Alkenes - Structure and Nomenclature

In this article we will learn about Structure and Nomenclature of Alkenes in organic chemistry. 

Key words: Alkenes, Unsaturated hydrocarbons, Carbon-carbon double bond, IUPAC Nomenclature. 

The Alkenes are known as unsaturated hydrocarbons. They are derivatives of alkanes and consist of one or more carbon-carbon double bond. Alkenes have general formula C_nH_2n.

Since alkenes are derivatives of alkanes; their names are assigned by the names of alkanes. To assign name of the alkene, just remove “ane” form the parent alkane and replace it with “ene”.

Few examples are shown in the table 1

Table 1

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

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

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

Therefore Alkene A should be named as “1-butene”

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

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 and D which are shown below. Here both of the molecules have same molecular formula. They are differing by the position of double bonds.

How to assign names for these molecules?

 Let’s consider molecule C first,

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

 

Step 2:  According to given numbering parent alkane is “butane” (a four carbon alkane). Therefore name of alkene is “butane”

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

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

Now let’s see molecule D

Step 1: Give numbering to the longest carbon chain. Here the numbering can be given in two ways (case 1 and 2). In both the cases double bond present in C2-C-3 carbons. But if you see case 1; CH₃ is present at C-3. And in case 2 CH₃ is at C-2. In this situation we need consider that branching should be at lowest carbon. Therefore case 2 is correct.

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

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

Step 4: There is methyl substituent at C-2. Hence correct name of the molecule D is “2-methyl-2-butene”.

Now let’s consider one more example E

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

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

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

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 is “5-ethyl-2-methyl-3-heptene”.

Now consider the situation where two or more double bonds are present in the molecule. In this case we need to add the suffix “diene” , “triene” or “tetraene” etc. (Table 2)

Number of double bonds	Suffix
one	ene
Two	diene
Three	Triene
Four	Tetraene
Five	Pentaene

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 “diene” since there are two double bonds in the molecule. Also the double bond present at C-2 and C-4. Therefore correct name of the molecule F is “2,4-hexadiene”

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

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

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

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

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

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

If there are more than one double bond the use the suffix “diene”; “triene” etc.

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