Propionaldehyde
Chemical compound
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Through Organic compound, Aldehyde and Sulfuric acid
Production
Propionaldehyde is mainly produced industrially by hydroformylation of ethylene:
- CO + H2 + C2H4 → CH3CH2CHO
In this way, several hundred thousand tons are produced annually.
Laboratory preparation
Propionaldehyde may also be prepared by oxidizing 1-propanol with a mixture of sulfuric acid and potassium dichromate. The reflux condenser contains water heated at 60 °C, which condenses unreacted propanol, but allows propionaldehyde to pass. The propionaldehyde vapor is immediately condensed into a suitable receiver. In this arrangement, any propionaldehyde formed is immediately removed from the reactor, thus it does not get over-oxidized to propionic acid.
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Reactions
Propionaldehyde exhibits the reactions characteristic of alkyl aldehydes, e.g. hydrogenation, aldol condensations, oxidations, etc. It is the simplest aldehyde with a prochiral methylene such that α-functionalized derivatives (CH3CH(X)CHO) are chiral. If water is available, propionaldehyde exists in equilibrium with 1,1-propanediol, a geminal diol.
2-Methylpentanal (CH3CH2CH2CH(CH3)CHO) arises by aldol condensation of propionaldehyde followed by dehydration and hydrogenation.
Uses
Both industrially and in the laboratory, propionaldehyde has primary application as a chemical building block.
It is predominantly used as a precursor to trimethylolethane (CH3C(CH2OH)3) through a condensation reaction with formaldehyde. This triol is an important intermediate in the production of alkyd resins. It is used in the synthesis of several common aroma compounds (cyclamen aldehyde, helional, lilial).
Reduction of propionaldehyde gives n‑propanol, and reductive amination gives propanamine. Rising demand for non-chlorocarbon solvents has caused some manufacturers to substitutively brominate n‑propanol to propyl bromide. However, the majority of applications use n‑propanol proper in esters or glycol ethers, or as a gentle alkylant for primary and secondary amines.
Oxidants instead give propionic acid and propionates, typically used as preservatives.
Laboratory uses
Many laboratory uses exploit its participation in condensation reactions. With tert-butylamine it gives CH3CH2CH=N-t-Bu, a three-carbon building block used in organic synthesis.
Extraterrestrial occurrence
Propionaldehyde along with acrolein has been detected in the molecular cloud Sagittarius B2 near the center of the Milky Way Galaxy, about 26,000 light years from Earth.
Measurements by the COSAC and Ptolemy instruments on comet 67/P's surface, revealed sixteen organic compounds, four of which were seen for the first time on a comet, including acetamide, acetone, methyl isocyanate and propionaldehyde.
Safety
With an LD50 of 1690 mg/kg (oral), propionaldehyde exhibits low acute toxicity, but is a lung and eye irritant and is a combustible liquid.