dehydration reaction

Examples of dehydration reaction in the following topics:

• Elimination Reactions of Alcohols

  • The elimination of water from an alcohol is called dehydration.
  • The last two reactions also demonstrate that the Zaitsev Rule applies to alcohol dehydrations as well as alkyl halide eliminations.
  • It should be noted that the acid-catalyzed dehydrations discussed here are the reverse of the acid-catalyzed hydration reactions of alkenes.
  • The dehydration reaction is shown by the blue arrows; the hydration reaction by magenta arrows.
  • The first equation shows the dehydration of a 3º-alcohol.

• Condensation Reactions

  • In a condensation reaction, two molecules or parts thereof combine, releasing a small molecule.
  • When this small molecule is water, it is known as a dehydration reaction.
  • Many condensation reactions follow a nucleophilic acyl substitution or an aldol condensation reaction mechanism (see previous concept for more information).
  • The condensation (dehydration) of two amino acids to form a peptide bond (red) with expulsion of water (blue).
  • Recognize the chemical principles of condensation reactions as they relate to polymerization.

• Elimination

  • Elimination is a type of organic reaction in which a leaving group and vicinal hydrogen are removed, leaving behind a double bond.
  • A common example of elimination is the dehydration reaction, which occurs in a unimolecular process:
  • Because E1 reactions are performed under thermodynamic control (at high temperature), the most thermodynamically stable product is favored.
  • All electron transfers in an E2 reaction occur by a concerted, one-step mechanism, which bears some similarity to SN2.
  • Predict whether a reaction will proceed via an E1 elimination mechanism or E2 elimination mechanism given the reaction conditions

• Other Reactions

  • One practical application of this behavior lies in the dehydration of 1º-amides to nitriles by treatment with thionyl chloride.
  • This reaction is also illustrated in the following diagram.
  • Other dehydrating agents such as P2O5 effect the same transformation.

• 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.
  • These products may then be modified or enhanced by further reactions.
  • In the second example, the absence of alpha-hydrogens on the aldehyde favors the mixed condensation, and conjugation of the double bond facilitates dehydration.
  • A concerted dehydrative-decarboxylation (shown by the magenta arrows) leads to the unsaturated carboxylic acid product.
  • Examples of how reaction sequences are used to prepare various compounds

• Addition Reactions

  • In organic chemistry, an addition reaction is, in its simplest terms, an organic reaction in which two or more molecules combine to form a larger molecule.
  • An addition reaction is the opposite of an elimination reaction.
  • For instance, an alkene's hydration reaction adds water to an alkene, and an alcohol's dehydration removes water from the alkene; these two reactions are opposites and are considered addition-elimination pairs.
  • In the related addition-elimination reaction, an addition reaction is followed by an elimination reaction; in most reactions, this involves addition to carbonyl compounds in nucleophilic acyl substitution.
  • An example of this type of reaction is:

• The Aldol Reaction

  • Three examples of the base-catalyzed aldol reaction are shown in the first diagram below, and equivalent acid-catalyzed reactions also occur.
  • In the case of aldehyde reactants (as in reactions #1 & 2 above), the aldol reaction is modestly exothermic and the yields are good.
  • Reaction #5 is an interesting example of an intramolecular aldol reaction; such reactions create a new ring.
  • The dehydration step of an aldol condensation is also reversible in the presence of acid and base catalysts.
  • Finally, reaction #4 has two reactive alpha-carbons and a reversible aldol reaction may occur at both.

• Reactions of Ylides

  • This reaction is illustrated by the first three equations below.
  • The Wittig reaction tolerates epoxides and many other functional groups, as demonstrated by reaction # 1.
  • A principal advantage of alkene synthesis by the Wittig reaction is that the location of the double bond is absolutely fixed, in contrast to the mixtures often produced by alcohol dehydration.
  • Reaction # 5 illustrates a double Wittig reaction, using a dialdehyde reactant (colored orange).
  • This is known as the Horner-Wadsworth-Emmons reaction.

• Preparation of Ethers

  • Reactions #1 and #2 below are two examples of this procedure.
  • Thus, reaction #1 gives a better and cleaner yield of benzyl isopropyl ether than does reaction #2, which generates considerable elimination product.
  • Reactions #3 and #4 are examples of this two-step procedure.
  • Acid-catalyzed dehydration of small 1º-alcohols constitutes a specialized method of preparing symmetrical ethers.
  • At 110º to 130 ºC an SN2 reaction of the alcohol conjugate acid leads to an ether product.

• Carbon-Carbon Bond Formation

  • The number of generally useful and well tested reactions for effecting carbon-carbon bond formation, ideally in a regio and stereospecific fashion, is relatively small, compared with reactions used to modify functional groups.
  • It should also be noted that more than half these reactions involve carbonyl reactants.
  • Unfortunately, the dehydration of the terminal 3º-alcohol (last step) was less selective and gave appreciable amounts of the gem.
  • Many of the essential reactions used here were not known at the time of Corey's work.
  • The preparation of isogeranic acid (top equation) makes use of a transition metal coupling reaction.