9.4.1. Nomenclature and Isomerism

In common system, alkynes are named as derivatives of acetylene. In IUPAC system, they are named as derivatives of the corresponding alkanes replacing ‘ane’ by the suffix ‘yne’.

Alkynes are a type of hydrocarbons that contain at least one triple bond between two carbon atoms. Their naming can be done using two systems: common and IUPAC. In the common naming system, alkynes are simply referred to as derivatives of acetylene, the simplest alkyne. In the IUPAC system, the names are derived from the parent alkane name by replacing the suffix 'ane' with 'yne,' which denotes the presence of a triple bond.

Think of naming pets in your neighborhood. Some people might name their dogs in a casual way (like calling a bulldog 'Bulldozer'), while others might use formal names like 'Maximillian the Third.' Similarly, alkynes like 'butyne' (for a four-carbon alkyne) follow a formal naming system just like pets with formal names.

You have already learnt that ethyne and propyne have got only one structure but there are two possible structures for butyne – (i) but-1-yne and (ii) but-2-yne. Since these two compounds differ in their structures due to the position of the triple bond, they are known as position isomers.

Isomerism occurs when compounds have the same molecular formula but different structures. For alkynes, the simplest isomers are found in compounds like butyne, which can exist in two forms based on the position of the triple bond: but-1-yne, where the triple bond is at the beginning of the chain, and but-2-yne, where the triple bond is situated in the middle. Since these variations are due to the position of the triple bond, they are classified as position isomers.

Consider a street with identical houses. If house number 1 has a fence on its front yard but house number 2 has a fence in the backyard, they might still be considered the same type of house but represent two different styles of layout. Similarly, but-1-yne and but-2-yne are like those two houses—they have the same elements but different arrangements.

In how many ways, you can construct the structure for the next homologue i.e., the next alkyne with molecular formula C5H8? Let us try to arrange five carbon atoms with a continuous chain and with a side chain.

For the molecular formula C5H8, there are multiple ways to arrange the carbon atoms to form different alkynes. For example, you can arrange them in a straight chain like pent-1-yne or pent-2-yne, where the triple bond is at the first or second carbon, respectively. Additionally, you can have branched structures like 3-methylbut-1-yne, showcasing different configurations leading to various isomers.

Imagine building with LEGO blocks. Each arrangement of the blocks can create a different structure. Just like how you can create various models with the same set of blocks, you can arrange carbon atoms in different configurations to create distinct alkynes.

Structures I and II are position isomers and structures I and III or II and III are chain isomers.

When comparing different alkynes, you often encounter two types of isomerism: position isomers and chain isomers. In this context, position isomers differ in the position of the triple bond in their skeletal structure, while chain isomers differ in how the carbon atoms are connected (whether in a straight or branched form). Structure I and II with the same carbon arrangement but different triple bond placements exemplify position isomers, while the branched forms demonstrate chain isomerism.

Think about arranging books on a shelf. If you place one book at the far left and another book just a bit to the right, they are like structure I and II—same books, different positions. If you then stack some books vertically on the shelf while others lay flat, you’re creating branches on your shelf—much like chain isomers that change how the structure is formed.

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