Alkynes - Structure and Nomenclature

In this article we will learn about Structure and Nomenclature of Alkynes.

Key words: Alkynes, Unsaturated hydrocarbons, carbon-carbon triple bond, IUPAC Nomenclature. 

Alkynes are the unsaturated hydrocarbons. They consist of at least one carbon-carbon triple bond.

Alkynes have general formula C_nH_2n-2 and general structure is shown below.

The systematic names of alkynes can be assigned by name of the alkane from which they derived.

To assign name of the alkyne, just remove “ane” form the parent alkane and replace it with “yne”.

Few examples are shown in the Table 1

Table 1

Alkynes are specifically named by referring the carbon number at which the triple bond is present. Consider the following example.

Both A and B molecules are having four carbons; so they are named as “butyne”.

But the position of triple bond is different. In structure A, triple bond is present at C1-C2, so the alkyne is referred by the lowest carbon number in which triple bond is present.

Therefore alkyne A should be named as “1-butyne”

Similarly, in molecule B the triple bond is present at C2-C3. Hence the molecule B is named as “2-butyne”

Since the molecule A and B have same molecular formula, but different structures. They are known as isomers of each other.

Now consider the molecule C which is shown below.

 

To assign the name of the molecule C, we need to follow few steps;

Step 1:  Give numbering to the longest carbon chain starting from the triple bond. Since the both CH₃ are attached to C-3; therefore both CH₃ are equivalent. 

Step 2:  The carbon chain is of four carbons.Therefore name of alkyne is “butyne”

Step 3: The double bond is present at C1, therefore name will be “1-butyne”

Step 4: Also there is methyl substituent at C-3. Hence correct name of the molecule C is “3-methyl-1-butyne”.

Take another example D,

Step 1:  Give numbering to the longest carbon chain.

Step 2:  According to given numbering parent alkane is “pentane” (a five carbon alkane). Therefore name of alkyne is “pentyne”

Step 3: The double bond is present at C2, therefore name will be “2- pentyne”

Step 4: Also there is “Chloro” substituent at C-1. Hence correct name of the molecule D is “1-chloro-2- pentyne”.

Now let’s consider the molecule E which is shown below.

Start with Step 1: Give numbering to the longest carbon chain.

Step 2:  According to given numbering parent alkane is “heptane”

Step 3: The triple bond is present at C-3, therefore name will be “3- heptyne”

Step 4: There is methyl substituent at C-2. Also ethyl substituent at C-5. Here the substituents are should be named in alphabetical order. Hence correct name of the molecule E is “5-ethyl-2-methyl-3-heptyne”.

Now consider the situation where two or more triple bonds are present in the molecule. In this case we need to add the suffix “diyne” , “triyne” or “tetrayne” etc. (Table 2)

Number of triple bonds	Suffix
one	yne
Two	diyne
Three	Triyne
Four	Tetrayne
Five	Pentayne

Table 2

For example see the molecule F and G,

In the molecule F; here the longest carbon chain is of six carbons. So parent alkane is “hexane”. Now remove “ne” and add “diyne” since there are two triple bonds in the molecule. Also the triple bond present at C-2 and C-4. Therefore correct name of the molecule F is “2,4-hexadiyne”

In the molecule G, longest carbon chain is of six carbons. Hence parent alkane is “hexane”. Now remove “ne” and add “triyne” since there are three triple bonds in the molecule. Also the triple bond present at C-1, C-3 and C-5. Therefore correct name of the molecule G is “1,3,5-hexatriyne”

Now consider a situation where a double and triple bond present in the same molecule. For example in molecule H there is one triple bond and one double bond present.

 

How to give numbering then?

In this case we need to always start the numbering from the end which nearest to the double bond. So give the numbering from alkene end.

 

Here the carbon chain is of six carbon so the parent alkane is hexane. Then remove “ane” and replace it with “ene”. So parent alkene is 1-hexene. Also there is a triple bond is present at C-5, so write the suffix “5-yne”. Therefore the correct name of the molecule H is “1-hexene-5-yne”.

To summarize this topic, for the nomenclature of alkynes we need to follow four steps.

Step 1: Give the numbering to the longest carbon chain which include the triple bond

Step 2: Based on the number of carbons in the selected chain; parent alkyne name should be assigned.

Step 3: The alkyne is referred by the lowest carbon number in which trilpe bond is present.

Step 4: The substituents should be written in alphabetical order before the name of alkyne.

If there are more than one triple bond the use the suffix “diyne”; “triyne” etc.

When there is situation where double and triple bonds present in same molecule. In that case consider alkene as parent alkene and start the numbering from the carbon which is nearest to the double bond.

That's all for this topic. If you have any questions please feel free to ask me. Also suggest me if any changes or additions are required. Thank you..!