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K223 / K225 / 2027
Organic Chemistry

Lesson 2 of 5

Alkanes: build a valid carbon skeleton

Use four bonds per carbon and one per hydrogen.

SEC G2 Science K223 / K225 (2027)

  • I can describe homologous series.Syllabus

    Syllabus K223 / K225, 8.2(a). Describe homologous series

  • I can describe alkanes.Syllabus

    Syllabus K223 / K225, 8.2(b). Describe alkanes

  • I can describe alkane reactions.Syllabus

    Syllabus K223 / K225, 8.2(d). Describe alkane reactions

  • I can draw and name alkanes.Syllabus

    Syllabus K223 / K225, 8.2(c). Draw and name alkanes

Alkanes: build a valid carbon skeleton

Use four bonds per carbon and one per hydrogen.

A homologous series has a common general formula, similar chemical properties and a gradual change in physical properties as molecule size and mass increase. Successive members differ by CH2. Alkanes are saturated hydrocarbons: all carbon-carbon bonds are single. For an unbranched or branched acyclic alkane, the general formula is CnH2n+2.

Shared alkane structures
NameMolecular formulaCondensed structure
MethaneCH4One C with four C-H bonds
EthaneC2H6CH3-CH3
PropaneC3H8CH3-CH2-CH3

To draw a displayed formula, draw the carbon-carbon connections first, then add individual C-H bonds until each carbon has four bonds. A condensed CH3 group stands for three separate hydrogens bonded to that carbon. Count atoms after drawing; moving the page or bending a chain does not create a new compound.

Four bonds around carbon

Methane has four single C-H bonds. Each ethene carbon has two C-H bonds and a double C=C bond, giving four bonds in total.

A double bond counts twice when checking carbon valency; flat drawings do not show molecular shape.

Displayed methane

Complete displayed structure with 1 carbon, 4 hydrogen atoms: CH4. Every bond and hydrogen is shown.

CH4. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed ethane

Complete displayed structure with 2 carbon, 6 hydrogen atoms: C2H6. Every bond and hydrogen is shown.

C2H6. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Displayed propane

Complete displayed structure with 3 carbon, 8 hydrogen atoms: C3H8. Every bond and hydrogen is shown.

C3H8. Count four bonds at every carbon, two at oxygen and one at hydrogen. The flat layout shows connectivity, not molecular shape.

Alkanes are generally unreactive, but burn and undergo substitution with chlorine in ultraviolet light. Complete combustion: CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l), showing cooled products. Limited oxygen can produce carbon monoxide and/or soot. Substitution: CH4(g) + Cl2(g) -> CH3Cl(g) + HCl(g), under UV. One H is replaced by Cl; further substitution is possible.

Check your understanding

A saturated, non-cyclic hydrocarbon has three carbon atoms. What is its formula?

  1. C3H12, from CnH4n
  2. C3H6, from CnH2n
  3. C3H8, from CnH2n+2
Show the answer

C3H8, from CnH2n+2

Alkanes follow CnH2n+2, so n = 3 gives C3H8. Check a drawing: each C has four bonds and there are eight H in total.

Check your understanding

Methane, ethane and propane are in the same homologous series. Which statement is not a feature of a homologous series?

  1. The members all have the same boiling point
  2. Each member differs from the next by CH2
  3. The members share the same general formula
  4. The members have similar chemical properties
Show the answer

The members all have the same boiling point

Boiling points rise gradually as molecules get larger and the attractions between them get stronger.