Reaction of Aromatic Compounds

Imagine a world without the dazzling colors of dyestuffs, the lifesaving capability of pharmaceuticals, or simply intoxicating perfumes. Each one of these marvels has stemmed from the incredible chemistry of aromatic compounds. Aromatic compounds, more popularly known as arenes, form a class of compounds consisting of benzene and its derivatives. This makes them frontline molecules in many industrial and pharmaceutical applications due to their unique chemical behavior and stability.

This Story also Contains

1. Reactions of Aromatic Compounds
2. Birch Reduction:
3. Oxidation of Aromatic Compounds:
4. Different Aspects and Types
5. Some Solved Examples
6. Summary

The benzene ring, for example, is a parent molecule to most medicines, plastics, and other chemical products synthesized from it. What has made these aromatic compounds very useful to chemists is the fact that these molecules are capable of a number of chemical reactions, allowing modification of their structure with the view of obtaining a whole range of useful substances. In the paper, we will focus on some major reactions of aromatic compounds, including reduction in aromatic compounds, addition by radicals, and Birch reduction and oxidation of aromatic compounds.

Reactions of Aromatic Compounds

Aromatic compounds have a stable ring structure with delocalized π-electrons accounting for their special kind of reactivity. These reactions can generally be divided into three classes: reduction, addition, and oxidation.

Reduction of Aromatic Compounds and Radical Addition: The reduction of an aromatic compound occurs most importantly by the addition of hydrogen atoms to the ring. The ring, hence, becomes less unsaturated. These can be done either by catalytic hydrogenation or by using reducing agents. Radical addition, on the other hand, is a process by which radicals get added to the ring of an aromatic compound. The structure may change radically.

Birch Reduction:

Birch reduction is a reduction of the aromatic ring by sodium or lithium in liquid ammonia with an alcohol. It reduces the aromatic ring partially to a 1,4-cyclohexadiene structure, which is useful in synthetic organic chemistry to modify an aromatic compound without totally saturating the ring.

Oxidation of Aromatic Compounds:

The main process involved in the oxidation of an aromatic compound is the introduction of oxygen atoms to the ring or its side chains to introduce various functional groups like carboxylic acid functional groups. This reaction is of relevance in producing more reactive and functionalized aromatics for further use in chemical synthesis.

Benzene is unreactive towards even strong oxidizing agents such as KMnO4K2Cr2O7 However, in drastic conditions, it can be oxidized slowly to CO2 and H2O. It can undergo a combustion reaction to give a luminous and smoky flame.

1. Controlled oxidation with air: Benzene on oxidation with air at 773 K in the presence of V₂O₅ as catalyst gives Maleic anhydride.
   
2. Oxidation of Alkylbenzene: Alkyl groups when attached to the benzene ring, are easily oxidized by an alkaline solution of potassium manganate.
   

A group that accumulates a positive charge during resonance is said to show a +M effect. A group that accumulates a negative charge during resonance is said to show the -M effect.

We can see that the group showing(+M) effect donates the electron density whereas the group showing (-M) effect withdraws the electron density towards it. For example, the Nitro group is an electron-withdrawing group and hence it takes the electron density towards it. Thus it shows the (-M) effect.

In general, the group exerting the +M effect has a lone pair of electrons on the atom connected to the ring while groups showing the -M effect have an unsaturation at the atom connected to the ring.

Some examples are given below:

MO˙H₂N¨H₂,−S¨H,−O¨R,−N~HCOCH₃,−F¨,−C¨

−M:−CHO,−COOH,−CN,−SO₃H,−COCl,−NO

It is to be noted that groups like Benzene and Alkenes can show both +M as well as -M effect depending upon the type of group which is attached to them.

Different Aspects and Types

Reduction and Radical Addition:

During reduction, introduction of hydrogen onto aromatic compounds like benzene is done using high-pressure catalysts such as platinum or palladium to yield cyclohexane. Radical additions involve reaction of the radicals with the aromatic ring to give new compounds like halogenated aromatics. These reactions are of immense importance in the production of a number of chemicals and some important intermediates in the pharmaceutical and polymer industries.

Birch Reduction:

What is unique about the Birch reduction is that it selectively reduces the aromatic ring, so in the resulting molecule, a portion remains aromatic while a part is reduced. Another of the classical examples is the one turning benzene into 1,4-cyclohexadiene through sodium in liquid ammonia. This reaction is especially useful in the synthesis of compounds that need a target reduction pattern unattainable through complete hydrogenation.

