Acetaldehyde

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Acetaldehyde
Acetaldehyde lewis.png
Properties
Miscible
Related compounds
Related compounds
Formaldehyde
Propionaldehyde
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

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[1]

During the synthesis of dioxane from ethylene glycol using conc. sulfuric acid as catalyst, small amounts of acetaldehyde will be produced. However you will need to convert large amounts of ethylene glycol into dioxane to get a significant amount of acetaldehyde.

Projects

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.

References

  1. Ullmann's Encyclopedia Of Industrial Chemistry, Wiley-VCH (2007)

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