Page 62 - Chemical and process design handbook

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ACETALDEHYDE

Acetaldehyde (ethanal, CH CH=O, melting point –123.5°C, boiling point:
3
o
o
o
20.1 C, density: 0.7780, flash point: –38 C, ignition temperature: 165 C)
is a colorless, odorous liquid.
Acetaldehyde has a pungent, suffocating odor that is somewhat fruity
and quite pleasant in dilute concentrations. Acetaldehyde is miscible in all
proportions with water and most common organic solvents, e.g., acetone,
benzene, ethyl alcohol, ether, gasoline, toluene, xylenes, turpentine, and
acetic acid.
Because of its versatile chemical reactivity, acetaldehyde is widely used
as a commencing material in organic syntheses, including the production of
resins, dyestuffs, and explosives. It is also used as a reducing agent, preser-
vative, and medium for silvering mirrors. In resin manufacture, paraldehyde
[(CH CHO) ] sometimes is preferred because of its higher boiling and flash
3 3
points.
Acetaldehyde was first prepared by Scheele in 1774, by the action of
manganese dioxide (MnO ) and sulfuric acid (H SO ) on ethyl alcohol
2 2 4
(ethanol, CH CH OH).
3 2
CH CH OH + [O] → CH CH=O + H O
3 2 3 2
Commercially, passing alcohol vapors and preheated air over a silver catalyst
o
at 480 C carries out the oxidation. With a multitubular reactor, conversions of
74 to 82 percent per pass can be obtained while generating steam to be used
elsewhere in the process.
The formation of acetaldehyde by the addition of water to acetylene was
observed by Kutscherow in 1881.

HC≡CH + H O → CH CH=O
2 3
In this hydration process, high-purity acetylene under a pressure of 15 psi
(103.4 kPa) is passed into a vertical reactor containing a mercury catalyst
o
dissolved in 18 to 25% sulfuric acid at 70 to 90 C. Fresh catalyst is fed to
+
the reactor periodically; the catalyst may be added in the mercurous (Hg )

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