What is Dehydration Reaction?

There are no countable limits to a number of chemical reactions. The world of chemistry is filled with so many chemical reactions that it is nearly impossible for us to study them individually. To study them, we categorize and label them based on certain characteristics. Dehydration is one of the major types that we often encounter.

What is Dehydration Reaction?

In a general sense, dehydration means loss or lack of water. Thus, dehydration reactions involve loss of water.

Dehydration reactions are reactions in which one or more water molecules are released as a byproduct.

A general chemical equation is as follows:

A → B + H2O

A is a reactant and B is the primary product.

The reaction may have more than one reactant and/or more than two products. But water (H2O) is always present on the right-hand side of the chemical equation.

A water molecule consists of one oxygen and two hydrogen atoms and usually splits into a hydroxyl (–OH) group and a hydrogen (–H) group. Therefore, reactants will contain the hydroxyl group in their chemical structures.

Examples of Dehydration Reaction

Some common dehydration reactions are described below:

Dehydration of alcohols to form alkenes

Alcohols are the hydroxyl group attached to alkyl. We can synthesize alkenes by removing the hydroxyl from alcohol itself.

R–OH + R–OH → R=R + 2H2O 

For example, the dehydration of ethanol (C2H5OH) gives ethene (C2H4).

CH3–CH2–OH → H2C=CH2 + H2O

This reaction is slow in the absence of a catalyst. At the industrial scale, ethene is produced by catalytic cracking and other petrochemical processes and not by the dehydration of alcohol.

Esterification

Esterification is a classic example of dehydration. In esterification (or Fischer esterification to be more precise), a carboxylic acid (R–COOH) and an alcohol (R'–OH) are refluxed in the presence of an acid catalyst (H2SO4) to give an ester (RCO–OR'), liberating water in the process.

R–COOH + R'–OH → RCOOR' + H2O

When R is ethyl and R' is methyl, we have ethanoic acid reacting with methanol to give methyl ethanoate.

Dehydration of amides

The dehydration of primary amides in the presence of phosphorus pentoxide (P2O5) or thionyl chloride (SOCl2) produces nitriles (–CN group). The method is one of the preferred methods employed in the production of nitriles. 

Making gypsum plaster

Gypsum plaster, or called plaster of Paris, is a building material applied to the surface of walls to make them smooth. Other uses include decorative ornaments, toys, and orthopedic casts.

Gypsum is a soft calcium sulfate mineral having two water molecules per calcium atom. It is represented as CaSO4·2H2O.

When this salt is subjected to high temperatures of around 120 °C to 180 °C in a kiln, it turns into gypsum plaster. And the water is liberated in the form of steam during the reaction. The chemical reaction is as follows:

CaSO4·2H2O(s) + heat → CaSO4·0.5H2O(s) + H2O(g)

Related terms

Many dehydration reactions are reversible. That means we will get reactants of dehydration reactions on the addition of water. For example, water plus ester gives carboxylic acid and alcohol. Or gypsum plaster can be converted back to gypsum upon exposure to water. This reversal is called hydration, i.e., addition of water to dehydrated chemicals. 

Another related term is condensation reaction. Condensation and dehydration have some conceptual overlap. In many cases, dehydration is a subset of condensation. In condensation, the two or more molecules combine to form one large molecule, expelling small molecules. When small molecules are water, the reaction can also be classified as dehydration.