Organic names, isomers and alkane fuels

What you’ll learn

• Interpret molecular, structural and skeletal formulae.
• Name simple hydrocarbons and distinguish structural isomers.
• Explain fuel processing and assess combustion emissions.

Understand the idea

A homologous series shares a functional group and a general formula; consecutive members differ by $\mathrm{CH_2}$. Choose the longest appropriate carbon chain, number it to give substituents the lowest available locants and use the correct ending. In skeletal formulae, unlabelled corners and line ends represent carbon, with enough implied hydrogens to complete four bonds.

Structural isomers share a molecular formula but differ in connectivity. Acyclic alkanes are saturated hydrocarbons with formula $\mathrm{C_nH_{2n+2}}$; a single saturated ring reduces the hydrogen count to $\mathrm{C_nH_{2n}}$. Rotating bonds or redrawing the same connectivity does not create a new structural isomer.

Fractional distillation separates crude-oil components by boiling range. Cracking breaks larger hydrocarbons into smaller products, often including an alkene; reforming changes carbon skeletons or forms rings to improve fuel performance. Complete combustion gives carbon dioxide and water. Limited oxygen can produce carbon monoxide and soot; high-temperature engines also form nitrogen oxides, while sulfur impurities give sulfur dioxide.

Carbon monoxide binds strongly to haemoglobin and impairs oxygen transport. Acidic nitrogen and sulfur oxides contribute to environmental damage. Evaluating an alternative fuel requires its whole life cycle: growing a biofuel can absorb carbon dioxide, but farming, processing and transport may still emit it. A hazard is an intrinsic harmful property; risk also depends on exposure, which controlled teaching laboratories reduce through suitable quantities, containment and supervision.

Key formulae

$$\text{acyclic alkane: }\mathrm{C_nH_{2n+2}}$$

$$\text{single-ring cycloalkane: }\mathrm{C_nH_{2n}}$$

$$\mathrm{C_3H_8+5O_2\rightarrow3CO_2+4H_2O}$$

$$\mathrm{C_{10}H_{22}\rightarrow C_8H_{18}+C_2H_4}$$

Worked example

Name $\mathrm{CH_3CH(CH_3)CH_2CH_3}$ and explain why it is an isomer of pentane.

1. The longest continuous chain contains four carbon atoms, so use butane as the parent name.
2. The remaining methyl group is on carbon 2 when numbered from the nearer end.
3. Both molecules have formula $\mathrm{C_5H_{12}}$, but the branched compound has different carbon connectivity.

Answer: 2-methylbutane; it is a structural isomer of pentane.