hydrogenation

Examples of hydrogenation in the following topics:

• Hydrogen Bonding

  • A hydrogen bond is a strong intermolecular force created by the relative positivity of hydrogen atoms.
  • A hydrogen atom attached to a relatively electronegative atom is a hydrogen bond donor.
  • This hydrogen atom is a hydrogen bond donor.
  • Greater electronegativity of the hydrogen bond acceptor will create a stronger hydrogen bond.
  • Where do hydrogen bonds form?

• Hydrogenation

  • Hydrogenation reactions, which involve the addition of hydrogen to substrates, have many important applications.
  • Hydrogenation refers to the treatment of substances with molecular hydrogen (H2), adding pairs of hydrogen atoms to compounds (generally unsaturated compounds).
  • Most hydrogenation reactions use gaseous hydrogen as the hydrogen source, but alternative sources have been developed.
  • The reverse of hydrogenation, where hydrogen is removed from the compounds, is known as dehydrogenation.
  • Hydrogen gas is the most common source of hydrogen used and is commercially available.

• The Hydrogen Economy

  • The hydrogen economy refers to using hydrogen as the next important source of fuel.
  • The hydrogen economy refers to a hypothetical future system of delivering energy through the use of hydrogen (H2).
  • There are two primary uses for hydrogen today.
  • Alternatively, liquid hydrogen or slush hydrogen (a combination of liquid and solid hydrogen) can be used.
  • Hydrogen can be stored as a chemical hydride or in some other hydrogen-containing compound.

• Hydrogen Bonding and Van der Waals Forces

  • Two weak bonds that occur frequently are hydrogen bonds and van der Waals interactions.
  • When polar covalent bonds containing hydrogen form, the hydrogen in that bond has a slightly positive charge because hydrogen’s one electron is pulled more strongly toward the other element and away from the hydrogen.
  • This interaction is called a hydrogen bond.
  • Hydrogen bonds are also responsible for zipping together the DNA double helix.
  • The slightly negative oxygen side of the water molecule and the slightly positive hydrogen side of the water molecule are attracted to each other and form a hydrogen bond.

• Properties of Hydrogen

  • Hydrogen is the smallest element, with one proton and one electron.
  • Hydrogen exists in two different spin isomers of hydrogen diatomic molecules that differ by the relative spin of their nuclei.
  • Hydrogen is available in different forms, such as compressed gaseous hydrogen, liquid hydrogen, and slush hydrogen (composed of liquid and solid), as well as solid and metallic forms.
  • Because hydrogen is buoyant in air, hydrogen flames ascend rapidly and cause less damage than hydrocarbon fires.
  • Oxidation of hydrogen removes its electron and yields the H+ ion.

• Hydrogen Bonding

  • Hydrogen forms polar covalent bonds to more electronegative atoms such as oxygen, and because a hydrogen atom is quite small, the positive end of the bond dipole (the hydrogen) can approach neighboring nucleophilic or basic sites more closely than can other polar bonds.
  • In the following diagram the hydrogen bonds are depicted as magenta dashed lines.
  • The molecule providing a polar hydrogen for a hydrogen bond is called a donor.
  • Also, O–H---O hydrogen bonds are clearly stronger than N–H---N hydrogen bonds, as we see by comparing propanol with the amines.
  • Comparison of Boiling Points of Methane, Ammonia, Water, and Hydrogen Fluoride

• Substitution of the Hydroxyl Hydrogen

  • As with the alcohols, the phenolic hydroxyl hydrogen is rather easily replaced by other substituents.

• Hydrogenation

  • Addition of hydrogen to a carbon-carbon double bond is called hydrogenation.
  • The simplest source of two hydrogen atoms is molecular hydrogen (H2), but mixing alkenes with hydrogen does not result in any discernible reaction.
  • Catalytic hydrogenation takes place in at least two stages, as depicted in the diagram.
  • First, the alkene must be absorbed on the surface of the catalyst along with some of the hydrogen.
  • This is often true, but the hydrogenation catalysts may also cause isomerization of the double bond prior to hydrogen addition, in which case stereoselectivity may be uncertain.

• Isotopes of Hydrogen

  • Hydrogen has three naturally occurring isotopes: protium, deuterium and tritium.
  • Hydrogen is generally found as diatomic hydrogen gas H2, or it combines with other atoms in compounds—monoatomic hydrogen is rare.
  • 2H, or deuterium (D), is the other stable isotope of hydrogen.
  • It is a highly unstable isotope of hydrogen.
  • 5H is another highly unstable heavy isotope of hydrogen.

• Emission Spectrum of the Hydrogen Atom

  • The emission spectrum of atomic hydrogen is divided into a number of spectral series.
  • Further series for hydrogen as well as other elements were discovered as spectroscopy techniques developed.
  • Typically, one can only observe these series from pure hydrogen samples in a lab.
  • Explain how the lines in the emission spectrum of hydrogen are related to electron energy levels.
  • The series limit where n2 is infinite and n1=1 corresponds to the ionization energy of hydrogen.