Dehydration Synthesis

Dehydration synthesis is a reaction where two smaller molecules are joined chemically by removing elements from each of these molecules. On the basis of the structure of hydrocarbons, organic compounds can be further classified as aliphatic and aromatic compounds. Organic compounds can give different kinds of chemical reactions which involve the formation of new compounds with the change in bonding between atoms.

Some of the chemical reactions form a resultant molecule whereas some simply show displacement or substitution of atoms. Synthesis or combination reactions involve a combination of two substances to form one molecule. For example, a combination of Mg and O2 forms MgO (magnesium oxide).

Unlike synthesis reactions, decomposition reactions can be defined as a reaction in which one substance decomposes to more than one substance like decomposition of the water molecule to H2 and O2. Another type of reaction is displacement reactions which can be single or double displacement. They involve displacement of one or more parts of compounds with each other. Some common organic reactions are addition, substitution, rearrangement, elimination and oxidation-reduction reactions.

Define Dehydration Synthesis

“Dehydration reactions can be defined as the chemical reactions in which a water molecule is eliminated from the reactant molecule. The process of combination of two molecules with the elimination of water molecule is called dehydration synthesis.”

This is because the term dehydration is used for ‘losing water’ and synthesis represents the formation of the new substance, therefore, dehydration synthesis is the elimination of water with the formation of new compounds. Definitely combination of two molecules will form a large compound and water molecule will eliminate and form as a by-product during the reaction.

Dehydration Synthesis Reaction

  • Dehydration synthesis can be defined as the synthesis reactions which involve the formation of a new compound with the elimination of water molecule.
  • Since these reactions result in the formation of a new compound with a large structure, therefore, they are called as synthesis reactions.
  • Here in dehydration synthesis reactions, since water molecule eliminates during the reaction, therefore, they are also a type of condensation reactions.
  • This is because during the condensation reaction two molecules condense to form a large molecule with loss of water molecule.

What is Substitution Reaction?

Substitution reactions are the most common reactions of alkanes, haloalkanes and other organic compounds. These reactions involve the substitution of one of the atoms of the molecule as leaving the group in the presence of either nucleophile or electrophile.

Unlike substitution reactions, elimination reactions involve the elimination of some of the atoms in the form of good leaving groups such as water, halogen etc. For example; elimination of water molecule from alcohol forms alkene. It is an example of a dehydration reaction.

Similarly, elimination of hydrogen molecule forms unsaturated compounds and called dehydrogenation reaction. Rearrangement reactions do not eliminate or add any atom but only rearrange the molecule to form a new one. Addition reactions involve the addition of some atoms over multiple chemical bonds.

Examples of Dehydration Synthesis Reaction

1. Biochemical reactions are one of the best examples of dehydration synthesis reactions. Amino acids are building blocks of proteins as they polymerise to form peptides and polypeptides. Amino acids have two functional groups; amino -NH2 and carboxylic group (-COOH). They react to form an amide linkage (-CO-NH-) with the elimination of water molecule.

Hence peptide formation is an example of a dehydration synthesis reaction.

Peptide Formation Dehydration Synthesis

2. Another example of dehydration synthesis is the formation of polysaccharides. The monomer units for polysaccharides are monosaccharides which are polyhydroxy carbonyl compounds. Monosaccharides are bonded with each other through glycosidic linkage. This linkage is formed by the reaction of –OH groups of two monomer units with the elimination of water molecule.

Formation of Maltose a Disaccharide Dehydration Synthesis

Formation of maltose is an example of a dehydration synthesis reaction. Two alpha-glucose units form a glycosidic linkage with elimination of water molecule to form one maltose molecule.

So we can say that a dehydration synthesis always involves two steps.

  1. Formation of a new product
  2. Loss of water molecule

What Happens in a Dehydration Synthesis Reaction?

Dehydration synthesis reactions are combination or synthesis reactions which occurs between the same or different monomer units with the elimination of water molecules. It is a kind of condensation reaction in which water molecule eliminate with the addition of two molecules.

Since each step occurs with the elimination of water molecules, therefore the reaction is called a dehydration reaction and it results in the formation of new substance so it is named as a dehydration synthesis reaction.

Many biochemical reactions occur through dehydration synthesis mechanism. For example formation of the peptide from amino acids is a dehydration synthesis reaction. It is a peptide bond formation reaction which occurs between two amino acid molecules.

The amino group of one molecule and carboxyl group of another molecule condenses with the elimination of water molecule and form an amide linkage in dipeptide.

Dehydration Synthesis and Hydrolysis

Dehydration synthesis involves the formation of new chemical bonds between two molecules which leads to the formation of new compounds.

  • A reaction occurs with the loss of water molecule at each step.
  • The loss of water molecule can occur due to reaction between two functional groups like –OH, -NH2 or –COOH. It is also called a condensation reaction.
  • Polymerization reactions are good examples of dehydration synthesis reaction in which monomer units condense together to form polymers.
  • Formation of disaccharides from monosaccharides in carbohydrates, the formation of lipids with one glycerol and three molecules of fatty acids are examples of dehydration synthesis.
  • Similarly, the formation of nucleic acid from nucleotide is also an example of dehydration synthesis.
  • Hydrolysis can be considered as the reverse reaction of a dehydration synthesis reaction. It involves the addition of water molecule with cleavage of bonds to form more than one substance from one substance.
  • In other words, hydrolysis is a chemical reaction which splits a molecule into smaller units by the addition of water molecules.
  • For example hydrolysis of polymers results in the formation of monomer units as it splits the connections between monomer units.
  • For example hydrolysis of disaccharide forms monosaccharide units. Similarly, the hydrolysis of lipids leads to the formation of glycerol and fatty acids.