In the previous section, we saw the naming procedure for hydrocarbons with only one branch. In this section, we will see the naming of hydrocarbons with more than one branch.
In the previous section we saw 5 steps. The same steps can be used here also with some modifications. Let us analyse using the example shown in fig.14.16 below:
Step 1:
• Identify the 'longest chain'. It should be considered as the main chain.
♦ The 'longest chain' is the chain with the maximum number of carbon atoms
• In our present case, the longest chain is highlighted in red color in the fig.14.17 below:
Step 2:
• Identify the positions from which the branches take off.
• In our present case, there are two branches. We have to identify the position from which they take off from the main chain.
• For that, we have to 'give numbers' to the carbon atoms in the main chain.
• Two possible ways of numbering are shown in fig 14.18(a) and (b) below. Let us analyse them:
(i) In fig(a), the numbering is done from left to right
(ii) In fig(b), the numbering is done from right to left.
(iii) So there are two possible ways to do the numbering.
♦ But only one is acceptable. Because, the numbering should satisfy a condition:
■ The 'first branch' should get the lowest possible number
• Let us see how this condition applies to our case:
♦ In fig(a), the numbering is done from left to right. Then the ethyl radical becomes the first branch. It gets a number 3
♦ In fig(b) the numbering is done from right to left. Then the methyl radical becomes the first branch. It gets a number 4
• So the numbering done in fig(a) is correct.
♦ Position of the first branch is 3
♦ Position of the second branch is 5
• Thus in this step 2, we accomplish two things:
(a) We finalize the number of carbon atoms in the 'main chain'
(b) We identify the positions of the branches
Step 3:
• From the 'finalized number of carbon atoms in the main chain', write down the word root.
♦ If the finalized number is 3, then word root is prop
♦ If the finalized number is 4, then word root is but
♦ If the finalized number is 5, then word root is pent
So on . . .
• The suffix is also written down in this step:
♦ The suffix is ane if all the carbon-carbon bonds in the molecule are single bonds
♦ The suffix is ene if any carbon-carbon double bonds are present in the molecule
♦ The suffix is yne if any carbon-carbon tripple bonds are present in the molecule
• In our present case, 'finalized number of carbon atoms in the main chain' is 8
♦ So the word root is oct
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane.
Step 4:
• In this step, we note down the name of the branches
• In the previous section, we have seen various radicals and their names
• In our present case, the name of the alkyl radicals which come as branches are:
♦ ethyl at position 3
♦ methyl at position 5
Step 5:
• In this step, we assemble the final name using the information obtained in the above four steps
■ The assembling is done as follows:
[Position number of branch+hyphen+name of radical]
+hyphen
+[Position number of branch+hyphen+name of radical]
+word root+suffix
• The items inside square brackets are related to branches. Those branches should be written in alphabetical order. So we must write the details of ethyl branch before methyl branch
• So in our present case, the IUPAC name is:
[3-ethyl]
+hyphen
+[5-methyl]
+octane
• In the above steps, the parts of name are written in separate lines for better understanding.
♦ So are the square brackets.
• Once we understand the basics, we will be able to write the name directly:
3-Ethyl-5-methyloctane
• An image of this molecule is shown below:
• The image is obtained from here:
https://pubchem.ncbi.nlm.nih.gov/compound/21358767#section=2D-Structure
• The green coloured 'H' atoms indicates the Methyl branch
♦ It is at the 5th junction from the right
• The blue coloured 'H' atoms indicates the Ethyl branch
♦ It is at the 3rd junction from the right
• The 'C' atoms at junctions are not shown in the image. This is to avoid clutter and obtain more clarity
Let us see another naming example:
Solution:
Step 1: The longest chain is highlighted in red colour in fig.14.20 below. It is the main chain:
Step 2:
• There are two branches. Methyl and propyl
• In fig.14.20(a), the numbering is done first from bottom to top and then towards the right
♦ In this numbering, the first branch is the methyl radical and it gets number 3
• In fig.14.20(b), the numbering is done first from right towards the left and then towards the bottom
♦ In this numbering, the first branch is the ethyl radical and it gets number 4
So the numbering in fig.14.20(a) is correct
♦ Position of the first branch is 3
♦ Position of the second branch is 5
Step 3:
• The 'finalized number of carbon atoms in the main chain' is 8
♦ So the word root is oct
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane.
