23 junio, 2024

Alkynes: what they are, structure, properties, uses and examples

What are alkynes?

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

Alkanes are saturated hydrocarbons, while alkynes are the most unsaturated with respect to the original structure. What does this mean? That an alkane H3C–CH3 (ethane) can be dehydrogenated to H2C=CH2 (ethene) and later to HC≡CH (ethyne, or better known as acetylene).

Notice how as additional bonds are formed between the carbons the number of hydrogens attached to them decreases. Carbon, due to its electronic characteristics, seeks to form four simple bonds, so the greater 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 (=) or 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, when they break they release a lot of heat. An example of this phenomenon can be seen when acetylene is burned with oxygen and the intense heat of the flame is used to weld or melt metals.

Acetylene is the simplest and smallest alkyne of all. Other hydrocarbons can be expressed from its chemical formula by substituting the H by alkyl groups (RC≡CR’). The same thing 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 => CaC2 + CO

CaC2 is calcium carbide, an inorganic compound that finally reacts with water to form acetylene:

CaC2 + 2H2O => Ca(OH)2 + HC≡CH

Physical and chemical properties of alkynes

Polarity

The triple bond distinguishes alkynes from alkanes and alkenes. All three types of hydrocarbons are nonpolar, 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 a 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 the 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

Also, this electronegativity causes hydrogen to HC≡CR is more acidic than any present in other hydrocarbons. Therefore, alkynes are more acidic species than alkenes and much more so than alkanes. However, its acidity remains 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 + NaNH2 => HC≡CNa + NH3

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

Alkyne 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 H2 molecule is very elusive and fast, so to increase the chances that it will add to the triple bond of alkynes, catalysts must be used.

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

RC≡CR’ + 2H2 => RCH2CH2R’

The result is that the hydrogen “anchors” to the carbons, breaking a bond, and so on until producing the corresponding alkane, RCH2CH2R’. This not only saturates the initial hydrocarbon, but also modifies its molecular structure.

Addition of 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’ + H2O => RCH2COR’

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.

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

Halogen addition

And regarding the additions, the diatomic molecules of the halogens (X2= F2, Cl2, Br2 or I2) can also be anchored to the carbons of the triple bond:

RC≡CR’ + 2X2 => RCX2–CX2R’

alkylation of acetylene

Other alkynes can be prepared from sodium acetylide solution by 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 + CH3I => HC≡CCH3 + NaX

HC≡CCH3 is propyne, also known as methylacetylene.

Chemical structure of alkynes

What is the structure of alkynes? An acetylene molecule is shown in the top 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 are sp3 hybridized and their bonds are 109° apart. They are actually a chain of covalently linked tetrahedrons. While alkenes are flat due to the sp2 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. Two sp hybrid orbitals bond to the H atoms in acetylene or to the alkyl groups in alkynes.

The distance that separates both 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 bonds 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 of this, the bond is stronger because the two π bonds help to 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 the H marks 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 sides of the chain.

alkyne nomenclature

How are alkynes named according to the rules dictated by 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≡CCH3 is called propyne, since it has three carbons, like propane (CH3CH2CH3). HC≡CCH2CH3 is 1-butyne, which is a terminal alkyne. But in the case of CH3C≡CCH3 it is 2-butyne, and in this the triple bond is not terminal but internal.

CH3C≡CCH2CH2(CH3)2 is 5-methyl-2-hexyne. Start counting the carbons from the side closest to the triple bond.

Another class of alkynes are cycloalkynes. For them it is enough to replace the suffix -ane by -yne of the corresponding cycloalkane. Thus the cyclopropane that has a triple bond is named as cyclopropine (which does not exist).

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

Uses of alkynes

acetylene or ethyne

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

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

natural alkynes

The only existing methods to prepare 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 of this, poisons, antidotes, medicines or any other compound that provides some benefit can be extracted from these sources; especially when it concerns health. There are many alternatives when it comes to modifying their original structures and using them as a support for new alkynes.

Examples of alkynes

Numerous examples of alkynes have been mentioned so far. 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

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 a nonpolar tail from a polar head; therefore it could be considered as an amphipathic molecule.

Histrionicotoxin

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

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