In the previous section, we saw the nomenclature of organic compounds carrying the amino group. In this section, we will see Isomers.
1. Consider the two compounds in fig.14.88 below. We have learned how to name them.
• Compound 1 is Propan-1-ol
• Compound 2 is Propan-2-ol
2. In this section we are going to see something more than just nomenclature. So let us write the properties of each:
Compound 1:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
Compound 2:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
3. We find that:
• The molecular formula is the same.
• The functional group is also the same.
• The only difference that we can see is in the position of the functional group.
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in the 'position of the functional group'.
■ Compounds having same molecular formula but different arrangement of atoms are called isomers. This phenomenon is called isomerism
5. We can say:
• Propan-1-ol is an isomer of Propan-2-ol
The reverse is also true:
• Propan-2-ol is an isomer of Propan-1-ol
Now we will see different types of isomerism
• Compound 1 is Butane
• Compound 2 is 2-Methylpropane
2. Let us write the details of each:
Compound 1:
• Molecular formula: C4H10
• Functional group in the compound: No functional group present
Compound 2:
• Molecular formula: C4H10
• Functional group in the compound: No functional group present
3. We find that:
• The molecular formula is the same.
• Both does not have any functional groups.
■ The only difference that we can see is in the chain structure.
• Compound 1 has an 'open chain structure'
• Compound 2 has a 'branched chain structure'
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in the 'chain structure'.
■ Compounds having same molecular formula but different chain structures are called chain isomers.
5. We can say:
• Butane is a chain isomer of 2-Methylpropane
The reverse is also true:
• 2-Methylpropane is a chain isomer of Butane
• CH3ㅡCH2 ㅡOH
• CH3ㅡOㅡCH3
We know how to name them.
• Compound 1 is Ethanol
♦ Ethanol does not require the position number (of it's functional group ㅡOH) to be written. This is because in ethanol, there is only one possible position that the ㅡOH group can take. We can number it from either left or right. The number should always be the lowest, which is 1. Since there is only one possible position, we need not write it's number.
• Compound 2 is Methoxymethane
2. Let us write the details of each:
Compound 1:
• Molecular formula: C2H6O
• Functional group in the compound: Hydroxyl
Compound 2:
• Molecular formula: C2H6O
• Functional group in the compound: Alkoxy
3. We find that:
• The molecular formula is the same.
■ The only difference that we can see is in the functional group
• Compound 1 has hydroxyl group
• Compound 2 has alkoxy group
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in functional groups.
■ Compounds having same molecular formula but different functional groups are called functional isomers.
5. We can say:
• Ethanol is a functional isomer of Methoxymethane
The reverse is also true:
• Methoxymethane is a functional isomer of Ethanol
Let us see another example for functional isomerism:
1. Consider the two compounds shown below
• CH3ㅡCOㅡCH3
• CH3ㅡCH2 ㅡCHO
We know how to name them.
• Compound 1 is Propanone
• Compound 2 is Propanal
2. Let us write the details of each:
Compound 1:
• Molecular formula: C3H6O
• Functional group in the compound: Keto
Compound 2:
• Molecular formula: C2H6O
• Functional group in the compound: Aldehyde
3. We find that:
• The molecular formula is the same.
■ The only difference that we can see is in the functional group
• Compound 1 has keto group
• Compound 2 has aldehyde group
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in functional groups.
5. We can say:
• Propanone is a functional isomer of Propanal
The reverse is also true:
• Propanal is a functional isomer of Propanone
1. Consider the two compounds in fig.14.88 below. We have learned how to name them.
We have already named them.
• Compound 1 is Propan-1-ol
• Compound 2 is Propan-2-ol
2. Let us write the details of each:
Compound 1:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
Compound 2:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
3. We find that:
• The molecular formula is the same.
• The functional group is also the same.
■ The only difference that we can see is in the position of the functional group.
• Compound 1 has hydroxyl group at position 1
• Compound 2 has hydroxyl group at position 2
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in 'position of same functional group'.
