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: Standard electrode potential (E°) is the tendency of an electrode to gain or lose electrons, expressed as a voltage against the standard hydrogen electrode, when every dissolved species is at 1 mol/L, the temperature is 298 K and any gas is at 1 bar.

Step 2: A bidentate ligand uses two lone-pair-bearing atoms to grip the metal at two points, forming a chelate ring. Common examples are ethane-1,2-diamine (en) and the oxalato ion.

Step 3 (a): RX reacting with sodium alkoxide NaOR' gives a mixed ether by nucleophilic substitution (Williamson synthesis); the product is R–O–R' with NaX as the by-product.

Step 4 (b): Hydrogen over the metal catalyst saturates the carbonyl of acetophenone, so the ketone becomes the secondary alcohol 1-phenylethanol, C₆H₅CH(OH)CH₃.

Step 5 (c): Passing CO and HCl into benzene with the AlCl₃/Cu₂Cl₂ catalyst formylates the ring (Gattermann-Koch reaction), giving benzaldehyde C₆H₅CHO.

Option 2:

Step 1: In the standard hydrogen electrode, a platinised platinum plate stands in 1 M acid while H₂ at 1 bar and 298 K flows past it; its potential is defined as 0.00 V, and every other electrode potential is quoted relative to it (see figure). Electrode reaction: 2H⁺ + 2e^- is in equilibrium with H₂.

Step 2: Optical isomerism is shown by species with non-superimposable mirror images, for example the octahedral chelate cis-[Co(en)₂Cl₂]⁺ or [Co(en)₃]³⁺.

Step 3 (a): Ethanol heated with concentrated sulphuric acid at 413 K undergoes intermolecular dehydration to diethyl ether, C₂H₅OC₂H₅, with elimination of water.

Step 4 (b): The Stephen reduction of a nitrile with SnCl₂/HCl stops at the aldimine stage; aqueous work-up then delivers the aldehyde RCHO.

Step 5 (c): Chromyl chloride in carbon disulphide attacks the methyl group of toluene (Etard reaction), and hydrolysis of the resulting complex yields benzaldehyde C₆H₅CHO.