Organic and Inorganic Compounds
In 1856, 18-year-old William Perkin accidentally synthesised the first synthetic dye, mauve, from coal tar, proving organic chemistry could build useful molecules from carbon chains.
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Q1 · Think about coal, wood, sugar, and petrol, in what ways do these seem similar, and in what ways are they completely different?
Q2 · Why do you think chemists separate compounds into "organic" (from living things) and "inorganic" (non-living origin) categories?
● Know
- The definition of organic and inorganic compounds
- Why CO₂ is classified as inorganic despite containing carbon
- The four main classes of organic compounds
● Understand
- Why carbon's four-bonding capacity creates enormous molecular diversity
- How carbon backbone length and branching affect compound properties
- Why the C–H bond is the defining feature of organic chemistry
● Can do
- Classify compounds as organic or inorganic based on their formula
- Identify carbon backbones in structural formulas
- Explain why diversity in organic compounds is possible
Pour a few drops of cooking oil into water: the oil floats in golden beads and refuses to mix, because organic molecules and inorganic molecules interact with each other very differently. Organic compounds are defined as compounds that contain a carbon–hydrogen (C–H) backbone. Carbon is unique: it has four valence electrons, allowing it to form four bonds simultaneously. It bonds to other carbon atoms in chains, branches, and rings of any length, from the two-carbon ethanol molecule to the 100,000-carbon polyethylene chains in plastic bags. This versatility is why there are over 10 million known organic compounds, far more than all inorganic compounds combined.
Inorganic compounds are everything else, salts, metals, minerals, most gases. The boundary has a few exceptions (CO₂ is usually treated as inorganic despite containing carbon), but the practical definition holds: organic = C–H backbone. This distinction matters in industry because organic compounds burn, can be fermented, often have biological activity, and can be polymerised into plastics, while most inorganic compounds are non-flammable and chemically stable in very different ways.
Ethanol (C₂H₅OH): organic, contains C–H bonds, is flammable, can be fermented from sugar. Sodium chloride (NaCl): inorganic, contains no C–H bonds, is not flammable, dissolves in water as ions. Both are used in food and medicine, but their chemistry is completely different.
CSIRO's Pharmaceutical Chemistry group in Melbourne synthesises new organic compounds for drug discovery. Australia's pharmaceutical industry produces over $4 billion worth of organic chemical products per year, from aspirin-like analgesics to complex biologic medicines derived from living cells.
A functional group is a specific arrangement of atoms within an organic molecule that gives it characteristic chemical properties. The carbon chain is the 'skeleton'; the functional group is what makes the molecule react in a particular way. Three important groups: the hydroxyl group (–OH) is found in alcohols like ethanol and makes them polar and soluble in water; the carboxyl group (–COOH) is found in carboxylic acids like acetic acid (vinegar) and makes them acidic; the carbonyl group (C=O) in the middle of a chain forms ketones like propanone (nail polish remover).
Understanding functional groups lets chemists predict how a compound will behave without testing it. A compound ending in –OH is likely to mix with water. One ending in –COOH will taste sour and neutralise bases. This predictive power is why pharmaceutical chemists deliberately attach specific functional groups to drug molecules to control how they dissolve in blood, where they bind in the body, and how quickly they are metabolised.
Aspirin (acetylsalicylic acid) contains both a carboxyl group (–COOH) making it slightly acidic, and an ester functional group that makes it less irritating to the stomach than plain salicylic acid. The functional groups were deliberately chosen to balance efficacy and side effects during drug design in the 1890s.
The University of Sydney's School of Chemistry synthesises novel functional group combinations for agrochemical companies, herbicides and pesticides used on Australian wheat and cotton farms. Attaching the right functional group determines whether the compound kills weeds selectively or non-selectively, a $400 million decision for Australian agriculture.
Every material made from carbon compounds, fuels, medicines, plastics, food molecules, is an organic compound. Ethanol (C₂H₅OH) is simultaneously a fuel additive (blended into petrol in Australian E10 fuel), a solvent (hand sanitiser), and a beverage ingredient, its function depends entirely on context. Glucose (C₆H₁₂O₆) is the primary energy molecule in every cell in your body; a single 100 g chocolate bar contains about 55 g of carbohydrate, most of which your body breaks down to glucose.
Aspirin (C₉H₈O₄) is one of the world's most-produced pharmaceutical organic compounds, about 40,000 tonnes manufactured globally per year. Polyethylene (–CH₂–CH₂–)ₙ is the most produced synthetic organic polymer: over 100 million tonnes per year. Yet all four examples share the same defining feature: a carbon–hydrogen backbone with specific functional groups that determine their exact chemical behaviour. The chemistry is the same at its core; the functional group changes the use case entirely.
E10 petrol sold at Australian service stations is 90% octane (C₈H₁₈) and 10% ethanol (C₂H₅OH), both organic compounds. The ethanol component comes partly from Queensland sugarcane fermentation, making Australian E10 fuel partially renewable and reducing net CO₂ emissions by about 3% compared to pure petrol.
Queensland sugarcane growers supply about 30 million tonnes of cane per year to mills, which produce both raw sugar and bioethanol. The ethanol is blended into E10 fuel sold across NSW and QLD, a direct link between Australian agriculture, organic chemistry, and everyday transport.
Every material made from carbon compounds, fuels, medicines, and plastics, is an compound. All organic compounds share a carbon– backbone. is used as a fuel additive, a solvent, and a beverage ingredient depending on context. is the primary energy molecule in every cell of your body. is the most-produced synthetic organic compound in the world.
At the start of this lesson, you heard that your body contains about 7 kg of carbon atoms, more than any other element, and that carbon's unique ability to bond to itself in chains of millions of atoms is what makes life, plastics, medicines, and fuels all possible from the same single element.
Now that you've worked through the lesson, how has your understanding of organic and inorganic compounds changed? Could you now explain why coal, sugar, and petrol are all organic despite looking so different, and give an example of an inorganic compound you encounter every day?
Q1. Explain why CO₂ is classified as an inorganic compound even though it contains carbon.
Q2. Classify each of the following as organic or inorganic and justify your decision: glucose (C₆H₁₂O₆), sodium chloride (NaCl), ethanol (C₂H₅OH), water (H₂O).
Q3. Explain why carbon, rather than silicon, is the basis of life on Earth. Refer to carbon's bonding capacity and the diversity of molecules it can form.