■ Compounds having same molecular formula and same functional group but different positions for that functional group are called position isomers.
5. We can say:
• Propan-1-ol is a position isomer of Propan-2-ol
The reverse is also true:
• Propan-2-ol is a position isomer of Propan-1-ol
So we have seen three types of isomerisms: Chain, functional and position.
We will now see some solved examples:
Solved example 14.16
Write all the possible position isomers of CH3ㅡCH2 ㅡCH2 ㅡCl.
Solution:
1. The given compound is shown in fig.14.89(a) below:
• It's IUPAC name is 1-Chloropropane
• It's molecular formula is C3H7Cl.
2. A position isomer of the given compound is shown in fig(b)
• It's IUPAC name is 2-Chloropropane
• It's molecular formula is C3H7Cl.
3. The compound in (2) is the only possible position isomer of the given compound
• In fig.(c), another structure is drawn. But it is in fact 1-Chloropropane.
♦ Because in it's case, numbering should be done from left to right
• So it is not a position isomer and hence, a '×' mark is given for it
4. Note that the position isomers in figs(a) and (b) have the same molecular formula
Solved example 14.17
Examine the compounds given in fig.14.90 below and find out the isometric pairs. To which type do they belong?
Solution:
Step 1. Write the molecular formula of each of the compounds:
1. C5H12 2. C4H10O 3. C5H12 4. C5H12O 5. C3H8O 6. C3H8O 7. C4H8O 8. C4H8O
Step 2. Group them into pairs. Pairs are formed in such a way that, both the members of any pair taken, will have the same molecular formula. Thus we get:
1&3 ⟹ [C5H12, C5H12]
5&6 ⟹ [C3H8O, C3H8O]
7&8 ⟹ [C4H8O, C4H8O]
Step 3. Write the names:
1&3 ⟹ [C5H12, C5H12] ⟹ [Pentane, 2,2-Dimethylpropane]
5&6 ⟹ [C3H8O, C3H8O] ⟹ [Propan-1-ol, Methoxyethane]
7&8 ⟹ [C4H8O, C4H8O] ⟹ [Propanal, Butanone]
Step 4. Write the type of isomerism:
1&3 ⟹ [Pentane, 2,2-Dimethylpropane] ⟹ [Alkane, Alkane] ⟹Chain isomerism
5&6 ⟹ [Propan-1-ol, Methoxyethane] ⟹ [Alcohol, Ether] ⟹Functional isomerism
7&8 ⟹ [Propanal, Butanone] ⟹ [Aldehyde, Ketone] ⟹Functional isomerism
■ From step 4, we can write a general information:
• An Alcohol may form a functional isomer with an Ether and vice versa
• An Aldehyde may form a functional isomer with a Ketone and vice versa
Solved example 14.18
(i) How many position isomers are possible for the compound
CH3ㅡCH2 ㅡCH2 ㅡCH2 ㅡCH2 ㅡOH ?
(ii) Write a functional isomer of this compound
(iii) Which functional group is present in that isomer
(iv) Write the structure and IUPAC name of that isomer
Solution:
Part (i): Two position isomers are possible for the given compound. They are shown in fig.14.91 (a) and (b) below:
The given compound is Pentan-1-ol. It's molecular formula is C5H12O
The compound in fig.14.91(a) is Pentan-2-ol. It's molecular formula is C5H12O
The compound in fig.14.91(b) is Pentan-3-ol. It's molecular formula is C5H12O
Part (ii): In the previous example 14.17, we saw that:
• An Alcohol may form a functional isomer with an Ether and vice versa
• An ether which is a functional isomer of the given alcohol is shown in fig(c)
Part (iii): The functional group in ether is Alkoxy group
Part (iv): The structure is shown in fig(c)
• The IUPAC name is Methoxybutane
Solved example 14.19
How many chain isomers are possible for the compound
CH3ㅡCH2 ㅡCH2 ㅡCH2 ㅡCH2 ㅡCH3 ?