Step 4:
• The names of the alkyl radicals which come as branches are:
♦ methyl at position 3
♦ propyl at position 5
Step 5:
Assembling the name:
[3-methyl]
+hyphen
+[5-propyl]
+octane
• The methyl group is written before propyl because alphabetically, methyl comes before propyl
• In the above steps, the parts of name are written in separate lines for better understanding.
♦ So are the square brackets.
• Once we understand the basics, we will be able to write the name directly:
3-Methyl-5-propyloctane
Let us see a solved example:
Solved example 14.3
Write the IUPAC names of the compound shown in fig.14.21 below:
Solution:
We will apply the 5 steps for the given compound
Step 1: The longest chain is highlighted in red colour in fig.14.22 below. It is the main chain:
Step 2:
• There are two branches. Methyl and propyl
• In fig.14.22(a), the numbering is done from left to right
♦ In this numbering, the first branch is the ethyl radical and it gets number 3
• In fig.14.22(b), the numbering is done from right to left
♦ In this numbering, the first branch is the methyl radical and it gets number 2
So the numbering in fig.14.22(b) is correct
♦ Position of the first branch is 2
♦ Position of the second branch is 3
Step 3:
• The 'finalized number of carbon atoms in the main chain' is 5
♦ So the word root is pent
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane
Step 4:
• The names of the alkyl radicals which come as branches are:
♦ methyl at position 2
♦ ethyl at position 3
Step 5:
Assembling the name:
[2-methyl]
+hyphen
+[3-ethyl]
+pentane
• The methyl group is written before ethyl because alphabetically, methyl comes before ethyl
• In the above steps, the parts of name are written in separate lines for better understanding.
♦ So are the square brackets.
• Once we understand the basics, we will be able to write the name directly:
2-Methyl-3-ethylpentane
So we know how to name the compounds which have more than one branch. In the next section, we will see a special case coming under this.
In the previous section we saw 5 steps. The same steps can be used here also with some modifications. Let us analyse using the example shown in fig.14.16 below:
 |
| Fig.14.16 |
• Identify the 'longest chain'. It should be considered as the main chain.
♦ The 'longest chain' is the chain with the maximum number of carbon atoms
• In our present case, the longest chain is highlighted in red color in the fig.14.17 below:
 |
| Fig.14.17 |
• Identify the positions from which the branches take off.
• In our present case, there are two branches. We have to identify the position from which they take off from the main chain.
• For that, we have to 'give numbers' to the carbon atoms in the main chain.
• Two possible ways of numbering are shown in fig 14.18(a) and (b) below. Let us analyse them:
 |
| Fig.14.18 |
(ii) In fig(b), the numbering is done from right to left.
(iii) So there are two possible ways to do the numbering.
♦ But only one is acceptable. Because, the numbering should satisfy a condition:
■ The 'first branch' should get the lowest possible number
• Let us see how this condition applies to our case:
♦ In fig(a), the numbering is done from left to right. Then the ethyl radical becomes the first branch. It gets a number 3
♦ In fig(b) the numbering is done from right to left. Then the methyl radical becomes the first branch. It gets a number 4
• So the numbering done in fig(a) is correct.
♦ Position of the first branch is 3
♦ Position of the second branch is 5
• Thus in this step 2, we accomplish two things:
(a) We finalize the number of carbon atoms in the 'main chain'
(b) We identify the positions of the branches
Step 3:
• From the 'finalized number of carbon atoms in the main chain', write down the word root.
♦ If the finalized number is 3, then word root is prop
♦ If the finalized number is 4, then word root is but
♦ If the finalized number is 5, then word root is pent
So on . . .
• The suffix is also written down in this step:
♦ The suffix is ane if all the carbon-carbon bonds in the molecule are single bonds
♦ The suffix is ene if any carbon-carbon double bonds are present in the molecule
♦ The suffix is yne if any carbon-carbon tripple bonds are present in the molecule
• In our present case, 'finalized number of carbon atoms in the main chain' is 8
♦ So the word root is oct
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane.
