In the previous section, we completed a discussion on alkynes. In this section, we will see aromatic hydrocarbons.
Three basic features of aromatic hydrocarbons are written below:
1. Aromatic hydrocarbons derived their name from the Greek word ‘aroma’ which means ‘pleasant smelling’.
• Another name for aromatic hydrocarbons is Arenes.
2. Recall that, we have seen some details about the benzene ring in the previous chapter [see fig.12.89 in section 12.14].
• Most of the aromatic compounds contain one or more benzene rings. We know that benzene contains double bonds. That means, benzene is an unsaturated hydrocarbon.
• We also know that, unsaturated hydrocarbons are easily attacked by other reagents. This is because, the loosely held π-electrons are readily available for the attacking reagents. So during the reaction, the double or triple bonds will be converted into single bonds.
• But in the case of aromatic hydrocarbons, the benzene ring is retained even after the reaction. That means, the π-electrons of the benzene ring are not easily available for the attacking reagents. We will see the reason in the next section.
3. Most aromatic hydrocarbons contain one or more benzene rings. But there are a few which do not contain any benzene rings.
• Aromatic compounds which contain benzene rings are known as benzenoids.
• Aromatic compounds which do not contain benzene rings are known as non-benzenoids.
Nomenclature and Isomerism
• We have seen the details about the nomenclature and isomerism of aromatic hydrocarbons in the previous chapter [see section 12.8].
• A few more details are written in 2 steps below:
1. All the six H atoms in benzene are equivalent. So if we want a monosubstituted benzene molecule, we will get only one type.
• ‘Monosubstituted’ means:
♦ one H atom is removed
♦ a group like -CH3 or -OH takes it’s place.
• We can remove any one of the six H atoms. Then we can put a suitable group in it’s place. We will always get the same product. This is because, all six H atoms are equivalent.
• We have already seen the explanation in the previous chapter. There we wrote:
When there is only one branch, the position of the C atom is not important [see the animation in fig.12.55 in section 12.8].
• A similar example is shown in fig.13.96 below.
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| Fig.13.96 |
♦ Consider the structure II
♦ Imagine an axis such that:
✰ It is perpendicular to the plane of the paper.
✰ It passes through the center of II.
♦ If we rotate II about that axis, we will get I.
♦ Both the structures in the fig., represent toluene.
• So we can write:
♦ If the benzene ring is monosubstituted, six different arrangements are possible.
♦ But all those six arrangements will represent the same molecule.
• We explained the 'equality of the six structures' by using the concept of rotation. However, a more scientific explanation can be given by using the concept of resonance. Due to resonance in benzene, there will not be any difference between single bonds and double bonds. All six bonds are identical. So all the six H atoms are in identical positions. We will see more details about resonance in the next section.
2. In the case of disubstituted benzene ring, five different arrangements are possible. But there will be only three different molecules. This can be explained in 4 steps:
(i) Fig.13.97(a) below shows two arrangements.
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| Fig.13.97 |
• In I, the substituents are in 1, 2 positions.
♦ The substituents are at the two ends of a double bond.
• In II, the substituents are in 1, 6 positions.
♦ The substituents are at the two ends of a single bond.
• But thanks to resonance, there is no difference between single bonds and double bonds in benzene. All bonds are equivalent. So, the substituents are in identical positions. Consequently, the two structures are the same.
• So if the substituents are in 1, 2 or 1, 6, we give a common name: ortho. It is abbreviated as: o-.
• So we can write:
♦ Both structures in fig.13.97(a) represent the same molecule.
♦ It’s common name is o-xylene.
• When we use the word ortho, what really matters is: The substituents are attached to adjacent C atoms.
♦ It does not matter whether it is 1,2 or 1,6.
• Based on the rules that we saw in the previous chapter, we can write the IUPAC name of this o-xylene. It is 1,2-Dimethylbenzene.
(ii) Fig.13.97(b) above shows two arrangements.
• In III, the substituents are in 1, 3 positions.
• In II, the substituents are in 1, 5 positions.
• Comparing I and II, we see that, the arrangement of single and double bonds between the two substituents is different.
•
But thanks to resonance, there is no difference between single bonds
and double bonds in benzene. All bonds are equivalent. So, the
substituents are in identical positions. Consequently, the two
structures are the same.
• So if the substituents are in 1, 3 or 1, 5, we give a common name: meta. It is abbreviated as: m-.
• So we can write:
♦ Both structures in fig.13.97(b) represent the same molecule.
♦ It’s common name is m-xylene.
• When we use the word meta, what really matters is: The substituents are attached to two C atoms with a C atom in between.
♦ It does not matter whether it is 1,3 or 1,5.
•
Based on the rules that we saw in the previous chapter, we can write
the IUPAC name of this o-xylene. It is 1,3-Dimethylbenzene.
(iii) Fig.13.97(c) above shows the last of the five arrangements.
• In V, the substituents are in 1, 4 positions.
• In this case, we give a common name: para. It is abbreviated as: p-.
• So we can write:
♦ The common name of the structure is p-xylene.
• When we use the word para, what really matters is: The substituents are attached to two C atoms with two C atoms in between.
•
Based on the rules that we saw in the previous chapter, we can write
the IUPAC name of this p-xylene. It is 1,4-Dimethylbenzene.
(iv) Consider the above three molecules again:
♦ 1,2-Dimethylbenzene
♦ 1,3-Dimethylbenzene
♦ 1,4-Dimethylbenzene
• It is clear that, the three are position isomers.
Structure of Benzene
Some basics about the structure can be written in 5 steps:
1. Benzene was first isolated by Michael Faraday in 1825. At that time, it’s structural details were not known.
2. We have seen the methods that scientists use to determine molecular formula of unknown compounds [see section 12.22]. Through those methods, the molecular formula of benzene was found to be C6H6.
• Six H atoms will not be able to satisfy the valencies of six C atoms through single bonds. So scientists assumed that, double or triple bonds are present in a benzene molecule.
3. To confirm the presence of double or triple bonds, bromine solution was added.
• We have seen test for unsaturation in previous sections [see fig.13.89 in section 13.15].
• We would expect the double or triple bonds to break and add Br atoms. This is shown in fig.13.98(a) below.
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| Fig.13.98 |
• But surprisingly, benzene was unreactive to bromine. No such product was obtained.
• Scientists came to this conclusion:
Though double or triple bonds are present, benzene has a stable structure.
4. Later scientists forced benzene to react with bromine. They used a catalyst for this purpose.
• The actual result obtained was unexpected. The double bond did not break. One H atom was removed and a Br atom was substituted for that H atom. This is shown in fig.13.98(b) above.
• We see that, by preferring substitution reaction, the benzene molecule is able to retain the double bonds.
5. After years of research, the German scientist August Kekule proposed the familiar structure (with alternate single and double bonds) that we see today in our basic courses in organic chemistry.
• But this structure was not able to explain why the double bond does not add bromine atoms.
• In other words, the Kekule structure was not able to explain the unusual stability of benzene.
Scientists were able to give a satisfactory explanation for the stability of benzene by using the concept of resonance. We will see it in the next section.
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