Write them down
Solution:
There are 5 possible chain isomers including the given compound. They are shown in the fig.14.92 below:
The IUPAC names are:
1. Hexane
2. 1-Methylpentane
3. 2-methylpentane
4. 2,3-Dimethylbutane
5. 2,2-Dimethylbutane
Solved example 14.20
The structural formulae of some compounds are given in fig.14.93 below. Tabulate them into different pairs of isomers. Write the IUPAC names and the functional groups of each compound.
Solution:
Step 1. Write the molecular formula of each of the compounds:
1. C4H8O 2. C6H14 3. C4H8O 4. C6H14 5. C3H8O 6. C3H8O
Step 2. Group them into pairs. Pairs are formed in such a way that, both the members of any pair taken, will have the same molecular formula. Thus we get:
1&3 ⟹ [C4H8O, C4H8O]
2&4 ⟹ [C6H14, C6H14]
5&6 ⟹ [C3H8O, C3H8O]
Step 3. Write the names:
1&3 ⟹ [C4H8O, C4H8O] ⟹ [Butanal, Butanone]
2&4 ⟹ [C6H14, C6H14] ⟹ [Hexane, 2-Methylpentane]
5&6 ⟹ [C3H8O, C3H8O] ⟹ [Methoxyethane, Propanol]
Step 4. Write the type of isomerism:
1&3 ⟹ [Butanal, Butanone] ⟹ [Aldehyde, Ketone] ⟹ Functional isomerism
2&4 ⟹ [Hexane, 2-Methylpentane] ⟹ [Alkane, Alkane] ⟹ Chain isomerism
5&6 ⟹ [Methoxyethane, Propanol] ⟹ [Ether, Alcohol] ⟹ Functional isomerism
■ From step 4, we can write a general information:
• An Alcohol may form a functional isomer with an Ether and vice versa
• An Aldehyde may form a functional isomer with a Ketone and vice versa
We saw this result in solved example 14.17 also
In the next section, we will see Isomerism in Alicyclic compounds.
1. Consider the two compounds in fig.14.88 below. We have learned how to name them.
 |
| Fig.14.88 |
• Compound 2 is Propan-2-ol
2. In this section we are going to see something more than just nomenclature. So let us write the properties of each:
Compound 1:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
Compound 2:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
3. We find that:
• The molecular formula is the same.
• The functional group is also the same.
• The only difference that we can see is in the position of the functional group.
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in the 'position of the functional group'.
■ Compounds having same molecular formula but different arrangement of atoms are called isomers. This phenomenon is called isomerism
5. We can say:
• Propan-1-ol is an isomer of Propan-2-ol
The reverse is also true:
• Propan-2-ol is an isomer of Propan-1-ol
Now we will see different types of isomerism
Chain Isomerism
1. Consider the two compounds in fig.14.89 below. We have learned how to name them. |
| Fig.14.89 |
• Compound 2 is 2-Methylpropane
2. Let us write the details of each:
Compound 1:
• Molecular formula: C4H10
• Functional group in the compound: No functional group present
Compound 2:
• Molecular formula: C4H10
• Functional group in the compound: No functional group present
3. We find that:
• The molecular formula is the same.
• Both does not have any functional groups.
■ The only difference that we can see is in the chain structure.
• Compound 1 has an 'open chain structure'
• Compound 2 has a 'branched chain structure'
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in the 'chain structure'.
■ Compounds having same molecular formula but different chain structures are called chain isomers.
5. We can say:
• Butane is a chain isomer of 2-Methylpropane
The reverse is also true:
• 2-Methylpropane is a chain isomer of Butane
Functional Isomerism
1. Consider the two compounds shown below• CH3ㅡCH2 ㅡOH
• CH3ㅡOㅡCH3
We know how to name them.
♦ Ethanol does not require the position number (of it's functional group ㅡOH) to be written. This is because in ethanol, there is only one possible position that the ㅡOH group can take. We can number it from either left or right. The number should always be the lowest, which is 1. Since there is only one possible position, we need not write it's number.
