Question

The molecular formula of \(n\) -decane \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8} \mathrm{CH}_{3} . \mathrm{De}-\) cane is not considered a polymer, whereas polyethylene is. What is the distinction?

Short answer

The distinction between n-decane and polyethylene lies in their molecular structures and sizes. n-decane is a low molecular weight, straight-chain hydrocarbon with no repeating monomer units, whereas polyethylene is a large molecule formed by the repetition of ethylene monomer units in a chain. Due to these differences, n-decane is not considered a polymer, while polyethylene is.

Step-by-step solution

Definition of a Polymer

A polymer is a large molecule, or macromolecule, composed of many repeated smaller units or monomers. They are typically organic (carbon-based) and have high molecular weights. Polymers can be found in nature, like proteins, cellulose, and starch, or could be man-made, like plastics and rubber.

Structure of n-decane

n-decane is a hydrocarbon with the molecular formula \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{8}\mathrm{CH}_{3}\). It is a straight-chain alkane, meaning all the carbon atoms are connected in a linear fashion, and consists of 10 carbon atoms and 22 hydrogen atoms. The structure can be illustrated as follows: \[CH_3-CH_2-CH_2-CH_2-CH_2-CH_2-CH_2-CH_2-CH_2-CH_3\]

Structure of Polyethylene

Polyethylene is a polymer made up of many ethylene monomers. Ethylene is a hydrocarbon with the chemical formula \(\mathrm{C}_{2}\mathrm{H}_{4}\) or \(CH_2=CH_2\). During the polymerization process, the double bond ( -C=C- ) opens up, and the ethylene monomers link to form long chains of -CH2-CH2- units. The polyethylene structure can be expressed as follows: \[(-CH_2-CH_2-)_{n}\] Where "n" represents the number of monomer units in the chain. In polyethylene, "n" can be a very large number.

Distinction between n-decane and Polyethylene

Now that we understand the structure of both n-decane and polyethylene let's identify the distinction between the two. 1. n-decane is a low molecular weight molecule with a simple, straight-chain structure consisting of only ten carbon atoms, making it a small molecule compared to polymers. In contrast, polyethylene is a large molecule with a much higher molecular weight, formed by the repetition of monomer units in a chain. 2. n-decane does not consist of any repeating monomer units, whereas polyethylene consists of many repeating ethylene units. Due to these distinctions, n-decane is not considered a polymer, while polyethylene is considered a polymer.

Key concepts

Polymer Definition

When we talk about polymers, we're referring to substances made up of long chains of repeating molecular units known as monomers. Think of a polymer as a train, with each car representing a monomer; when connected, they form a long and sometimes complex structure. This arrangement gives polymers unique properties that make them versatile and useful in a wide range of applications.

Polymers can be categorized into two groups: natural and synthetic. Natural polymers include proteins, which are vital for our body's functions, and cellulose, which is the main component of plant cell walls. Synthetic polymers, on the other hand, are human-made and include materials such as plastics, which are integral to modern life. The key to polymers is their size and the repeating structure of their building blocks that create materials with diverse characteristics.

Structure of n-Decane

n-Decane showcases a simplicity that is quite the opposite of complex polymer structures. It’s a hydrocarbon, specifically an alkane, that follows a straight-chain configuration. To envision this, picture a line of ten conjoined circles with two little arms of hydrogen flanking each circle except for those at the ends, which have three. This simplistic linear carbon chain denoted as \(CH_3(CH_2)_8CH_3\), consists of a sequence of carbon and hydrogen atoms. There are no branches or loops — just a straight line of atoms from end to end.

What makes n-decane stand out is its lack of repeating units. Unlike polymers that have monomers lined up like dominoes, n-decane is like a solitary tile standing on its own. This structure means that n-decane is not a polymer, because polymers require a repeating sequence of monomers, which n-decane does not possess.

Polyethylene Polymerization

Polyethylene is a prime example of a synthetic polymer and it emerges from a process called polymerization. Imagine each molecule of ethylene (\(C_2H_4\)) as a link that connects to others to form a chain. When these links, or monomers, chemically bond together by opening up their double bonds (found in the ethylene), a lengthy molecular chain develops. This is not unlike connecting a series of paperclips to form a long strand.

The resulting polymer, polyethylene, resembles a series of -CH2-CH2- units connected in an extensive chain, symbolically written as \[(-CH_2-CH_2-)_n\]. Here, 'n' can be a very large number, depicting the malleability of this material in creating everything from plastic bags to bulletproof vests. This transformation from a monomer to a polymer involves not just the physical linking but also chemical changes, which imbue polyethylene with properties significantly different from the original ethylene monomer.