Alkanes
Alkanes
- Alkanes are saturated hydrocarbons ($\text{C}_n\text{H}_{2n+2}$).
- They are useful fuels but generally unreactive.
- We make them and burn them.
Practice
Alkanes are:
Alkanes have only single C–C bonds (saturated) and follow CₙH₂ₙ₊₂.
Making alkanes
- hydrogenation: add hydrogen to an alkene (Pt or Ni catalyst, heat).
- cracking: break a long-chain alkane into shorter ones (heat with $\text{Al}_2\text{O}_3$).
Practice
Cracking is used to:
Cracking turns heavy crude-oil fractions into smaller, more useful alkanes and alkenes.
Combustion
- complete (plenty of oxygen) → carbon dioxide + water.
- incomplete (not enough oxygen) → water + toxic carbon monoxide (CO) + soot.
Practice
Incomplete combustion of an alkane (not enough oxygen) produces:
With too little oxygen you get CO (toxic) and carbon (soot) as well as water.
Unreactivity and environment
- Alkanes are unreactive towards polar reagents: the C–H and C–C bonds are strong and have little polarity, so there's no charge to attract attack.
- Burning them in engines gives CO, NOₓ and unburnt hydrocarbons — a catalytic converter removes these.
Practice
Alkanes are unreactive towards polar reagents because their bonds are:
Non-polar, strong C–H and C–C bonds give no charged site for an attacking species.
You've got it
Key idea
- alkanes = saturated hydrocarbons ($\text{C}_n\text{H}_{2n+2}$); made by hydrogenation or cracking
- complete combustion → CO₂ + water; incomplete → toxic CO + soot
- unreactive (strong, non-polar bonds); engine pollutants removed by a catalytic converter