In the previous section, we completed the discussion on alkynes. So far we have seen three homologous series. Alkanes, alkenes and alkynes. A very large number of compounds are present in each of those series. We can note one peculiarity in all those compounds. That is., they contain only two elements: Carbon and Hydrogen.
• The compounds which contain only carbon and hydrogen are called Hydrocarbons.
• So any compound that we pick from the alkanes, alkenes or alkynes will be a hydrocarbon.
Let us analyse the reason for such a name:
• Take a hydrocarbon in which all the carbon-carbon bonds are single bonds
• In that hydrocarbon, pick any carbon atom
• Each of it’s 4 valence electrons will be locked in a single bond.
[This is because, a hydrocarbon will contain only carbon and hydrogen, and if the carbon-carbon bond is a single bond, the other bonds with hydrogen atoms will also be single bonds, as hydrogen can enter into single bond only]
• Any carbon atom that we pick, all around it, there will be single bonds only. This is a kind of 'saturation.'.
■ So we can write the definition of a saturated hydrocarbon:
Any hydrocarbon is a saturated hydrocarbon if it satisfies the following condition:
• All carbon-carbon bonds should be single bonds
The opposite of saturation is ‘unsaturation’. So The other category is: Unsaturated hydrocarbons. We can write it’s definition easily:
■ Any hydrocarbon is an unsaturated hydrocarbon if it satisfies any one of the following conditions:
• Any one carbon-carbon bond is a double bond
• Any one carbon-carbon bond is a triple bond
Based on the above definitions, we can write this:
■ Alkanes come under the category of Saturated hydrocarbons
■ Alkenes and Alkynes come under the category of unsaturated hydrocarbons
We can draw a flow chart like diagram:
IUPAC (International Union of Pure and Applied Chemistry) is an international organisation that strives to carry forward the new trends in the field of chemical sciences. Thus the developments in chemistry can be effectively utilised for the service of mankind. This organisation was founded in 1919. It’s headquarters is in Zurich, Switzerland. IUPAC takes the lead role in naming of elements and compounds. It standardises the atomic weights and physical constants. It also recognises new terms in chemistry.
Let us see the rules:
• Consider a group of hydrocarbons. We want to name each one of them.
• What is the first difference that we notice among them?
Ans: It is of course, the number of carbon atoms. Different hydrocarbons will be having different number of carbon atoms. So we give a root name based on this number. The table below shows the root name:
C1 - Meth
C2 - Eth
C3 - Prop
C4 - But
C5 - Pent
C6 - Hex
C7 - Hept
C8 - Oct
C9 - Non
C10 - Dec
• Let us see a few examples which show the application of the above root names:
♦ If a hydrocarbon has 3 carbon atoms, it’s root name will be ‘Prop’
♦ If a hydrocarbon has 8 carbon atoms, it’s root name will be ‘Oct’
• It may be noted that, from 'C5' onwards, the root names can be connected to the names of ‘polygons’ that we see in maths classes.
♦ A pentagon has 5 sides
♦ A hexagon has 6 sides
♦ A heptagon has 7 sides
So on...
• Now we know how to fix root names. But another problem arises:
Compounds with same number of carbon atoms are present in the three homologous series.
• For example, C3H8 belongs to the alkane series. C3H6 belongs to the alkene series. C3H4 belongs to the alkyne series.
♦ They all have the same number of carbon atoms which is 3
♦ So they all will have the same root name ‘prop’
♦ In fact, any number from C2 in the alkane series, will have a ‘cousin’ in alkene and alkyne series
Thus it is clear that, a root name is not enough.
• To solve this problem, we use a suffix after the root name. This suffix is derived from the name of the series.
