In the previous section, we saw how branches are formed and also the names of the branches. In this section, we will see the naming of branched chain hydrocarbons.
• The chain in fig(a) is a simple 'open chain'. The chain in fig(b) is a 'open chain', but with a 'branch'
• For the chain in fig(b), we will fill up the valencies using hydrogen atoms. We will get:
• When the valencies are filled up, it is a stable molecule. We must give it a name. The procedure given by IUPAC is explained below in 5 steps:
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.14.7(a) below:
Step 2:
• Identify the positions from which the branches take off.
• In our present case, there is only one branch. We have to identify the position from which it takes 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 14.7(b) and (c) above. Let us analyse them:
(i) In fig(b), the numbering is done from left to right
(ii) In fig(c), 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 'carbon atom carrying the branch' should get the lowest possible number
• Let us see how this condition applies to our case:
♦ In fig(b) the 'carbon atom carrying the branch gets the number 3
♦ In fig(c) the 'carbon atom carrying the branch gets the number 2
• So the numbering done in fig(c) is correct.
♦ Position of the branch is 2
• 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
♦ If the finalized number is 6, then word root is hex
♦ If the finalized number is 7, then word root is sept
♦ If the finalized number is 8, then word root is oct
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 4
♦ So the word root is but
• 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 branch
• In the previous section, we have seen various radicals and their names
• In our present case, the name of the alkyl radical which comes as branch is: methyl
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+word root+suffix
♦ A hyphen (-) is used to separate numerals and alphabets while writing the IUPAC name.
In our present case, the IUPAC name is: 2-methylbutane.
Let us see some solved examples:
Solved example 14.1
Write the IUPAC names of the four compounds shown in fig.14.8 below:
Solution:
We will apply the 5 steps for each of the given 4 compounds
Compound 1:
Step 1: The longest chain is highlighted in red colour in fig.14.9(a) below. It is the main chain:
Step 2:
• When the numbering is done from left to right, the position of the branch is 4. See fig.14.9(b)
• When the numbering is done from right to left, the position of the branch is 2. See fig.14.9(c)
• So the numbering in fig.14.9(c) is correct.
♦ Number of carbon atoms in the main chain is 5
♦ Position of the branch is 2
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:
• Name of the alkyl radical which comes as branch is: methyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 2-methylpentane.
Compound 2:
Step 1: The longest chain is highlighted in red colour in fig.14.10(a) below. It is the main chain:
Step 2:
• When the numbering is done from left to right, the position of the branch is 3. See fig.14.10(b)
• When the numbering is done from right to left, the position of the branch is 2. See fig.14.10(c)
• So the numbering in fig.14.10(c) is correct.
♦ Number of carbon atoms in the main chain is 4
♦ Position of the branch is 2
Step 3:
• The 'finalized number of carbon atoms in the main chain' is 54
♦ So the word root is but
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane
Step 4:
• Name of the alkyl radical which comes as branch is: methyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 2-methylbutane.
Compound 3:
Step 1: The longest chain is highlighted in red colour in fig.14.11(a) below. It is the main chain:
Step 2:
• When the numbering is done from left to right, the position of the branch is 3. See fig.14.11(b)
• When the numbering is done from right to left also, the position of the branch is 3. See fig.14.11(c)
Note: A 'five carbon chain' can be obtained in another way also. This is shown in fig.14.12 below:
• The numbering can be done first to the right and then downwards as in fig.14.12(a). Then also, we get the position of branch as 3
• The numbering can be done first to the top and then towards the left as in fig.14.12(b). Then also, we get the position of branch as 3
• So we get:
♦ Number of carbon atoms in the main chain is 5
♦ Position of the 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:
• Name of the alkyl radical which comes as branch is: ethyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 3-ethylpentane.
Compound 4:
Step 1: The longest chain is highlighted in red colour in fig.14.13(a) below. It is the main chain:
Step 2:
• When the numbering is done first to the left and then downwards, the position of the branch is 3. See fig.14.13(b)
• When the numbering is done first towards the top and then towards the right, the position of the branch is 3. See fig.14.13(c)
• So we get:
♦ Number of carbon atoms in the main chain is 5
♦ Position of the 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:
• Name of the alkyl radical which comes as branch is: methyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 3-methylpentane.
Solved example 14.2
Write the IUPAC names of the four compounds shown in fig.14.14 below:
Solution:
The fig.14.15 below shows the required markings. Based on that, the naming can be done easily.
The compounds are:
1. 3-Methylhexane
2. 3-Methylhexane
3. 3-Methylheptane
4. 2-Methylbutane
• The reader may write the steps in his/her own notebooks.
In the next section, we will see hydrocarbons with more than one branch.
