Physical Properties of Alkynes

Introduction

Alkynes are the most basic organic molecules that are made of carbon and hydrogen. Hydrocarbon is a thing that can be found in nature and that is too in anything that people usually use on daily basis. That is the reason they are considered the parent organic compounds. Any compound can be made by the replacement of hydrogen atom.

What are alkynes?

Alkynes can be defined as non-polar, unsaturated hydrocarbons that have the triple connection between the carbon and carbon particles, which can be represented as -C=C-. The general chemical formula of it is $\mathrm{C_nH_{2n–2}}$, where the value of n is changed; it can be 2, 3, 4, 5, and many more.

Figure 1: The alkyne: Ethyne

The simplest form of alkynes had at least one triple bond. Alkynes are also known as acetylenes. However, the name sometimes refers to only one kind of alkynes, which is C₂H₂. The alkynes are also referred to as acetyls and also hydrophobic.

Structure of Alkynes

The bond of alkynes means the H–C≡C bond angles forms 180° angles and because of the angle, they look like a rod. The triple bond of it is so strong that the “bond strength is 839 kJ/mol. However, the bonding is discussed based on the molecular orbital theory, in which the triple bond is recognized as arising from the overlap of s and p orbitals.

Figure 2: Structure of Alkynes

As the number n in the $\mathrm{C_nH_{2n–2}}$ changes, as per the IUPAC framework, the names of each alkyne form in a specific way. These all have a postfix -yne and the prefix depends on carbon particle numbers. For example, the ethyne’s carbon atom number is 2 and the formula is $\mathrm{C_2H_2}$, the same thing happens with the “Butyne, where the carbon atom number is 4 and the formula is $\mathrm{C_4H_6}$.

Some other alkynes are- Propyne, Pentyne, Hexyne, Heptyne, Octyne, and their carbon atom number are 3, 5, 6, 7, 8 respectively.

Isomerism in Alkynes

The structural isomerism of alkynes is mainly of four types and they are −

• Chain Isomerism: In this kind of isomerism occurs when the alkynes have five or more carbon atoms that owe to various carbon chain configurations.

• Position Isomerism: In this kind of isomerism, the isomers differ based on the triple bond location.

• Functional isomerism: In this isomerism, the functional isomers of the dienes the compounds contain two of the double bonds.

• Ring chain isomerism: In this kind of Isomerism, those show ring chain isomerism with the cycloalkanes.

Physical Properties of Alkynes

The most common physical properties of the alkynes are somewhat similar to the alkanes and alkenes −

The physical State of Alkynes

They are the unsaturated carbon that has triple security at the carbon site. The first three members of the group are colorless and present in the gas form while the remaining eight members are liquid and other than that, higher alkynes are solid in the form. The alkynes are the slightest polar in nature.

Smell or Odour

All the members of alkynes are odourless but the acetylene has a garlic-like smell. The reason behind it is the presence of impurity phosphine. Other than that, the ethylene has the slightest odour.

Boiling and Melting points of Alkynes

The melting and the boiling point of the alkynes depend on the molecular mass. Both points increase with the rise of the molecular mass. If the melting and boiling point of the alkynes are compared with the alkanes and alkenes, these are higher.

Solubility

The alkynes show a soluble nature in the case of non-polar solvent and the case of polar solvents are insoluble, such as water.

Density

The density of the alkynes increases with the increase of the molecular weight. If they can be compared with water, they are light. The density range of the alkynes is 0.69–0.77 g/cm³.

Uses of Alkyne

The alkynes are used in many sectors, such as −

• The ethyne has exceptionally fire that is the reason they are used in the oxyacetylene gas for welding and cutting.

• They are used as fuel.

• It is used to make natural mixtures such as ethanol, ethanoic corrosive, and acrylic corrosive.

• They are used as gas lights.

• Acetylene is used for making the fruits ripen.

• They are used to make cis and trans alkenes.

Conclusion

The alkyne has a unique structure that makes it different from the other compounds and essential in the field of organic chemistry. They are majorly two types, that are terminal and internal alkynes. As with any other hydrocarbon, the alkynes are also hydrophobic by nature.

FAQs

Q1. What are the main properties of alkynes?

Ans. The alkynes are the unsaturated hydrocarbons and their physical properties are similar to the same of alkanes and alkenes. The most important of them are -dissolve in organic solvents, and have the slightly dissolvable in the polar solvents. However, they do not get dissolved in the water.

Q2. Do alkynes have acidic nature?

Ans. Alkynes are by nature acidic and the reason behind it is that they can “release the hydrogen atoms and then can form the alkyne ions. Because of the acidic nature of alkynes, they can be used in the form of Brönsted-Lowry acid”.

Q3. What is the main use of the alkynes?

Ans. Alkynes are used as the lanterns that help in the cave exploration. It is also used in the oxyacetylene torches as fuel. Because of its capability of producing high temperature, it is preferred in the welding and cutting metals.

Q4. Why do alkynes have a higher boiling point?

Ans. The boiling point of the alkynes rises with the increase of molecular mass. Not only that, but the melting point also rises as an effect of that.