In the previous section,
we completed a discussion on electrophilic substitution reactions of aromatic compounds. In this section, we will see addition reactions. Later in this section, we will see the directive influence of functional groups.
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We have seen that, benzene has unusual stability. It prefers to undergo substitution reactions rather than addition reactions. This is to retain the ring structure.
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But benzene can be forced to undergo addition reaction by providing suitable conditions.
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We have to learn about two addition reactions.
I. Addition of H atoms to the benzene ring
This can be written in 3 steps:
1. For addition of H atoms, we must provide high temperature and/or pressure.
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Nickel should be present as a catalyst.
2. Three hydrogen molecules add to one benzene ring.
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The product is cyclohexane. The reaction is shown in the fig.13.127 below:
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| Fig.13.127 |
3. This process is known as hydrogenation of benzene.
II. Addition of Cl atoms to the benzene ring
This can be written in 2 steps:
1. For addition of Cl atoms, we must provide ultraviolet light.
2. Three chlorine molecules add to one benzene ring.
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The product is benzene hexachloride.
♦ Another name for benzene hexachloride is gammaxane.
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The reaction is shown in the fig.13.128 below:
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| Fig.13.128 |
Combustion of benzene
This can be written in 2 steps:
1. When benzene is heated in air, it burns with a sooty flame.
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The equation is:
C6H6 + 15/2 O2 ⟶ 6CO2 + 3H2O
2. The general equation for the combustion of any hydrocarbon can be written as:
CxHy + (x+y/4) O2 ⟶ xCO2 + y/2 H2O
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Recall that, we have already seen this general equation when we saw the quantitative analysis of hydrocarbons [see section 12.22].
Directive influence of a functional group
This can be explained in 6 steps:
1. Consider a monosubstituted benzene molecule.
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Monosubstituted indicates that, there is only one functional group in the benzene ring.
2. Now, a second functional group wants to get attached to the ring. In such a situation, there are three possibilities:
(i) The incoming group gets attached to the second C atom.
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This creates a 1,2-disubstituted product.
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We know that, a 1,2-disubstituted product is also known as ortho product. See fig.12.58 of section 12.8.
(ii) The incoming group gets attached to the third C atom.
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This creates a 1,3-disubstituted product.
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We know that, a 1,3-disubstituted product is also known as meta product.
(iii) The incoming group gets attached to the fourth C atom.
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This creates a 1,4-disubstituted product.
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We know that, a 1,4-disubstituted product is also known as para product.
3. The above three are the only possibilities.
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So we would expect one third of the product to be ortho, another one third to be meta and the remaining one third to be para.
4. But in reality, we never get such equal products.
(i) In some reactions, ortho and para products are the major products. The meta product will be formed only in small quantities.
(ii) In some reactions, meta product is the major product. The ortho and para products will be formed only in small quantities.
5. This is because, the incoming group is directed towards particular positions.
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In 4(i), the incoming group is directed towards ortho and para positions.
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In 4(ii), the incoming group is directed towards meta position.
6. The incoming group has no role in deciding the positions.
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The functional group already present in the benzene ring is responsible for directing the incoming group towards particular positions.
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This is known as directive influence of functional groups.
◼ We have seen that:
♦ The group which is already present
♦ is able to direct the incoming group
♦ towards particular positions.
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Let us see how such an ability is acquired. It can be written in 6 steps:
1. Consider the molecule of phenol.
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Phenol is a resonance hybrid as shown in the fig.13.129 below:
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| Fig.13.129 |
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The reader must be able to realize the significance of each curved arrow in the above fig. The details related to fig.12.89(a) in section 12.14 can be used as a guide.
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The reason for the charges in various C atoms also must be verified. For example, the second C atom in II has five electrons around it. An independent C atom will have only four electrons. So this second C atom has a -ve charge.
2. We see that:
♦ In II, the electron density is greater at the ortho position.
♦ In III, the electron density is greater at the para position.
♦ In IV, the electron density is greater at the ortho position.
3. So we can write:
Due to the presence of the -OH group, electron density is greater at the ortho and para positions. As a consequence, substitution takes place mainly at these positions.
4. The -OH group has a tendency to pull electrons towards itself. As a result, the successive C atoms gain small +ve charges δ+, δδ+, δδδ+ so on.
