Question

An organic compound (A) with molecular formula $\mathrm{C_3H_5N}$ on reaction with $\mathrm{C_6H_5MgBr}$ followed by hydrolysis, gives a compound (B). Compound (B) forms an orange-red precipitate with 2,4-DNP reagent and does not give iodoform test. It neither reduces Tollens' or Fehling's reagent nor does it decolourise bromine water. On drastic oxidation with chromic acid it gives a carboxylic acid (C) having molecular formula $\mathrm{C_7H_6O_2}$. Identify the compounds (A), (B) and (C). Write the reactions of compound (A) with $\mathrm{C_6H_5MgBr}$ followed by hydrolysis to give compound (B).

Hint:

Nitrile + Grignard → ketone; ketone tests + oxidation to benzoic acid fix A, B, C.

Answer 1

Concept: Some ligands can grab the metal at more than one point and form a ring. These ring-forming ligands give complexes that are unusually stable, and that extra stability has a special name.

Step 1: What the chelate effect means
The chelate effect is the extra stability that a complex gets when it contains a polydentate ligand, that is a ligand that holds the metal at two or more points and so forms a ring (a chelate). Such a complex is much more stable than a similar complex made with several separate monodentate ligands (each holding on at only one point). The simple reason is that one ring-forming ligand grips the metal tightly and is far less likely to let go.

Step 2: An example
Compare $\mathrm{[Ni(en)_3]^{2+}}$ with $\mathrm{[Ni(NH_3)_6]^{2+}}$. Both have nitrogen donors attached to nickel, but 'en' (ethylenediamine) is bidentate and forms rings, while $\mathrm{NH_3}$ is monodentate. The chelated complex $\mathrm{[Ni(en)_3]^{2+}}$ is clearly more stable than $\mathrm{[Ni(NH_3)_6]^{2+}}$.

Answer: The chelate effect is the extra stability of a complex caused by ring-forming (chelating) ligands; for example $\mathrm{[Ni(en)_3]^{2+}}$ is more stable than $\mathrm{[Ni(NH_3)_6]^{2+}}$.