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UPSC Combined Geo-Scientist

Chemist

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

Mains Paper I covers inorganic chemistry.

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

Q1(f). Dimethylberyllium bridges

Question

Draw the structure of dimethyl beryllium and explain the bonding.

Answer in brief

Solid dimethylberyllium forms a chain polymer with bridging methyl groups and electron-deficient Be–C–Be three-centre, two-electron bonds.

Key points

  • Each pair of adjacent Be centres is linked by two methyl bridges.
  • A bridge uses an orbital associated with carbon and suitable orbitals on two Be atoms to form a delocalised bonding combination occupied by two electrons.
  • The repeat composition remains Be(CH₃)₂, because each bridging methyl is shared. Monomeric vapour is represented by linear CH₃-Be-CH₃.

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(f), 2017 Mains Paper I: solution pages 3 and 4
Solution pages 3 and 4 of 16
Read the whole 2017 solution2017 question paper

CoversDimethylberyllium bridgesElectron-deficient bonding

All parts of Question 1