♦ If the hydrocarbon is from the alkane series, the suffix will be ‘ane’
♦ If the hydrocarbon is from the alkene series, the suffix will be ‘ene’
♦ If the hydrocarbon is from the alkyne series, the suffix will be ‘yne’
• So how do we apply the suffix? We will see with the help of an example:
(i) Pick any one hydrocarbon from the alkane series. Say C4H10
(ii) It has 4 carbon atoms. So the root name is ‘But’
(iii) It is from the alkane series. So the suffix will be ‘ane’
(iv) Now we can assemble the name:
Root name + suffix ⇒ But + ane → Butane
(v) Butane is the name of C4H10. It’s cousins in the alkene and alkyne series are C4H8 and C4H6 respectively.
Their names will be:
But + ene → Butene
But + yne → Butyne
So we now know how to name the hydrocarbons. The names of hydrocarbons upto C10, in the three homologous series are given here.
We may get a different type of problem. We will see it as solved examples:
Solved example 8.1
Write the IUPAC name of C5H10
Solution:
1. In this problem, only the molecular formula is given. Structural formula or condensed formula are not given.
2. So we do not know whether it is an alkane, alkene or alkyne. But we can find out.
3. Number of carbon atoms ‘n’ = 5
4. Number of hydrogen atoms = 10. This is ‘2n’
5. So the general formula is CnH2n. It belongs to the alkene series. Now we can name it.
6. The root name is ‘pent’ (since number of carbon atoms = 5)
7. The suffix is ‘ene’. (since it belongs to the alkene series)
8. So the name is pent + ene → pentene
Solved example 8.2
Write the IUPAC name of C3H4
Solution:
1. In this problem, only the molecular formula is given. Structural formula or condensed formula are not given.
2. So we do not know whether it is an alkane, alkene or alkyne. But we can find out.
3. Number of carbon atoms ‘n’ = 3
4. Number of hydrogen atoms = 4. This is ‘2n-2’
5. So the general formula is CnH2n-2. It belongs to the alkyne series. Now we can name it.
6. The root name is ‘prop’ (since number of carbon atoms = 3)
7. The suffix is ‘yne’. (since it belongs to the alkyne series)
8. So the name is prop + yne → propyne
In the next section, we will learn about Cyclic hydrocarbons.
• The compounds which contain only carbon and hydrogen are called Hydrocarbons.
• So any compound that we pick from the alkanes, alkenes or alkynes will be a hydrocarbon.
Classification of hydrocarbons
If in a hydrocarbon, all the ‘carbon-carbon bonds’ are single bonds, that hydrocarbon will come under the category of Saturated hydrocarbons.Let us analyse the reason for such a name:
• Take a hydrocarbon in which all the carbon-carbon bonds are single bonds
• In that hydrocarbon, pick any carbon atom
• Each of it’s 4 valence electrons will be locked in a single bond.
[This is because, a hydrocarbon will contain only carbon and hydrogen, and if the carbon-carbon bond is a single bond, the other bonds with hydrogen atoms will also be single bonds, as hydrogen can enter into single bond only]
• Any carbon atom that we pick, all around it, there will be single bonds only. This is a kind of 'saturation.'.
■ So we can write the definition of a saturated hydrocarbon:
Any hydrocarbon is a saturated hydrocarbon if it satisfies the following condition:
• All carbon-carbon bonds should be single bonds
The opposite of saturation is ‘unsaturation’. So The other category is: Unsaturated hydrocarbons. We can write it’s definition easily:
■ Any hydrocarbon is an unsaturated hydrocarbon if it satisfies any one of the following conditions:
• Any one carbon-carbon bond is a double bond
• Any one carbon-carbon bond is a triple bond
Based on the above definitions, we can write this:
■ Alkanes come under the category of Saturated hydrocarbons
■ Alkenes and Alkynes come under the category of unsaturated hydrocarbons
We can draw a flow chart like diagram:
Nomenclature of hydrocarbons
We have seen that a large number of hydrocarbons are present in the three homologous series. It is difficult to find appropriate names for each of them. IUPAC has put forward some rules for the naming of hydrocarbons.IUPAC (International Union of Pure and Applied Chemistry) is an international organisation that strives to carry forward the new trends in the field of chemical sciences. Thus the developments in chemistry can be effectively utilised for the service of mankind. This organisation was founded in 1919. It’s headquarters is in Zurich, Switzerland. IUPAC takes the lead role in naming of elements and compounds. It standardises the atomic weights and physical constants. It also recognises new terms in chemistry.
