Nomenclature and Preparation of Alkynes

Natural gas has many uses but do you know it acts as a precursor of one of the most important and commercially dominant alkyne. Acetylene, commercially dominant alkyne, production is possible by partial oxidation of natural gas. What is the first naturally occurring acetylenic substance? The compound dehydromatricaria ester is the first naturally occurring acetylenic substance. It acts as the precursor for many compounds like acrylates as well as it acts as fuel. It was obtained from Artemisia species in the year 1826, according to Ferdinand Bohlmann. Isolation of acetylene is possible from natural sources such as plant species, marine sponges, corals, bacteria, and fungi. One such example is polyynes. In this topic, we will discuss nomenclature of alkynes and preparation of alkynes.

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Alkynes

Alkynes are unsaturated hydrocarbons containing a triple bond between two carbon atoms. The least number of hydrogen atoms is present in alkynes among alkanes, alkenes, and alkynes. The general formula of this class of organic compound is C_nH_2n–2. The first stable of an alkyne is ethyne or acetylene.

Acetylene is essential alkyne and the compound finds its use for many different purposes such as arc welding. Acetylene in this example is present in the flame as oxyacetylene flame (mixing acetylene with oxygen). It is also the precursor for many other compounds. Therefore, it is essential and interesting to learn about the nomenclature and preparation of this class of organic compounds.

In the case of alkynes understand the isomerism along with nomenclature is very important because it plays an important role in the naming of the compound. Therefore, we will study the isomerism and preparation of alkynes along with the nomenclature.

Browse more Topics under Hydrocarbons

• Alkanes
• Conformation of Alkanes
• Nomenclature and Preparation of Alkenes
• Nomenclature and Preparation of Aromatic Hydrocarbons
• Properties of Alkenes
• Properties of Alkynes
• Properties of Aromatic Hydrocarbons

Nomenclature of Alkynes

• Common system:  According to the common system, different members of alkynes are named in the form of derivatives of acetylene.
• IUPAC System:  According to the IUPAC system, the nomenclature of alkynes is very similar to the nomenclature of alkanes. Hence, the naming is similar just the suffix “yne” replaces the suffix “ane” of the corresponding alkanes. Moreover, the numbering of position starts with the first carbon having the triple bond.

Refer to the below diagram to observe the structure of alkynes along with their respective common name and IUPAC name.

Isomerism

Isomerism is the phenomenon of two or more compounds demonstrating same molecular formula but different physical and chemical properties. We have studied about geometric isomerism and stereochemistry in the chapter chemical reactions of haloalkanes. However, to understand the proper nomenclature of alkynes it is important to understand the structural isomerism.

Isomerism can be divided into two different categories

• Structural Isomerism
• Stereoisomerism

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Structural Isomerism

Compounds demonstrating same molecular formula but differs in the structures. Thus, the structural isomerism has six different categories. They are chain isomerism, position isomerism, functional isomerism, metamerism, tautomerism, and ring chain isomerism. Let us discuss them one by one.

• Chain Isomerism: Compounds exhibiting same molecular formula but separate skeletons of carbon refer to as chain isomers. This phenomenon refers to as chain isomerism. Let’s take an example of the hydrocarbon pentane (C₅H₁₂). There can be three different possibilities of representation. Refer to the diagram below

• Position Isomerism: The phenomenon in which the compounds exhibit same molecular formula but have different types of bonds or functional group is position isomerism. The compounds exhibiting this phenomenon are position isomers. Let’s take an example of propanol (C₃H₈O).

• Functional Isomerism: Compounds exhibiting same molecular formula but separate functional groups are functional isomers and the phenomenon is functional isomerism. Let’s take an example C₂H₆O. It has the possibility of different functional groups. Refer to the example below.

• Metamerism: Metamers are isomers having the same molecular formula but they differ in the alkyl chain present on the sides of the functional group of the compounds. Therefore, this phenomenon is known as metamerism. For example, let’s take the compound C₄H₁₀O

• Tautomerism: This is a special type of isomerism phenomenon which occurs inside the same molecule because of the migration taking place in the 1,3-migration of a hydrogen atom. One of the classic examples of tautomerism is keto-enol tautomerism. In this example of tautomerism, one of the forms will have enol group and the other form will have the keto group. Let’s take the example of acetaldehyde and ethanol

• Ring-Chain Isomerism: Ring chain isomerism is the phenomenon where compounds will have the same molecular formula but will have different structures. In this type of isomerism, one of the compounds will have an open chain. On the other hand, the other compound will have ring-chain, hence this name ring-chain isomerism.

• Stereoisomerism: In this type of isomerism, the compounds will have the same molecular formula but will vary in the spatial arrangement of atoms/group of atoms. It consists of two different subclasses. They are conformational isomerism and configurational isomerism. Further classification of configurational isomerism is geometrical and optical isomerism.

Isomerism of Alkynes

Ethyne and propyne, members of the alkyne group will have just one structure. However, the higher members of this group will exhibit more than a single structure. For example, the compound butyne exhibit two different structures.

Similarly, pentyne will have three different isomers. Therefore, the compound pentyne exhibits chain isomerism and position isomerism. The three isomers of pentyne are pent-1-yne, pent-2-yne, and 3-Methylbut-1-yne. Among these three, pent-1-yne and pent-2-yne will exhibit position isomerism; pent-2-yne and 3-Methylbut-1-yne will exhibit chain isomerism; pent-1-yne and 3-Methylbut-1-yne will exhibit chain isomerism.

• Alkynes Exhibit Functional Isomerism: Alkynes exhibit functional isomerism consisting of dienes such as but-1-yne and but-1,2-diene exhibit functional isomerism. Similarly, but-1-yne and but-1,3-diene are also examples of functional isomerism.
• Alkynes are Ring-Chain Isomers: Alkynes exhibits ring-chain isomerism with cycloalkanes. For example, propyne and cyclopropene.

Preparation of Alkynes

Alkynes from Calcium Carbide

This is the industrial method for the preparation of ethyne. In this method, calcium carbide undergoes treatment with water to form ethyne. Preparation of calcium carbide is possible by heating quicklime and coke. We can obtain quicklime by heating limestone.

Alkynes from Vicinal Dihalides

Treatment of vicinal dihalides with potassium hydroxide (alcoholic) results in the dehydrohalogenation reaction. In this reaction, removal of a hydrogen halide molecule takes place thereby forming alkenyl halide. Furthermore, treatment of Alkenyl halide with sodamide produces alkyne.

Solved Question for You

Question: Write the structures and IUPAC names of the different types of isomers corresponding to the fifth alkyne member. Identify the type of isomerism demonstrated by the compounds.

Solution: The formula for the fifth member of alkyne series is C₆H₁₀. There is the possibility of 7 different isomeric forms.

Hence, different pairs of isomers will exhibit position as well as chain isomers.