In the previous section, we saw the reasons for formation of very large number of carbon compounds. In this section we will see the arrangement of atoms in Carbon compounds. Based on that we will discuss about Homologous series.
Consider the molecule C2H6. It is a molecule of 'ethane'. We want to know how the atoms are arranged in the molecule. Let us find out:
Step 1: In C2H6, there are two carbon atoms. The first step is to connect those two, using a single bond. It is shown in fig.8.6(a) below:
Step 2: • Consider the carbon atom on the left. It is in single bond with the right side carbon atom. So it has acquired one electron through this single bond. Three more electrons are required.
• So bond it to three hydrogen atoms. A bond between a carbon atom and a hydrogen atom is possible only through a single bond. This is because hydrogen has only one electron to share.
• Thus the left side carbon atom is bonded to three hydrogen atoms through single bonds. With this step, the left side carbon atom attains octet. The three hydrogen atoms also attain octet. This step is shown in fig.8.6(b)
Step 3: The same situation we saw in step 2, exists with the right side carbon atom. But we will write the steps again:
• Consider the carbon atom on the right. It is in single bond with the left side carbon atom. So it has acquired one electron through this single bond. Three more electrons are required.
• So bond it to the remaining three hydrogen atoms. As mentioned above, a bond between a carbon atom and a hydrogen atom is possible only through a single bond. This is because hydrogen has only one electron to share.
• Thus the right side carbon atom is bonded to three hydrogen atoms through single bonds. With this step, the right side carbon atom attains octet. The three hydrogen atoms also attain octet. This step is shown in fig.c
■ With the above three steps, we get the final arrangement of atoms in a molecule of C2H6. The electron dot diagram is shown in the fig.8.7 below:
Note that the 'pair of electrons between the two carbon atoms' are both green in colour. This is different from all other pairs. The reader may write the reason for this 'difference in colour' in his/her own notebooks.
Step 2: • Consider the left most carbon atom. It is in single bond with the middle carbon atom. So it has acquired one electron through this single bond. Three more electrons are required.
• So bond it to three hydrogen atoms. A bond between a carbon atom and a hydrogen atom is possible only through a single bond. This is because hydrogen has only one electron to share.
• Thus the left most carbon atom is bonded to three hydrogen atoms through single bonds. With this step, the left most carbon atom attains octet. The three bonded hydrogen atoms also attain octet. This step is shown in fig.b
Step 3: Consider the middle carbon atom. It is already bonded to two carbon atoms. They are two single bonds. So it has already acquired two electrons (one from each bond). Two more electrons are required.
• So this carbon atom is bonded to two hydrogen atoms. One at top and the other at bottom. With this step, the middle carbon atom attains octet. The two hydrogen atoms bonded to it also attains octet. This step is shown in fig.c.
Step 4: Now the only remaining carbon atom is the right most one. It is in the same situation as in the left most carbon atom. But we will write the steps again:
• Consider the right most carbon atom. It is in single bond with the middle carbon atom. So it has acquired one electron through this single bond. Three more electrons are required.
• For that, bond it to the three remaining hydrogen atoms. With this step, the right most carbon atom attains octet. The three hydrogen atoms which remained also attain octet. This step is shown in fig.d
■ With the above four steps, we get the final arrangement of atoms in a molecule of C3H8. The electron dot diagram is shown in the fig.8.9 below:
(ii) Molecule with 3 carbon atoms and 8 hydrogen atoms
• A pattern is beginning to emerge. The next two cases will be:
(iii) Molecule with 4 carbon atoms and 10 hydrogen atoms
(iv) Molecule with 5 carbon atoms and 12 hydrogen atoms
• Note that number of carbon atoms is increasing by 1. Also, number of hydrogen atoms is increasing by 2
• It is like a series. Let us tabulate the arrangement for the first six members of the series:
With the above table, the analysis about the series becomes easy. Let us learn the properties of the series:
Property 1:
• The 1st member of the series has 1 carbon atom
And 4 hydrogen atoms
• The 2nd member of the series has 2 carbon atoms
And 6 hydrogen atoms
• The 3rd member of the series has 3 carbon atoms
And 8 hydrogen atoms
So on...
■ So there exists a definite relation between the two quantities below:
• No. of carbon atoms
• No. of hydrogen atoms
■ If the no. of carbon atoms is 'n', then the no. Of hydrogen atoms will be (2n+2)
So we can represent the members of the series by a general formula: CnH(2n+2)
Property 2:
• Consider the second row. Consider the item in the 'Condensed formula' column in this row. We have: CH3 – CH3
A Condensed formula is a system of writing molecules in the same line as other text. It shows all atoms, but omits the vertical bonds. It also omits most or all the horizontal single bonds.
• Now consider the third row, same column. We have: CH3 – CH2 – CH3
The difference from the previous second row is a CH2 group
• Now consider the fourth row, same column. We have: CH3 – CH2 – CH2 – CH3
Here also, the difference between the previous third row is a CH2 group
• We will find the same difference through out the series. That is:
■ Successive members differ by a CH2 group
• All members of the series can be represented by a general formula
• Successive members differ by a CH2 group
• There is a regular gradation in their physical properties. That is., as we move down the series, the melting point, boiling point etc., increases.
So now we know what a 'homologous series' is. The series that we saw just above is given a special name: Alkanes. In the next section, we will see another homologous series.