Let us see the rules:
• Consider a group of hydrocarbons. We want to name each one of them.
• What is the first difference that we notice among them?
Ans: It is of course, the number of carbon atoms. Different hydrocarbons will be having different number of carbon atoms. So we give a root name based on this number. The table below shows the root name:
C1 - Meth
C2 - Eth
C3 - Prop
C4 - But
C5 - Pent
C6 - Hex
C7 - Hept
C8 - Oct
C9 - Non
C10 - Dec
• Let us see a few examples which show the application of the above root names:
♦ If a hydrocarbon has 3 carbon atoms, it’s root name will be ‘Prop’
♦ If a hydrocarbon has 8 carbon atoms, it’s root name will be ‘Oct’
• It may be noted that, from 'C5' onwards, the root names can be connected to the names of ‘polygons’ that we see in maths classes.
♦ A pentagon has 5 sides
♦ A hexagon has 6 sides
♦ A heptagon has 7 sides
So on...
• Now we know how to fix root names. But another problem arises:
Compounds with same number of carbon atoms are present in the three homologous series.
• For example, C3H8 belongs to the alkane series. C3H6 belongs to the alkene series. C3H4 belongs to the alkyne series.
♦ They all have the same number of carbon atoms which is 3
♦ So they all will have the same root name ‘prop’
♦ In fact, any number from C2 in the alkane series, will have a ‘cousin’ in alkene and alkyne series
Thus it is clear that, a root name is not enough.
• To solve this problem, we use a suffix after the root name. This suffix is derived from the name of the series.
♦ If the hydrocarbon is from the alkane series, the suffix will be ‘ane’
♦ If the hydrocarbon is from the alkene series, the suffix will be ‘ene’
♦ If the hydrocarbon is from the alkyne series, the suffix will be ‘yne’
• So how do we apply the suffix? We will see with the help of an example:
(i) Pick any one hydrocarbon from the alkane series. Say C4H10
(ii) It has 4 carbon atoms. So the root name is ‘But’
(iii) It is from the alkane series. So the suffix will be ‘ane’
(iv) Now we can assemble the name:
Root name + suffix ⇒ But + ane → Butane
(v) Butane is the name of C4H10. It’s cousins in the alkene and alkyne series are C4H8 and C4H6 respectively.
Their names will be:
But + ene → Butene
But + yne → Butyne
So we now know how to name the hydrocarbons. The names of hydrocarbons upto C10, in the three homologous series are given here.
We may get a different type of problem. We will see it as solved examples:
Solved example 8.1
Write the IUPAC name of C5H10
Solution:
1. In this problem, only the molecular formula is given. Structural formula or condensed formula are not given.
2. So we do not know whether it is an alkane, alkene or alkyne. But we can find out.
3. Number of carbon atoms ‘n’ = 5
4. Number of hydrogen atoms = 10. This is ‘2n’
5. So the general formula is CnH2n. It belongs to the alkene series. Now we can name it.
6. The root name is ‘pent’ (since number of carbon atoms = 5)
7. The suffix is ‘ene’. (since it belongs to the alkene series)
8. So the name is pent + ene → pentene
Solved example 8.2
Write the IUPAC name of C3H4
Solution:
1. In this problem, only the molecular formula is given. Structural formula or condensed formula are not given.
2. So we do not know whether it is an alkane, alkene or alkyne. But we can find out.
3. Number of carbon atoms ‘n’ = 3
4. Number of hydrogen atoms = 4. This is ‘2n-2’
5. So the general formula is CnH2n-2. It belongs to the alkyne series. Now we can name it.
6. The root name is ‘prop’ (since number of carbon atoms = 3)
7. The suffix is ‘yne’. (since it belongs to the alkyne series)
8. So the name is prop + yne → propyne
In the next section, we will learn about Cyclic hydrocarbons.
Homologous series and characteristics of homologous series
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