Dehydration Synthesis: Reaction, Types & Examples

Dehydration synthesis refers to the reaction wherein two smaller molecules are joined together chemically by removing elements from each of these molecules in order to form a different compound or molecule. This process of synthesis is referred to as dehydration synthesis as there is a loss of water molecules involved. Many reactions which involve dehydration synthesis are majorly associated with the formation of biological polymers (polymerization) where in on the addition of each monomer to the polymer chain, there is the elimination of one molecule of water.

Key Takeaways: Dehydration Synthesis, Polymerisation, Condensation Reaction, Glycosidic Bond, Monomer, Polymer, Acetic acid

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What is Dehydration Synthesis?

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Dehydration reactions refer to a subset of condensation reactions in which the two functional groups inherently combine in order to form a covalent bond which is accompanied by the release of a small molecule, for instance, water, Hydrochloric acid, methanol or acetic acid. Though these above mentioned small molecules are commonly observed in the large scale industrial synthesis of organic molecules, in biological systems, water is the most frequent byproduct of a condensation reaction.

Dehydration Synthesis Reaction

Since each step occurs with the elimination of water molecules, the reaction is called a dehydration reaction and it results in the formation of a new substance. Many biochemical reactions occur through the mechanism of dehydration synthesis.

For instance, the formation of the peptide from amino acids is a dehydration synthesis reaction. It is a reaction which leads to the formation of peptide bonds occurring between two amino acid molecules. Here, the amino group of one molecule and carboxyl group of another molecule condenses with the elimination of water molecules and form an amide linkage in dipeptide.

One common dehydration reaction which involves a simple molecule is the formation of symmetric ethers from alcohol condensation. This is a reaction which is catalysed by the presence of an acid and thus, occurs at a pH < 7 (less than 7).

The following equation represents the formation of ethoxyethane from ethanol through dehydration synthesis.

2 C₂H₅OH ↔ C₄H₁₀O + H₂O

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Types of Dehydration Synthesis Reactions

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Dehydration synthesis can be classified according to a number of different criteria. 

Criteria 1: They can be grouped on the basis of the nature of the reactants taking part in the reaction. For instance, some reactants are molecules which possess two functional groups and can react with one another. as in the case of amino acids which contain an amine group and a carboxylic acid group attached to the same carbon atom.

Amino Acid

The amine group (-NH3) of one amino acid can react with the acid group (-COOH) of another in order to form an amide bond. This also results in releasing one molecule of water per reaction between the functional groups. Furthermore, the newly formed amino acid dimer again undergoes the same reaction as it contains one free amine group and one free carboxylic acid group, thereby allowing the reaction to proceed with more amino acids and leading to formation of a polypeptide chain.

Dehydration reaction occurring as a result of peptide bond formation between two amino acids.

Bi-functional monomers, give rise to linear products wherein the constituent monomers are attached to each other end-to-end. Moreover, the reactants can also have multiple functional groups, which can result in the creation of a branched product, for instance, the formation of glycogen from the constituent glucose molecules.

Criteria 2: Dehydration reactions can also be classified on the nature of the catalyst. For instance, in the formation of symmetrical ethers, the catalyst present is a hydrogen ion. However, for reactions occurring within a living organism, the pH, salt concentrations and temperature cannot be altered. Under such situations, the presence of some other catalyst plays a critical role in driving a particular reversible reaction in one direction. 

Alternatively, the reactions of the process of dehydration can be further classified on the basis of the products which they produce. In biological systems, most dehydration reactions create polymers. Therefore, these reactions can be further grouped on the basis of the fact that whether they create complex carbohydrates from simpler monosaccharides, lead to formation of fatty acids from acetyl coA or result in the synthesis of proteins from amino acids.

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Examples of Dehydration Synthesis Reactions

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Dehydration reactions are those which are involved in the industrial production of polyester fibres and polyethylene terephthalate (PET). PET is formed by dehydration synthesis from two monomers – ethylene glycol and terephthalic acid.

