Dehydration Synthesis 

Dehydration synthesis involves the combination of two reactant molecules to form a covalent bond, accompanied by the elimination of a water molecule. This reaction is integral in the formation of various organic compounds, including alkenes, ethers, and esters. In biological systems, dehydration synthesis is crucial for the synthesis of macromolecules such as proteins, carbohydrates, and nucleic acids.

1.0Dehydration Synthesis Reaction Mechanism

The mechanism generally follows these steps:

1. Activation of Molecules: Two reactants approach each other.
2. Water Removal: A hydroxyl group (–OH) from one molecule and a hydrogen atom (–H) from another are removed.
3. Bond Formation: The remaining atoms form a covalent bond, joining the two molecules together.
4. Water Molecule Formation: The removed –OH and –H combine to form water.

Example:
Formation of an ether from two alcohols:

$R-\text{OH}+\text{HO}-R^{\prime }\stackrel{{\text{H}}_2{\text{SO}}_4}{\to }R-\text{O}-R^{\prime }+{\text{H}}_2\text{O}$

General Equation for Dehydration Synthesis

The generalized equation is:

$A-\text{H}+B-\text{OH}\to A-B+{\text{H}}_2\text{O}$

Here, two reactant molecules (A–H and B–OH) combine to form a larger molecule (A–B) and water.

2.0Preparation and Synthesis Methods

The mechanism of dehydration synthesis varies depending on the type of alcohol involved. The process generally proceeds through either the E1 (unimolecular elimination) or E2 (bimolecular elimination) pathway.

Primary Alcohols and the E2 Mechanism

Primary alcohols typically undergo dehydration via the E2 mechanism. In this pathway:

1. Protonation of the Alcohol: The hydroxyl group (-OH) of the alcohol is protonated by an acid catalyst (e.g., sulfuric acid), forming an alkyloxonium ion.
2. Formation of the Alkene: A base abstracts a proton from a β-carbon atom, leading to the simultaneous elimination of the leaving group (water) and the formation of a double bond, resulting in an alkene.

This concerted mechanism requires a strong base and typically occurs at high temperatures.

3.0Role in Organic Chemistry

Formation of Alcohols and Ethers

• Alcohols undergo intermolecular dehydration to form ethers.
• Example: Ethanol dehydration gives diethyl ether:

$2{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{{\text{H}}_2{\text{SO}}_4}{\to }{\text{C}}_2{\text{H}}_5-\text{O}-{\text{C}}_2{\text{H}}_5+{\text{H}}_2\text{O}$

Formation of Alkenes

• Alcohols undergo intramolecular dehydration in the presence of concentrated H₂SO₄ to form alkenes.
• Example: Ethanol to ethene:

${\text{C}}_2{\text{H}}_5\text{OH}\stackrel{{\text{H}}_2{\text{SO}}_4,\Delta }{\to }{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$

Dehydration in Carbohydrates and Biomolecules

• Monosaccharides (like glucose) combine through dehydration synthesis to form disaccharides (like maltose).

$\text{Glucose}+\text{Glucose}\to \text{Maltose}+{\text{H}}_2\text{O}$

• Similarly, amino acids form peptide bonds via dehydration synthesis.

4.0Properties of Dehydration Synthesis

Physical Properties 

Dehydration synthesis reactions are typically endothermic and require heat to proceed. The physical state of the reactants and products can vary; for instance, alcohols are often liquids, while the resulting alkenes can be gases or liquids depending on their molecular weight. The elimination of water during the reaction can also affect the solubility and boiling points of the products relative to those of the reactants.

Chemical Properties and Reactions

Several factors affect the efficiency and outcome of dehydration reactions:

• Alcohol Structure: Tertiary alcohols dehydrate more readily than secondary or primary alcohols due to the stability of the corresponding carbocations.
• Reaction Conditions: Higher temperatures favour elimination reactions. The required temperature decreases with increasing substitution of the hydroxyl-containing carbon:
• Primary alcohols: 170°–180°C
• Secondary alcohols: 100°–140°C
• Tertiary alcohols: 25°–80°C
• Catalysts: Strong acids such as sulfuric or phosphoric acid are commonly used to protonate the hydroxyl group, facilitating the formation of a good leaving group.
• Carbocation Stability: In E1 mechanisms, the formation of a stable carbocation intermediate is essential. Rearrangements may occur to form more stable carbocations, influencing the final product distribution.

5.0Uses in Real Life

Dehydration synthesis is pivotal in both laboratory and industrial settings:

• Alkene Production: Dehydration of alcohols is a common method for synthesizing alkenes, which are fundamental building blocks in organic synthesis.
• Esterification: Carboxylic acids react with alcohols in a dehydration synthesis reaction to form esters, a process widely used in the production of fragrances and plastics.
• Polymer Formation: In biological systems, dehydration synthesis reactions form complex carbohydrates, proteins, and nucleic acids by linking monomer units.

6.0Solved Examples

Example 1: Dehydration of Ethanol

Calculate the mass of ethene formed when 92 g of ethanol undergoes complete dehydration.

Solution:
Molecular mass of ethanol = 46 g/mol
Moles of ethanol = 92 / 46 = 2 mol
From the reaction:

${\text{C}}_2{\text{H}}_5\text{OH}\to {\text{C}}_2{\text{H}}_4+{\text{H}}_2\text{O}$

1 mol ethanol gives 1 mol ethene.
So, 2 mol ethanol → 2 mol ethene.
Mass of ethene = 2 × 28 = 56 g

Answer: 56 g of ethene formed.

Example 2: Formation of Maltose

Two glucose molecules (C₆H₁₂O₆) combine to form maltose (C₁₂H₂₂O₁₁). Write the balanced equation.

Solution:

${\text{C}}_6{\text{H}}_{12}{\text{O}}_6+{\text{C}}_6{\text{H}}_{12}{\text{O}}_6\to {\text{C}}_{12}{\text{H}}_{22}{\text{O}}_{11}+{\text{H}}_2\text{O}$

This illustrates dehydration synthesis in biomolecules.

Example 3: Formation of Ether

Show the dehydration synthesis of diethyl ether.

Solution:

$2{\text{C}}_2{\text{H}}_5\text{OH}\stackrel{{\text{H}}_2{\text{SO}}_4}{\to }{\text{C}}_2{\text{H}}_5-\text{O}-{\text{C}}_2{\text{H}}_5+{\text{H}}_2\text{O}$