• Compound 2 is Methoxymethane
2. Let us write the details of each:
Compound 1:
• Molecular formula: C2H6O
• Functional group in the compound: Hydroxyl
Compound 2:
• Molecular formula: C2H6O
• Functional group in the compound: Alkoxy
3. We find that:
• The molecular formula is the same.
■ The only difference that we can see is in the functional group
• Compound 1 has hydroxyl group
• Compound 2 has alkoxy group
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in functional groups.
■ Compounds having same molecular formula but different functional groups are called functional isomers.
5. We can say:
• Ethanol is a functional isomer of Methoxymethane
The reverse is also true:
• Methoxymethane is a functional isomer of Ethanol
Let us see another example for functional isomerism:
1. Consider the two compounds shown below
• CH3ㅡCOㅡCH3
• CH3ㅡCH2 ㅡCHO
We know how to name them.
• Compound 2 is Propanal
2. Let us write the details of each:
Compound 1:
• Molecular formula: C3H6O
• Functional group in the compound: Keto
Compound 2:
• Molecular formula: C2H6O
• Functional group in the compound: Aldehyde
3. We find that:
• The molecular formula is the same.
■ The only difference that we can see is in the functional group
• Compound 1 has keto group
• Compound 2 has aldehyde group
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in functional groups.
5. We can say:
• Propanone is a functional isomer of Propanal
The reverse is also true:
• Propanal is a functional isomer of Propanone
Position Isomerism
1. Consider the two compounds that we saw in fig.14.88 at the beginning of this section. They are shown again below:1. Consider the two compounds in fig.14.88 below. We have learned how to name them.
|
| Fig.14.88 |
• Compound 1 is Propan-1-ol
• Compound 2 is Propan-2-ol
Compound 1:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
Compound 2:
• Molecular formula: C3H8O
• Functional group in the compound: Hydroxyl
3. We find that:
• The molecular formula is the same.
• The functional group is also the same.
■ The only difference that we can see is in the position of the functional group.
• Compound 1 has hydroxyl group at position 1
• Compound 2 has hydroxyl group at position 2
4. They are two different compounds having different physical and chemical properties.
• The difference in properties is due to the difference in 'position of same functional group'.
■ Compounds having same molecular formula and same functional group but different positions for that functional group are called position isomers.
5. We can say:
• Propan-1-ol is a position isomer of Propan-2-ol
The reverse is also true:
• Propan-2-ol is a position isomer of Propan-1-ol
So we have seen three types of isomerisms: Chain, functional and position.
We will now see some solved examples:
Solved example 14.16
Write all the possible position isomers of CH3ㅡCH2 ㅡCH2 ㅡCl.
Solution:
1. The given compound is shown in fig.14.89(a) below:
 |
| Fig.14.89 |
• It's molecular formula is C3H7Cl.
2. A position isomer of the given compound is shown in fig(b)
• It's IUPAC name is 2-Chloropropane
• It's molecular formula is C3H7Cl.
3. The compound in (2) is the only possible position isomer of the given compound
• In fig.(c), another structure is drawn. But it is in fact 1-Chloropropane.
♦ Because in it's case, numbering should be done from left to right
• So it is not a position isomer and hence, a '×' mark is given for it
4. Note that the position isomers in figs(a) and (b) have the same molecular formula
Solved example 14.17
Examine the compounds given in fig.14.90 below and find out the isometric pairs. To which type do they belong?
