Chapter 18 · Organic Chemistry
18.5Alkanes: Saturated Hydrocarbons
9 min · two checks
Predict
What are the names of the first four straight-chain alkanes?
The idea
- Name the smallest alkanes and write their formulas.
- Describe combustion of alkanes.
Methane, ethane, propane, and butane are gases at room temperature that are familiar as fuel. Pentane through about hexane and octane are liquids and show up in gasoline. The melting and boiling points rise with chain length because London dispersion forces grow. Alkanes are relatively unreactive: the C–C and C–H bonds are strong and the molecules are nonpolar. Their characteristic reaction in this course is combustion. Complete combustion gives CO₂ and H₂O. Limited oxygen gives CO or soot, which is why ventilation matters around a heater.
Drawings of chains as flat zigzags are a convention. Each carbon is tetrahedral, and the chain can twist around single bonds. Those twists are conformations, not different compounds, because the atoms are not locked. A different connectivity — a branch — is a different compound. That distinction is isomerism, next.
Keep these
- C1 to C4: methane, ethane, propane, butane.
- Boiling point rises as the chain gets longer.
- Complete combustion: hydrocarbon + O₂ → CO₂ + H₂O.