Consider the molecule C2H6. It is a molecule of 'ethane'. We want to know how the atoms are arranged in the molecule. Let us find out:
Step 1: In C2H6, there are two carbon atoms. The first step is to connect those two, using a single bond. It is shown in fig.8.6(a) below:
 |
| Fig.8.6 |
• So bond it to three hydrogen atoms. A bond between a carbon atom and a hydrogen atom is possible only through a single bond. This is because hydrogen has only one electron to share.
• Thus the left side carbon atom is bonded to three hydrogen atoms through single bonds. With this step, the left side carbon atom attains octet. The three hydrogen atoms also attain octet. This step is shown in fig.8.6(b)
Step 3: The same situation we saw in step 2, exists with the right side carbon atom. But we will write the steps again:
• Consider the carbon atom on the right. It is in single bond with the left side carbon atom. So it has acquired one electron through this single bond. Three more electrons are required.
• So bond it to the remaining three hydrogen atoms. As mentioned above, a bond between a carbon atom and a hydrogen atom is possible only through a single bond. This is because hydrogen has only one electron to share.
• Thus the right side carbon atom is bonded to three hydrogen atoms through single bonds. With this step, the right side carbon atom attains octet. The three hydrogen atoms also attain octet. This step is shown in fig.c
■ With the above three steps, we get the final arrangement of atoms in a molecule of C2H6. The electron dot diagram is shown in the fig.8.7 below:
 |
| Fig.8.7 |
Now we will consider another example: C3H8. We will write the required steps as in the previous example.
Step 1: In C3H8, there are 3 carbon atoms. The first step is to connect the three using single bonds. It is shown in fig.8.8(a) below: |
| Fig.8.8 |
• So bond it to three hydrogen atoms. A bond between a carbon atom and a hydrogen atom is possible only through a single bond. This is because hydrogen has only one electron to share.
• Thus the left most carbon atom is bonded to three hydrogen atoms through single bonds. With this step, the left most carbon atom attains octet. The three bonded hydrogen atoms also attain octet. This step is shown in fig.b
Step 3: Consider the middle carbon atom. It is already bonded to two carbon atoms. They are two single bonds. So it has already acquired two electrons (one from each bond). Two more electrons are required.
• So this carbon atom is bonded to two hydrogen atoms. One at top and the other at bottom. With this step, the middle carbon atom attains octet. The two hydrogen atoms bonded to it also attains octet. This step is shown in fig.c.
Step 4: Now the only remaining carbon atom is the right most one. It is in the same situation as in the left most carbon atom. But we will write the steps again:
• Consider the right most carbon atom. It is in single bond with the middle carbon atom. So it has acquired one electron through this single bond. Three more electrons are required.
• For that, bond it to the three remaining hydrogen atoms. With this step, the right most carbon atom attains octet. The three hydrogen atoms which remained also attain octet. This step is shown in fig.d
■ With the above four steps, we get the final arrangement of atoms in a molecule of C3H8. The electron dot diagram is shown in the fig.8.9 below:
 |
| Fig.8.9 |
So we determined the arrangement of atoms in the following two molecules:
(i) Molecule with 2 carbon atoms and 6 hydrogen atoms(ii) Molecule with 3 carbon atoms and 8 hydrogen atoms
• A pattern is beginning to emerge. The next two cases will be:
(iii) Molecule with 4 carbon atoms and 10 hydrogen atoms
(iv) Molecule with 5 carbon atoms and 12 hydrogen atoms
• Note that number of carbon atoms is increasing by 1. Also, number of hydrogen atoms is increasing by 2
• It is like a series. Let us tabulate the arrangement for the first six members of the series:
Property 1:
• The 1st member of the series has 1 carbon atom
And 4 hydrogen atoms
• The 2nd member of the series has 2 carbon atoms
And 6 hydrogen atoms
• The 3rd member of the series has 3 carbon atoms
And 8 hydrogen atoms
So on...
■ So there exists a definite relation between the two quantities below:
• No. of carbon atoms
• No. of hydrogen atoms
■ If the no. of carbon atoms is 'n', then the no. Of hydrogen atoms will be (2n+2)
So we can represent the members of the series by a general formula: CnH(2n+2)
Property 2:
• Consider the second row. Consider the item in the 'Condensed formula' column in this row. We have: CH3 – CH3
A Condensed formula is a system of writing molecules in the same line as other text. It shows all atoms, but omits the vertical bonds. It also omits most or all the horizontal single bonds.
• Now consider the third row, same column. We have: CH3 – CH2 – CH3
The difference from the previous second row is a CH2 group
• Now consider the fourth row, same column. We have: CH3 – CH2 – CH2 – CH3
Here also, the difference between the previous third row is a CH2 group
• We will find the same difference through out the series. That is:
■ Successive members differ by a CH2 group
A series of compounds having the above two properties is called a homologous series. We can write the official definition:
■ A Homologous Series is a group of chemical compounds satisfying the following conditions:• All members of the series can be represented by a general formula
• Successive members differ by a CH2 group
• Members of a homologous series show similarity in chemical properties
• There is a regular gradation in their physical properties. That is., as we move down the series, the melting point, boiling point etc., increases.
So now we know what a 'homologous series' is. The series that we saw just above is given a special name: Alkanes. In the next section, we will see another homologous series.
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