Poison dart frogs are considered the most poisonous frog species there is. They got their name because they were used by Native Americans as arrow poison. These frogs are poisonous because their skin releases histrionicotoxin. This substance consists of two alkyne groups that frogs use to protect themselves from reptiles and mammals.
The aliphatic hydrocarbons alkanes, alkenes and alkynes
Alkynes are aliphatic carbons with at least one triple bond. Like the alkenes, they belong to the group of unsaturated hydrocarbons. The general molecular formula of alkynes is CnH2n-2.
Like alkanes and alkenes, alkynes are aliphatic hydrocarbons. Like alkenes with their double bonds, alkynes are unsaturated hydrocarbons because of their triple bonds. You can also distinguish between the various aliphatic hydrocarbons based on their different molecular formulas.
alkanes
alkenes
alkynes
CnH2n
CnH2n+2
CnH2n-2
The homologous series of alkynes
Due to the general empirical formula, there is a homologous series for the unbranched alkynes. The homologous series of alkynes applies to alkynes with a triple bond at the first carbon atom.
Surname
molecular formula
Simplified structural formula
ethin
C2H2
CHCH
propyne
C3H4
CHC-CH3
But-1-in
C4H6
CHC-CH2-CH3
Pent 1 in
C5H8
CHC-CH2-CH2-CH3
Hex-1-in
C6H10
CHC-CH2-CH2-CH2-CH3
hept-1-yne
C7H12
CHC-CH2-CH2-CH2-CH2-CH3
Oct-1-in
C8H14
CHC-CH2-CH2-CH2-CH2-CH2-CH3
Non 1 in
C9H16
CHC-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Dec-1-in
C10H18
CHC-CH2-CH2-CH2-CH2-CH2-CH2-CH2-CH3
Naming the alkynes
The alkynes are named by deriving the alkane root name by adding the suffix -in instead of the suffix -. With their triple bonds, the alkynes belong to the functional groups.
The position of the triple bonds of the alkynes is given with a number before the -in suffix, which represents on which carbon atom the triple bond is located. If there are several triple bonds in an alkyne, the respective Greek number, e.g. di-, tri-, etc., is added in front of the name.
The arrangement of the triple bonds in the alkynes can be conjugated, but never cumulated. Since a carbon atom can only form 4 bonds, no two triple bonds can be attached to it. For this reason, a cumulative triple bond is not possible. However, triple bonds can alternate with single bonds, i.e. be conjugated, in alkynes.
The properties of alkynes
The alkynes are highly combustible due to the hydrogen and carbon atoms, which produce carbon dioxide and water. In the homologous series of alkynes, the boiling temperature continues to rise. Since the alkynes are non-polar molecules, they are also difficult to dissolve in water.
When the triple bonds of the alkynes are in the middle, they are much more stable than when the triple bond is at the end of the chain. The physical state of the alkynes from but-2-yne is liquid. In contrast, ethyne, propyne and but-1-yne are gases in their normal state.
Since alkynes have at least one triple bond connecting two carbon atoms, the hybridization is sp. There is an increased proportion of s orbitals and thus a high probability of the electrons being present on the carbon atom. When an alkyne has a triple bond at the end of its chain, it has a lower pKa value and hence high CH acidity.
The triple bonds of alkynes can be shifted by tautomerization and addition of a proton to give constitutional isomers.
The alkyne ethyne
Ethine is a colorless gas. The reactive properties of ethyne are particularly popular. Ethyne is the simplest alkyne and is also known as acetylene. This alkyne has a linear structure due to the triple bond between the two carbon atoms.
Due to its high instability, ethyne is very reactive and can therefore explode with heat or a shock. For this reason, this alkyne is often used in welding as a welding gas up to 3000 °C.
The natural occurrence of alkynes
Alkynes are very rare in nature. The alkynes are mainly found in nature as a defensive poison or as an irritant to the mucous membranes of animals. For example, two alkyne groups can be found in the skin of poison dart frogs. The frog protects itself from other creatures with its skin substance.
The reactions of the alkynes
The triple bonds make alkynes particularly suitable for addition reactions. These are mostly electrophilic additions, but nucleophilic additions also occur. Polymerization is also possible.
The hydrogenation
Hydrogenation often occurs with the alkynes. Two hydrogen atoms are added with the help of a catalyst. First, alkenes are hydrogenated from the alkynes. The Lindlar catalyst is used to prevent further hydrogenation of the alkenes to alkanes. Calcium carbonate is the carrier and palladium is the catalyst.
The hydration
In the reaction with water, also called hydration, and in mercury catalysis, an alkyne reacts to form the enol. The ketone is formed by tautomerism.
Halogenation and hydrohalogenation
Hydrohalogenation is an electrophilic addition of hydrogen halides. The triple bond is attacked, whereupon an alkenyl cation, an alkene, a halogen and a hydrogen atom are formed.
Halogenation is also an electrophilic addition. An alkene is formed and then added with a halogen or hydrogen halide. This is due to the higher reactivity of the double bond compared to the triple bond.
The nucleophilic addition
Nucleophilic addition can act on the alkynes under extreme conditions. The alkyne reacts with alcohols to form aldehydes or ketones or with thiols to form thioenol ethers.
The Polymerization
Alkynes are also used in the polymerization. During this process, polymers with double bonds are formed. For example, ethyne can react with hydrochloric acid to form vinyl chloride, or with hydrogen cyanide to form acrylonitrite. When these intermediates are polymerized, acrylic fibers are formed.
How can alkyne be detected?
The detection of the alkynes is possible with the bromine water test. The alkynes are mixed with brown bromine water, whereupon bromine atoms are added to the triple bonds. The brown bromine water then becomes colorless. However, this detection generally applies to multiple bonds, which is why not only alkynes are detected with it, but also, for example, alkenes with their double bonds.
The alkynes – everything you need to know at a glance!
If you have read the article this far, you now know the essential facts about alkynes. In the following you will find the most important points summarized again in bullet points for you.
- Alkynes are aliphatic carbons with at least one triple bond
- The general molecular formula of alkynes is CnH2n-2
- Alkynes are flammable and not water-soluble
- Electrophilic addition reactions are often used with alkynes
- The alkynes are detected by the bromine water test
Excellent! If you want to learn more about functional groups, check out the alkanes and alkenes articles on .