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QUESTION

# What is the equation to convert ethanol to propanoic acid ? And what conditions needed for the conversion ?

If we start from ethanol, then we can use "PBr"_3 in pyridine to turn ethanol into ethyl bromide, which can then be converted to a Grignard reagent by reacting with magnesium solid in dry ether, followed by a reaction with carbon dioxide.

1. \mathbf("PBr"_3) acts as an electrophile for the alcohol to attack, while pyridine acts as a proton acceptor. A ""^(-)"O"-"PBr"_2 (bromophosphite) group leaves when the bromide that left from "PBr"_3 attacks again as a nucleophile (Organic Chemistry, Bruice, Ch. 10.2).
2. Formation of a Grignard reagent; you should know this if you have taken second-semester organic chemistry for a few weeks. It must be dry to prevent deactivation of the nucleophile.
3. Grignard reagents can react in an "S"_N2 reaction with \mathbf("CO"_2).
4. A bit of aqueous acid finishes up the reaction by protonating the carboxylate and hydrolyzing the "MgBr"^(+), neutralizing any remaining Grignard reagent as well by (easily) protonating the ethyl anion.