Physical Properties of Alkynes

Alkynes Physical Properties

Alkynes are the unsaturated hydrocarbons that consist of at least one triple bond between the carbon atoms. There are two types of alkynes named terminal alkynes and internal alkynes.  Terminal alkynes are the triple bonded compounds in which the carbon atom shares a triple bond with the carbon which lies at the end of the chain. Internal alkynes are the compounds in which the triple bond lies in between the two carbon atoms, neither of which are terminal. The general molecular formula of alkynes is given by CₙH₂ₙ₋₂. the physical properties of alkynes are quite similar to those of the physical properties of alkenes. In this article, we will learn about the alkynes physical properties in detail.

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The structure of the alkyne has a uniqueness in it due to the presence of hybridization. The acidity of alkynes, the non-polar bonding strength, and the linearity in them is because of the triple bonds in the alkynes. The alkyne compounds are slightly soluble in the polar solvents and are completely insoluble in water. Alkynes have the capacity to dissolve in the organic solvents since the density of the solution is less, which is yet a characteristic feature of alkenes. For example, alkynes have the capability for dissolving in the ether solution.

These triple bonded alkyne compounds possess a boiling point which is slightly higher than those of the alkanes and alkenes. For example, the boiling point of ethane is -88.6 C whereas that of ethene is -103.7 C. Ethyne has a slightly higher boiling point of -84 C. The boiling point of alkynes increases with an increase in the number of carbon atoms.

The acidity of these triple alkyne bonded compounds is higher than its counterparts, which are alkanes and alkenes. The sp hybridized alkynes are the most acidic. They can be deprotonated only with the use of strong bases. For example, ethane consists of a pKa value of 62 which makes it the least acidic, whereas ethene has a pKa value of 45. Ethyne is the most acidic amongst them all with a pKa value of 26.

Because of the repulsion of electrons, alkynes are relatively known to be unstable compounds having a high energy. The amount of energy that is present in the alkyne molecules is responsible for the generation of a large amount of heat.

Structure and Bonding of Alkynes

In the compound acetylene, the H–C≡C bond angles are 180°. By virtue of this bond angle, alkynes are rod-like. Correspondingly, the cyclic alkynes are rare. Benzyne is highly unstable. The C≡C bond distance of 121 picometers is shorter than the C=C distance in alkenes (134 pm) or the C–C bond in the alkanes (153 pm).

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The triple bond is much stronger having a bond strength of 839 kJ/mol. The sigma bond contributes to 369 kJ/mol, the first pi-bond contributes to 268 kJ/mol and the second pi-bond of 202 kJ/mol bond strength. The bonding usually is discussed in the context of the molecular orbital theory that recognizes the triple bond as arising from the overlap of the s and p orbitals. In the language of the valence bond theory, the carbon atoms in an alkyne bond are sp hybridized, which means that they each have two unhybridized p orbitals and two sp hybrid orbitals. The overlap of the sp orbital from every atom forms one sp–sp sigma bond. Each p orbital on one atom overlaps the other on the other atom, forms two pi bonds, and gives a total of three bonds. The remaining sp orbital on each of the atom can form a sigma bond to another atom, for example, to the hydrogen atoms in the parent acetylene. The two sp orbitals project on the opposite sides of the carbon atom.

FAQ (Frequently Asked Questions)

1. What do you Mean by Alkynes?

In organic chemistry, an alkyne refers to an unsaturated hydrocarbon that contains at least one carbon-carbon triple bond. The simplest acyclic alkynes that have only one triple bond and no other functional groups tend to form a homologous series with the general chemical formula CₙH₂ₙ₋₂. Alkynes are traditionally called as acetylenes, although the name acetylene is also used specifically for C₂H₂. Like the other hydrocarbons, alkynes are usually hydrophobic in nature but tend to be more reactive.

2. Where are Alkynes Used?

The most important alkyne C₂H₂ which is commonly called as acetylene. It is easily produced by placing the calcium carbide or just carbide, which is a white solid having the formula Ca₂C₂, which is often sold in the hardware stores, in contact with water. This carbide reacts much easily with water such that it should be stored in tightly closed cans for avoiding any contact with the humidity of the air. The burning of acetylene generates a very hot and luminous flame.

For the same reason, acetylene is often used in the lanterns for the cave exploration. Today, it is known to be the fuel that is used in oxyacetylene torches which is capable of producing a very high temperature of the order 3,000°C, thereby lending itself for the uses of welding and cutting metals. In addition to this, many derivatives of it are also produced from it that serve as raw materials for the plastics, synthetic rubbers, textiles, etc.