Hydrogenation Reactions: Videos & Practice Problems

In hydrogenation reactions that convert unsaturated compounds into alkanes, the metal catalyst most commonly used is platinum (Pt). However, other metals like palladium (Pd) and nickel (Ni) are also frequently used for this purpose. These catalysts help to facilitate the addition of hydrogen (H₂) across the double or triple bonds of unsaturated hydrocarbons, such as alkenes and alkynes, to yield saturated alkanes. The catalyst provides a surface where the reactants can adsorb, and the hydrogen molecules dissociate into atoms and become more reactive. This process allows the hydrogen atoms to add to the carbon atoms with multiple bonds more efficiently, resulting in the formation of single-bonded carbon atoms, or alkanes, with a higher saturation of hydrogen.

Hydrogenation reactions are chemical processes where hydrogen molecules are added to other compounds, typically unsaturated organic molecules like alkenes and alkynes. This addition of hydrogen generally occurs in the presence of a catalyst, such as palladium, platinum, or nickel, and often requires the application of heat and pressure.

During hydrogenation, the double or triple bonds between carbon atoms in the unsaturated compounds are converted into single bonds as hydrogen atoms are added. This process results in the saturation of the molecule, turning alkenes into alkanes for instance. A common example of a hydrogenation reaction is the conversion of vegetable oils, which contain unsaturated fats, into semi-solid fats like margarine, a process that increases the melting point of the oil.

Hydrogenation is widely used in the food industry, as well as in the production of chemicals and pharmaceuticals, where controlling the saturation level of molecules is crucial for the properties and functionality of the final products.

To answer your question on drawing the major organic product for hydrogenation reactions, I'll provide a general explanation since I don't have the specific reactions you're referring to.

In hydrogenation reactions, an alkene or alkyne is typically converted into an alkane through the addition of hydrogen (H₂) in the presence of a catalyst, usually palladium (Pd), platinum (Pt), or nickel (Ni). The reaction typically proceeds through a syn addition, where both hydrogen atoms add to the same side of the double or triple bond.

For alkenes:

• The hydrogenation of a simple alkene will yield a single product, an alkane, with no stereoisomers since alkanes do not have chiral centers or geometric isomerism.
• If the alkene is disubstituted and has cis/trans (E/Z) isomerism, the product will still be a single alkane without stereoisomers.

For alkynes:

• The hydrogenation of an alkyne can proceed in two stages. The first stage adds one molecule of hydrogen to give a cis-alkene. The second stage adds another molecule of hydrogen to give an alkane.
• If the reaction is stopped at the alkene stage, you may have cis/trans (E/Z) isomerism depending on the substituents attached to the carbon-carbon double bond.