Organic chemistry is fundamental to understanding the chemical nature of living organisms and the substances they interact with in agriculture. For agriculture and extension professionals in Kenya, knowledge of organic compounds is essential for managing soil fertility, crop protection, and livestock nutrition. This chapter explores the concepts of organic compounds, focusing on their definitions and classifications, which underpin many agricultural applications such as pesticide formulation, fertilizer development, and soil organic matter analysis.

3.1 Organic compounds

Organic compounds are central to biological systems and agricultural inputs. These compounds, primarily composed of carbon atoms bonded with other elements, form the basis of life and influence many agricultural processes. Understanding their properties and types helps extension officers advise farmers on using organic fertilisers, pesticides, and soil conditioners effectively.

3.1.1 Definition of terms

Precise understanding of key terms related to organic compounds is critical for agricultural professionals. These terms clarify the nature, composition, and behaviour of organic substances encountered in farming environments. The terms below are the working vocabulary for the classes of compounds studied in the next section.

Organic compounds

Organic compounds are chemical substances that contain carbon atoms covalently bonded to hydrogen, oxygen, nitrogen, and other elements. Unlike inorganic compounds, organic compounds are typically associated with living organisms or their by-products. A few carbon compounds, such as carbon dioxide, carbonates and carbon monoxide, are treated as inorganic. Example: carbohydrates and proteins in soil organic matter influence nutrient cycling in tea plantations.

Functional groups

Functional groups are specific groupings of atoms within molecules that determine the chemical reactivity and properties of organic compounds. The presence of hydroxyl (-OH), carboxyl (-COOH), or amino (-NH2) groups can influence how organic fertilisers interact with soil microbes in maize farming. Compounds that share a functional group react in similar ways, so the group is used to place a compound in its class.

Isomers

Isomers are compounds with the same molecular formula but different structural arrangements, resulting in distinct properties. For example, butane and methylpropane both have the formula C4H10, yet they have different boiling points because their carbon chains are arranged differently. In the same way, cis and trans isomers in certain pesticide molecules can have varying effectiveness in controlling crop pests.

Organic reactions

Organic reactions involve the transformation of organic compounds into new substances. These reactions underpin the manufacturing of agrochemicals and the degradation of organic matter in soils. The main types are:

• Substitution: one atom or group in a molecule is replaced by another. Replacing a hydrogen atom with chlorine is a step in making some chlorinated pesticides.
• Addition: atoms are added across a double or triple bond, converting an unsaturated compound into a more saturated one. Hardening vegetable oil with hydrogen is an example.
• Elimination: a small molecule such as water is removed, leaving a double bond. Ethanol loses water to form ethene.
• Polymerisation: many small molecules (monomers) join into a long chain. Glucose units join to form starch and cellulose in plants.
• Combustion: an organic compound burns in oxygen to give carbon dioxide and water. Methane from a farm biogas digester burns this way when used for cooking.
• Fermentation and decomposition: microbes break down organic matter into simpler compounds. This is how compost heaps and silage form.

3.1.2 Classes of organic compounds

Organic compounds are diverse and classified based on their structure and functional groups. For agricultural applications, recognising these classes aids in selecting appropriate substances for crop and soil management. The classes covered here are carbohydrates, proteins, lipids and hydrocarbons, followed by nucleic acids as an additional class.

3.1.2.1 Carbohydrates

Carbohydrates are organic molecules consisting of carbon, hydrogen, and oxygen, primarily serving as energy sources for plants and soil microbes. Their general formula is Cx(H2O)y, which is why the name means "hydrated carbon". Plants make them by photosynthesis, and farmers harvest them as maize grain, sugarcane, potatoes and cassava.

Carbohydrates are grouped by size. Monosaccharides, such as glucose and fructose, are single sugar units found in fruit and honey. Disaccharides, such as sucrose in sugarcane, are two units joined together. Polysaccharides are long chains: starch stores energy in grains and tubers, while cellulose forms the cell walls of plants. Polysaccharides like cellulose contribute to soil organic matter, enhancing soil structure in coffee farms, and are the main fibre fed to ruminants such as dairy cows.

