In the previous section, we completed a discussion on Production of metals. In this section, we will see Nomenclature of Organic compounds. We have seen some basics about Nomenclature here. Now we continue that discussion.
• We have seen that thousands of compounds can be formed by the combination of carbon atoms. It will be very difficult to give different names for each one of them.
• Let us see an example of how the naming may become confusing. We will do an analysis in steps:
1. The structural formula of butane is given below:
• It has 4 carbon atoms and 10 hydrogen atoms. So it's molecular formula is: C4H10
2. Now consider the structural formula given below:
• It has 4 carbon atoms and 10 hydrogen atoms. So it's molecular formula is: C4H10
3. So we find that, the molecular formula is same in (1) and (2). But structural formula is different.
• The first molecule is 'straight chain' or 'open chain'
• The second molecule is 'branched chain'
4. Because of the difference in the structure, they have different chemical and physical properties.
■ Because of this difference in properties, we cannot call the second compound 'butane'. It is very important to give it a different name.
Let us see another example. Here also we will do the analysis in steps:
1. The structural formula of pentane is given below:
• It has 5 carbon atoms and 12 hydrogen atoms. So it's molecular formula is: C5H12
2. Now consider the structural formula given below:
• It has 5 carbon atoms and 12 hydrogen atoms. So it's molecular formula is: C5H12.
3. Consider another structural formula given below:
• It has 5 carbon atoms and 12 hydrogen atoms. So it's molecular formula is also: C5H12.
4. So we find that, the molecular formula is same in (1), (2) and (3). But structural formula is different.
• The first molecule is 'straight chain' or 'open chain'
• The second and third molecules are 'branched chains'
5. Because of the difference in the structure, they have different chemical and physical properties.
■ Because of this difference in properties, we cannot call the second and third molecules 'pentane'. It is very important to give them different names.
• For giving appropriate names, the IUPAC has specified certain rules.
• We have already seen a small portion of those rules when we named 'straight chain' hydrocarbons (Details here).
• Let us refresh our memory:
1. Consider the structural formula given below:
• There are 6 carbon atoms. So the word root is hex
2. All the carbon-carbon bonds are single bonds. So the suffix is ane
3. Thus the name of the compound is:
[Word root+suffix] = [hex+ane] = hexane
Another example:
1. Consider the structural formula given below:
• There are 8 carbon atoms. So the word root is oct
2. All the carbon-carbon bonds are single bonds. So the suffix is ane
3. Thus the name of the compound is:
[Word root+suffix] = [oct+ane] = octane
Another example:
1. Consider the structural formula given below:
• There are 10 carbon atoms. So the word root is dec
2. All the carbon-carbon bonds are single bonds. So the suffix is ane
3. Thus the name of the compound is:
[Word root+suffix] = [dec+ane] = decane
Above steps are already known to us. The reader should have a good knowledge about all those basic details that we discussed in a previous chapter on hydrocarbons. Such a knowledge is essential for our present discussion.
• We are going to discuss the procedure for naming 'branched chains'.
• For that, first we have to learn some basics about those 'branches'.
We will learn them in steps:
1. Consider the saturated hydrocarbons. That is., alkanes.
• In them, all the valency requirements of the carbon atoms are satisfied by hydrogen atoms. This we saw in fig.8.4(a). It is shown again below:
2. In methane, we have all single bonds as in fig.8.1.a above. So any one methane molecule will be as shown in fig.14.1 below:
• The green dots represent the four valence electrons already possessed by the carbon atom
• The red dots represent the two valence electrons already possessed by the hydrogen atom
• The two electrons in a bond belong to both the carbon and hydrogen atoms
Note the following two points:
(i) The carbon atom has the required 8 electrons around it.
• So the carbon has attained octet configuration.
♦ In other words, valency of carbon atom is satisfied
(ii) Any hydrogen atom that we take, has two electrons around it.
• So every hydrogen atom has attained octet configuration.
♦ In other words, valency of hydrogen atoms are satisfied
3. But if we remove one hydrogen atom, the situation will change. The 'new form' of methane will be as shown in fig.14.2 below:
• One hydrogen atom has left the methane molecule.
• While leaving, it took it's electron also with it.
4. Now the carbon atom is in need of one electron to fill it's valency.
• So the remaining portion which is CH3, as a whole, will change to a 'reactive group of atoms'.
• They are said to be a 'reactive group' because, they tend to enter into reactions and obtain one electron from other sources.
5. Such reactive groups are called radicals.
• The radical formed by the removal of one hydrogen atom from a methane molecule is called methyl radical. It is represented as: CH3⎼
• Similarly ethane can become ethyl radical: CH3⎼CH2⎼
• Propane can become propyl radical: CH3⎼CH2⎼CH2⎼
6. In general, they are known as the alkyl radicals, and are represented as: R⎼.
• Now we will see how these radicals gets themselves attached to the 'straight chain' hydrocarbons.
7. Consider fig.14.3 below:
• On the left most end, we have a stable 'straight chain' molecule.
8. But a methyl radical enters into reaction with this stable molecule.
• As a result, a hydrogen atom is removed and the radical takes it's place.
9. The newly formed molecule is stable because, all the valencies are satisfied as shown in fig.14.4(a) below:
In fig.a, note the two points:
(i) Any carbon atom that we take has the required 8 electrons around it.
• So every carbon has attained octet configuration.
♦ In other words, valency of carbon atoms are satisfied
(ii) Any hydrogen atom that we take has two electrons around it.
• So every hydrogen atom has attained octet configuration.
