From Models to Materials

Homologous Series and Structural Representation

A homologous series, also referred to as a class, represents a collection of organic compounds that share the same general formula, such as C_nH_2n. The defining characteristic of compounds within a homologous series is that they differ from one another by a consistent structural unit, typically a methylene group (CH₂). Organic molecules can be represented using various structural formulas, including full structural formulas that show all atoms and bonds, and condensed structural formulas that abbreviate repeating units or groups. Within these molecules, specific arrangements of atoms, known as functional groups, are responsible for the characteristic chemical reactions of the compound.

Classes of Organic Compounds and Their Functional Groups

Organic chemistry categorizes compounds into various classes based on the presence of specific functional groups, which are the reactive centers of the molecules. The following images illustrate some common functional groups and their typical representations. The table below provides a comprehensive overview of different classes of organic compounds, detailing their characteristic functional groups, the names associated with these groups, their prefixes or suffixes used in nomenclature, an example compound, and their general formula where applicable.

Class	Functional group	F.group name	Prefix/suffix	Example	General formula
Alkane	C-C	Alkyl	-ane	ethane	CnH2n+2
Alkene	C=C	Alkenyl	-ene	ethene	CnH2n
Alkyne	C⫢C	Alkynyl	-yne	ethyne	CnH2n-2
Alcohol	R-OH	Hydroxyl	-ol / hydroxy-	ethanol	CnH2n+1OH
Ether	R-O-R	Ether	oxy -	methoxymethane
Ketone		Carbonyl	-one	propanone	CnH2nO
Aldehyde		Aldehyde	-al	ethanal	CnH2nO
Carboxylic acid		Carboxyl	-oic acid	Ethanoic acid	Cn-1H2n-1COOH
Ester		Ester	-yl -anoate	Methyl ethanoate
Amide	R-CONH2	Amide	-amide	propanamide
Amine	R-NH2	Amine	-amine	propanamine
Nitrile	R-C⫢N	Nitrile	-nitrile	propanenitrile
Halogenoalkane	R - X	Halogeno	halo-	chloroethane

The Process of Addition Polymerization

During addition polymerization, an alkene undergoes an addition reaction with itself. This process is characterized by the fact that all the atoms present in the original alkene monomers are incorporated into the resulting polymer chain. The outcome is the formation of long hydrocarbon chains. The chemical equation for this reaction typically illustrates the transformation from the monomer to the repeating unit within the polymer, where 'n' signifies a large, indeterminate number of repeating units. For instance, ethene monomers polymerize to form poly(ethene).

Common Monomers and Their Corresponding Addition Polymers

A variety of alkene monomers can undergo addition polymerization to produce different polymers, each with distinct properties. The table below illustrates some common examples, showing the monomer and its corresponding polymer, along with any common names or abbreviations.

Monomer	Polymer	Common Name/Abbreviation
Ethene	Poly(ethene)
Propene	Poly(propene)
Chloroethene	Poly(chloroethene)	Polyvinylchloride (PVC)
Tetrafluoroethene	Poly(tetrafluoroethene)	PTFE ("Teflon")

Preparation and Properties of Addition Polymers

Many addition polymers are prepared  through a free radical process, which typically requires high pressure, high temperature, and a catalyst such as oxygen (O₃) or peroxides. These catalysts readily decompose to form radicals, which then initiate a chain reaction leading to polymerization. The physical properties of addition polymers can be significantly varied by adjusting the reaction conditions, such as pressure and temperature, during their synthesis. Chemically, the properties of these polymers are determined by the functional groups present in their structure. For example, poly(ethene), being a long chain of saturated hydrocarbons, is generally resistant to chemical attack and is non-biodegradable due to the absence of reactive functional groups.

Environmental Challenges and Solutions for Polymer Waste

While polymers derived from alkenes are indispensable in modern society, their disposal presents significant environmental challenges. A major issue is their resistance to degradation; they are unreactive to most chemicals and bacteria, making them non-biodegradable. Consequently, discarded polymers contribute substantially to landfill problems. Addressing these issues involves several strategies:

• Recycling: Although effective, recycling often faces challenges due to the high cost associated with collecting and reprocessing polymer waste.
• Incineration: Burning polymer waste can reduce landfill volume and generate energy. However, burning chlorinated polymers, such as PVC, can produce toxic fumes like hydrogen chloride (HCl), which necessitates specialized scrubbers for removal.
• Feedstock Use: This approach involves converting polymer waste into useful organic compounds. New technologies are emerging that can transform waste polymers back into hydrocarbons, which can then be used as raw materials to synthesize new polymers, creating a more circular economy.