Question

Like any catalyst, palladium, platinum, and nickel catalyze both directions of a reaction: the addition of hydrogen to (hydrogenation) and its elimination from (dehydrogenation) carbon double bonds.

(a) Which variable determines whether an alkene will be hydrogenated or dehydrogenated?

(b) Which reaction requires a higher temperature?

(c) How can all-trans fats arise during the hydrogenation of fats that contain some cis-double bonds?

Short answer

1. Based on the rate of the reaction
2. Dehydrogenation
3. Formation of trans fats:

Step-by-step solution

Hydrogenation and Dehydrogenation

Hydrogenation is the process of adding the hydrogens to the double bond to convert it into a saturated form, whereas dehydrogenation is the reverse of hydrogenation which leads to the elimination of hydrogen and the formation of the unsaturated bond from the saturated bonds.

Hydrogenation is done to make the molecule stable. It is down to convert the liquid into solid that is oil to fat which increases the melting point of the fat and is even used to terminate the coke formatting reactions. In contrast, dehydrogenation is the process used to improve the reaction rate when the rate of the reaction is quite slow. For example, in polymer formation, akene is dehydrogenated to form alkene and increases the rate of the reaction. Even dehydrogenation can occur in the absence of a catalyst, whereas for hydrogenation catalyst is necessary.

Hydrogenation and Dehydrogenation temperature

Hydrogenation occurs at 400°C, whereas dehydrogenated occurs at 500°C and above. A dehydrogenation occurs at high temperatures because the reactions are highly endothermic, and the reactions are used to form aromatic compounds. Even nonoxidative dehydrogenation requires high temperatures to overcome unfavorable thermodynamic conditions

Hydrogenation to form trans fats

On hydrogenation of cis fatty acids, trans fats are formed in the presence of a metal catalyst as they are the result of the side chain with the catalyst. The cis bond is converted to s the transform due to the energy provided by the catalyst during hydrogenation and trans fats are not good for health as they increase the LDL cholesterol.