Organic chemistry principles form a foundational pillar in understanding water quality, especially in the context of water engineering. The classification of organic compounds, rooted in the periodic table and atomic structure, provides insight into the behavior and interaction of elements and molecules within water systems. For water engineers in Kenya, knowledge of these principles assists in diagnosing contamination, designing treatment processes, and ensuring the safety of water supplies. This chapter explores the classification of organic compounds through the lens of elemental properties and atomic configurations relevant to water quality management.

2.1 Classification of organic compounds

The classification of organic compounds depends heavily on the properties and arrangements of elements, primarily carbon, hydrogen, oxygen, and nitrogen. These elements’ positions on the periodic table, their atomic structures, and electronic configurations dictate the types of organic compounds formed and their reactivity. Understanding these fundamentals enables water engineers to identify pollutants and design effective treatment methods for organic contaminants in water sources such as rivers, wells, and reservoirs in Kenya.

Organic compounds are compounds of carbon, usually combined with hydrogen and often with oxygen, nitrogen, sulphur, phosphorus or halogens. There are millions of them, so chemists group them into classes according to the shape of their carbon chain and the functional group they contain. Compounds in the same class behave in similar ways, which lets an engineer predict how a pollutant will dissolve, break down or respond to treatment once its class is known. Example: a county water laboratory that detects phenols in a river below a factory knows from the class that they will give a medicinal taste with chlorine and need removal by activated carbon or oxidation.

2.1.1 Meaning and characteristics of organic compounds

Organic compounds are built on chains or rings of carbon atoms joined by covalent bonds. Carbon can form four bonds and can bond to itself repeatedly, a property called catenation, which is why it forms so many compounds of different sizes and shapes. Organic compounds include fuels, food substances, detergents, pesticides, plastics and many of the pollutants found in wastewater.

2.1.2 Classification by carbon skeleton

Organic compounds can first be grouped by the shape of their carbon framework. Open chains and rings behave differently: aromatic rings, for example, are very stable and resist breakdown, which is why aromatic pollutants persist in water.

• Acyclic (open-chain) compounds: Carbon atoms form straight or branched chains with no rings, for example butane and ethanol.
• Straight-chain compounds: Carbons are joined one after another in a single line, as in pentane.
• Branched-chain compounds: Side chains branch off the main chain, as in methylpropane; branching affects boiling point and biodegradability.
• Alicyclic compounds: Carbon atoms form a ring that is not aromatic, for example cyclohexane.
• Aromatic compounds: Contain one or more benzene rings, for example benzene, toluene and phenol; many are toxic and persistent.
• Heterocyclic compounds: Rings that contain an atom other than carbon, such as nitrogen, oxygen or sulphur, for example pyridine.

2.1.3 Classification by functional group

A functional group is the atom or group of atoms that gives a compound its characteristic reactions. Compounds with the same functional group form a homologous series and are named and treated as one class.

Characteristics of a homologous series

• Same general formula: All members fit one general formula, such as CnH2n+2 for alkanes.
• Same functional group: All members contain the same functional group and so react similarly.
• Successive difference of CH₂: Each member differs from the next by a −CH₂− unit.
• Gradual change in physical properties: Boiling point and density rise steadily with chain length, while solubility in water falls.
• Similar methods of preparation: Members are made by similar reactions.

Worked example: identifying a compound's class

Example: Identify the class of C₄H₁₀ and confirm it with the general formula.

For n = 4, CnH2n+2 gives H = 2 × 4 + 2 = 10

C₄H₁₀ fits CnH2n+2

Answer: C₄H₁₀ (butane) is an alkane

2.1.4 Saturated and unsaturated compounds

Saturated compounds contain only single bonds between carbon atoms, so each carbon holds the maximum number of other atoms; alkanes are saturated. Unsaturated compounds contain at least one double or triple carbon-carbon bond; alkenes and alkynes are unsaturated. The difference matters because unsaturated compounds are more reactive: they decolourise bromine water and react readily with oxidants such as chlorine and ozone used in water treatment.

2.1.5 Classification of organic pollutants in water

For water engineering, organic compounds are also grouped by their source and their behaviour in water and treatment. This helps engineers decide whether a contaminant will be removed by biological treatment or needs special processes such as activated carbon, advanced oxidation or source control.

• Natural organic matter: Humic and fulvic substances from decaying plants that colour water brown and react with chlorine to form by-products.
• Biodegradable organics: Carbohydrates, proteins and fats in sewage and food industry effluent that microorganisms break down, measured as BOD.
• Petroleum hydrocarbons: Fuels, oils and grease from garages and spills that form films, block oxygen transfer and are toxic.
• Pesticides and herbicides: Organochlorine and organophosphate compounds from farms that are toxic at very low concentrations and often persistent.
• Detergents and surfactants: Cleaning agents that cause foaming and carry phosphates.
• Phenols and industrial chemicals: From industries such as tanneries, paint and plastics, causing taste and odour problems and toxicity.
• Disinfection by-products: Compounds such as trihalomethanes formed when chlorine reacts with natural organic matter.
• Emerging contaminants: Pharmaceuticals, hormones and personal care products that pass through conventional treatment.

Biodegradable and non-biodegradable organics

Chapter Summary

This chapter began with an exploration of the periodic table, detailing the organization of elements according to their atomic number and properties. It then examined the nature of elements and the fundamental atomic structure, including protons, neutrons, and electrons. The electronic configuration of the first 30 elements was explained to illustrate how electrons are arranged in shells and subshells. The concepts of groups, periods, and blocks were discussed to describe the classification and trends within the periodic table. The chapter also covered the physical and chemical properties of elements, highlighting how these characteristics influence their behavior in various contexts. Further, the classification of organic compounds was introduced, emphasizing the diverse structures and functional groups found in organic chemistry. The identification of plastics was addressed, focusing on their chemical composition and relevance in environmental management. Finally, the chapter examined aerobic and anaerobic processes in wastewater treatment and discussed eutrophication as a significant environmental consequence of nutrient pollution.