condensation

Examples of condensation in the following topics:

• Condensation Reactions

  • When two separate molecules react, their condensation is termed intermolecular.
  • A simple example is the condensation of two amino acids to form a peptide.
  • When a condensation is performed between different parts of the same molecule, the reaction is termed intramolecular condensation; in many cases, this leads to ring formation.
  • Many condensation reactions follow a nucleophilic acyl substitution or an aldol condensation reaction mechanism (see previous concept for more information).
  • Other condensations, such as the acyloin condensation, are triggered by radical conditions.

• Condensation Polymers

  • The polyester Dacron and the polyamide Nylon 66, shown in the first diagram, are two examples of synthetic condensation polymers, also known as step-growth polymers.
  • Examples of naturally occurring condensation polymers are cellulose, the polypeptide chains of proteins, and poly(β-hydroxybutyric acid), a polyester synthesized in large quantity by certain soil and water bacteria.

• Claisen Condensation

  • Esters undergo a similar transformation called the Claisen Condensation.
  • The last equation shows a mixed condensation between two esters, one of which has no alpha-hydrogens.
  • The Claisen condensation differs from the aldol reaction in several important ways.
  • (i) The aldol reaction may be catalyzed by acid or base, but most Claisen condensations require base.
  • Esters usually serve as the electrophilic acceptor component of the condensation.

• Modification of Condensation Products

  • The Claisen condensation produces beta-ketoesters.
  • In the second example, the absence of alpha-hydrogens on the aldehyde favors the mixed condensation, and conjugation of the double bond facilitates dehydration.
  • The doubly-activated methylene group of malonic and acetoacetic acids or esters makes them good donors in any condensation, as is demonstrated by the third aldol-like reaction.
  • The fourth reaction demonstrates that the conjugate base of the beta-ketoester products from Claisen or Dieckmann condensation may be alkylated directly.
  • A large family of vinylagous reactions, related to the condensations, acylations and alkylations described here, increase the bond forming options available to the synthetic chemist.

• The Evaporating Atmosphere

  • At equilibrium, evaporation and condensation processes exactly balance and there is no net change in the volume of either phase.
  • Likewise, every once in a while a vapor molecule collides with the liquid surface and condenses into the liquid.
  • At equilibrium, evaporation and condensation processes exactly balance and there is no net change in the volume of either phase.
  • Collisions between water molecules in the atmosphere allows some to condense and some to remain in vapor.

• Characteristics of Condensation Polymers

  • Condensation polymers form more slowly than addition polymers, often requiring heat, and they are generally lower in molecular weight.
  • The following examples of condensation polymers are illustrative.
  • Many polymers, both addition and condensation, are used as fibers the chief methods of spinning synthetic polymers into fibers are from melts or viscous solutions.
  • Bisphenol A is prepared by the acid-catalyzed condensation of acetone with phenol.

• Applications of Condensation Reactions to Synthesis

  • This will be illustrated for aldol and Claisen condensations in the following section.

• Biosynthetic Mechanisms

  • The complex organic compounds found in living organisms on this planet originate from photosynthesis, an endothermic reductive condensation of carbon dioxide requiring light energy and the pigment chlorophyll.
  • Claisen condensation of ethyl acetate (or other acetate esters) forms an acetoacetate ester, as illustrated by the top equation in the following diagram.
  • Nature's solution to carrying out a Claisen-like condensation in a living cell is shown in the bottom equation of the diagram.
  • Depending on the enzymes involved, the condensation product may be reduced and then further elongated so as to produce fatty acids (as shown), or elongated by further condensations to polyketone intermediates that are precursors to a variety of natural phenolic compounds.
  • The second diagram below shows examples of polyketone condensations.

• The Aldol Reaction

  • Hence the following examples are properly referred to as aldol condensations.
  • The dehydration step of an aldol condensation is also reversible in the presence of acid and base catalysts.
  • The previous examples of aldol reactions and condensations used a common reactant as both the enolic donor and the electrophilic acceptor.
  • Aldol condensations between different carbonyl reactants are called crossed or mixed reactions, and under certain conditions such crossed aldol condensations can be effective.
  • The aldol condensation of ketones with aryl aldehydes to form α,β-unsaturated derivatives is called the Claisen-Schmidt reaction.

• Block Copolymerization

  • Several different techniques for preparing block copolymers have been developed, many of which use condensation reactions (next section).