Crotonaldehyde

CAS Number: 123-73-9. A clear to yellow liquid, whose vapor (at
high concentrations) can be very irritable to the respiratory
system. It is used as an intermediate in the production of other
industrial chemicals. It also occurs naturally in foods, and is
formed during the combustion of fossil fuels (including engine
exhausts), wood and tobacco and in heated cooking oils. Chemical
formula = C4H6O. Molecular weight = 70.09 g/mol.