Arenes: Benzene and Phenol: แบบฝึกหัด

The enthalpy change for the hydrogenation of cyclohexene is \( -120 \text{ kJ mol}^{-1} \). The experimental enthalpy change for the hydrogenation of benzene to cyclohexane is \( -208 \text{ kJ mol}^{-1} \).

Which of the following is the resonance energy (delocalisation energy) of benzene?

During the nitration of benzene, a "nitrating mixture" of concentrated nitric acid and concentrated sulfuric acid is used at \( 50^\circ\text{C} \).

Which statement correctly describes the role of the acids and the structure of the intermediate formed in this mechanism?

The enthalpy change of hydrogenation for cyclohexene is \(-120\text{ kJ mol}^{-1}\). Explain, in terms of structure and bonding, why the experimental enthalpy change of hydrogenation for benzene is \(-208\text{ kJ mol}^{-1}\) instead of the theoretical \(-360\text{ kJ mol}^{-1}\) predicted for a cyclohexa-1,3,5-triene structure.

Write a balanced equation for the generation of the electrophile used in the nitration of benzene, and identify the role of the concentrated sulfuric acid in this specific step.

Benzene, \(C_6H_6\), is the simplest member of the arene family. Historically, Kekulé proposed a structure consisting of alternating single and double bonds. However, modern evidence supports a delocalised model based on p-orbital overlap.

(a) Describe the delocalised model of benzene in terms of p-orbital overlap and state how this affects the stability of the molecule. (2 points)

(b) The experimental enthalpy of hydrogenation of benzene is \(-208 \text{ kJ mol}^{-1}\). The enthalpy of hydrogenation of cyclohexene is \(-120 \text{ kJ mol}^{-1}\). Use this data to calculate the 'resonance energy' (stability) of benzene compared to the theoretical cyclohexa-1,3,5-triene and explain why this value differs from a simple sum of three double bonds. (2 points)

(c) State the expected observation when benzene is shaken with bromine water and explain why benzene does not undergo an addition reaction like alkenes do. (1 point)

The nitration of benzene is a key electrophilic substitution reaction used in the synthesis of organic nitrogen compounds.

(a) State the reagents and specific temperature range required for the mononitration of benzene. Write an equation to show how the electrophile, \(NO_2^+\), is generated. (2 points)

(b) Describe the mechanism for the mononitration of benzene. Your answer should describe the movement of electrons, the structure of the intermediate formed, and how the catalyst is regenerated. (3 points)

(c) Phenol, \(C_6H_5OH\), reacts much more readily with bromine water than benzene does. Explain the role of the lone pair of electrons on the oxygen atom in increasing the reactivity of the benzene ring. (2 points)