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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 · 2016 · Paper I

Q7(c). cis- and trans-[PtCl₂(NH₃)₂]: synthesis by the trans effect

  • 10 marks
  • Answered
Question

Starting with [PtCl₄]²-, prepare diammine dichloro platinum (II). Name the isomer formed. Choose the relevant starting material and write all the steps to get the other geometrical isomer of the above product.

Answer in brief

[PtCl₄]²- + 2NH₃ gives cis-[PtCl₂(NH₃)₂] (cisplatin). The trans isomer is made from [Pt(NH₃)₄]²+ + 2Cl⁻. Both follow from the trans effect, Cl⁻ > NH₃.

Key points

  • [PtCl₄]²- + NH₃ → [PtCl₃(NH₃)]⁻ + Cl⁻: all four Cl are alike; any one is replaced.
  • In [PtCl₃(NH₃)]⁻ two Cl are trans to Cl and one is trans to NH₃. Cl is the stronger trans director, so a Cl trans to Cl is replaced: 2pt] [PtCl₃(NH₃)]⁻ + NH₃ → cis-[PtCl₂(NH₃)₂] + Cl⁻
  • cis-diamminedichloridoplatinum(II) (cisplatin), yellow
  • [PtCl₄]²- + 4NH₃ (excess) → [Pt(NH₃)₄]²+ + 4Cl⁻.

The full model answer

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

A ligand labilises the group trans to itself (CN⁻ > NO₂⁻ > I⁻ > Br⁻ > Cl⁻ > NH₃).

cis isomer from [PtCl₄]²-

  1. [PtCl₄]²- + NH₃ → [PtCl₃(NH₃)]⁻ + Cl⁻: all four Cl are alike; any one is replaced.
  2. In [PtCl₃(NH₃)]⁻ two Cl are trans to Cl and one is trans to NH₃. Cl is the stronger trans director, so a Cl trans to Cl is replaced: 2pt] [PtCl₃(NH₃)]⁻ + NH₃ → cis-[PtCl₂(NH₃)₂] + Cl⁻
  3. product: cis-diamminedichloridoplatinum(II) (cisplatin), yellow

trans isomer: start from [Pt(NH₃)₄]²+

  1. Make the tetraammine first: [PtCl₄]²- + 4NH₃ (excess) → [Pt(NH₃)₄]²+ + 4Cl⁻.
  2. [Pt(NH₃)₄]²+ + Cl⁻ → [PtCl(NH₃)₃]⁺ + NH₃: any NH₃ is replaced.
  3. In [PtCl(NH₃)₃]⁺ the NH₃ trans to Cl is the labile one (Cl > NH₃): 2pt] [PtCl(NH₃)₃]⁺ + Cl⁻ → trans-[PtCl₂(NH₃)₂] + NH₃ (heat with HCl)
  4. product: trans-diamminedichloridoplatinum(II), pale yellow (no dipole moment; cis has one)

Application. cisplatin is an anticancer drug: its two cis Cl are replaced by two adjacent guanine N7 atoms of DNA; the trans isomer cannot form this cross-link and is inactive.

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Model answer to Q7(c), 2016 Paper I: solution page 40
Solution page 40 of 46
Read the whole 2016 solution2016 question paper

All parts of Question 7