Characteristic organic reactions — common mistakes

Free-radical substitution is not a 'clean' reaction. It produces a mixture of products because substitution can occur at any position on a longer alkane chain, and multiple substitutions can occur on the same molecule (e.g., forming dichloromethane, trichloromethane, etc.).

For primary alcohol oxidation, the conditions are crucial. If a question asks for the aldehyde, you MUST mention distillation. If it asks for the carboxylic acid, you MUST mention reflux. These keywords are often worth a mark.

Alkanes contain only sigma (σ) bonds, which are strong and have low polarity. There are no lone pairs or regions of high electron density to attract attacking species (electrophiles or nucleophiles). Alkenes, however, have a π-bond, which is a region of high electron density and is weaker than a σ-bond, making it susceptible to attack by electrophiles.

Homolytic fission is the symmetrical breaking of a covalent bond where each atom gets one electron from the shared pair, forming two free radicals. This is common in non-polar bonds initiated by UV light. Heterolytic fission is the unsymmetrical breaking of a bond where one atom takes both electrons, forming a positive ion (cation) and a negative ion (anion). This is common in polar bonds.

The primary factor is the structure of the halogenoalkane. Tertiary (3°) halogenoalkanes favour the Sₙ1 mechanism because they can form a relatively stable tertiary carbocation intermediate. Primary (1°) halogenoalkanes favour the Sₙ2 mechanism because there is little steric hindrance, allowing the nucleophile to attack the carbon atom directly. Secondary (2°) halogenoalkanes are intermediate and can undergo both mechanisms.