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Organic Chemistry

Topic 2 of 7

Alkanes: build a valid carbon skeleton

Use four bonds per carbon and one per hydrogen.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

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 understandingA saturated acyclic hydrocarbon contains three carbon atoms. What formula should it have, and how do you check a drawing?Think it through, then reveal the answer
C3H8, from CnH2n+2. Draw three connected carbons and fill remaining valencies with H; each C must have four bonds and the total hydrogen count must be eight.