Kekulé suggested this structure for benzene. Benzene is now represented by this structure. Figure 3 shows the relative stability of $$C_6H_6(g)$$ compared to $$C_6H_6(g)$$ Enthalpy $$ \Delta H_f = +83 \text{ kJ mol}^{-1} $$ Use Figure 3 and the data shown in Table 1 to calculate $$\Delta H_{z}$$. Explain, in terms of structure and bonding, why $$C_6H_6$$ is more thermodynamically stable than $$C_6H_6$$. A mixture of concentrated nitric acid and concentrated sulfuric acid reacts with benzene. Figure 4 shows the incomplete mechanism for this reaction. Complete the mechanism in Figure 4 by adding any lone pairs of electrons involved in each step two curly arrows in step 1 a curly arrow in step 2 a curly arrow in step 3 a curly arrow in step 4. Name of mechanism.

A-Level AQA Chemistry Alkenes: Kekulé suggested this structure

Kekulé suggested this structure for benzene - AQA - A-Level Chemistry - Question 4 - 2021 - Paper 2

Question 4

Kekulé suggested this structure for benzene. Benzene is now represented by this structure. Figure 3 shows the relative stability of $$C_6H_6(g)$$ compared to $$C_... show full transcript

Worked Solution & Example Answer:Kekulé suggested this structure for benzene - AQA - A-Level Chemistry - Question 4 - 2021 - Paper 2

Step 1

Use Figure 3 and the data shown in Table 1 to calculate $$\Delta H_{z}$$.

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Answer

To calculate $\Delta H_{z}$ , we can use the bond enthalpies provided in Table 1. The bond enthalpies for the relevant bonds are:

Bond enthalpy (C–C) = 612 kJ/mol
Bond enthalpy (C–H) = 412 kJ/mol

For benzene, we have 6 C–C bonds and 6 C–H bonds:

\Delta H_{z} = (6 \times 612) + (6 \times 412) - 83 = 5332 kJ/mol - 83 kJ/mol = 5256 kJ/mol

Step 2

Explain, in terms of structure and bonding, why $$C_6H_6$$ is more thermodynamically stable than $$C_6H_6$$.

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Answer

Benzene ( $C_6H_6$ ) is more thermodynamically stable due to its resonance structure. The electrons in benzene are delocalized over the carbon atoms, which leads to a lower energy state compared to structures with localized double bonds. This delocalization results in stronger and more uniform C–C bonds throughout the ring, contributing to its overall stability.

Step 3

Complete the mechanism in Figure 4 by adding any lone pairs of electrons involved in each step.

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Answer

Step 1:
- Add lone pair on the nitrogen to show the nucleophilic attack.
- Draw two curly arrows from the N lone pair to the sulfuric acid framework.
Step 2:
- Draw a curly arrow from the C–H bond towards the positive charge on the nitrogen, showing the formation of the intermediate.
Step 3:
- Add a curly arrow from the bond between the benzene ring and the nitrogen to the benzene's carbon ring.
- Show the N substituent moving off.
Step 4:
- Draw a curly arrow from the bond to the sulfur atom back into the hexagon, indicating a new bond formation.

Step 4

Name of mechanism.

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Answer

The mechanism is called Electrophilic substitution.

Kekulé suggested this structure for benzene - AQA - A-Level Chemistry - Question 4 - 2021 - Paper 2

Worked Solution & Example Answer:Kekulé suggested this structure for benzene - AQA - A-Level Chemistry - Question 4 - 2021 - Paper 2

Use Figure 3 and the data shown in Table 1 to calculate $$\Delta H_{z}$$.

Explain, in terms of structure and bonding, why $$C_6H_6$$ is more thermodynamically stable than $$C_6H_6$$.

Complete the mechanism in Figure 4 by adding any lone pairs of electrons involved in each step.

Name of mechanism.

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