ALKYNES: PROPERTIES, STRUCTURE, USES AND EXAMPLES - CHEMISTRY - 2025

The alkynes are hydrocarbons or organic compounds containing in their structures a triple bond between two carbons. This triple bond (≡) is considered a functional group since it represents an active site of the molecule, and therefore is responsible for their reactivity.

Although alkynes do not differ much from alkanes or alkenes, they exhibit greater acidity and polarity due to the nature of their bonds. The precise term to describe this slight difference is what is known as unsaturation.

By jason.kaechler (Flickr: Oxygen / Acetylene Torch), via Wikimedia Commons

Alkanes are saturated hydrocarbons, while alkynes are the most unsaturated with respect to the original structure. What does this mean? That an alkane H ₃ C-CH ₃ (ethane) can be dehydrogenated to H ₂ C = CH ₂ (ethene) and subsequently to HC≡CH (ethyne, or better known as acetylene).

Note how as additional bonds are formed between the carbons the number of hydrogens bonded to them decreases. Carbon, due to its electronic characteristics, seeks to form four simple bonds, so the higher the unsaturation, the greater the tendency to react (with the exception of aromatic compounds).

On the other hand, the triple bond is much stronger than the double bond (=) or the single bond (-), but at a high energy cost. Hence most hydrocarbons (alkanes and alkenes) can form triple bonds at elevated temperatures.

As a consequence of the high energies of these, and when they break, they release a lot of heat. An example of this phenomenon is seen when acetylene is burned with oxygen and the intense heat of the flame is used to weld or melt metals (top image).

Acetylene is the simplest and smallest alkyne of all. Other hydrocarbons can be expressed from its chemical formula by substituting the H for alkyl groups (RC≡CR '). The same happens in the world of organic synthesis through a large number of reactions.

This alkyne is produced from the reaction of calcium oxide from limestone and coke, a raw material that provides the necessary carbons in an electric furnace:

CaO + 3C => CaC ₂ + CO

CaC ₂ is calcium carbide, an inorganic compound that finally reacts with water to form acetylene:

CaC ₂ + 2H ₂ O => Ca (OH) ₂ + HC≡CH

Physical and chemical properties of alkynes

Polarity

The triple bond distinguishes alkynes from alkanes and alkenes. The three types of hydrocarbons are apolar, insoluble in water, and very weak acids. However, the electronegativity of the double and triple bond carbons is greater than that of the single carbons.

According to this, the carbons adjacent to the triple bond give it negative charge density by inductive effect. For this reason, where the C≡C or C = C bonds are, there will be a higher electron density than in the rest of the carbon skeleton. As a consequence, there is a small dipole moment whereby molecules interact by dipole-dipole forces.

These interactions are very weak if their dipole moments are compared with those of the water molecule or any alcohol. This is reflected in their physical properties: alkynes generally have higher melting and boiling points compared to their less unsaturated hydrocarbons.

Likewise, due to their low polarity, they are less insoluble in water, but they are soluble in nonpolar organic solvents such as benzene.

Acidity

Likewise, this electronegativity causes the hydrogen HC ≡CR to be more acidic than any present in other hydrocarbons. Therefore, alkynes are more acidic species than alkenes and much more acidic than alkanes. However, its acidity is still negligible when compared to that of carboxylic acids.

Since alkynes are very weak acids, they only react with very strong bases, such as sodium amide:

HC≡CR + NaNH ₂ => HC≡CNa + NH ₃

From this reaction a solution of sodium acetylide is obtained, raw material for the synthesis of other alkynes.

Reactivity

The reactivity of alkynes is explained by the addition of small molecules to their triple bond, decreasing their unsaturation. These may well be hydrogen molecules, hydrogen halides, water or halogens.

Hydrogenation

The small molecule of H ₂ is very elusive and fast, so to increase the probabilities that they are added to the triple bond of the alkynes, catalysts must be used.

These are usually metals (Pd, Pt, Rh or Ni) finely divided to increase the surface area; and thus, the contact between hydrogen and alkyne:

RC≡CR '+ 2H ₂ => RCH ₂ CH ₂ R'

The result is that the hydrogen is "anchored" to the carbons by breaking a bond, and so on until the corresponding alkane, RCH ₂ CH ₂ R ', is produced. This not only saturates the initial hydrocarbon, but also modifies its molecular structure.

Adding hydrogen halides

Here the inorganic molecule HX is added, where X can be any of the halogens (F, Cl, Br or I):

RC≡CR '+ HX => RCH = CXR'

Hydration

The hydration of alkynes is when they add a water molecule to form an aldehyde or a ketone:

RC≡CR '+ H ₂ O => RCH ₂ COR'

If R 'is an H, it is an aldehyde; if it is an alkyl, then it is a ketone. In the reaction, a compound known as enol (RCH = C (OH) R ') is formed as an intermediate.

This undergoes a conversion from the enol form (C – OH) to the ketonic form (C = O) in an equilibrium called tautomerization.

Adding halogens

And regarding the additions, the diatomic molecules of halogens (X ₂ = F ₂, Cl ₂, Br ₂ or I ₂) can also be anchored to the carbons of the triple bond:

RC≡CR '+ 2X ₂ => RCX ₂ –CX ₂ R'

Acetylene alkylation

Other alkynes can be prepared from the sodium acetylide solution using an alkyl halide:

HC≡CNa + RX => HC≡CR + NaX

For example, if it were methyl iodide, then the resulting alkyne would be:

HC≡CNa + CH ₃ I => HC≡CCH ₃ + NaX

HC≡CCH ₃ is propyne, also known as methyl acetylene.

Chemical structure

By Ben Mills, from Wikimedia Commons

What is the structure of alkynes? An acetylene molecule is shown in the upper image. From it, the linear geometry of the C≡C bond can be clearly observed.

