Question

Polyacetylene is an electrically conducting polymer of acetylene, \(\mathrm{HC} \equiv \mathrm{CH}\). Now used in batteries, the polymer was discovered accidentally in 1970 when far too much catalyst was used in an experimental attempt to polymerize acetylene into synthetic rubber. Write a plausible Lewis structure for a portion of a polyacetylene molecule.

Short answer

A plausible Lewis structure is \( [-\mathrm{HC}=\mathrm{CH}-]_n \).

Step-by-step solution

Understand the basic structure of acetylene

Acetylene is a simple hydrocarbon molecule with the formula \( \mathrm{HC} \equiv \mathrm{CH} \). It consists of two carbon atoms triple-bonded to each other, and each carbon is also bonded to a hydrogen atom.

Imagine the polymerization process

When acetylene polymerizes, the triple bonds between the carbon atoms participate in forming longer chains. The electrons from these bonds align to allow the continuation of carbon-carbon bonding throughout the polymer chain.

Form the repeat unit of polyacetylene

The process involves each acetylene molecule (\( \mathrm{C_2H_2} \)) linking up to form a repeated structure. Each triple bond contributes to two double bonds in sequence within the chain, making it an alternate pattern like: \( [-\mathrm{HC}=\mathrm{CH}-]_n \).

Draw the plausible Lewis structure

A Lewis structure of a segment of polyacetylene shows alternating single and double bonds along the carbon chain. Represent a portion of the chain as: \( \text{H-C=C-C=C-C-H} \). This can continue indefinitely depending on the polymer length.

Key concepts

Polymerization

Polymerization is a chemical process where small molecules, called monomers, join together to form a larger molecule known as a polymer. In the case of polyacetylene, the monomer is acetylene, denoted as \( \mathrm{HC} \equiv \mathrm{CH} \). The process starts with the activation of these monomers, often using a catalyst, enabling them to link up. During polymerization, the triple bond present in acetylene opens up. As these bonds rearrange, they form new links between the carbon atoms of different monomers. This continuation of chain formation results in a long, repeating structure. The unique aspect of polyacetylene is that it features alternating single and double bonds, which greatly contribute to its properties as a polymer. This process can be envisioned as a series of links being fused together, creating an extensive chain with unique characteristics like conductivity and flexibility, depending on how the monomers align and bond.

Lewis Structure

Lewis structures are a way to represent the bonding in a molecule using dots to represent electrons. For polyacetylene, the goal is to showcase the arrangement of alternating single and double bonds that characterizes this polymer. To create a Lewis structure for polyacetylene, we start by noting that each carbon atom originally involved in a triple bond uses its electrons to form new bonds. The transformation results in a new pattern throughout the chain:

• Each acetylene unit \( \mathrm{C_2H_2} \) contributes to forming alternating single and double bonds along the chain.
• The structure presents a repeating unit of: \( -\mathrm{H-C=C-} \).
• This pattern can extend indefinitely, as shown in the example \( \text{H-C=C-C=C-C-H} \), depicting the potential continuous repetition.

Visualizing the Lewis structure helps in understanding the molecule's stability and reactivity, as it clarifies the sharing and distribution of electrons between atoms.

Electrically Conducting Polymers

Electrically conducting polymers, like polyacetylene, are unique materials that combine the properties of polymers with the ability to conduct electricity. Typically, polymers are known for their insulating properties; however, polyacetylene is an exception due to its unique bonding structure. The structure of alternating single and double bonds in polyacetylene allows for the movement of electrons along the polymer chain. This electron delocalization means that electrons can move along the chain, creating pathways for electrical current. This is similar to how electricity flows in metals. Some important aspects of conducting polymers include:

• Electron Delocalization: The alternating bonds allow electrons to move freely, which is crucial for conductivity.
• Applications: These polymers are used in various electronic devices, including batteries, flexible electronics, and sensors.
• Transitioning Polymers: Upon doping (adding impurities), these polymers can enhance their conductivity, often reaching levels comparable to metals.

Understanding the conductive properties of polymers like polyacetylene opens up a world of possibilities in materials science and electronic engineering.