Step 4:
• In this step, we note down the name of the branches
• In the previous section, we have seen various radicals and their names
• In our present case, the name of the alkyl radicals which come as branches are:
♦ ethyl at position 3
♦ methyl at position 5
Step 5:
• In this step, we assemble the final name using the information obtained in the above four steps
■ The assembling is done as follows:
[Position number of branch+hyphen+name of radical]
+hyphen
+[Position number of branch+hyphen+name of radical]
+word root+suffix
• The items inside square brackets are related to branches. Those branches should be written in alphabetical order. So we must write the details of ethyl branch before methyl branch
• So in our present case, the IUPAC name is:
[3-ethyl]
+hyphen
+[5-methyl]
+octane
• In the above steps, the parts of name are written in separate lines for better understanding.
♦ So are the square brackets.
• Once we understand the basics, we will be able to write the name directly:
3-Ethyl-5-methyloctane
• An image of this molecule is shown below:
https://pubchem.ncbi.nlm.nih.gov/compound/21358767#section=2D-Structure
• The green coloured 'H' atoms indicates the Methyl branch
♦ It is at the 5th junction from the right
• The blue coloured 'H' atoms indicates the Ethyl branch
♦ It is at the 3rd junction from the right
• The 'C' atoms at junctions are not shown in the image. This is to avoid clutter and obtain more clarity
Let us see another naming example:
 |
| Fig.14.19 |
Step 1: The longest chain is highlighted in red colour in fig.14.20 below. It is the main chain:
 |
| Fig.14.20 |
• There are two branches. Methyl and propyl
• In fig.14.20(a), the numbering is done first from bottom to top and then towards the right
♦ In this numbering, the first branch is the methyl radical and it gets number 3
• In fig.14.20(b), the numbering is done first from right towards the left and then towards the bottom
♦ In this numbering, the first branch is the ethyl radical and it gets number 4
So the numbering in fig.14.20(a) is correct
♦ Position of the first branch is 3
♦ Position of the second branch is 5
Step 3:
• The 'finalized number of carbon atoms in the main chain' is 8
♦ So the word root is oct
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane.
Step 4:
• The names of the alkyl radicals which come as branches are:
♦ methyl at position 3
♦ propyl at position 5
Step 5:
Assembling the name:
[3-methyl]
+hyphen
+[5-propyl]
+octane
• The methyl group is written before propyl because alphabetically, methyl comes before propyl
• In the above steps, the parts of name are written in separate lines for better understanding.
♦ So are the square brackets.
• Once we understand the basics, we will be able to write the name directly:
3-Methyl-5-propyloctane
Let us see a solved example:
Solved example 14.3
Write the IUPAC names of the compound shown in fig.14.21 below:
 |
| Fig.14.21 |
We will apply the 5 steps for the given compound
Step 1: The longest chain is highlighted in red colour in fig.14.22 below. It is the main chain:
 |
| Fig.14.22 |
• There are two branches. Methyl and propyl
• In fig.14.22(a), the numbering is done from left to right
♦ In this numbering, the first branch is the ethyl radical and it gets number 3
• In fig.14.22(b), the numbering is done from right to left
♦ In this numbering, the first branch is the methyl radical and it gets number 2
So the numbering in fig.14.22(b) is correct
♦ Position of the first branch is 2
♦ Position of the second branch is 3
Step 3:
• The 'finalized number of carbon atoms in the main chain' is 5
♦ So the word root is pent
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane
Step 4:
• The names of the alkyl radicals which come as branches are:
♦ methyl at position 2
♦ ethyl at position 3
Step 5:
Assembling the name:
[2-methyl]
+hyphen
+[3-ethyl]
+pentane
• The methyl group is written before ethyl because alphabetically, methyl comes before ethyl
• In the above steps, the parts of name are written in separate lines for better understanding.
♦ So are the square brackets.
• Once we understand the basics, we will be able to write the name directly:
2-Methyl-3-ethylpentane
So we know how to name the compounds which have more than one branch. In the next section, we will see a special case coming under this.
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