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

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Kekulé suggested this structure for benzene. Benzene is now represented by this structure. Figure 3 shows the relative stability of $ ext{C}_6 ext{H}_6$ compared to... 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 ΔH_rx.

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Answer

To calculate the enthalpy change for the reaction, we can use the given data from Figure 3 and Table 1.

  1. Calculate the total bond enthalpy of the products:

    • Benzene (6 ext{C-H} bonds) and (6 ext{C-C} bonds):

      extTotalBondEnthalpy(Products)=6(412)+6(348)=4932extkJ ext{Total Bond Enthalpy (Products)} = 6(412) + 6(348) = 4932 ext{ kJ}

  2. Calculate the total atomisation enthalpy of the reactants:

    • Atomisation of carbon (6 ext{C}) and hydrogen (6 ext{H}):

      extTotalAtomisation(Reactants)=6(715)+6(218)=5586extkJ ext{Total Atomisation (Reactants)} = 6(715) + 6(218) = 5586 ext{ kJ}

  3. Finally, calculate ΔH_rx:

    extΔHrx=extTotalBondEnthalpy(Products)−extTotalAtomisation(Reactants)=4932−5586=−654extkJmol−1 ext{ΔH_rx} = ext{Total Bond Enthalpy (Products)} - ext{Total Atomisation (Reactants)} = 4932 - 5586 = -654 ext{ kJ mol}^{-1}

Step 2

Explain, in terms of structure and bonding, why C6H6 is more thermodynamically stable than C6H6.

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Answer

The structure of benzene (extC6extH6 ext{C}_6 ext{H}_6) is characterized by a resonance hybrid of multiple Lewis structures, leading to delocalized electrons across the carbon atoms. This delocalization reduces the overall energy of the molecule, making it more stable compared to a structure that would have localized electrons, such as its hypothetical open-chain form. The symmetry and planar structure contribute to the stability, as any bond breaking is more energetically unfavorable due to the resonance stabilization.

Step 3

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

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Answer

  1. Step 1:

    • Add lone pair from extO2extN ext{O}_2 ext{N}.
    • Add two curly arrows indicating electron donation from the nitrogen's lone pair to form a bond with the electrophile.

  2. Step 2:

    • Add a curly arrow from the bond extH−extO ext{H}- ext{O} to extO2extN ext{O}_2 ext{N}, indicating the formation of the extN+ ext{N}^+ ion.

  3. Step 3:

    • A curly arrow from the bond within benzene that forms a new extC−H ext{C-H} bond, showing how the benzene ring reforms after allowing for substitution.

  4. Step 4:

    • A curly arrow from the bond used to regenerate the aromaticity to the ring and back into the nitrogen group.

Step 4

Name of mechanism.

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Answer

Electrophilic substitution

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