| Family | General formula | Key reactions | |--------|----------------|----------------| | Alkanes | CₙH₂ₙ₊₂ | Combustion, free radical substitution (limited) | | Alkenes | CₙH₂ₙ | Electrophilic addition, polymerisation (addition polymers) | | Alcohols | CₙH₂ₙ₊₁OH | Combustion, dehydration (→alkene), oxidation (primary→aldehyde→acid; secondary→ketone; tertiary no oxidation) | | Halogenoalkanes | CₙH₂ₙ₊₁X | Nucleophilic substitution (OH⁻, CN⁻, NH₃), elimination (strong base/heat → alkene) | | Aldehydes | RCHO | Nucleophilic addition, mild oxidation (Tollens’/Fehling’s → carboxylic acid) | | Ketones | RCOR’ | Nucleophilic addition (slower than aldehydes), no oxidation (except strong oxidisers break C–C) | | Carboxylic acids | RCOOH | Acidic (weak), esterification (with alcohol + H⁺), reduction (LiAlH₄ → primary alcohol) | | Esters | RCOOR’ | Hydrolysis (acid or base – saponification) | | Amines | RNH₂ | Basic, nucleophilic, acylation (with acyl chloride → amide) | | Acyl chlorides | RCOCl | Highly reactive – nucleophilic acyl substitution (→ acid, ester, amide, anhydride) |
To understand chemical spontaneity, one must first address the concept of enthalpy ($\Delta H$). Historically, chemists believed that all spontaneous reactions were exothermic ($\Delta H < 0$). This alignies with the intuitive " downhill" principle—systems naturally lower their potential energy, much like a ball rolling down a hill. While this holds true for many reactions, such as the combustion of hydrocarbons, it fails to explain endothermic processes that occur spontaneously, such as the melting of ice at room temperature or the dissolving of ammonium nitrate in water. These reactions absorb heat from the surroundings, yet they proceed without external intervention. This anomaly suggests that the minimisation of energy is not the sole criterion for spontaneity. chemistry advanced level