Crotonaldehyde

Crotonaldehyde

Crotonaldehyde (but-2-en-1-al), b.p. 104^oC, may be prepared by heating aldol alone, or better, with a dehydrating agent, e.g., zinc chloride. The best yield is obtained by distilling aldol with acetic acid as catalyst:

CH₃CHOHCH₂CHO    (-H₂O) ⟶    CH₃CH=CHCHO

Crotonaldehyde closely resembles acraldehyde in its chemical properties. It exists, however, in two geometrical isomeric forms, cis and trans:

A number of unsaturated aldehydes occur naturally in the essential oils, many accompanying the corresponding unsaturated alcohol.

The simplest acetylenic aldehyde is propargylaldehyde (propiolaldehyde, propynal), b.p. 60^oC. This may be prepared from acraldehyde, the aldehyde group of which is ‘protected by acetal formation:

It may also be obtained by the controlled oxidation of propargyl alcohol.

Propargylaldehyde undergoes cleavage when treated with sodium hydroxide (cf. acraldehyde, above):