catalysis

Examples of catalysis in the following topics:

• Homogeneous Catalysis

  • Homogeneous catalysis is a class of catalysis in which the catalyst occupies the same phase as the reactants.
  • Typically, heterogeneous catalysis involves the use of solid catalysts placed in a liquid reaction mixture.
  • Acid catalysis, organometallic catalysis, and enzymatic catalysis are examples of homogeneous catalysis.
  • Most often, homogeneous catalysis involves the introduction of an aqueous phase catalyst into an aqueous solution of reactants.
  • However, unlike with heterogeneous catalysis, the homogeneous catalyst is often irrecoverable after the reaction has run to completion.

• Heterogeneous Catalysis

  • Heterogeneous catalysis is a type of catalysis in which the catalyst occupies a different phase than the reaction mixture.
  • The process of speeding up a reaction by using a catalyst is known as catalysis.
  • Heterogeneous catalysis has a number of benefits.
  • However, one limitation of heterogeneous catalysis has to do with the available surface area of the catalyst.
  • Despite this, the overall benefits of heterogeneous catalysis often outweigh its disadvantages, in that the catalyzed reaction is still much faster than the uncatalyzed reaction.

• Enzyme Catalysis

  • Electrostatic catalysis: electrostatic attractions between the enzyme and the substrate can stabilize the activated complex.
  • Covalent catalysis: covalent bonding to side chains or cofactors can lower the energy of the transition state.

• Regioselectivity and Lewis Acid Catalysis

  • The two examples of Diels-Alder catalysis in the the first diagram below illustrate the improvement in yield and regioselectivity that often accompanies such catalysis.
  • In some cases Lewis acid catalysis may change the regioselectivity of a Diels-Alder reaction.
  • Even intramolecular Diels-Alder reactions may benefit from catalysis of this kind, as is demonstrated by the third diagram above.
  • In many cases, this analysis of HOMO and LUMO orbital coefficients also provides a good explanation for the beneficial influence of Lewis acid catalysis.
  • If the two ends of the dienophile each have a carbonyl substituent, as in the case of quinones and anhydrides, then Lewis acid catalysis may change the regioselectivity of the cycloaddition.

• Acid-Base Catalysis

  • Fortunately, these retarding factors can often be overcome by acid or base catalysis, which in general enhances electrophilicity (acids) or nucleophilicity (bases).
  • Equation #9 shows the former approach, which is an example of base catalysis.
  • Acid catalysis of the reaction also works well.

• Ziegler-Natta Catalytic Polymerization

• The Effect of a Catalyst

• Hydrogenation

  • For homogeneous catalysis, the metal binds to hydrogen to give a dihydride complex via oxidative addition.
  • The hydrogenation of ethylene (C2H4) on a solid support is an example of heterogeneous catalysis.

• Elimination Reactions of Alcohols

  • Recalling that water is a much better leaving group than hydroxide ion, it is sensible to use acid-catalysis rather than base-catalysis to achieve such reactions.

• Enzymes Used in Industry

  • Synthetic molecules, called artificial enzymes, also display enzyme-like catalysis.