Reaction of Aromatic Compounds - Practice Questions & MCQ

A group which accumulates positive charge during resonance is said to show +M effect. And a group which accumulates negative charge during resonance is said to show -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, Nitro group is an electron-withdrawing group and hence it takes the electron density towards it. Thus it shows (-M) effect.

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

Some examples are given below:

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.

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 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 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  which is also called as Benzene Hexachloride or Gammaxane.

The birch reduction is an organic reaction which is used to convert aromatic compounds into cyclohexadienes. In this reaction, organic reduction of aromatic rings in liquid ammonia with sodium, potassium or 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

Benzene is unreactive towards even strong oxidising agents such as KMnO₄/K₂Cr₂O₇. However, in drastic conditions, it can be oxidised slowly to CO₂ and H₂O. It can undergo combustion reaction to give luminous and smoky flame.

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