Acetic acid (ethanoic acid, vinegar acid, CH3CO2H, melting point 16.6o C, boiling point: 117.9o C, density: 1.0490, flash point: 43o C, ignition temperature 465o C) is a colorless, pungent liquid that is miscible with water, alcohol, and ether in all proportions.
Acetic acid is available commercially in several concentrations: (1) glacial acetic is approximately 99.7% glacial acetic acid with water the principal impurity, (2) reagent grade acetic acid generally contains 36% acetic acid by weight, and (3) commercial aqueous solutions are usually 28, 56, 70, 80, 85, and 90% acetic acid.
Acetic acid is the active ingredient in vinegar, in which the content ranges from 4 to 5% acetic acid. Acetic acid is classified as a weak, monobasic acid (-CO2H) but the three hydrogen atoms linked to the carbon atom (CH3) are not replaceable by metals.
Acetic acid is manufactured by three processes: acetaldehyde oxidation, n-butane oxidation, and methanol carbonylation.Ethylene is the exclusive organic raw material for making acetaldehyde, 70 percent of which is further oxidized to acetic acid or acetic anhydride. The single-stage (Wacker) process for making acetaldehyde involves cupric chloride and a small amount of palladium chloride in aqueous solution as a catalyst.
CH2=CH2 + H2O + PdCl2 ? CH3CHO + 2HCl + Pd0
The yield is 95 percent and further oxidation of the acetaldehyde produces acetic acid (Fig. 1).
2CH3CHO + O2 ? 2CH3CO2H
A manganese or cobalt acetate catalyst is used with air as the oxidizing agent in the temperature and pressure ranges of 55 to 80o C and 15 to 75 psi; the yield is 95 percent.
The second manufacturing method for acetic acid utilizes butane from the C4 petroleum stream rather than ethylene. A variety of products is formed but conditions can be controlled to allow a large percentage of acetic acid to be formed. Cobalt, manganese, or chromium acetates are catalysts with temperatures of 50 to –250oC and a pressure of 800 psi.
C4H10 + O2 ? CH3CO2H + HCO2H + CH3CH2OH + CH3OH
The third and preferred method of acetic acid manufacture is the carbonylation of methanol, involving reaction of methanol and carbon monoxide (both derived from methane) with rhodium and iodine as catalysts at 175oC and 1 atm (Fig. 2).
CH3OH + CO ? CH3CO2H
The yield of acetic acid is 99 percent based on methanol and 90 percent based on carbon monoxide.
Acetic acid is used for the manufacture of methyl acetate (Fig. 3) and acetic anhydride (Fig. 4), vinyl acetate, ethyl acetate, terephthalic acid, cellulose acetate, and a variety of acetic esters.
Vinyl acetate is used mainly as a fiber in clothing. Ethyl acetate is a common organic solvent. Acetic acid is used in the manufacture of terephthalic acid, which is a monomer for the synthesis of poly (ethylene terephthalate), the polyester of the textile industry.