♦ In other words, valency of hydrogen atoms are satisfied
10. The same structure in fig.14.4(a) is drawn again in fig.14.4(b).
• But a portion is specially marked within dotted lines. This is our 'portion of interest'.
• This is where the methyl radical got itself attached to the 'straight chain'.
• Thus a 'branch' is formed.
■ So now we know the basics about 'branches'.
• We know some of their names and how they are attached to the 'straight chains'.
• So we can learn the method of naming 'branched chains'. We will see it in the next section.
• We have seen that thousands of compounds can be formed by the combination of carbon atoms. It will be very difficult to give different names for each one of them.
• Let us see an example of how the naming may become confusing. We will do an analysis in steps:
1. The structural formula of butane is given below:
2. Now consider the structural formula given below:
3. So we find that, the molecular formula is same in (1) and (2). But structural formula is different.
• The first molecule is 'straight chain' or 'open chain'
• The second molecule is 'branched chain'
4. Because of the difference in the structure, they have different chemical and physical properties.
■ Because of this difference in properties, we cannot call the second compound 'butane'. It is very important to give it a different name.
Let us see another example. Here also we will do the analysis in steps:
1. The structural formula of pentane is given below:
2. Now consider the structural formula given below:
3. Consider another structural formula given below:
4. So we find that, the molecular formula is same in (1), (2) and (3). But structural formula is different.
• The first molecule is 'straight chain' or 'open chain'
• The second and third molecules are 'branched chains'
5. Because of the difference in the structure, they have different chemical and physical properties.
■ Because of this difference in properties, we cannot call the second and third molecules 'pentane'. It is very important to give them different names.
• For giving appropriate names, the IUPAC has specified certain rules.
• We have already seen a small portion of those rules when we named 'straight chain' hydrocarbons (Details here).
• Let us refresh our memory:
1. Consider the structural formula given below:
2. All the carbon-carbon bonds are single bonds. So the suffix is ane
3. Thus the name of the compound is:
[Word root+suffix] = [hex+ane] = hexane
Another example:
1. Consider the structural formula given below:
2. All the carbon-carbon bonds are single bonds. So the suffix is ane
3. Thus the name of the compound is:
[Word root+suffix] = [oct+ane] = octane
Another example:
1. Consider the structural formula given below:
2. All the carbon-carbon bonds are single bonds. So the suffix is ane
3. Thus the name of the compound is:
[Word root+suffix] = [dec+ane] = decane
Above steps are already known to us. The reader should have a good knowledge about all those basic details that we discussed in a previous chapter on hydrocarbons. Such a knowledge is essential for our present discussion.
• We are going to discuss the procedure for naming 'branched chains'.
• For that, first we have to learn some basics about those 'branches'.
We will learn them in steps:
1. Consider the saturated hydrocarbons. That is., alkanes.
• In them, all the valency requirements of the carbon atoms are satisfied by hydrogen atoms. This we saw in fig.8.4(a). It is shown again below:
 |
| Fig.8.1 |
• The green dots represent the four valence electrons already possessed by the carbon atom
• The red dots represent the two valence electrons already possessed by the hydrogen atom
• The two electrons in a bond belong to both the carbon and hydrogen atoms
 |
| Fig.14.1 |
(i) The carbon atom has the required 8 electrons around it.
• So the carbon has attained octet configuration.
♦ In other words, valency of carbon atom is satisfied
(ii) Any hydrogen atom that we take, has two electrons around it.
• So every hydrogen atom has attained octet configuration.
♦ In other words, valency of hydrogen atoms are satisfied
3. But if we remove one hydrogen atom, the situation will change. The 'new form' of methane will be as shown in fig.14.2 below:
 |
| Fig.14.2 |
• While leaving, it took it's electron also with it.
4. Now the carbon atom is in need of one electron to fill it's valency.
• So the remaining portion which is CH3, as a whole, will change to a 'reactive group of atoms'.
• They are said to be a 'reactive group' because, they tend to enter into reactions and obtain one electron from other sources.
5. Such reactive groups are called radicals.
• The radical formed by the removal of one hydrogen atom from a methane molecule is called methyl radical. It is represented as: CH3⎼
• Similarly ethane can become ethyl radical: CH3⎼CH2⎼
• Propane can become propyl radical: CH3⎼CH2⎼CH2⎼
6. In general, they are known as the alkyl radicals, and are represented as: R⎼.
• Now we will see how these radicals gets themselves attached to the 'straight chain' hydrocarbons.
7. Consider fig.14.3 below:
 |
| Fig.14.3 |
8. But a methyl radical enters into reaction with this stable molecule.
• As a result, a hydrogen atom is removed and the radical takes it's place.
9. The newly formed molecule is stable because, all the valencies are satisfied as shown in fig.14.4(a) below:
 |
| Fig.14.4 |
(i) Any carbon atom that we take has the required 8 electrons around it.
• So every carbon has attained octet configuration.
♦ In other words, valency of carbon atoms are satisfied
(ii) Any hydrogen atom that we take has two electrons around it.
• So every hydrogen atom has attained octet configuration.
♦ In other words, valency of hydrogen atoms are satisfied
10. The same structure in fig.14.4(a) is drawn again in fig.14.4(b).
• But a portion is specially marked within dotted lines. This is our 'portion of interest'.
• This is where the methyl radical got itself attached to the 'straight chain'.
• Thus a 'branch' is formed.
■ So now we know the basics about 'branches'.
• We know some of their names and how they are attached to the 'straight chains'.
• So we can learn the method of naming 'branched chains'. We will see it in the next section.
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