecular formula with difference in nature of functional group are called as functional group isomers. Alkynes are functional group isomers for cyclo alkenes. For example pentyne and cyclo pentene are functional group isomers.
CH3-CH2-CH2-C ≡ CH and
4. Geometrical isomers
The bond between carbon atoms in alkyne is rigid and cannot be rotated. Hence there should be possibility of geometrical isomers. But the orientation of atoms cannot be determined. Hence there is no possibility of geometrical isomerism in alkynes.
- Alkynes are colorless gases with specific odor. They are insoluble in water and soluble in most of the organic solvents like benzene and carbon tetra chloride.
- The first four members are colorless gases and next five to six members are liquids. The branched alkynes will have low boiling points then linear alkynes due to compact packing efficiency.
- The H/C ratio is least in alkynes and hence they will burn with sooty flame due to incomplete combustion.
- The alkynes having triple bond in the last carbon atom are called as terminal alkynes.
- The hydrogen atom attached with carbon in terminal alkyne is polarized due to high electronegative nature of carbon.
- The hydrogen gets partial positive charge and can be removed easily by strong bases like soda amide. Hence alkynes are more acidic then alkenes and alkanes.
CH ≡ CH and CH2=CH2 and CH3-CH3
pKa 46 54 65
- Acetylene is used in oxy-acetylene torch. When acetylene is burnt in sufficient oxygen we can get the temperature of around 3000oC which can be used to melt and join the metal parts.
- Chlorinated acetylene is used as solvent. Similarly acetylene is a starting material in the synthesis of polyvinyl chloride and polyacrylo nitrile which are used as polymers.
- Alkyne carbanions are very good nucleophiles and can be used to attack carbonyl compounds, alkyl halides to give addition product and thus they are important material in the chain expansion of organic compound.