A-LevelChemistryIntroduction to organic chemistry9701/43 · May/June 2022 · Q414 Marks

(d) The ligand bipyridine consists of two pyridine rings. Pyridine, C₅H₅N, and benzene, C₆H₆, have similar planar, cyclic structures. [Figure 4.2] (e) Pyridine reacts with Cl₂ in the presence of AlCl₃ as shown in Fig. 4.3. [Figure 4.3] The mechanism of this reaction is similar to that of the chlorination of benzene. AlCl₃ reacts with chlorine to generate an electrophile, Cl⁺. Complete the diagram to show the mechanism for the reaction of pyridine with Cl⁺. Include all relevant charges, dipoles, lone pairs of electrons and curly arrows as appropriate. [Diagram for reaction mechanism]

(a)The 3d orbitals in an isolated Fe²⁺ ion are degenerate. Complete the diagram to show the splitting of the 3d orbital energy levels in an isolated Fe²⁺ ion and when Fe²⁺ forms an octahedral complex.[2]
(b)(i)Bipyridine, bipy, is a bidentate ligand. Explain what is meant by bidentate ligand.[2]
(b)(ii)The complex [Fe(bipy)₃]²⁺ exists as two stereoisomers. Complete the three-dimensional diagrams to show the two stereoisomers of [Fe(bipy)₃]²⁺. State the type of stereoisomerism shown. Use [figure of bipy] to represent bipy in your diagrams. [Figure with Fe in center, 2 incomplete diagrams][3]
(c)Standard electrode potentials can be used to compare the stability of different complex ions for a given transition element. Table 4.1 lists electrode potentials for some electrode reactions for Fe³⁺/Fe²⁺ systems. Table 4.1 electrode reaction E/V [Fe(H2O)6]3+ + e¯ [Fe(H2O)6]2+ +0.77 [Fe(CN)6]3- + e¯ [Fe(CN)6]4- +0.36 [Fe(bipy)3]³+ + e¯ [Fe(bipy)3]2+ +0.96 Use relevant data from Table 4.1 to state which iron(III) complex is hardest to reduce. Explain your choice. iron(III) complex explanation[1]
(d)By reference to the hybridisation of the carbon atoms and the nitrogen atom, and orbital overlap, suggest how the σ and π bonds are formed in a pyridine molecule.[3]
(e)Complete the diagram to show the mechanism for the reaction of pyridine with Cl⁺. Include all relevant charges, dipoles, lone pairs of electrons and curly arrows as appropriate.[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[2] • five 3d orbitals in the isolated Fe²⁺ ion of same energy
• splitting two higher and three lower d orbitals
• energy of non-degenerate d orbitals in the complex more than degenerate d orbitals in isolated ion
two for one mark, three for two marks
[2] M1 (a species) that donates / uses two lone pairs
M2 to form dative / coordinate bonds to a metal atom / metal ion / TM/TE/metal
[3] M1 one correct 3D diagram with three bipy ligands
M2 both diagrams correct
M3 optical isomerism
[1] [Fe(CN)₆]³⁻ AND equilibrium lies most to the left / lowest E° value
[3] M1 N sp² AND C sp²
M2 σ bonds are formed by end-on-end overlap orbitals between C-H / C-C / C-N
M3 π bonds are formed by sideways overlap of p orbitals between C-N/ C-C
[3] M1 first curly arrow to Cl⁺
M2 correct intermediate
M3 second curly arrow AND H⁺ formed / lost

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