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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2017 · Paper I

Q5(d). Ferrocene: electrophilic substitution and oxidation

  • 10 marks
  • Answered
Question

Explain the following properties of Ferrocene:
(i) Easy electrophilic substitution of its Cp ring.
(ii) Oxidation is difficult in comparison to that of cobaltocene.

Answer in brief

(i) Each Cp ring is an aromatic 6π ring made electron rich by the iron, so electrophiles attack it much faster than benzene. (ii) Ferrocene (18 e) loses an electron from the non-bonding a_(1g) orbital; cobaltocene (19 e) loses its extra electron from an antibonding e_(1g)* orbital and becomes the stable 18-electron [Co(Cp)₂]⁺.

Key points

  • Each C₅H₅⁻ ring has 6 π electrons in a planar ring: it is aromatic, so it undergoes substitution (not addition), like benzene.
  • The rings carry negative charge and receive electron density from the Fe(II) centre: the π cloud is electron rich. Electrophilic aromatic substitution is much more rapid than in benzene.
  • Friedel–Crafts acylation with CH₃COCl/AlCl₃ or acetic anhydride/H₃PO₄ gives acetylferrocene, (C₅H₅)Fe(C₅H₄COCH₃), under mild conditions.
  • HNO₃ and halogens oxidise the iron to ferrocenium instead.

The full model answer

Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.

FERROCENEFe(II), d⁶, 18 eCOBALTOCENECo(II), d⁷, 19 eREACTIONFriedel–Crafts acylation

(i) Easy electrophilic substitution

  1. Each C₅H₅⁻ ring has 6 π electrons in a planar ring: it is aromatic, so it undergoes substitution (not addition), like benzene.
  2. The rings carry negative charge and receive electron density from the Fe(II) centre: the π cloud is electron rich. Electrophilic aromatic substitution is much more rapid than in benzene.
  3. Example: Friedel–Crafts acylation with CH₃COCl/AlCl₃ or acetic anhydride/H₃PO₄ gives acetylferrocene, (C₅H₅)Fe(C₅H₄COCH₃), under mild conditions.
  4. Nitration and halogenation fail: HNO₃ and halogens oxidise the iron to ferrocenium instead.

(ii) Oxidation harder than for cobaltocene

  1. Ferrocene: 8 (Fe) + 10 (two Cp) = 18 valence electrons. All bonding and non-bonding MOs are filled; the HOMO is the non-bonding a_(1g) (mainly d_(z²)). Oxidation to the 17-electron ferrocenium ion [Fe(Cp)₂]⁺ is possible but needs a fair oxidant.
  2. Cobaltocene: 9 + 10 = 19 electrons. The extra electron sits in the antibonding e_(1g)* orbital; it is high in energy and is lost very easily, giving the 18-electron cobaltocenium ion [Co(Cp)₂]⁺.
  3. Hence cobaltocene is a strong reducing agent that oxidises in air, while ferrocene is air-stable. The Co–C bonds become shorter on oxidation because an antibonding electron is removed.

Application. the ferrocenium/ferrocene couple is a reversible one-electron reference in non-aqueous electrochemistry, and cobaltocene is used as a one-electron reducing agent.

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Model answer to Q5(d), 2017 Paper I: solution page 21
Solution page 21 of 38
Read the whole 2017 solution2017 question paper

All parts of Question 5