VKC ScienceChemistry for written exams Telegram

UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper III covers analytical and organic chemistry.

UPSC Combined Geo-Scientist, Mains · 2017 · Mains Paper III

Q1(c). Solvent effect on aniline basicity

Question

Suggest a solvent in which aniline behaves as a stronger base than in water and explain.

Answer in brief

An acidic solvent such as glacial acetic acid enhances its proton-accepting behaviour and permits nonaqueous acidimetric titration.

Key points

  • Basicity is relative to the proton-donating ability of the medium. Aniline is only weakly protonated by water.
  • In an acidic solvent the equilibrium PhNH₂ + HOAc ⇌ PhNH₃⁺ + OAc⁻ is shifted towards protonation compared with water.
  • Glacial acetic acid is commonly used to titrate weak organic bases with perchloric acid; a suitable strong-acid medium can level bases toward their protonated forms.
  • This does not mean an aqueous Kb can be transferred unchanged between solvents or that aniline becomes a strong aqueous base.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Model answer to Q1(c), 2017 Mains Paper III: solution pages 1 and 2
Solution pages 1 and 2 of 18
Read the whole 2017 solution2017 question paper

CoversSolvent effect on aniline basicityNonaqueous acid–base chemistry

All parts of Question 1