 |
| Fig.14.90 |
Step 1. Write the molecular formula of each of the compounds:
1. C5H12 2. C4H10O 3. C5H12 4. C5H12O 5. C3H8O 6. C3H8O 7. C4H8O 8. C4H8O
Step 2. Group them into pairs. Pairs are formed in such a way that, both the members of any pair taken, will have the same molecular formula. Thus we get:
1&3 ⟹ [C5H12, C5H12]
5&6 ⟹ [C3H8O, C3H8O]
7&8 ⟹ [C4H8O, C4H8O]
Step 3. Write the names:
1&3 ⟹ [C5H12, C5H12] ⟹ [Pentane, 2,2-Dimethylpropane]
5&6 ⟹ [C3H8O, C3H8O] ⟹ [Propan-1-ol, Methoxyethane]
7&8 ⟹ [C4H8O, C4H8O] ⟹ [Propanal, Butanone]
Step 4. Write the type of isomerism:
1&3 ⟹ [Pentane, 2,2-Dimethylpropane] ⟹ [Alkane, Alkane] ⟹Chain isomerism
5&6 ⟹ [Propan-1-ol, Methoxyethane] ⟹ [Alcohol, Ether] ⟹Functional isomerism
7&8 ⟹ [Propanal, Butanone] ⟹ [Aldehyde, Ketone] ⟹Functional isomerism
■ From step 4, we can write a general information:
• An Alcohol may form a functional isomer with an Ether and vice versa
• An Aldehyde may form a functional isomer with a Ketone and vice versa
Solved example 14.18
(i) How many position isomers are possible for the compound
CH3ㅡCH2 ㅡCH2 ㅡCH2 ㅡCH2 ㅡOH ?
(ii) Write a functional isomer of this compound
(iii) Which functional group is present in that isomer
(iv) Write the structure and IUPAC name of that isomer
Solution:
Part (i): Two position isomers are possible for the given compound. They are shown in fig.14.91 (a) and (b) below:
 |
| Fig.14.91 |
The compound in fig.14.91(a) is Pentan-2-ol. It's molecular formula is C5H12O
The compound in fig.14.91(b) is Pentan-3-ol. It's molecular formula is C5H12O
Part (ii): In the previous example 14.17, we saw that:
• An Alcohol may form a functional isomer with an Ether and vice versa
• An ether which is a functional isomer of the given alcohol is shown in fig(c)
Part (iii): The functional group in ether is Alkoxy group
Part (iv): The structure is shown in fig(c)
• The IUPAC name is Methoxybutane
Solved example 14.19
How many chain isomers are possible for the compound
CH3ㅡCH2 ㅡCH2 ㅡCH2 ㅡCH2 ㅡCH3 ?
Write them down
Solution:
There are 5 possible chain isomers including the given compound. They are shown in the fig.14.92 below:
 |
| Fig.14.92 |
1. Hexane
2. 1-Methylpentane
3. 2-methylpentane
4. 2,3-Dimethylbutane
5. 2,2-Dimethylbutane
Solved example 14.20
The structural formulae of some compounds are given in fig.14.93 below. Tabulate them into different pairs of isomers. Write the IUPAC names and the functional groups of each compound.
 |
| Fig.14.93 |
Step 1. Write the molecular formula of each of the compounds:
1. C4H8O 2. C6H14 3. C4H8O 4. C6H14 5. C3H8O 6. C3H8O
Step 2. Group them into pairs. Pairs are formed in such a way that, both the members of any pair taken, will have the same molecular formula. Thus we get:
1&3 ⟹ [C4H8O, C4H8O]
2&4 ⟹ [C6H14, C6H14]
5&6 ⟹ [C3H8O, C3H8O]
Step 3. Write the names:
1&3 ⟹ [C4H8O, C4H8O] ⟹ [Butanal, Butanone]
2&4 ⟹ [C6H14, C6H14] ⟹ [Hexane, 2-Methylpentane]
5&6 ⟹ [C3H8O, C3H8O] ⟹ [Methoxyethane, Propanol]
Step 4. Write the type of isomerism:
1&3 ⟹ [Butanal, Butanone] ⟹ [Aldehyde, Ketone] ⟹ Functional isomerism
2&4 ⟹ [Hexane, 2-Methylpentane] ⟹ [Alkane, Alkane] ⟹ Chain isomerism
5&6 ⟹ [Methoxyethane, Propanol] ⟹ [Ether, Alcohol] ⟹ Functional isomerism
■ From step 4, we can write a general information:
• An Alcohol may form a functional isomer with an Ether and vice versa
• An Aldehyde may form a functional isomer with a Ketone and vice versa
We saw this result in solved example 14.17 also
In the next section, we will see Isomerism in Alicyclic compounds.
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