3.1.2.2 Proteins

Proteins are complex organic compounds made from amino acids linked by peptide bonds (-CO-NH-). Each amino acid carries an amino group (-NH2) and a carboxyl group (-COOH), and proteins therefore contain nitrogen, and sometimes sulphur. They are vital for plant and animal growth, influencing livestock nutrition and the decomposition process of organic fertilisers in dairy farming.

Proteins perform several roles in farming systems:

• Structural: proteins build muscle, hair, hooves and milk solids. Casein in milk is an example.
• Enzymes: enzymes speed up chemical reactions such as digestion in the rumen and decay in compost.
• Storage: legume seeds such as beans and soya store protein for the germinating seedling, which is why they are valued as feed and food.
• Transport: haemoglobin in blood carries oxygen around the animal's body.
• Defence and regulation: antibodies protect livestock against disease, and some hormones are proteins.

When protein-rich residues such as legume leaves decompose, microbes release nitrogen as ammonium, which crops can take up. Feed rations for poultry and dairy cattle are therefore formulated for protein content.

3.1.2.3 Lipids

Lipids are hydrophobic organic molecules including fats, oils, and waxes. Most fats and oils are triglycerides, formed from one glycerol molecule joined to three fatty acids. In agriculture, lipids play roles in seed storage, plant cuticle formation, and as carriers in pesticide formulations for effective leaf adherence.

Fats and oils differ in the fatty acids they contain. Saturated fatty acids have no double bonds and give solid fats such as butter from milk. Unsaturated fatty acids have one or more double bonds and give liquid oils such as sunflower and groundnut oil. Waxes form the protective cuticle on leaves and fruits, reducing water loss, and phospholipids make up cell membranes. Oil seeds store energy as lipid, which is why sunflower and groundnut are grown for oil, and the cake left after pressing is used as livestock feed.

3.1.2.4 Hydrocarbons

Hydrocarbons are organic compounds composed exclusively of carbon and hydrogen atoms. They serve as the building blocks for more complex molecules like pesticides. Alkanes, alkenes, and alkynes are common hydrocarbon types relevant in agrochemical synthesis.

The three families differ in their carbon-to-carbon bonds:

• Alkanes: saturated hydrocarbons with single bonds only, general formula CnH2n+2. Methane (CH4) is the main gas produced in a farm biogas digester, and diesel for tractors is a mixture of larger alkanes.
• Alkenes: unsaturated hydrocarbons with at least one double bond, general formula CnH2n. Ethene (C2H4) is a plant hormone that ripens fruit such as bananas.
• Alkynes: unsaturated hydrocarbons with at least one triple bond, general formula CnH2n-2. Ethyne (C2H2) is the simplest member and is used as a starting material in chemical synthesis.

Hydrocarbons are the parent compounds of many derivatives that carry functional groups. Alcohols, ethers, and halides have functional groups that modify chemical behaviour, making them useful in formulating pesticides and herbicides.

Nucleic acids

Nucleic acids, such as DNA and RNA, store and transmit genetic information in plants and animals. They are long chains of nucleotides, and each nucleotide contains a sugar, a phosphate group and a nitrogen-containing base. Understanding their chemistry supports advances in crop breeding and genetic modification aimed at improving disease resistance. Example: selecting maize varieties for drought tolerance relies on differences in the DNA of the plants.

Chapter Summary

This chapter explored the fundamental concepts of organic compounds, beginning with clear definitions and classifications that include carbohydrates, proteins, lipids, and hydrocarbons. It then examined the physical properties of these compounds such as colour, hardness, mass, and solubility, highlighting how these characteristics influence their behaviour and applications. The chemical properties were also discussed, focusing on factors like pH, chemical stability, radioactivity, flammability, and heat of combustion, which determine the compounds' reactivity and safety considerations. The chapter further addressed the importance of purifying synthesized organic compounds, emphasizing their uses in various industries and research. Practical steps for carrying out purification were outlined to ensure the quality and effectiveness of these compounds. Overall, the chapter provided a comprehensive understanding of organic chemistry principles essential for practical applications in diverse scientific and industrial contexts.