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Ethane (CH3-CH3) is to be converted to propanoic anhydride (CH3CH2-CO-O-CO-CH2CH3) as the major product. Which of the following reagent sequences achieves this transformation?

1. (i) Br2 (1 eq), hv; (ii) Mg/dry ether; (iii) CO2, then H3O+; (iv) Conc. H2SO4
2. (i) Cl2 (1 eq), hv; (ii) KCN; (iii) H3O+; (iv) Conc. H3PO4
3. (i) Br2 (1 eq), hv; (ii) AgCN; (iii) H3O+; (iv) Conc. H2SO4
4. (i) Cl2 (1 eq), hv; (ii) Mg/dry ether; (iii) HCHO then HOH; (iv) H+, KMnO4; (v) Conc. H2SO4

Correct answer: (i) Br2 (1 eq), hv; (ii) Mg/dry ether; (iii) CO2, then H3O+; (iv) Conc. H2SO4

Solution

Route A: EtBr (from Br2/hv) -> EtMgBr (Mg/ether) -> EtMgBr + CO2 -> EtCOOMgBr -> H3O+ -> EtCOOH (propanoic acid). Two moles of EtCOOH + conc. H2SO4 (dehydration) -> (EtCO)2O (propanoic anhydride). This is correct. Route C uses AgCN which gives isocyanide (isonitrile) not nitrile, so hydrolysis does not give propanoic acid — incorrect. Routes B and D can also work but A is the most direct and unambiguous.

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