Therefore, where there is a triple bond, the structure of the molecule should be linear. This is another of the notable differences between them and the rest of the hydrocarbons.

Alkanes are usually represented as zigzags, because they have sp ³ hybridization and their bonds are 109º apart. They are actually a chain of covalently linked tetrahedra. While alkenes are flat due to sp ² hybridization of their carbons, more specifically forming a trigonal plane with bonds separated by 120º.

In alkynes the orbital hybridization is sp, that is, they have 50% s character and 50% p character. There are two sp hybrid orbitals that bind to H atoms in acetylene or to alkyl groups in alkynes.

The distance between the two H or R is 180º, in addition to the fact that only in this way the pure p orbitals of the carbons can form the triple bond. For this reason the bond –C≡C– is linear. Looking at the structure of any molecule, –C≡C– stands out in those regions where the skeleton is very linear.

Distance of the links and terminal alkynes

The carbons in the triple bond are closer together than in the double or single bond. In other words, C≡C is shorter than C = C and C – C. As a result, the bond is stronger because the two π bonds help stabilize the single σ bond.

If the triple bond is at the end of a chain, then it is a terminal alkyne. Therefore, the formula of said compound must be HC≡CR, where H highlights the end or beginning of the chain.

If, on the other hand, it is an internal triple bond, the formula is RC≡CR ', where R and R' are the right and left side of the chain.

Nomenclature

How are alkynes named according to the rules dictated by the IUPAC? In the same way as alkanes and alkenes have been named. To do this, the suffix –ano or –eno is changed to the suffix –ino.

For example: HC≡CCH ₃ is called propyne, since it has three carbons, like propane (CH ₃ CH ₂ CH ₃). HC≡CCH ₂ CH ₃ is 1-butyne, which is a terminal alkyne. But in the case of CH ₃ C≡CCH ₃ it is 2-butyne, and in this the triple bond is not terminal but internal.

CH ₃ C≡CCH ₂ CH ₂ (CH ₃) ₂ is 5-methyl-2-hexyne. The carbons are counted from the side closest to the triple bond.

Another type of alkynes are cycloalkines. For them it is enough to replace the suffix –ano with –ino of the corresponding cycloalkane. Thus cyclopropane that has a triple bond is named as cyclopropino (which does not exist).

When there are two triple links, the prefix di- is added to the name. Examples are HC≡C-C≡H, diacetylene or propadino; and HC≡C – C – C≡H, butadiino.

Applications

Acetylene or ethyne

The smallest of the alkynes thickens the possible number of uses for these hydrocarbons. From it through alkylations other organic compounds can be synthesized. Likewise, it undergoes oxidative reactions to obtain ethanol, acetic acid, acrylic acid, among others.

Another of its uses consists of providing the heat source to excite the electrons of the atoms; more specifically of metal cations in atomic absorption-emission determinations, a widely used spectroscopic technique.

Natural alkynes

The only existing methods for preparing alkynes are not only synthetic or with the application of heat in the absence of oxygen, but also biological.

These use enzymes called acetylenases, which can dehydrogenate a double bond. Thanks to this, many natural sources of alkynes are obtained.

As a result, poisons, antidotes, medicines or any other compound that provides some benefit can be extracted from these sources; especially when it concerns health. The alternatives are many when modifying their original structures and having them as a support for new alkynes.

Examples of alkynes

So far numerous examples of alkynes have been mentioned. However, some come from very specific sources or have particular molecular structures: they are polyacetylenes.

This means that there can be more than one triple bond that is part of a very large structure, and not just a single carbon chain.

Tariric acid

By Yikrazuul, from Wikimedia Commons

Tariric acid comes from a plant located in Guatemala called Picramnia tariri. It is specifically extracted from the oil of its seeds.

In its molecular structure, a single triple bond can be observed that separates an apolar tail from a polar head; therefore it could be considered as an amphipathic molecule.

Histrionicotoxin

By Meodipt and Rolf Kolasch

at en.wikipedia, from Wikimedia Commons

Histrionicotoxin is a poison secreted by the skin of frogs inhabiting Colombia, Brazil and other Latin American countries. It has two triple bonds conjugated to one double bond. Both are terminal and are separated by a six-carbon ring and a cyclic amine.

Cicutoxin

By Giorgiogp2, from Wikimedia Commons

From the molecular structure of Cicutoxin, where are the triple bonds? If the double bonds are flat, as seen on the right, and the single bonds are tetrahedral, as at the ends, the triples are linear and on the slope ().

This compound consists of a neurotoxin found primarily in the water hemlock plant.

Capillina

By Klever, from Wikimedia Commons

It is an alkyne present in the essential oil of mugwort plants that is used as an antifungal agent. Two consecutive triple bonds can be observed, more correctly conjugated.

What does it mean? That triple bonds resonate throughout the entire carbon chain and involves the C = O double bond opening to C – O ^-.

Pargyline

By Harbin, from Wikimedia Commons

It is an alkyne with antihypertensive activity. Analyzing its structure in parts, we have: a benzyl group on the left, a tertiary amine in the middle, and a propynyl on the right; that is, a terminal propyne group.

References

1. Francis A. Carey. Organic Chemistry. Carboxylic acids. (sixth ed., pages 368-397). Mc Graw Hill.
2. Brennan, John. (March 10, 2018). Examples of Alkynes. Sciencing. Taken from: sciencing.com
3. BYJU'S. (2018). Triple Bond in Alkynes. Taken from: byjus.com
4. Encyclopedia of Examples (2017). Alkynes. Recovered from: examples.co
5. Kevin A. Boudreaux. Alkynes. Taken from: angelo.edu
6. Robert C. Neuman, Jr. Alkenes and Alkynes.. Taken from: chem.ucr.edu