dehydration reaction

(noun)

an elimination (condensation) reaction in which the small molecule that is removed is water

Related Terms

  • condensation reaction
  • condensation polymerization
  • monomer
  • condensation

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.
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