Acetaldehyde

Acetaldehyde is the organic compound with chemical formula CH₃CHO. It is the second-simplest aldehyde and finds use as a building block in organic synthesis.

Properties

Chemical

Depending on reaction conditions, the oxidation of acetaldehyde by oxygen variously co-produces acetic anhydride and acetic acid via the intermediate peracetic acid. The process relies on a catalyst containing metal ions and is typically conducted by introduction of gaseous oxygen into liquid acetaldehyde.

Physical

Acetaldehyde is a transparent, volatile, and extremely flammable liquid at room temperature that boils at only 20.2°C (68.4°F), making it difficult to store. The characteristic odor of acetaldehyde is sweet and reminiscent of green apple. Acetaldehyde has a flash point of only −39°C, making it absolutely crucial that any significant amount is kept away from possible ignition sources.

Availability

No over-the-counter source of acetaldehyde is known, and sources are unlikely due to the difficulty and danger of prolonged storage.

Preparation

Industrially, acetaldehyde is produced via the oxidation of ethene over a copper-palladium catalyst.

2 CH₂=CH₂ + O₂ → 2 CH₃CHO

Two routes exist from ethanol, the first an exothermic, self-sustaining oxidation reaction using a copper or silver catalyst at a temperature of 500-650°C:

2 CH₃CH₂OH + O₂ → 2 CH₃CHO + 2 H₂O

The second method of preparing acetaldehyde from ethanol involves its dehydrogenation at a temperature of of 260-290 °C, again over a catalyst of copper. This route is endothermic, requiring constant and uniform heating, but has the advantage of not requiring oxygen input, which also reduces the risk of fire.

CH₃CH₂OH → CH₃CHO + H₂

Projects

• Pentaerythritol synthesis

Handling

Safety

Acetaldehyde is designated a probable carcinogen and is many times more toxic than ethanol, which it is a metabolite of. With a high chance of vaporization and a flash point of only −39 °C, acetaldehyde represents a significant fire hazard, and care should be made that all potential sources of ignition are kept away.

Storage

Acetaldehyde must be stored in a highly temperature-controlled environment given its low boiling point, and should also be kept in a separate cabinet or secondary container in a well-ventilated space, always away from potential sources of ignition.

Another option is to store it into a metal pressurized container, such as a lecture bottle.

Disposal

Acetaldehyde can be safely burned.