Question

Polyethylene terephthalate (PET) is used to make synthetic fibers, such as Dacron; thin films, such as Mylar; and bottles for carbonated beverages. PET is produced from ethylene glycol and either of two monomers, depending on whether the reaction proceeds by dehydration-condensation or by displacement. Write equations for the two syntheses. (Hint: The displacement is reversed by adding methanol to PET at high \(T\) and \(P .\) )

Short answer

Reactions for PET synthesis: 1. Dehydration-condensation: \[ n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{H})_2) \rightarrow \text{[-O-CH}_2-\text{CH}_2-\text{O-CO-C}_6\text{H}_4-\text{CO-]}_n + 2n \text{H}_2\text{O}\] 2. Displacement: \[ n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{CH}_3)_2) \rightarrow \text{[-O-CH}_2-\text{CH}_2-\text{O-CO-C}_6\text{H}_4-\text{CO-]}_n + 2n \text{CH}_3\text{OH}\]

Step-by-step solution

Understanding the Components

Identify the main components: ethylene glycol (HO-CH2-CH2-OH) and the monomers for the synthesis of PET. The monomers are either terephthalic acid (C6H4(CO2H)2) for the dehydration-condensation method or dimethyl terephthalate (C6H4(CO2CH3)2) for the displacement method.

Dehydration-Condensation Reaction

In the dehydration-condensation method, ethylene glycol reacts with terephthalic acid. The reaction removes water (H2O) to form PET:\[ n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{H})_2) \rightarrow \text{[-O-CH}_2-\text{CH}_2-\text{O-CO-C}_6\text{H}_4-\text{CO-]}_n + 2n \text{H}_2\text{O}\]

Displacement Reaction

In the displacement method, ethylene glycol reacts with dimethyl terephthalate. The reaction removes methanol (CH3OH) to form PET:\[ n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{CH}_3)_2) \rightarrow \text{[-O-CH}_2-\text{CH}_2-\text{O-CO-C}_6\text{H}_4-\text{CO-]}_n + 2n \text{CH}_3\text{OH}\]

Understanding the Reverse Reaction

Adding methanol (CH3OH) to PET at high temperature (T) and pressure (P) reverses the displacement reaction, breaking PET back into ethylene glycol and dimethyl terephthalate.

Key concepts

Dehydration-Condensation Reaction

A dehydration-condensation reaction is a process where two molecules react to form a larger molecule, while losing a small molecule, typically water (\text{H}_2\text{O}).

In the synthesis of Polyethylene Terephthalate (PET), ethylene glycol (HO-CH_2-CH_2-OH) reacts with terephthalic acid (C_6H_4(CO_2H)_2).

Here, water molecules are removed as a byproduct.

This reaction can be summarized by the equation:
\[ n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{H})_2) \rightarrow \text{[-O-CH}_2-\text{CH}_2-\text{O-CO-C}_6\text{H}_4-\text{CO-]}_n + 2n \text{H}_2\text{O} \]

The key steps in understanding a dehydration-condensation reaction:
* Two reactants combine to form a larger molecule.
* A small molecule (usually water) is eliminated.
* The resulting larger molecule is often a polymer in the case of PET.

This method is widely used in the production of various polymers.

Displacement Reaction

A displacement reaction involves the replacement of one group in a molecule by another.

In the case of PET synthesis, ethylene glycol (HO-CH_2-CH_2-OH) reacts with dimethyl terephthalate (C_6H_4(CO_2CH_3)_2).

The primary byproduct here is methanol (CH_3OH), which is displaced from the dimethyl terephthalate.

The overall reaction is represented by:
\[ n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{CH}_3)_2) \rightarrow \text{[-O-CH}_2-\text{CH}_2-\text{O-CO-C}_6\text{H}_4-\text{CO-]}_n + 2n \text{CH}_3\text{OH} \]

Key points to understand:
* One group in a molecule is replaced by another.
* Methanol serves as a byproduct.
* It allows another efficient route for PET production.

Polymer Chemistry

Polymer chemistry involves the study of large molecules made up of repeating structural units (monomers).

PET is a prime example of a polymer, formed by monomer reactions.

Important things to remember about polymer chemistry:
* Monomers repeat in a chain to produce polymers which have various properties.
* The specific arrangement and type of monomers determine the polymer's characteristics.
* For PET, the repeating unit is formed from the reaction of ethylene glycol and terephthalic acid or dimethyl terephthalate.
* Polymers can be natural (like DNA) or synthetic (like PET).

An understanding of polymer chemistry is essential to grasp how materials like PET, with specific desirable properties for things like plastic bottles and fibers, are made.

Reverse Reaction

In chemical reactions, a reverse reaction is one where the products can convert back to the reactants.

For PET, the reverse of the displacement reaction can occur by adding methanol (CH_3OH) to PET under high temperature (T) and pressure (P).

This reverse reaction breaks down PET back into its monomers:
* Ethylene glycol
* Dimethyl terephthalate

Understanding reverse reactions is key in processes like recycling of PET bottles. Here is how this reverse reaction is represented:
\[ \text{PET} + 2\text{CH}_3\text{OH} \rightarrow n(\text{HO-CH}_2-\text{CH}_2-\text{OH}) + n(\text{C}_6\text{H}_4(\text{CO}_2\text{CH}_3)_2) \]

Important factors include:
* The need for high temperature and pressure.
* The potential for repeatability making polymers recyclable.
* Helps in reducing waste and promoting sustainable practices.