Chapter 16: Halogenoalkanes

Halogenoalkanes chemistry chapter, titled "Halogenoalkanes," provides an in-depth exploration of the synthesis, structure, reactivity, and reaction pathways of organic compounds where one or more hydrogen atoms in an alkane are replaced by a halogen. Halogenoalkanes are fundamentally more reactive than simple alkanes because the carbon-halogen covalent bond is polarized, causing the carbon atom to carry a partial positive charge, which makes it highly susceptible to attack by electron-rich species known as nucleophiles. Historically, the characteristic chemical inertness of some halogenoalkanes under normal conditions allowed their use in industrial processes, such as flame retardants and anaesthetics like trichloromethane (chloroform) and 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane), the latter of which is considered safe due to three very strong carbon-fluorine bonds. Conversely, chlorofluorocarbons (CFCs), once widely used as refrigerants and propellants due to their non-flammable and non-toxic properties, demonstrate that this stability can pose an environmental hazard; high in the stratosphere, ultraviolet light breaks the carbon-chlorine bonds, releasing chlorine free radicals that catalytically destroy the protective ozone layer. Synthesis of halogenoalkanes primarily involves three methods: free-radical substitution of alkanes, electrophilic addition to alkenes, and substitution of alcohols using reagents like hydrogen halides or phosphorus halides. The chapter focuses heavily on nucleophilic substitution reactions. Reactivity is governed by the strength of the carbon-halogen bond, with the weakest carbon-iodine bond making iodoalkanes the fastest reacting, while fluoroalkanes are the slowest due to the strongest carbon-fluorine bond. Nucleophilic substitution reactions include hydrolysis, where the halogen is replaced by a hydroxyl group to form an alcohol, proceeding faster with the fully charged hydroxide ion (OH-) than with neutral water; reaction with ethanolic potassium cyanide (CN-) to form a nitrile, which is essential for extending the carbon chain by one atom; and reaction with excess ethanolic ammonia (NH3) under pressure to yield primary amines. The mechanism of substitution varies with the structure of the halogenoalkane: Primary halogenoalkanes follow the single-step, bimolecular SN2 mechanism, where the nucleophile attacks simultaneously as the carbon-halogen bond breaks, meaning the reaction rate depends on both reactants. Tertiary halogenoalkanes follow the two-step, unimolecular SN1 mechanism, where the rate-determining step is the heterolytic fission of the carbon-halogen bond to form a stable tertiary carbocation intermediate before the nucleophile attacks. Tertiary carbocations are more stable than primary ones because alkyl groups reduce the positive charge density through the positive inductive effect. Finally, halogenoalkanes also undergo elimination reactions, which remove a hydrogen halide molecule to produce an alkene. Crucially, the choice of solvent dictates the reaction type: aqueous sodium hydroxide leads to substitution (alcohol product), whereas ethanolic sodium hydroxide leads to elimination (alkene product).