The alkynes or acetylenic hydrocarbons They are hydrocarbons that are characterized by having a carbon-carbon triple bond.
Non-cyclic alkynes respond to the molecular formula CnH2n-2, where n represents the number of carbon atoms.
They have a degree of unsaturation even higher than that of alkenes (hydrocarbons formed by at least one carbon-carbon double bond). For instance: ethyne, propyne, 2-butyne, 3-octyne.
Characteristics of alkynes
- Between the physical properties Of the alkynes, the fact that they are compounds of low polarity stands out, therefore, they are insoluble in water but quite soluble in common organic solvents such as ether, benzene or carbon tetrachloride.
- The boiling and melting points of alkynes are very similar to those of alkanes or alkenes of equal carbon number. The boiling and melting point increases the more carbon atoms the alkyne has.
- The simplest alkyne is acetylene, followed by propylene (or propyne) and butyne, which can be 1-butyne (triple bond at the end of the molecule) or 2-butyne (triple bond in the center of the molecule) . These three are gases and those with the highest number of carbon atoms are liquid or solid.
- The triple links that characterize alkynes give great chemical reactivity to these substances and make them very prone to undergo addition reactions (hydrogen, halogens, water) and others. However, the three bonds that join one carbon atom with another are not equivalent: one of them (which is called the sigma bond) is stronger and acts as the main responsible for the union. When an alkyne is hydrogenated, the resulting compound can become double bonds or just single bonds.
- Compounds that have the triple bond at one end of the chain are called terminal alkynes. These compounds are characterized by their marked acidity, in fact, terminal alkynes are the most acidic simple hydrocarbons.
- The length between carbon atoms in the bond is 1.20 pm (picometers), even less than that of alkenes (1.34 pm) and that of alkanes (1.54 pm). Single, double and triple carbon-carbon bonds can coexist in the same molecule. When this happens, the hydrocarbon is named as an alkyne and the position of the double bond is marked with the ending “ene”, inserting it where appropriate.
Alkynes in everyday life
The simplest alkyne and one of the most used in everyday life and in the industrial field is the ethyne or acetylene. Ethine is a highly flammable gas that produces a large flame at a very high temperature, which makes it a useful substance in many processes and is characterized by its efficiency. It is used in the welding process, to manufacture lighting fixtures and for cutting materials. Other alkyne gases, such as propyne and 1-butyne, are also used in welding and cutting procedures.
Many alkynes are used in intermediate stages within the material production process like PVC, polyethylene, artificial rubber, neoprene and other plastics. Others are used in the production of solvents and as additives for fuels, paints and coatings.
Alkynes are also present in the pharmaceutical industry, are used for the production of cytostatic drugs (which are used in cancer treatments) and other medicines such as contraceptives.
Examples of alkynes
- ethyne
- propyne
- 2-butyne
- 2-pentyne
- 1-butyne
- 3-methyl-1-pentyne
- cyclobutyne
- tert-butylacetylene
- 6,6-diethyl-4-nonyne
- hex-1-en-4-yne
- 2-nonyne
- 5,6-dimethyl-3-heptyne
- 7,7-dimethyl-3,5-nonadiyne
- 4-methylcyclohexyne
- 5-methyl-2-hexyne
- deca-3,5,7-triyne
- cyclohexyne
- 2-methyl-2,4-heptadiyne
- hept-3-ene-1,6-diyne
- cyclopropyne
Diagrams of the chemical compounds of these alkynes:
