Problem 9 - Arihant Objective Approach to Organic Chemistry for IIT JEE and NEET all Medical Entrances Part 4

Question

Replacement of \(\mathrm{Cl}\) of chlorobenzene to give phenol requires drastic conditions but chlorine of 2,4 -dinitrochlorobenzene is readily replaced. This is because: |CESE 1997| (a) NO, makes the ring electron rich at ortho and para positions (b) NO, withdraw electrons from meta position (c) NO, donates electron at meta position (d) \(\mathrm{NO}_{2}\), withdraws electrons from ortho/para positions.

Step-by-Step Solution

Answer

(d) \(\mathrm{NO}_2\) withdraws electrons from ortho/para positions.

Step 1: Understand the Structure Involved

The question deals with chlorobenzene and 2,4-dinitrochlorobenzene. Chlorobenzene is a benzene ring with one chlorine atom attached. In 2,4-dinitrochlorobenzene, there are two nitro groups attached to the benzene ring at the 2nd and 4th positions, in addition to chlorine.

Step 2: Analyze the Effect of Nitro Groups

Nitro groups (\(\mathrm{NO}_2\)) are strong electron-withdrawing groups through resonance and inductive effects. When they are attached to the benzene ring, they withdraw electron density from ortho and para positions due to their resonance effect.

Step 3: Determine the Reactivity of the Chlorine Atom

In 2,4-dinitrochlorobenzene, the electron-withdrawing effect of the nitro groups makes the benzene ring electron-deficient, particularly at the ortho and para positions where the nitro groups are located. This electron deficiency makes the chlorine atom more susceptible to nucleophilic substitution reaction (such as replacement by hydroxy group to form phenol) under milder conditions compared to chlorobenzene where no electron-withdrawing groups are present.

Step 4: Compare with Options

The correct description matches option (d): \(\mathrm{NO}_{2}\) groups withdraw electrons from the ortho/para positions, making it easier for the chlorine to be replaced in 2,4-dinitrochlorobenzene.

Key Concepts

Electron Withdrawing Groups

Electron withdrawing groups (EWGs) are organic or inorganic groups that pull electron density away from a benzene ring or any other aromatic system. They play a crucial role in influencing the chemical reactivity of molecules. These groups decrease the electron density on a ring, making it electron-deficient.
This electronic impact is predominantly due to two key effects:

• Resonance Effect: EWGs can delocalize electrons away from the ring through resonance, thus reducing electron density especially at the ortho and para positions.
• Inductive Effect: Inductive effects occur by EWGs pulling electron density through sigma bonds, further depleting the aromatic system's electron cloud.

These effects make aromatic compounds with EWGs more reactive towards reactions that require an electron-deficient center, such as nucleophilic aromatic substitution (NAS). Nitro groups are a common example of strong electron withdrawing groups.

Reactivity of Haloarenes

Haloarenes, such as chlorobenzene, contain a halogen atom directly bonded to an aromatic ring. The presence of a halogen affects the molecule's reactivity. In simple haloarenes without electron withdrawing groups, the halogen is typically less reactive to nucleophilic substitution due to electron-rich conditions.
However, once EWGs like nitro groups are introduced, as in 2,4-dinitrochlorobenzene, the reactivity changes significantly. The electron-withdrawing action of the nitro groups creates electron-deficient sites on the benzene ring, especially at positions directly across or beside the nitrogen groups.
This increases the likelihood of the halogen being actively displaced by another nucleophile, such as a hydroxide ion ( ext{OH}^-), thereby facilitating substitution reactions under much milder conditions compared to haloarenes without such groups.

Nitro Groups Impact on Reactivity

Nitro groups ( ext{NO}_2) have a profound impact on the reactivity of aromatic compounds due to their strong electron-withdrawing characteristics. They enhance the tendency of the aromatic ring to undergo nucleophilic aromatic substitution by creating electron-deficient areas on the ring.
When a nitro group is attached to a benzene ring, it pulls electron density away, especially at the ortho (adjacent) and para (opposite) positions. This withdrawal increases the electron deficiency at these sites, facilitating an easier and faster nucleophilic attack on the halogen.
In the case of 2,4-dinitrochlorobenzene, the two nitro groups greatly enhance the reactivity compared to chlorobenzene. This allows the chlorine atom to be readily replaced by a nucleophile like a hydroxide ion to form phenol, something not easily achievable in chlorobenzene without harsh conditions. Thus, nitro groups play a critical role in modifying the reactivity pattern of haloarenes.