Question

When nitrobenzene is treated with \(\mathrm{Br}_{2}\) in presence of \(\mathrm{FeBr}_{3}\), the major product formed is m-bromonitrobenzene. The statements which are related to obtain the \(\mathrm{m}\)-isomer are 1\. The electron density on meta carbon is more than at ortho and para positions 2\. the intermediate carbonium ion formed after initial attack of \(\mathrm{Br}^{+}\)at the meta positions is least destabilized 3\. loss of aromaticity when \(\mathrm{Br}^{+}\)attacks at the ortho and para positions and not at m-position 4\. easier loss of \(\mathrm{H}+\) to regain aromaticity from the meta position than from ortho and para positions (a) 1,2 (b) 1,3 (c) \(1,3,4\) (d) 1,4

Short answer

Answer: b (1, 3)

Step-by-step solution

Understand the Reaction

The reaction involves the nitration of benzene to form nitrobenzene, which is then treated with Br2 in the presence of FeBr3. The result is bromination where a Br atom is added to the benzene ring of nitrobenzene. The presence of the nitro group (-NO2) directs the incoming Br+ electrophile to the meta position.

Electrophilic Aromatic Substitution Mechanism

The presence of the -NO2 group on benzene is deactivating and meta-directing due to its electron-withdrawing nature. The mechanism involves the formation of a sigma complex (arenium ion) where the Br+ attacks the benzene ring. Positions with lower electron density are less favorable for attack, which generally is the case at ortho and para positions when the substituent is an electron-withdrawing group like -NO2.

Analyze Statements for Meta Direction

1. A nitro group reduces electron density at ortho and para more than meta, making meta more favorable electronically. 2. The sigma complex formed when Br+ attacks the meta position is relatively more stable than when it attacks ortho or para. 3. There is a loss of aromatic stabilization in all positions upon attack, but ortho and para are more destabilized due to the nitro group. 4. Loss of H+ is a step to regain aromaticity; it is not necessarily 'easier' from any specific position here but more about the relative stability of sigma complexes.

Compare with Options

Option 1 states that meta carbon has more electron density, supporting meta attack. Option 2 mentions the stability of the sigma complex, supporting meta formation. Option 3 implies ortho and para attack reduce stability due to aromaticity loss, applicable here. Option 4 is less directly relevant since regain of aromaticity is more about sigma complex stabilization, not easier H+ loss.

Determine Correct Answer

Both 1 and 3 align well with our analysis regarding electron density and loss of stabilization. Option 4’s focus on H+ loss doesn’t align well with the primary stabilization reason. The statement set (1, 3) best aligns with our understanding.

Key concepts

Meta-Directing Groups

When we talk about meta-directing groups, like nitro (-NO2) on a benzene ring, it is important to understand how they influence research reactions in organic chemistry. A meta-directing group is a substituent that, when present on a benzene ring, tends to guide incoming electrophiles to the meta position.
The nitro group, being a classic example of a meta-director, strongly pulls electrons away from the benzene ring through resonance and inductive effects.

• Resonance involves the delocalization of electrons, causing the electron density at the ortho and para positions to be lowered relative to the meta position.
• This is due to the withdrawal of electron density by the -NO2 group, making the meta position relatively richer in electrons.

Therefore, when electrophilic aromatic substitution occurs, the meta position becomes the favorable site for substitution because the electron density is slightly higher there compared to the ortho and para positions. This makes it a preferred site for external electrophiles like Br+.

Electron Withdrawing Groups

Electron withdrawing groups (EWGs) like the nitro group (-NO2) play a crucial role in electrophilic aromatic substitution (EAS) reactions. They lower the electron density in the aromatic ring, especially at ortho and para positions.
Through induction and resonance, these groups can pull electron density away from the carbon atoms of the benzene ring, impacting the likelihood of electrophilic attack.

• Such groups are often electronegative or have polar bonds, enabling electron withdrawal from the ring.
• Through resonance, EWGs can also engage in electron delocalization, further depleting electrons from specific positions on the ring.
• This overall reduction makes the aromatic ring less reactive toward electrophiles, yet directs them to attack the meta position.

The electron-withdrawing effect of these groups reduces reactivity but tunes the site of attack on the benzene ring, demonstrating how chemical structures can influence reactivity significantly.

Sigma Complex Formation

The sigma complex, or arenium ion, is a temporary and critical intermediate step in the electrophilic aromatic substitution process. When an electrophile, such as Br+, attacks a benzene ring that carries an EWG like a nitro group, a sigma complex is formed.
In this complex, the cyclic aromatic structure temporarily loses its aromaticity as one of the carbon atoms forms a new sigma bond with the electrophile.

• The formation of the sigma complex causes the aromatic system to become a charged, less stable entity compared to its original structure.
• Stability of this intermediate is crucial, as it influences the ultimate outcome of the substitution reaction.
• The meta position forms a more stable sigma complex in the presence of an EWG because this position is less destabilized by the powerfully electron-withdrawing effects of such groups.

This added stability leads to easier reformation of the aromatic system by loss of a proton, allowing the substituted product to regain full aromaticity efficiently. Understanding sigma complexes is key to predicting and rationalizing the direction of substitution on an aromatic ring.