The compound \(\mathrm{CH}_{3}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH}_{3}\) is hydrogenated to an alkene using platinum as the catalyst. Predict whether the product is the pure trans isomer, the pure cis isomer, or a mixture of cis and trans isomers. Based on your prediction, comment on the mechanism of the heterogeneous catalysis.

Hydrogenation is a chemical reaction where hydrogen molecules are added across double or triple bonds in unsaturated compounds, such as alkenes and alkynes, resulting in a more saturated molecule. This reaction is commonly conducted in the presence of a catalyst, such as platinum, palladium, or nickel. The process is widely used in the food industry to convert unsaturated fats to saturated fats, increasing shelf-life of products.

For alkyne chemistry, hydrogenation can stop at the alkene stage if controlled properly. When an alkyne is hydrogenated, it can form either a cis or trans alkene, depending on the conditions and the mechanism used. Controlling these conditions allows for selective synthesis, which is valuable in industrial applications and synthesis of target molecules in medicinal chemistry.

Cis-trans isomerism is a type of stereoisomerism found in alkenes due to the inflexibility of the carbon-carbon double bond. Cis isomers have substituents located on the same side of the double bond, while trans isomers have them on opposite sides. This geometric difference can significantly affect the physical and chemical properties of the molecules.

The importance of cis-trans isomerism is evident in biological systems, pharmaceuticals, and material science where the isomers can have drastically different biological activities, solubilities, and melting points. For example, the cis configuration in fatty acids results in a lower melting point, contributing to the liquid nature of unsaturated fats at room temperature.

Heterogeneous catalysis occurs when the reactants and the catalyst are in different phases, such as a solid catalyst and gaseous or liquid reactants. The benefits of this type of catalysis include ease of separation after the reaction and the ability to reuse the catalyst. In heterogeneous catalysis, the reactant molecules like the alkyne and hydrogen in this textbook exercise adsorb onto the catalyst’s surface. This adsorption weakens the bonds, facilitating the addition of hydrogen in a controlled manner.

For educators and students focusing on the study of catalysts, understanding the surface chemistry and the adsorption-desorption phenomena is crucial. Platinum, the catalyst mentioned in the exercise, is a common heterogeneous catalyst renowned for its ability to facilitate reactions under relatively mild conditions.

Syn addition is a type of chemical addition reaction where two substituents are added to the same side of a double or triple bond in a molecule. This process contrasts with anti addition, where the substituents are added on opposite sides. In the context of alkyne chemistry, when an alkyne undergoes syn addition of hydrogen, the result is usually the formation of a cis alkene.

Understanding syn addition is critical for predicting the outcome of a hydrogenation reaction. Catalysts like platinum often lead to syn addition, ensuring that both hydrogen atoms are added from the same direction onto the alkyne, paving the path from a triple bond to a cis double bond, rather than trans.

Alkyne chemistry involves the reactions and properties of alkynes, hydrocarbons that contain at least one carbon-carbon triple bond. These compounds are linear and feature sp-hybridized carbon atoms, providing unique reactivity compared to alkenes or alkanes. Alkynes can undergo various reactions, including hydrogenation, halogenation, and hydration, often resulting in the formation of new functional groups.

The flexibility of alkyne chemistry makes it a critical area of study for organic synthesis. Due to their reactivity, alkynes are valuable starting materials for synthesizing complex molecules. For instance, as seen in the exercise, the controlled hydrogenation of an alkyne can lead to selective alkene production, which is a foundational concept in organic chemistry.