A) Suggest one evidence that shows that the chemistry of benzene cannot be explained on the basis of the Kekulé structures. b) Benzene undergoes electrophilic substitution to produce nitrobenzene. b)(

Benzene, despite having three double bonds in its Kekulé structures, does not readily undergo addition reactions like typical alkenes. Instead, it primarily undergoes substitution reactions, maintaining its stable ring structure.

One evidence is that benzene is much more stable than expected for a compound with three double bonds. Its enthalpy of hydrogenation is significantly less exothermic than three times the enthalpy of hydrogenation of cyclohexene. Also, all carbon-carbon bond lengths in benzene are identical, intermediate between single and double bonds, which contradicts the alternating single and double bonds in Kekulé structures.

• Evidence: Benzene undergoes substitution reactions rather than addition reactions with reagents like bromine, unlike typical alkenes. Additionally, all carbon-carbon bond lengths in benzene are equal, which is not consistent with alternating single and double bonds.

The electrophile in the nitration of benzene is the nitronium ion.

• Electrophile: $\boxed{\text{NO}_2^+}$

b)(ii) Give the reagents and reaction conditions for the nitration of benzene.

The reagents required are concentrated nitric acid and concentrated sulfuric acid.

The reaction is typically carried out at a controlled temperature, usually between $50^\circ\text{C}$ and $60^\circ\text{C}$, to prevent further nitration or oxidation.

• Reagents: Concentrated $\text{HNO}_3$ and concentrated $\text{H}_2\text{SO}_4$.
• Conditions: Temperature between $50^\circ\text{C}$ and $60^\circ\text{C}$ (or reflux below $60^\circ\text{C}$).

b)(iii) Outline the mechanism for the nitration reaction.

Concentrated sulfuric acid protonates nitric acid, which then loses a water molecule to form the nitronium ion ($\text{NO}_2^+$).

The nitronium ion acts as an electrophile and attacks the electron-rich benzene ring, forming a sigma complex (carbocation intermediate) where the aromaticity is temporarily disrupted.

(This is represented by the student's drawing of the benzene ring with $\text{NO}_2$ attached and a positive charge delocalized over the ring, with a hydrogen atom also attached to the carbon bearing the $\text{NO}_2$ group).

A proton is removed from the sigma complex by a base (e.g., $\text{HSO}_4^-$ or $\text{H}_2\text{O}$), restoring the aromaticity of the ring and forming nitrobenzene.

(This is represented by the student's drawing showing the loss of $\text{H}^+$ and the formation of nitrobenzene).

b)(vi) Further nitration to the dinitro-product is difficult. Give the structure of the possible product on further nitration and explain why this is difficult.

The nitro ($\text{-NO}_2$) group is a meta-director and a strong deactivating group. This means it directs incoming electrophiles to the meta position and makes the benzene ring less reactive towards electrophilic substitution.

When nitrobenzene undergoes further nitration, the second nitro group will attach at the meta position relative to the first nitro group, forming 1,3-dinitrobenzene.

(This is represented by the student's drawing of a benzene ring with two $\text{NO}_2$ groups in meta positions).

The nitro group is a strong electron-withdrawing group. It withdraws electron density from the benzene ring, making the ring less nucleophilic (less electron-rich). This deactivates the ring, making it much less susceptible to attack by the electrophile ($\text{NO}_2^+$). Therefore, harsher conditions (higher temperature, more concentrated acids) are required for further nitration.

• Structure of possible product: $\boxed{\text{1,3-dinitrobenzene}}$
• Explanation: The existing nitro ($\text{-NO}_2$) group is a strong electron-withdrawing group. It deactivates the benzene ring by reducing its electron density, making it less attractive to the electrophilic nitronium ion ($\text{NO}_2^+$). This makes further electrophilic substitution significantly more difficult, requiring more vigorous reaction conditions.