Question

i) Define standard electrode potential. (1)
ii) Define bidentate ligand. (1)
iii) Complete the following reactions (1+1+1):
a) RX + NaOR' → ?
b) C₆H₅COCH₃ (acetophenone) + H₂/Pd-BaSO₄ → ?
c) C₆H₆ (benzene) + CO + HCl → (AlCl₃/Cu₂Cl₂) ?
OR
i) Explain standard hydrogen electrode potential with diagram. (1)
ii) Give an example of optical isomerism. (1)
iii) Complete the following reactions (1+1+1):
a) CH₃CH₂OH → (H₂SO₄, 413 K) ?
b) RCN + SnCl₂ + HCl → ?
c) C₆H₅CH₃ (toluene) + CrO₂Cl₂ + CS₂ → ?

Hint:

Standard electrode potential is measured against the standard hydrogen electrode at 1 M, 298 K and 1 bar. The named reactions here are the Williamson ether synthesis, the Gattermann-Koch reaction, ethanol dehydration to ether at 413 K, the Stephen reduction and the Etard reaction.

Answer 1

Option 1:

Step 1: i) Standard electrode potential. It is the electrode potential of an electrode measured with respect to the standard hydrogen electrode under standard conditions, that is, all ions at unit concentration (1 M / unit activity), temperature 298 K and any gas at 1 bar pressure. It is denoted E°.

Step 2: ii) Bidentate ligand. A ligand that has two donor atoms, each carrying a lone pair, through which it binds to the central metal atom or ion at the same time. Examples: ethylenediamine (en, two N donor atoms) and the oxalate ion C₂O₄^2- (two O donor atoms).

Step 3: iii-a) Williamson ether synthesis. The alkoxide ion of NaOR' displaces the halide of the alkyl halide by nucleophilic substitution:
RX + NaOR' → R–O–R' + NaX (an ether).

Step 4: iii-b) Reduction of the ketone. Catalytic hydrogenation adds H₂ across the C=O group of acetophenone, reducing the ketone to a secondary alcohol:
C₆H₅COCH₃ + H₂ → C₆H₅CH(OH)CH₃ (1-phenylethanol).

Step 5: iii-c) Gattermann-Koch reaction. Benzene reacts with carbon monoxide and hydrogen chloride in the presence of anhydrous AlCl₃ and Cu₂Cl₂ to give benzaldehyde:
C₆H₆ + CO + HCl → C₆H₅CHO + HCl.

Option 2:

Step 1: i) Standard hydrogen electrode (SHE). It is the reference electrode whose potential is fixed at exactly zero volt. A platinum foil coated with platinum black is dipped in a 1 M H⁺ (HCl) solution, and pure H₂ gas at 1 bar is bubbled over it at 298 K (see diagram above). The electrode reaction is 2H⁺ + 2e^- → H₂. All other electrode potentials are measured against it.

Step 2: ii) Example of optical isomerism. The complex [Co(en)₃]³⁺ exists as two non-superimposable mirror images (d and l forms) and so shows optical isomerism. (Lactic acid is a simple organic example.)

Step 3: iii-a) Dehydration to an ether. With concentrated H₂SO₄ at 413 K, two molecules of ethanol lose one molecule of water to give diethyl ether:
2C₂H₅OH → C₂H₅OC₂H₅ + H₂O. (At the higher temperature 443 K the product would be ethene instead.)

Step 4: iii-b) Stephen reaction. A nitrile is reduced by stannous chloride and HCl to an aldimine, which on hydrolysis gives an aldehyde:
RCN + SnCl₂ + HCl → RCH=NH, then + H₂O → RCHO + NH₃.

Step 5: iii-c) Etard reaction. Toluene is oxidised by chromyl chloride (CrO₂Cl₂) in CS₂ to a brown chromium complex, which on hydrolysis gives benzaldehyde:
C₆H₅CH₃ + CrO₂Cl₂, then H₂O → C₆H₅CHO.