Example 1: Formation of Glycosidic Bond

Glycosidic bonds are those covalent bonds which are formed between a carbohydrate and any other molecule. Many of these glycosidic bonds involve the process of a dehydration reaction. For instance, when maltose is formed from glucose, there is formation of a glycosidic bond between the two constituent glucose molecules which is accompanied by the release of one molecule of water.

Similarly, long polymers of glucose can be formed in a manner which is similar, as summarised above, by undergoing through a series of dehydration reactions in order to give rise to starch, cellulose or glycogen-based chains based on the position of the glycosidic bonds.

Other disaccharides such as sucrose and lactose are also formed through dehydration reactions between two monosaccharides.

Glycosidic bonds are also involved when a carbohydrate is modified.

Example 2. Triglyceride Formation 

One of the intermediates of the process of carbohydrate and fat metabolism is Acetyl CoA (a molecule in which a two-carbon acetyl group is attached to coenzyme A). Though it is primarily intended to become a part of the Krebs cycle in the mitochondria, it can also be used to generate chains of long-chain fatty acids. These fatty acids then subsequently lead to formation of triglycerides (energy storage molecules).

Triglycerides derive their name as the three hydroxyl groups on glycerol undergo esterification with fatty acids in order to form the triglyceride. Furthermore, each of the three fatty acids undergoes a dehydration reaction with the alcohol moieties on glycerol in order to generate one molecule of triglyceride.

One of the predominant reasons why triglycerides are considered a better storage medium than carbohydrates is because of their high energy density. Triglycerides have a much larger proportion of carbon atoms which can undergo oxidation and contain fewer oxygen molecules as a result of which the fatty acids are generated from hydrocarbons. Furthermore, the removal of three water molecules in the process of forming a triglyceride additionally increases the energy density of the molecule.

Triglyceride Formation

 

In the above structure, R1, R2 and R3 are referring to the long chain hydrocarbons, each of which is further attached to a carboxylic acid functional group. They form ester linkages with the hydroxyl groups which are attached to the α, β, and α’ of glycerol which additionally gives rise to a triglyceride.

Example 3. Hydrolysis

Hydrolysis is also referred to as the reverse of a dehydration reaction. This is because it involves the breaking of a covalent bond through the addition of a molecule of water. Moreover, hydrolysis is catalysed by a group of enzymes which are called hydrolases. Among them, the most commonly known hydrolases are the digestive enzymes which are present in living organisms. 

The path can be traced as the digestion begins from the mouth, where salivary amylase breaks down starch molecules. This is the reason why extended chewing of starchy foods gives rise to a sweet taste in the mouth. The action of the enzyme salivary amylase generates monosaccharides. This is subsequently followed by the action of proteases in the stomach that initiates the process of breaking the peptide bonds in proteins.

The process of digestion is continued by hydrolytic enzymes from the pancreas and small intestine wherein various enzymes act on the lipids, carbohydrates, nucleic acids and proteins present in the food. Similar enzymes which are involved in intracellular digestion are present within the lysosomes.

Additionally, there are specific enzymes which reverse the post-translational modifications of proteins, such as phosphatases. These enzymes are capable of removing the phosphate groups which are attached to the protein via a hydrolysis reaction. In a similar manner, ATPase enzymes catalyse the hydrolysis of the terminal phosphodiester bond in ATP. These ATP molecules play a critical role in releasing the energy stored in the molecule.

Numerous enzymes which are involved in the process of hydrolysis contain a serine residue in their active site and are therefore known as serine hydrolases. These serine hydrolases include the majority of the digestive enzymes and are those which are involved in the major metabolic pathways occurring within the cell.

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Things To Remember 

• Dehydration synthesis is a reaction which is accompanied by the loss of a molecule of water at each step. 
• In addition to dehydration synthesis, the loss of a water molecule also occurs as a result of condensation reaction ( reaction between two functional groups, such as -OH, -COOH and so on.
• Polymerization reactions represent dehydration synthesis reactions wherein the monomers condense in order to form polymers. 
• Hydrolysis is referred to as the reverse of the reaction of the dehydration synthesis process wherein there is addition of a water molecule accompanied by the cleavage of the bonds of the reacting molecules.

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