Horiuti-Polanyi Mechanism

Definition

The Horiuti-Polanyi mechanism is a model that describes the step-by-step process of the catalytic hydrogenation of alkenes. It provides a detailed explanation of how hydrogen gas interacts with an alkene substrate in the presence of a metal catalyst to reduce the carbon-carbon double bond.

5 Must Know Facts For Your Next Test

1. The Horiuti-Polanyi mechanism describes the step-by-step process of the catalytic hydrogenation of alkenes, which is a fundamental reaction in organic chemistry.
2. The mechanism involves the adsorption of hydrogen gas and the alkene substrate onto the surface of a metal catalyst, typically platinum or palladium.
3. The adsorbed hydrogen molecules are then activated, allowing for the stepwise addition of hydrogen atoms to the carbon-carbon double bond of the alkene.
4. The formation of a transition state intermediate is a key step in the mechanism, where the double bond is partially broken and the hydrogen atoms are partially transferred.
5. The Horiuti-Polanyi mechanism provides a detailed understanding of the kinetics and thermodynamics of the hydrogenation reaction, which is crucial for optimizing the reaction conditions and improving the efficiency of the process.

Review Questions

• Explain the role of the metal catalyst in the Horiuti-Polanyi mechanism for the hydrogenation of alkenes.
  • In the Horiuti-Polanyi mechanism, the metal catalyst, typically platinum or palladium, plays a crucial role. The catalyst provides a surface for the adsorption and activation of both the hydrogen gas and the alkene substrate. The adsorbed hydrogen molecules undergo heterolytic cleavage, where the hydrogen atoms become partially charged, making them more reactive towards the alkene. The catalyst also helps to stabilize the transition state intermediate, where the double bond is partially broken, facilitating the stepwise addition of the hydrogen atoms to the carbon-carbon double bond and the formation of the final alkane product.
• Describe the step-by-step process of the Horiuti-Polanyi mechanism for the hydrogenation of an alkene.
  • The Horiuti-Polanyi mechanism for the hydrogenation of alkenes involves the following steps: 1) Adsorption of hydrogen gas and the alkene substrate onto the surface of the metal catalyst. 2) Activation of the adsorbed hydrogen molecules, leading to the formation of partially charged hydrogen atoms. 3) Formation of a transition state intermediate, where the carbon-carbon double bond is partially broken and the hydrogen atoms are partially transferred. 4) Stepwise addition of the hydrogen atoms to the double bond, resulting in the formation of the final alkane product. 5) Desorption of the alkane from the catalyst surface, completing the catalytic cycle.
• Analyze the importance of the Horiuti-Polanyi mechanism in understanding the kinetics and thermodynamics of the catalytic hydrogenation of alkenes.
  • The Horiuti-Polanyi mechanism provides a detailed understanding of the kinetics and thermodynamics of the catalytic hydrogenation of alkenes, which is crucial for optimizing the reaction conditions and improving the efficiency of the process. By elucidating the step-by-step mechanism, including the adsorption of reactants, the formation of transition state intermediates, and the kinetics of the individual steps, the Horiuti-Polanyi mechanism allows for the prediction of reaction rates, the identification of rate-limiting steps, and the optimization of catalyst design and reaction parameters. This knowledge is essential for the development of more efficient and cost-effective hydrogenation processes in the chemical industry, with applications ranging from the production of pharmaceuticals to the synthesis of fuels and other important chemicals.

Related terms

Heterogeneous Catalysis: A type of catalysis where the catalyst is in a different phase (usually solid) than the reactants (usually liquid or gas).

Adsorption: The process by which molecules or atoms from a gas or liquid adhere to the surface of a solid, forming a thin film.

Transition State: An unstable, high-energy intermediate that forms during the course of a chemical reaction, representing the point at which the reactants are converted to products.