Question

The transestification reaction between PET and \(\mathrm{CH}_{3} \mathrm{OH}\) forms dimethyl terephthalate and ethylene glycol. If the methanol used in this reaction is labeled with oxygen-18 \(\left(^{18} \mathrm{O}\right),\) in which of the products will the label be found?

Short answer

The label ^{18} O is found in dimethyl terephthalate.

Step-by-step solution

Understanding the Reaction

First, recognize the transesterification reaction between polyethylene terephthalate (PET) and methanol. PET is a polymeric ester, and when it reacts with methanol ( CH_3 OH ), it undergoes a reaction that breaks its ester bonds to form dimethyl terephthalate and ethylene glycol.

Identifying the Labeled Atom

In the given reaction, methanol is labeled with the isotope oxygen-18 ( ^{18}O ). This label indicates that the oxygen atom in methanol is distinct from typical oxygen-16 due to the additional isotopic mass.

Tracing the Labeled Atom in the Products

During transesterification, the original CH_3 group from methanol becomes part of the ester groups in dimethyl terephthalate. As a result, the labeled oxygen ( ^{18}O ) in the methanol is incorporated into the ester linkage, specifically in the carbon-oxygen single bond with the methyl group, one for each end of the dimethyl terephthalate molecule.

Confirming the Location of the Oxygen-18

The labeled oxygen ( ^{18}O ) becomes part of the carbonyl ester group in dimethyl terephthalate, forming dimethyl terephthalate ( C_6H_4(CO_2CH_3)_2 ) and not in ethylene glycol ( HOCH_2CH_2OH ) which remains unaffected by the labeled methanol.

Key concepts

Polyethylene Terephthalate (PET)

Polyethylene terephthalate, commonly known as PET, is a type of plastic widely used in packaging, textiles, and various other materials. It is known for its strength, thermo-stability, and transparency. At the molecular level, PET is a polymer formed from repeating units that consist of terephthalic acid and ethylene glycol. These monomers are linked through ester bonds, which define PET as a polyester.

• PET is recyclable, making it a popular choice in environmentally conscious applications.
• Commonly found in drink bottles, PET offers excellent barrier properties to gas and moisture, preserving the content's freshness.

The durability and versatility of PET make it a staple in consumer products. During chemical reactions like transesterification, its ester bonds are susceptible to breaking and reforming, which is pivotal in recycling processes.

Dimethyl Terephthalate

Dimethyl terephthalate (DMT) is an organic compound that serves as an intermediate in the synthesis of PET. It is a white solid with a crystalline structure that participates in ester exchange reactions, such as transesterification. DMT is typically formed by the esterification of terephthalic acid with methanol, replacing the hydroxyl groups of the acid with methyl ester groups.

• DMT is valued for its high purity and stability.
• The presence of ester groups makes DMT reactive with different alcohols and acids, enabling further polymerization or modification.

This compound is significant in industrial applications due to its role in producing polyesters. In the transesterification reaction mentioned, DMT is formed when PET interacts with methanol, integrating the methyl groups from methanol into the structure of DMT.

Ethylene Glycol

Ethylene glycol is a crucial component in the manufacture of PET, existing in its foundational structure. In its pure form, ethylene glycol is a clear, syrupy liquid with a sweet taste (though toxic when ingested). It is widely used as antifreeze and in hydraulic fluids due to its properties.

• Ethylene glycol acts as a monomer for polyester plastics.
• Its ability to lower freezing points is utilized in coolant applications.

During the transesterification reaction discussed, ethylene glycol is one of the products. It is not labeled with oxygen-18 since the reaction utilizes methanol labeled with oxygen, which transfers its labeled atom to only the forming ester linkage, not to ethylene glycol.

Oxygen-18 Labeling

Oxygen-18 is a stable isotope of oxygen, distinguishable by its atomic mass compared to the more common oxygen-16 isotope. In scientific research, oxygen-18 labeling serves as a tracer to follow chemical processes and reactions. By substituting an isotope for typical oxygen, scientists can trace atomic movement within molecular transformations.

• It is a non-radioactive marker, safe for various laboratory applications.
• Oxygen-18 is invaluable in studying reaction pathways and mechanisms.

In the case of the transesterification reaction between PET and methanol, the methanol is oxygen-18 labeled. As the reaction proceeds, the oxygen-18 is found in the ester linkage of the resulting dimethyl terephthalate. This tracing allows scientists to pinpoint where atoms end up after the reaction, providing insights into the reaction dynamics and atom conservation.