Understanding Alkynes: Melting Point, Boiling Point, and Solubility
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.
FAQs on Physical Properties of Alkynes Explained
1. What are alkynes in simple terms?
Alkynes are a type of unsaturated hydrocarbon in organic chemistry. Their defining feature is the presence of at least one carbon-carbon triple bond (C≡C) in their molecular structure. The simplest alkyne is ethyne, also commonly known as acetylene.
2. What are the main physical properties of alkynes?
The key physical properties of alkynes are related to their structure and molecular mass. Here are the main points:
- Physical State: The first three members of the alkyne series (ethyne, propyne, and butyne) are gases at room temperature. The next eight are liquids, and any with more than 11 carbon atoms are solids.
- Solubility: They are non-polar molecules and are therefore insoluble in water, which is a polar solvent. However, they readily dissolve in non-polar organic solvents like benzene and ether.
- Boiling Point: The boiling points of alkynes increase as the number of carbon atoms (molecular mass) increases.
- Density: All alkynes are less dense than water.
- Colour and Odour: In their pure form, alkynes are colourless and odourless.
3. Why are alkynes generally insoluble in water?
Alkynes are insoluble in water due to a fundamental principle in chemistry: "like dissolves like." Water is a polar molecule, meaning it has a slight positive and negative charge on its ends. Alkynes, on the other hand, are almost completely non-polar. Because their molecular structures and charge distributions are so different, they cannot form strong attractions with water molecules, and therefore do not dissolve.
4. How does the boiling point of alkynes change as the carbon chain gets longer?
The boiling point of alkynes steadily increases as the carbon chain gets longer. This happens because larger molecules have a greater surface area, which leads to stronger temporary attractions between them called van der Waals forces. More energy (in the form of heat) is needed to overcome these stronger forces and turn the liquid into a gas, resulting in a higher boiling point.
5. What are some common uses of alkynes in industry?
The most important alkyne is ethyne (acetylene). It is widely used in:
- Welding and Cutting: In oxy-acetylene torches, burning acetylene produces an extremely hot flame (around 3300°C), which can easily cut and weld metals.
- Chemical Manufacturing: It serves as a starting material for producing a variety of other important chemicals, such as vinyl chloride for making PVC plastic, and acetaldehyde.
- Artificial Ripening: Acetylene gas is also used to artificially ripen fruits like bananas and mangoes.
6. Why do alkynes have slightly higher boiling points than alkanes with the same number of carbons?
Alkynes have a linear, rod-like shape around their triple bond. This straight structure allows alkyne molecules to pack together more closely and efficiently than the zigzag shape of alkanes. This close packing increases the strength of the intermolecular van der Waals forces, which requires more energy to break, leading to a slightly higher boiling point for the alkyne.
7. What is the key difference between the physical and chemical properties of alkynes?
The main difference lies in whether the alkyne's chemical identity is changed:
- Physical properties are characteristics you can observe without changing the substance itself. This includes its boiling point, solubility, density, and physical state (gas, liquid, or solid).
- Chemical properties describe how an alkyne behaves in a chemical reaction, where it transforms into a new substance. For alkynes, this includes their tendency to undergo addition reactions and the unique acidity of terminal alkynes.