Oxidation Reactions:

Aromatic compounds may undergo oxidation of the ring or directly on the side chains. Potassium permanganate or chromium trioxide are common and active reagents that might be considered as oxidizing agents in the conversion of alkyl side chains to carboxylic acids. This occurs in the process of oxidation of toluene to benzoic acid. Such reactions become very useful in regard to providing functional groups to enhance the reactivity and applicability of aromatic compounds in further chemical processes.

Reduction and Radical Addition

Hydrogenation of the aromatics comes in very handy in building cyclohexane, an important nylon precursor. On the other hand, radical addition is paramount in the synthesis of halogenated aromatics applied in agrochemicals and pharmaceuticals; for instance, the generation of chlorobenzene used in the synthesis of herbicides requires it.

Reduction of Aromatic compounds
Reduction of the aromatic compounds is not easy as they are very stable due to their aromaticity and hence they have high resonance energy. So, we need to supply a greater amount of energy to reduce them.

Benzenes can be reduced by hydrogen in the presence of a Ni catalyst under stronger reaction conditions

For example:

If in case there is unsaturation present in the compound not involved in aromaticity, these unsaturations would be reduced first. Subsequent reduction of the benzene ring would take place only when these have all been reduced.

In the case of Catalytic hydrogenation of Phenanthrene, the middle ring gets reduced first and on subsequent reduction becomes completely saturated.

Benzene also shows free radical addition under UV light and adds three molecules of Chlorine to form C6H6Cl which is also called as Benzene Hexachloride or Gammaxane.

Birch Reduction:

This selective reduction capability makes Birch reduction almost indispensable in synthetic organic chemistry. With the capacity of the chemist to synthesize partially reduced aromatic compounds, it serves key synthetic intermediate in the synthesis of complex molecules—from pharmaceuticals such as cortisone and vitamins to other uses.

The birch reduction is an organic reaction that is used to convert aromatic compounds into cyclohexadiene. In this reaction, the organic reduction of aromatic rings in liquid ammonia with sodium, potassium, lithium, and alcohol occurs.

For example

Mechanism

Birch Reduction when an Electron Withdrawing group is present on the Benzene ring

Birch Reduction when an Electron Donating group is present on the Benzene ring

It is important that you remember the products of the Birch Reduction when an Electron donating or a withdrawing group is present on the Benzene Ring

Oxidation Reactions:

Oxidation reactions are central to the transformation of simple aromatic compounds into more complex and functionalized molecules. One such example of an important step in the production of a very widely applied polymer is the oxidation of p-xylene to terephthalic acid for the production of PET. To this end, understanding such oxidation processes is key in academics to appreciate organic synthesis and the functionalization of aromatic compounds.

Recommended topic video on(Reaction of aromatic compound)

Some Solved Examples

Example 1
Question:
The correct order of reactivity towards electrophilic substitution is:
1. phenol > benzene > chlorobenzene > benzoic acid
2. benzoic acid > chlorobenzene > benzene > phenol
3. phenol > chlorobenzene > benzene > benzoic acid
4. benzoic acid > phenol > benzene > chlorobenzene

Solution:
Groups showing the (+M) effect donate the electron density to the benzene ring, whereas the groups showing the (-M) effect withdraw the electron density from the benzene ring towards it. Groups exerting the +M effect activate the benzene ring, while those with the -M effect deactivate the benzene ring. Thus, the order of reactivity is:

phenol > benzene > chlorobenzene > benzoic acid

Hence, the correct answer is option (1).

Example 2
Question:
Which of the following is correct with respect to the Mesomeric effect?
1. It operates in unsaturated conjugated compounds
2. In the Mesomeric effect, the sum of lone pairs and bond pairs remains constant
3. It involves the delocalization of (pi) electrons
4. It is distance dependent

Solution:
The mesomeric effect is not distance-dependent. It depends on the extent of conjugation. The inductive effect, on the other hand, is distance-dependent.

Hence, the correct answer is option (4).

Example 3
Question: Select an incorrect combination of the functional group and the electronic effect exerted by it:

1)−CHO:−I,−M

2)−NH₂:−I,+M

3)−O−:+I,+M

4) (correct)−SO₃H:+I,−M

Solution

−SO₃H group exerts -I and -M effect.

Hence, it is incorrectly matched

Hence, the answer is the option (4).

Summary

Some of the exemplary reactions forming most of the industrial processes and academic studies related to aromatic compounds include reduction, radical addition, Birch reduction, and oxidation. Such reactions can convert very stable aromatic rings into versatile intermediates and functionalized products; therefore, they open up avenues for further improvements in important areas like pharmaceuticals, polymers, or synthetic organic chemistry.