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Related Concept Videos

Reduction of Alkenes: Catalytic Hydrogenation02:13

Reduction of Alkenes: Catalytic Hydrogenation

Alkenes undergo reduction by the addition of molecular hydrogen to give alkanes. Because the process generally occurs in the presence of a transition-metal catalyst, the reaction is called catalytic hydrogenation.
Metals like palladium, platinum, and nickel are commonly used in their solid forms — fine powder on an inert surface. As these catalysts remain insoluble in the reaction mixture, they are referred to as heterogeneous catalysts.
The hydrogenation process takes place on the surface of...
Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation02:24

Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Introduction
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
Hybridization of Atomic Orbitals II03:35

Hybridization of Atomic Orbitals II

sp3d and sp3d 2 Hybridization
π Molecular Orbitals of 1,3-Butadiene01:24

π Molecular Orbitals of 1,3-Butadiene

Conjugated dienes have lower heats of hydrogenation than cumulated and isolated dienes, making them more stable. The enhanced stabilization of conjugated systems can be understood from their π molecular orbitals.
The simplest conjugated diene is 1,3-butadiene: a four-carbon system where each carbon is sp2-hybridized and has an unhybridized p orbital that contains an unpaired electron. According to molecular orbital theory, atomic orbitals combine to form molecular orbitals such that the number...
Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation01:28

Reduction of Benzene to Cyclohexane: Catalytic Hydrogenation

Unlike the easy catalytic hydrogenation of an alkene double bond, hydrogenation of a benzene double bond under similar reaction conditions does not take place easily. For example, in the reduction of stilbene, the benzene ring remains unaffected while the alkene bond gets reduced. Hydrogenation of an alkene double bond is exothermic and a favorable process. In contrast, to hydrogenate the first unsaturated bond of benzene, an energy input is needed; that is, the process is endothermic. This is...
Reduction of Alkenes: Asymmetric Catalytic Hydrogenation02:17

Reduction of Alkenes: Asymmetric Catalytic Hydrogenation

Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an...

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Catalytic Reactions at Amine-Stabilized and Ligand-Free Platinum Nanoparticles Supported on Titania During Hydrogenation of Alkenes and Aldehydes
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Pyrene: hydrogenation, hydrogen evolution, and π-band model.

Jakob Arendt Rasmussen1, Graeme Henkelman, Bjørk Hammer

  • 1Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, Ny Munkegade, Building 1520, Aarhus University, DK-8000 Aarhus C, Denmark.

The Journal of Chemical Physics
|May 3, 2011
PubMed
Summary

Hydrogenation of pyrene is exothermic, with edge carbon atoms being most reactive. Molecular hydrogen evolution from dihydrogenpyrene has a significant energy barrier, suggesting stability.

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Area of Science:

  • Physical Chemistry
  • Computational Chemistry
  • Materials Science

Background:

  • Polycyclic aromatic hydrocarbons (PAHs) like pyrene are abundant in various environments.
  • Understanding their reactivity with hydrogen is crucial for astrochemistry and materials science.
  • Previous studies have explored PAH hydrogenation with varying theoretical approaches.

Purpose of the Study:

  • To theoretically investigate the hydrogenation of pyrene by atomic hydrogen.
  • To determine the energetics and kinetics of hydrogen binding to pyrene.
  • To explore the subsequent molecular hydrogen evolution from hydrogenated pyrene.

Main Methods:

  • Density Functional Theory (DFT) at the GGA-PBE level was employed for primary calculations.
  • Hybrid-DFT (PBE0) was used to complement specific DFT calculations.
  • A tight-binding model was developed to analyze electronic structure and predict binding sites.

Main Results:

  • Hydrogenation of pyrene is exothermic (up to 1.6 eV) with a strong site dependence, favoring edge carbon atoms.
  • Low activation barriers (down to 0.06 eV) were found for initial hydrogen binding, with second hydrogen binding barrierless at many sites.
  • The most stable dihydrogenpyrene structure is energetically favored over pyrene plus molecular hydrogen, with a substantial 3.7 eV barrier for hydrogen evolution.

Conclusions:

  • Edge carbon atoms in pyrene are the most reactive sites for hydrogenation.
  • Hydrogenated pyrene species exhibit significant stability against molecular hydrogen evolution.
  • The projected density of π-states is a useful descriptor for predicting hydrogen binding sites on PAHs.