Organic Synthesis Revision Notes for Leaving Cert Chemistry

Organic Synthesis

What is Organic Synthesis?

Organic synthesis refers to the process of constructing complex organic molecules from simpler ones through a series of chemical reactions.

It plays a crucial role in pharmaceuticals, materials science, and the production of various consumer goods. Organic synthesis requires a deep understanding of reaction mechanisms, the ability to predict the outcome of chemical reactions, and the strategic selection of reagents.

Principles of Organic Synthesis

Chemical synthesis involves two fundamental processes:

Bond Breaking: Existing chemical bonds in the starting materials are broken, typically through the action of reagents or catalysts.
Bond Forming: New chemical bonds are formed to produce the desired product. In organic synthesis, the challenge often lies in controlling the reaction conditions to ensure that the correct bonds are broken and formed in a stepwise manner to create the target molecule.

Designing Reaction Schemes

A reaction scheme is a sequence of reactions used to convert a starting material into a desired product.

Organic synthesis may involve multiple steps, each carefully designed to achieve specific transformations.
Chemists must be familiar with common reaction types, such as addition, substitution, elimination, and oxidation-reduction, to plan these sequences effectively.

Steps in Developing a Synthesis Scheme

Identify the Target Molecule: Determine the structure of the final product.
Work Backwards (Retrosynthetic Analysis): Break down the target molecule into simpler precursor molecules, considering how it could be made from available starting materials.
Plan the Synthesis: Choose appropriate reactions for each step that can convert starting materials to intermediates and eventually to the target molecule.
Consider Reagents and Conditions: Select suitable reagents, catalysts, and conditions to ensure each step proceeds efficiently.

infoNote

Example: Synthesis of PVC from Ethene One common example of organic synthesis is the production of polyvinyl chloride (PVC), a widely used plastic, from ethene ( $C₂H₄$ ).

Step 1: Chlorination of Ethene

Ethene undergoes an addition reaction with chlorine ( $Cl₂$ ) to form 1,2-dichloroethane ( $C₂H₄Cl₂$ ):

\text{C₂H₄} + \text{Cl₂} → \text{C₂H₄Cl₂}

This is an electrophilic addition reaction where the chlorine atoms add across the double bond of ethene.

Step 2: Dehydrochlorination of 1,2-Dichloroethane

1,2-Dichloroethane is heated to remove a molecule of hydrogen chloride ( $HCl$ ), producing vinyl chloride ( $C₂H₃Cl$ ), the monomer of PVC:

\text{C₂H₄Cl₂} → \text{C₂H₃Cl} + \text{HCl}

Step 3: Polymerization of Vinyl Chloride

Vinyl chloride undergoes polymerization, where multiple vinyl chloride molecules link together to form long chains of polyvinyl chloride (PVC):

n \text{C₂H₃Cl} → \text{(C₂H₃Cl)}_n

PVC is used extensively in pipes, cables, and other construction materials due to its durability and resistance to chemicals.

Examples of Multi-Step Synthesis

Organic synthesis often involves multiple steps to achieve the desired transformation.

Here are a few example reactions that may be used in a reaction scheme:

infoNote

Example: Synthesis of Ethanol from Ethene

Hydration of Ethene:

Ethene reacts with water in the presence of an acid catalyst ( $H₃PO₄$ ) to form ethanol.

\text{C₂H₄} + \text{H₂O} → \text{C₂H₅OH}

infoNote

Example 2: Conversion of Ethanol to Ethanoic Acid

Oxidation of Ethanol:

Ethanol can be oxidised to ethanal (acetaldehyde) using an oxidising agent such as potassium dichromate ( $K₂Cr₂O₇$ ).

\text{C₂H₅OH} → \text{CH₃CHO} + 2\text{H⁺} + 2\text{e⁻}

Oxidation of Ethanal to Ethanoic Acid:

Ethanal is further oxidised to ethanoic acid.

\text{CH₃CHO} + \text{H₂O} → \text{CH₃COOH} + 2\text{H⁺} + 2\text{e⁻}

infoNote

Example 3: Synthesis of Aspirin

Esterification:

Salicylic acid ( $C₇H₆O₃$ ) reacts with acetic anhydride ( $C₄H₆O₃$ ) to form acetylsalicylic acid (aspirin, $C₉H₈O₄$ ) and acetic acid ( $CH₃COOH$ ).

\text{C₇H₆O₃} + \text{C₄H₆O₃} → \text{C₉H₈O₄} + \text{CH₃COOH}

Key Concepts in Organic Synthesis

Bond breaking and forming: Each step of the synthesis involves carefully breaking and forming specific bonds.
Multi-step processes: Many syntheses require multiple reaction steps to achieve the desired product.
Reaction design: Synthesis schemes are designed based on a deep understanding of reaction mechanisms, such as substitution, addition, elimination, and oxidation-reduction reactions.

Organic Synthesis (Leaving Cert Chemistry): Revision Notes