2

Acetaldehyde is used a starting material for the manufacture of 1,3-butadiene. The route has three steps: acetaldehyde is dimerised to acetaldol, which is reduced to 1,3-butanediol, from which two molecules of water are eliminated.

Industrial Organic Chemistry, 3rd. Ed. K. Weissermel, H.-J. Arpe, VCH (1997)

Reaction defined as:
Condensation
Elimination
Hydrogenation, catalytic
Industrial Process: Organic