3.11 - Hydrocarbons & Homologous Series

Hydrocarbons and homologous series

• Organic chemistry is the chemistry of compounds that contain carbon.
• A hydrocarbon is a compound made up of hydrogen and carbon only.
  • Methane (CH₄) is a hydrocarbon. Ethanol is not, because it contains oxygen.

Why there are so many carbon compounds

• A carbon atom forms four covalent bonds, and it bonds readily to other carbon atoms.
• Carbon atoms join end to end into chains (short or long) and into rings.
• Each carbon uses its remaining bonds to join to hydrogen atoms or atoms of other elements.

Homologous series

• A homologous series is a family of similar compounds.
• Members of the same homologous series share:
  • The same general formula.
  • Each member differs from the next by a CH₂ unit.
  • A gradual change in physical properties (melting and boiling points rise as molecules get bigger, because larger molecules have stronger intermolecular forces).
  • Similar chemical properties (they react in similar ways).

The alkanes

• Alkanes are the simplest homologous series of hydrocarbons.
• Every carbon-carbon bond is a single bond, and all other bonds are to hydrogen atoms.
• An alkane holds as many hydrogen atoms as its carbon framework can, making it saturated.

The first four alkanes

The first four alkanes are all gases at room temperature and pressure.

Each alkane has one more carbon atom and two more hydrogen atoms than the one before it (the CH₂ step).

General formula

The general formula for all alkanes is:

C_nH_2n+2

• $n$ is the number of carbon atoms.
• For propane ($n$ = 3): hydrogens = (2 × 3) + 2 = 8, giving C₃H₈.
• For an alkane with 8 carbons ($n$ = 8): hydrogens = (2 × 8) + 2 = 18, giving C₈H₁₈.

Ways to write an alkane

• Molecular formula - Shows the actual number of atoms of each element in a molecule (e.g. propane is C₃H₈).
• Structural formula (also called a shortened structural formula) - Shows the arrangement of atoms along the chain without drawing every bond (e.g. propane is CH₃CH₂CH₃).
• Displayed formula (also called a full structural formula) - Shows every atom and every covalent bond as a line. Some questions call this fully drawn form simply a structural formula, so when a question asks you to draw a structural formula, draw every atom and every bond.

When drawing a displayed formula, check that every carbon atom has four bonds and every hydrogen atom has one.

Recognising an alkane

• A name ending in -ane.
• A molecular formula that fits C_nH_2n+2.
• A structure containing only carbon and hydrogen with single C-C bonds only.

Alkanes are insoluble in water and are widely used as fuels.