Dehydration Reaction – Definition and Examples


Dehydration Reaction
A dehydration is a synthesis reaction that joins together two small molecules while releasing water.

A dehydration reaction, also known as a dehydration synthesis reaction, is a key process in chemistry and biology, playing a crucial role in the formation of complex molecules from simpler ones. This article explores the definition, importance, mechanisms, and examples of dehydration reactions.

Dehydration Reaction Definition

A dehydration reaction or dehydration synthesis reaction is a chemical reaction in which two molecules join together via a covalent bond, with the removal of a water molecule. In simple terms, it involves the synthesis of a new compound by combining two reactants and the simultaneous elimination of water. A dehydration reaction is a specific type of condensation reaction and synthesis reaction.

Importance of Dehydration Reactions

Dehydration reactions are essential in numerous biological and chemical processes:

  1. Formation of Polymers: They are key in forming polymers like carbohydrates, proteins, and nucleic acids by linking monomers (basic units).
  2. Metabolic Pathways: They play a vital role in metabolic pathways and energy production.
  3. Synthetic Applications: In industrial chemistry, these reactions synthesize various materials, including plastics and small compounds.

Mechanism of a Dehydration Reaction

The general mechanism of a dehydration reaction involves the following steps:

  1. Nucleophilic Attack: One reactant’s nucleophile (electron-rich site) attacks the electrophile (electron-deficient site) of another reactant.
  2. Water Formation: Concurrently, a hydroxyl group (-OH) from one reactant and a hydrogen atom from another combine to form a water molecule.
  3. Bond Formation: The removal of water facilitates the formation of a new covalent bond between the reactants.

Examples of Dehydration Reactions

Dehydration reactions occur between molecules that have functional groups like -NH2, -OH, and -COOH. So, they are common reactions in biochemistry in the formation of disaccharides from monosaccharides, lipids from glycerol and fatty acids, nucleic acids from nucleotides, and peptides from amino acids. Here are some specific examples:

Between Amino Acids

In biochemistry, amino acids undergo dehydration synthesis to form dipeptides:

H2​N−CHR−COOH + H2​N−CHR′−COOH → H2​N−CHR−CO−NH−CHR′−COOH + H2​O

Here, the carboxyl group (-COOH) of one amino acid reacts with the amino group (-NH2) of another, forming a peptide bond and releasing water.

Between Monosaccharides

Dehydration synthesis is common in carbohydrate chemistry:

C6​H12​O6 ​+ C6​H12​O6​ → C12​H22​O11 ​+ H2​O

Two glucose molecules, for instance, combine to form maltose, a disaccharide, with the elimination of water.

Tips for Recognizing a Dehydration Reaction

Dehydration reactions can be recognized by:

  1. Formation of a new covalent bond between two molecules.
  2. Release of water as a byproduct.
  3. Involvement of hydroxyl (-OH) and hydrogen (H) groups in the reactants.

Look for reactions between two reactants with functional groups, where one larger molecule and water are products.

Comparing Dehydration with Other Reactions

Dehydration vs. Substitution Reaction

In a substitution reaction (also known as a replacement or displacement reaction), an atom or a group in a molecule is replaced by another atom or group. Unlike dehydration reactions, substitution reactions do not necessarily involve the formation of water. They are common in organic chemistry, especially in halogenated compounds.

Dehydration vs. Hydrolysis Reaction

Hydrolysis is essentially the reverse of a dehydration reaction. It involves breaking a covalent bond in a molecule by the addition of a water molecule, with the water’s hydrogen and hydroxyl groups integrating into the product molecules. This reaction is vital in digesting and breaking down polymers into monomers.

References

  • Besson, Michèle; Gallezot, Pierre; Pinel, Catherine (2014-02-12). “Conversion of Biomass into Chemicals over Metal Catalysts”. Chemical Reviews. 114 (3): 1827–1870. doi:10.1021/cr4002269
  • Carey, Francis A.; Sundberg, Richard J. (2013). Advanced Organic Chemistry Part B: Reactions and Synthesis. Springer.
  • Kolbe, H. (1845). “Beiträge zur Kenntniss der gepaarten Verbindungen”. Annalen der Chemie und Pharmacie. 54 (2): 145–188. doi:10.1002/jlac.18450540202
  • Vogel, A.I.; Tatchell, A.R.; Furnis, B.S.; Hannaford, A.J.; Smith, P.W.G. (1996). Vogel’s Textbook of Practical Organic Chemistry (5th ed.). Prentice Hall. ISBN 0-582-46236-3.