condensation reaction

Examples of condensation reaction in the following topics:

• Condensation Reactions

  • Condensation is a chemical reaction in which one molecule is formed and one small molecule (often water) is lost.
  • In a condensation reaction, two molecules or parts thereof combine, releasing a small molecule.
  • Many condensation reactions follow a nucleophilic acyl substitution or an aldol condensation reaction mechanism (see previous concept for more information).
  • Small molecules are usually liberated in these condensation steps, unlike polyaddition reactions.
  • Recognize the chemical principles of condensation reactions as they relate to polymerization.

• Applications of Condensation Reactions to Synthesis

  • The construction of complex molecules by a series of suitable reactions carried out from simple starting compounds is called synthesis.
  • Nevertheless, it is useful to classify the various reactions we have studied with respect to their ability to (i) enlarge or expand a given structure, (ii) transform or relocate existing functional groups, and (iii) do both of these in a stereoselective fashion.
  • A useful assortment of carbon-carbon bond forming reactions have been described in this and earlier chapters.
  • With the exception of Friedel-Crafts alkylation these reactions all give products having one or more functional groups at or adjacent to the bonding sites.
  • This will be illustrated for aldol and Claisen condensations in the following section.

• Claisen Condensation

  • Intramolecular reactions, such as #3, lead to rings (usually five or six-membered) and are referred to as Dieckmann Condensations.
  • 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.
  • (ii) In contrast to the catalytic base used for aldol reactions, a full equivalent of base (or more) must be used for the Claisen condensation.
  • (iii) The aldol reaction may be catalyzed by hydroxide ion, but the Claisen condensation requires that alkoxide bases be used, in order to avoid ester hydrolysis.

• Modification of Condensation Products

  • The aldol reaction produces beta-hydroxyaldehydes or ketones, and a number of subsequent reactions may be carried out with these products.
  • The Claisen condensation produces beta-ketoesters.
  • 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.

• Dehydration Synthesis

  • This type of reaction is known as dehydration synthesis, which means "to put together while losing water. " It is also considered to be a condensation reaction since two molecules are condensed into one larger molecule with the loss of a smaller molecule (the water.)
  • In a dehydration synthesis reaction between two un-ionized monomers, such as monosaccharide sugars, the hydrogen of one monomer combines with the hydroxyl group of another monomer, releasing a molecule of water in the process.
  • As additional monomers join via multiple dehydration synthesis reactions, the chain of repeating monomers begins to form a polymer.
  • Three of the four major classes of biological macromolecules (complex carbohydrates, nucleic acids, and proteins), are composed of monomers that join together via dehydration synthesis reactions.
  • These three are polysaccharides, classified as carbohydrates, that have formed as a result of multiple dehydration synthesis reactions between glucose monomers.

• The Aldol Reaction

  • A useful carbon-carbon bond-forming reaction known as the Aldol Reaction or the Aldol Condensation is yet another example of electrophilic substitution at the alpha carbon in enols or enolate anions.
  • Reactions in which a larger molecule is formed from smaller components, with the elimination of a very small by-product such as water are termed Condensations.
  • 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.

• Biosynthetic Mechanisms

  • The multitude of lipid structures described here are constructed from acetate by enzymatic reactions that in many respects correspond to reactions used by chemists for laboratory syntheses of similar compounds.
  • Consequently, the condensation, alkylation, oxidation and reduction reactions that accomplish the biosynthesis of lipids will not make use of the very strong bases, alkyl halides, chromate oxidants or metal hydride reducing agents that are employed in laboratory work.
  • 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.

• Thermodynamics of Redox Reactions

  • The thermodynamics of redox reactions can be determined using their standard reduction potentials and the Nernst equation.
  • Q is the reaction quotient $\frac{C^cD^d}{A^aB^b}$.
  • If T is held constant at 298K, the Nernst equation can be condensed using the values for the constants R and F:
  • The relationship between the Gibbs free energy change and the standard reaction potential is:
  • Translate between the equilibrium constant/reaction quotient, the standard reduction potential, and the Gibbs free energy change for a given redox reaction

• Non-Ionic Reactions

  • With few exceptions, the multitude of reactions discussed in this and other introductory texts are classified as ionic reactions.
  • Here we shall consider two other classes of organic reactions: Free-Radical Reactions & Pericyclic Reactions.
  • One type of "free-radical reaction", alkane halogenation has already been described.
  • In contrast to ionic reactions, both free radical and pericyclic reactions may occur in the gas phase, as well as in solution in various solvents.
  • Also, these nonionic reactions are more tolerant of spectator functional groups than are many ionic reactions.

• 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.
  • Also, the polymerization reactions for Nylon 6 and Spandex do not proceed by elimination of water or other small molecules.
  • Here the covalent bonding occurs by an SN2 reaction between a nucleophile, usually an amine, and a terminal epoxide.
  • Bisphenol A is prepared by the acid-catalyzed condensation of acetone with phenol.