Hydrogenation typically refers to processes through which liquid vegetable
oils are converted to solid or semi-solid
fats, such as
margerine. It refers to a
chemical reaction in which "
unsaturated"
bonds between
carbon atoms are "
reduced" by attachment of a
hydrogen atom to each carbon. The process thus results in the "saturation" of the atoms and, when carried to completion, converts unsaturated
fatty acids to saturated ones. Processes accomplishing the reverse are called "dehydrogenation" or "partial dehydrogenation."
Hydrogenation typically uses hydrogen gas as a reactant[?] and an undissolved (or "heterogeneous") metal catalyst, such as nickel, palladium or platinum. Otherwise, the "homogeneous" rhodium-based catalyst known as Wilkinson's catalyst is often used. Such reactions belong to organic chemistry.