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This is known as -ve inductive effect (-I effect). See fig.12.88 in section 12.13. Due to this effect, the electron density is slightly reduced at ortho and para positions.
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But the resonance effect predominates the -I effect. Due to resonance, the electron density will be greater at ortho and para positions.
5. Recall that, an ordinary benzene ring has the same electron density at all the six C atoms.
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But now we see that, when the -OH group is present in the benzene ring, electron density increases at the ortho and para positions.
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We can say that, the -OH group activates the benzene ring in such a way that, the ring becomes vulnerable to attack by an electrophile.
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Like the -OH group, other groups like -NH2, -NHR, -NHCOCH3, -OCH3, -CH3, -C2H5 etc., also activates the benzene ring. They are all ortho/para directing groups.
6. If instead of the above groups, suppose that, a halogen atom is present.
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Halogen atoms are highly electronegative. So the -I effect will be higher.
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That means, the δ+, δδ+, δδδ+ charges acquired by successive C atoms will be higher. As a consequence, the electron density will decrease at the various C atoms.
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However, due to resonance, the density will be greater (when compared to meta position) at ortho and para positions. So halogens are also ortho/para directing groups.
◼ Now we will see the meta directing groups. It can be written in 6 steps:
1. Consider the molecule of nitromethane.
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Nitromethane is a resonance hybrid as shown in the fig.13.130 below:
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| Fig.13.130 |
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The reader must be able to realize the significance of each curved arrow in the above fig. The details related to fig.12.89(a) in section 12.14 can be used as a guide.
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The reason for the charges in various C atoms also must be verified.
For example, the second C atom in II has only three electrons around it. An
independent C atom will have four electrons. So this second C atom
has a +ve charge.
2. We see that:
♦ In II, the electron density is lesser at the ortho position.
♦ In III, the electron density is lesser at the para position.
♦ In IV, the electron density is lesser at the ortho position.
3. So we can write:
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Due
to the presence of the -NO2 group, electron density is lesser at the
ortho and para positions.
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Conversely, due
to the presence of the -NO2 group, electron density is greater at the
meta position (3 and 5).
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As a consequence, substitution takes place
mainly at meta position.
4. The -NO2 group has a strong tendency to pull electrons towards itself. As a result, the successive C atoms gain small +ve charges δ+, δδ+, δδδ+ so on.
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This is known as -ve inductive effect (-I effect). See fig.12.88 in section 12.13. Due to this effect, the electron density is slightly reduced at meta position.
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But the resonance effect predominates the -I effect. Due to resonance,
the electron density will be greater at meta position.
5. Recall that, an ordinary benzene ring has the same electron density at all the six C atoms.
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When the -NO2 group is present in the benzene ring, electron density decreases at various C atoms in the ring. This is due to the strong -I effect.
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Due to this decrease in electron densities, the electrophiles will find it very difficult to get attached to the benzene ring.
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We can say that, the -NO2 group deactivates the benzene ring in such a way
that, the ring becomes stable against attack by an electrophile.
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Like the -NO2 group, other groups like -CN, -CHO, -COR, -COOH,
-COOR, -SO3H etc., also deactivates the benzene ring.
6. Even when such deactivation takes place, the electron density at meta position will be greater (when compared to ortho/para positions).
So the electrophile attacks the meta position, resulting in meta substitution. So these groups are meta directing groups.
Carcinogenicity and Toxicity
This can be explained in 4 steps:
1. We know that a benzene molecule contains only one ring.
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In some molecules, two or more benzene rings will be fused together. Such molecules are called polynuclear aromatic hydrocarbons (PAH).
2. Benzene and polynuclear aromatic hydrocarbons are carcinogens.
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A carcinogen is a substance, organism or agent that cause cancer.
3. Carcinogenic substances are produced during incomplete combustion of tobacco, coal, petroleum etc.,
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They enter into human body and undergo various bio chemical reactions. Such reactions will damage the DNA and cause cancer.
4. A few carcinogens are shown in fig.13.131 below:
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| Fig.13.131 |
The link below gives more solved examples related to the topics that we saw in this chapter.
Exercises on chapter 13
We have completed a discussion on hydrocarbons. In the next chapter, we will see environmental chemistry.
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