A carbon compound is a compound that contains carbon, usually bonded to hydrogen and often oxygen. Most are organic compounds, sourced from living things and petroleum.
Carbon forms an enormous number of compounds because each carbon atom forms four strong covalent bonds and can bond to other carbon atoms in chains and rings.
This page covers the first content standard of the Form 5 Carbon Compounds chapter: the introduction to carbon compounds. It sets up every idea that follows, homologous series, alkanes, alkenes, alcohols, carboxylic acids and esters, so getting the definitions and classifications right here makes the rest of the chapter far easier.
What is a carbon compound?
A carbon compound is a compound whose molecules contain carbon atoms, almost always bonded to hydrogen and often to oxygen as well. The vast majority of carbon compounds are called organic compounds because, historically, they were first obtained from living (organic) things, plants, animals and the fossil fuels formed from them. Today we know organic compounds can also be made in the laboratory, but the name has stuck.
Not every compound of carbon is organic. A small group of simple carbon compounds is treated as inorganic by convention: carbon dioxide (CO₂), carbon monoxide (CO), carbonates such as calcium carbonate (CaCO₃), and hydrogencarbonates. You should be able to state that these are the well-known exceptions, because a common exam question asks you to pick out the inorganic carbon compound from a list.
Key definition. An organic compound is a compound of carbon (with hydrogen, and often other elements such as oxygen) that is generally obtained from living things or from petroleum, and that burns to produce carbon dioxide and water.
Where carbon compounds come from
The syllabus wants you to know the main sources of organic compounds:
- Living things, carbohydrates, proteins, fats and oils in plants and animals are all organic compounds.
- Petroleum (crude oil) and natural gas, the fossil sources from which fuels such as petrol, diesel and cooking gas, and raw materials for plastics, are obtained by fractional distillation.
Why carbon forms so many compounds
There are millions of known carbon compounds, far more than the compounds of all the other elements combined. Two properties of the carbon atom explain this:
- Carbon forms four covalent bonds. A carbon atom has four valence electrons, so it shares four pairs of electrons and forms four strong covalent bonds. This lets each carbon join to as many as four other atoms.
- Carbon atoms bond to one another (catenation). Carbon–carbon bonds are strong and stable, so carbon atoms can link into long straight chains, branched chains and rings. Different chain lengths and arrangements give different compounds.
Together, these two properties mean carbon can build an almost unlimited variety of stable molecules.
Hydrocarbons and non-hydrocarbons
Organic compounds are first split into two classes:
- A hydrocarbon is a compound that contains only carbon and hydrogen. Alkanes (for example methane, CH₄) and alkenes (for example ethene, C₂H₄) are hydrocarbons.
- A non-hydrocarbon organic compound contains carbon and hydrogen plus at least one other element, usually oxygen. Alcohols, carboxylic acids and esters are non-hydrocarbons because they contain oxygen.
Saturated and unsaturated compounds
A second useful classification describes the carbon–carbon bonding:
- A saturated compound contains only single carbon–carbon bonds (C–C). Alkanes are saturated.
- An unsaturated compound contains at least one double (C=C) or triple carbon–carbon bond. Alkenes are unsaturated.
This distinction matters throughout the chapter, because unsaturated compounds react in ways (addition reactions, decolourising bromine water) that saturated compounds do not.
Worked example
Question. From the following list, classify each substance as an organic hydrocarbon, an organic non-hydrocarbon, or an inorganic carbon compound: ethane (C₂H₆), ethanol (C₂H₅OH), calcium carbonate (CaCO₃).
Step 1, Ethane (C₂H₆). It contains only carbon and hydrogen. A compound of only C and H is a hydrocarbon, and hydrocarbons are organic. So ethane is an organic hydrocarbon.
Step 2, Ethanol (C₂H₅OH). It contains carbon and hydrogen and oxygen. Because it has an element other than C and H, it is not a hydrocarbon; but it is a compound of carbon obtained from living/plant sources, so it is an organic non-hydrocarbon.
Step 3, Calcium carbonate (CaCO₃). Although it contains carbon, carbonates are one of the recognised exceptions, so it is an inorganic carbon compound.
Answer. Ethane, organic hydrocarbon; ethanol, organic non-hydrocarbon; calcium carbonate, inorganic carbon compound.
Practice question
Question. Explain why carbon is able to form a very large number of different compounds. Give two reasons.
Answer. First, a carbon atom has four valence electrons and forms four covalent bonds, so it can join to up to four other atoms. Second, carbon atoms can bond to other carbon atoms through strong, stable C–C bonds to form long straight chains, branched chains and rings (catenation). Because chains of different lengths and shapes give different molecules, these two properties allow an enormous variety of stable compounds to exist.
Exam tip
Two answers are examined again and again in this opening standard. When asked why carbon forms so many compounds, always give both reasons, four covalent bonds and carbon-to-carbon bonding (catenation), because one reason alone usually loses a mark. When asked to identify an inorganic carbon compound, remember the short list of exceptions: CO₂, CO, carbonates and hydrogencarbonates. Keep the words hydrocarbon (only C and H) and non-hydrocarbon (C, H plus another element such as O) precise, and never call an alcohol or acid a hydrocarbon.
How this connects
Everything in this chapter builds on the classifications set out here. Hydrocarbons split into the alkane and alkene families you will meet next; non-hydrocarbons include the alcohols, carboxylic acids and esters later in the chapter. The saturated/unsaturated idea explains why alkenes are reactive while alkanes are not. Because these categories reappear in every experiment across SPM Chemistry, fixing them now pays back through the whole of Form 5.
Where this fits
This is content standard 10.1 of the Carbon Compounds chapter, the foundation on which the rest of the chapter stands. Reinforce the definitions with the chapter revision notes and practise sorting compounds into the right class with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers make sure students can define an organic compound, list the inorganic exceptions, and give both reasons carbon forms so many compounds without hesitation.
Quick recap
- A carbon compound contains carbon, usually with hydrogen and often oxygen.
- Most are organic; the inorganic exceptions are CO₂, CO, carbonates, hydrogencarbonates.
- Sources: living things and petroleum / natural gas.
- Carbon forms many compounds because it makes four covalent bonds and bonds to other carbon atoms (catenation).
- Hydrocarbon = only C and H; non-hydrocarbon = also contains another element (usually O).
- Saturated = only C–C single bonds; unsaturated = has a C=C double bond.
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