Chapter 12.2 - Solved Examples on Bond-Line Formula

In the previous section, we saw complete structural formula, condensed structural formula and bond-line formula. We saw some solved examples also. In this section we will see a few more solved examples.

Solved example 12.5
For each of the following compounds, write a condensed formula and also their bond-line formula.
(a) HOCH₂CH₂CH₂CH(CH₃)CH(CH₃)CH₃
(b) CH₃CH=CH(CH₂)₃CH₃
Solution:
Part (a):
• The complete structural formula is shown in figs.12.12 (a) below. This will help us to check valencies of all atoms.

Fig.12.12

• The bond-line formula can be drawn in 3 steps:
1. First we write the condensed formula in which all the C-C bonds are shown using '-'.
• This is shown in fig. 12.12(b) above.
• We must check and ensure that, valencies of all atoms are satisfied.
2. Next we write the parent chain (also known as main chain or root chain) in a zigzag manner. This is shown in fig.c
• Here also, we must check and ensure that, valencies of all atoms are satisfied.
3. Finally, we remove all C and H atoms, while retaining the functional group OH.
• The result is shown in fig.d. This is the required bond-line formula.
• Here also, we must check and ensure that, valencies of all atoms are satisfied.

Part (b):
• The complete structural formula is shown in figs.12.13 (a) below. This will help us to check valencies of all atoms.

Fig.12.13

• The bond-line formula can be drawn in 3 steps:
1. First we write the condensed formula in which all the C-C bonds are shown using '-'.
• This is shown in fig. 12.13(b) above.
• We must check and ensure that, valencies of all atoms are satisfied.
2. Next we write the parent chain (also known as main chain or root chain) in a zigzag manner. This is shown in fig.c
(Note that in the present case, there is only one C atom in the main chain)
• Here also, we must check and ensure that, valencies of all atoms are satisfied.
3. Finally, we remove all C and H atoms, while retaining the functional group CN.
• The result is shown in fig.d. This is the required bond-line formula.
• Here also, we must check and ensure that, valencies of all atoms are satisfied.
Note: After solving a large number of problems, we will be able to write the bond-line formula directly, with out any intermediate steps.

Solved example 12.6
Expand each of the following bond-line formulas to show all the atoms including carbon and hydrogen.

Fig.12.14

Solution:
Part (a):
1. Consider fig.12.15(i) below:

Fig.12.15

• At junctions 1, 2, 3, 4, and 6, two lines meet. That means, at these junctions, two valencies of C are already satisfied. The remaining two valencies have to be satisfied by two H atoms.So we will be having CH₂ at those five junctions.
2. At junctions 5 and 7, three lines meet. So we will be having CH at those two junctions.
3. The points 8 and 9 are terminal points. We will be having CH₃ at those terminal points.
4. Based on the above steps, we can draw the condensed formula as shown in fig.(ii)
5. Based on the condensed formula, we can draw the complete structural formula as shown in fig.(iii)
Note: After solving a large number of problems, we will be able to write the complete structural formula directly, with out any intermediate steps.

Part (b):
1. Consider fig.12.16(i) below:

Fig.12.16

• At junctions 2, 3, 4, 5, 6 and 7, two lines meet. That means, at these junctions, two valencies of C are already satisfied. The remaining two valencies have to be satisfied by two H atoms.So we will be having CH₂ at those five junctions.
2. The points 1 and 8 are terminal points. We will be having CH₃ at those terminal points.
3. Based on the above steps, we can draw the condensed formula as shown in fig.(ii)
4. Based on the condensed formula, we can draw the complete structural formula as shown in fig.(iii)
Note: After solving a large number of problems, we will be able to write the complete structural formula directly, with out any intermediate steps.

Part (c):
1. Consider fig.12.17(i) below:

Fig.12.17

• At junction 2, four lines meet. That means, at that junction, four valencies of C are already satisfied. So we will be having C only at that junction.
2. At junction 3, two lines meet. That means, at these junctions, two valencies of C are already satisfied. The remaining two valencies have to be satisfied by two H atoms.So we will be having CH₂ at that junction.
3. At junction 4, three lines meet. So we will be having CH at that junction.
4. The point 1 is a terminal point. There are three lines at that point. That means, at that point, three valencies of C are already satisfied. We will be having CH at that terminal point.
• The point 5 is an ordinary terminal point. We will be having a CH₃ at that terminal point.
5. Based on the above steps, we can draw the condensed formula as shown in fig.(ii)
6. Based on the condensed formula, we can draw the complete structural formula as shown in fig.(iii)
Note: After solving a large number of problems, we will be able to write the complete structural formula directly, with out any intermediate steps.

Part (d):
1. Consider fig.12.18(i) below:

Fig.12.18

• At junctions 2 and 5, three lines meet. That means, at these junctions, three valencies of C are already satisfied. The remaining one valency have to be satisfied by one H atoms.So we will be having CH at those two junctions.
2. The points 1, 3, 4 and 6 are terminal points. We will be having CH₃ at those points.
3. Based on the above steps, we can draw the condensed formula as shown in fig.(ii)
4. Based on the condensed formula, we can draw the complete structural formula as shown in fig.(iii)
Note: After solving a large number of problems, we will be able to write the complete structural formula directly, with out any intermediate steps.

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Three dimensional representation of organic molecules

• We know that a molecule is formed by the combination of different atoms.
• In many molecules, the atoms are not aligned along a single plane.
• For those molecules where all atoms are on a single plane, it is easy to draw the molecular structure.
   ♦ For example, the three atoms of H₂O lie on a plane.
   ♦ We can easily draw the ‘V’ shape of the H₂O molecule.
• For molecules with more than three atoms, one of them will be out of plane. That means, the molecule will be having a 3D structure. It is not easy to draw the structure of such molecules on the plane surface of paper.
• So scientists have developed some conventions. If every body follow those conventions, the idea about the 3D structure of any molecule can be easily conveyed among readers.
• We have already seen those conventions when we discussed the structure of NH₃ molecule. (see fig. 4.73 of section 4.11)
• Based on that, let us now draw the representation of CH₄ molecule.
It can be explained in steps:
1. The CH₄ molecule has a tetrahedral structure.
• The four H atoms are arranged around the C atom.
   ♦ The C atom is at the center of the tetrahedron.
   ♦ The H atoms are at the four corners of the tetrahedron.
(See fig.8.17 of section 8.6 of our earlier chemistry classes)
2. A plane can be made to pass through the C and any two H atoms.
• But unfortunately, the other two H atoms will be out of plane.
   ♦ One of those 'out of plane' H atoms will be in front of the plane.
   ♦ The other 'out of plane' H atom will be behind the plane.
• Thus the 2D representation will be as shown in fig.12.19 below:

Fig.12.19

3. This type of representation is known as Solid and Dashed Wedge Formula.
• The solid wedge indicates a bond which projects out of the plane of the paper, towards the observer.
• The dashed wedge indicates a bond which projects out of the plane of the paper, away from the observer.
• The solid line indicates a bond which lies on the plane of the paper.

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In the next section, we will see classification of organic compounds.


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