Nomenclature of Branched Hydrocarbons
• Consider two carbon chains shown in fig.14.5(a) and (b) below: |
| Fig.14.5 |
• For the chain in fig(b), we will fill up the valencies using hydrogen atoms. We will get:
 |
| Fig.14.6 |
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.14.7(a) below:
 |
| Fig.14.7 |
• Identify the positions from which the branches take off.
• In our present case, there is only one branch. We have to identify the position from which it takes 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 14.7(b) and (c) above. Let us analyse them:
(i) In fig(b), the numbering is done from left to right
(ii) In fig(c), 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 'carbon atom carrying the branch' should get the lowest possible number
• Let us see how this condition applies to our case:
♦ In fig(b) the 'carbon atom carrying the branch gets the number 3
♦ In fig(c) the 'carbon atom carrying the branch gets the number 2
• So the numbering done in fig(c) is correct.
♦ Position of the branch is 2
• 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
♦ If the finalized number is 6, then word root is hex
♦ If the finalized number is 7, then word root is sept
♦ If the finalized number is 8, then word root is oct
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 4
♦ So the word root is but
• 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 branch
• In the previous section, we have seen various radicals and their names
• In our present case, the name of the alkyl radical which comes as branch is: methyl
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+word root+suffix
♦ A hyphen (-) is used to separate numerals and alphabets while writing the IUPAC name.
In our present case, the IUPAC name is: 2-methylbutane.
Let us see some solved examples:
Solved example 14.1
Write the IUPAC names of the four compounds shown in fig.14.8 below:
 |
| Fig.14.8 |
We will apply the 5 steps for each of the given 4 compounds
Compound 1:
Step 1: The longest chain is highlighted in red colour in fig.14.9(a) below. It is the main chain:
 |
| Fig.14.9 |
• When the numbering is done from left to right, the position of the branch is 4. See fig.14.9(b)
• When the numbering is done from right to left, the position of the branch is 2. See fig.14.9(c)
• So the numbering in fig.14.9(c) is correct.
♦ Number of carbon atoms in the main chain is 5
♦ Position of the branch is 2
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:
• Name of the alkyl radical which comes as branch is: methyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 2-methylpentane.
Compound 2:
Step 1: The longest chain is highlighted in red colour in fig.14.10(a) below. It is the main chain:
 |
| Fig.14.10 |
• When the numbering is done from left to right, the position of the branch is 3. See fig.14.10(b)
• When the numbering is done from right to left, the position of the branch is 2. See fig.14.10(c)
• So the numbering in fig.14.10(c) is correct.
♦ Number of carbon atoms in the main chain is 4
♦ Position of the branch is 2
Step 3:
• The 'finalized number of carbon atoms in the main chain' is 54
♦ So the word root is but
• All the carbon-carbon bonds in the molecule are single bonds
♦ So the suffix is ane
Step 4:
• Name of the alkyl radical which comes as branch is: methyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 2-methylbutane.
Compound 3:
Step 1: The longest chain is highlighted in red colour in fig.14.11(a) below. It is the main chain:
 |
| Fig.14.11 |
• When the numbering is done from left to right, the position of the branch is 3. See fig.14.11(b)
• When the numbering is done from right to left also, the position of the branch is 3. See fig.14.11(c)
Note: A 'five carbon chain' can be obtained in another way also. This is shown in fig.14.12 below:
 |
| Fig.14.12 |
• The numbering can be done first to the top and then towards the left as in fig.14.12(b). Then also, we get the position of branch as 3
• So we get:
♦ Number of carbon atoms in the main chain is 5
♦ Position of the 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:
• Name of the alkyl radical which comes as branch is: ethyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 3-ethylpentane.
Compound 4:
Step 1: The longest chain is highlighted in red colour in fig.14.13(a) below. It is the main chain:
 |
| Fig.14.13 |
• When the numbering is done first to the left and then downwards, the position of the branch is 3. See fig.14.13(b)
• When the numbering is done first towards the top and then towards the right, the position of the branch is 3. See fig.14.13(c)
• So we get:
♦ Number of carbon atoms in the main chain is 5
♦ Position of the 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:
• Name of the alkyl radical which comes as branch is: methyl.
Step 5:
Assembling the name: Position number of branch+hyphen+name of radical+word root+suffix
We get: 3-methylpentane.
Solved example 14.2
Write the IUPAC names of the four compounds shown in fig.14.14 below:
 |
| Fig.14.14 |
The fig.14.15 below shows the required markings. Based on that, the naming can be done easily.
 |
| Fig.14.15 |
1. 3-Methylhexane
2. 3-Methylhexane
3. 3-Methylheptane
4. 2-Methylbutane
• The reader may write the steps in his/her own notebooks.
In the next section, we will see hydrocarbons with more than one branch.
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