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Chemistry Unit 10: Organic chemistry basics

Organic compounds are carbon-based; their structure and functional groups determine their names, properties and reactions.

Covers: Hydrocarbons: alkanes, alkenes and alkynes, naming and formulas, isomers, functional groups, organic reactions

Study guide

Organic compounds contain carbon, usually bonded to hydrogen. Carbon forms four covalent bonds and can bond to itself in chains, branches and rings, which is why there are millions of organic compounds. Hydrocarbons contain only carbon and hydrogen. Alkanes have only single bonds and the general formula CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀); they are saturated, holding the maximum number of hydrogens. Alkenes have one double bond (CₙH₂ₙ, like ethene C₂H₄), and alkynes have one triple bond (CₙH₂ₙ₋₂, like ethyne C₂H₂); both are unsaturated. Names use prefixes for the number of carbons (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non-, dec-) and endings for bond type (-ane, -ene, -yne). Benzene is a ring of six carbons, the basis of aromatic compounds.

Isomers have the same molecular formula but different structures, and therefore different properties; for example, butane and methylpropane are both C₄H₁₀. Structural formulas show every bond. Functional groups are specific atoms or groups that give compounds their properties. Key groups include halides (–F, –Cl, –Br, –I, as in chloromethane), alcohols (–OH, ending -ol, like methanol and ethanol), ethers (–O– between two carbons), aldehydes (–CHO at the end of a chain, ending -al), ketones (C=O within the chain, ending -one, like propanone or acetone), organic acids (–COOH, ending -oic acid, like ethanoic acid), esters (–COO–, ending -oate, often with fruity smells), amines (–NH₂), amides (–CONH₂) and amino acids, which contain both an amine and an acid group.

Organic reactions are usually slower than inorganic ones because covalent bonds must break. In substitution, an atom (usually H) on a saturated hydrocarbon is replaced, as when chlorine replaces a hydrogen in an alkane. In addition, atoms add across a double or triple bond of an unsaturated compound, like H₂ or Br₂ adding to an alkene. Combustion burns hydrocarbons in oxygen to produce CO₂ and H₂O. Fermentation turns sugar into ethanol and CO₂ using yeast. Esterification combines an organic acid and an alcohol to form an ester and water. Saponification breaks down fats with a base to make soap. Polymerization links many small monomers into long polymers, either by addition (like polyethylene) or by condensation, which releases water (like nylon and proteins).

Key ideas

• Carbon forms four bonds and can form chains, branches and rings.
• Alkanes are saturated; alkenes and alkynes are unsaturated.
• Isomers share a formula but differ in structure and properties.
• Functional groups determine a compound's name and properties.
• Substitution happens on saturated compounds; addition on unsaturated ones.

Formulas and rules

Common mistakes

• Confusing substitution (saturated compounds) with addition (unsaturated compounds).
• Mixing up aldehydes (C=O at the end of a chain) and ketones (C=O in the middle).

Words to know

Organic compound

A carbon-based compound, usually containing hydrogen.

Hydrocarbon

A compound of only carbon and hydrogen.

Alkane

A hydrocarbon with only single bonds.

Alkene

A hydrocarbon with one double bond.

Alkyne

A hydrocarbon with one triple bond.

Saturated

Having only single bonds between carbons.

Unsaturated

Having at least one double or triple bond.

Isomers

Compounds with the same formula but different structures.

Functional group

An atom or group that gives a compound its properties.

Alcohol (organic)

A compound with an –OH group.

Organic acid

A compound with a –COOH group.

Ester

A compound formed from an acid and an alcohol, often fruity-smelling.

Ketone

A compound with a C=O group within the carbon chain.

Amine

A compound with an –NH₂ group.

Substitution reaction

Replacing an atom in a saturated hydrocarbon.

Addition reaction

Adding atoms across a double or triple bond.

Fermentation

Converting sugar into ethanol and CO₂.

Polymerization

Linking monomers into a long polymer.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.