How is #"propionic acid"#, #"H"_3"CCH"_2"CO"_2"H"#, synthesized?
Industrially, propanoic acid is formed by the addition of carbon monoxide and water to ethylene. The process employs catalysis.
In the laboratory, propionic acid could be formed the reaction of ethyl magnesium chloride with dry ice, or cyanide with ethyl chloride.
If we start from ethanol, then we can use
#\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).
- 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.
- Grignard reagents can react in an
#"S"_N2#reaction with #\mathbf("CO"_2)#.
- 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.