halogens

• Halogenoalkanes are alkanes that have one or more halogens.
• Halogenoalkanes can be produced from free-radical substitution of alkanes, electrophilic addition of alkenes, and substitution of an alcohol.
• Ultraviolet light (UV) is required for the reaction to start.
• A free-radical substitution reaction is a three-step reaction consisting of initiation, propagation and termination steps.
• In the initiation step of a free-radical substitution reaction, the halogen bond is broken by energy from the UV light to produce two radicals in a homolytic reaction.
• The propagation step of a free-radical substitution reaction refers to the progression (growing) of the substitution reaction in a chain type reaction.
• The termination step of a free-radical substitution reaction is when the chain reaction terminates (stops) due to two free radicals reacting together and forming a single unreactive molecule.
• Halogenoalkanes can also be produced from the addition of hydrogen halides (HX) or halogens (X) at room temperature to alkenes.
• In hydrogen halides, the hydrogen acts as the electrophile and accepts a pair of electrons from the C-C bond in the alkene.
• The formation of the silver fluoride is the slowest, indicating the slowest nucleophilic substitution reaction.
• The C-I bond requires the least energy to break, making it the weakest carbon-halogen bond.
• Reacting halogenoalkanes with aqueous silver nitrate solution results in the formation of a precipitate.
• This confirms that uoroalkanes are the least reactive and iodoalkanes are the most reactive halogenoalkanes.
• Substitution reactions involve breaking the carbon-halogen bond, hence the bond energies can be used to explain their different reactivities.
• During substitution reactions, the C-I bond will heterolytically break as follows: R C-I + OH → R C-OH + I.
• The rate of formation of these precipitates can be used to determine the reactivity of the halogenoalkanes.
• The formation of the pale yellow silver iodide is the fastest, indicating the fastest nucleophilic substitution reaction.
• Fluoroalkanes will be less likely to undergo substitution reactions.
• The C-F bond requires the most energy to break, making it the strongest carbon-halogen bond.
• The halogenoalkanes have different rates of substitution reactions.
• If NaOH(aq) is used, a nucleophilic substitution reaction takes place to form an alcohol from a halogenoalkane.
• The S2 mechanism is a one-step reaction.
• The major product in the addition of hydrogen halides to alkenes is the one in which the halide is bonded to the most substituted carbon atom (Markovnikov’s rule).
• At the same time, the C-X bond is breaking and the halogen (X) takes both electrons in the bond (heterolytic elimination).
• The halogenoalkanes have different rates of hydrolysis, so this reaction can be used as a test to identify halogens in a halogenoalkane by measuring how long it takes for the test tubes containing the halogenoalkane and aqueous silver nitrate solutions to become opaque.
• A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than with aqueous alkali.
• Hydrogen bromide is eliminated to form ethene.
• The reaction conditions in a reaction are extremely important.
• In primary halogenoalkanes, the carbon that is attached to the halogen is bonded to one alkyl group.
• The halogenoalkanes are heated with ethanolic sodium hydroxide causing the C-X bond to break heterolytically, forming an X ion and leaving an alkene as an organic product.
• The halogen leaves the halogenoalkane as an X ion.
• These halogenoalkanes undergo nucleophilic substitution by an S2 mechanism.
• In an elimination reaction, an organic molecule loses a small molecule.
• The nucleophile donates a pair of electrons to the δ+ carbon atom to form a new bond.
• These reactions can occur in two different ways (known as S2 and S1 reactions) depending on the structure of the halogenoalkane involved.
• In the case of halogenoalkanes, the small molecule is a hydrogen halide (eg HCl).
• In nucleophilic substitution reactions involving halogenoalkanes, the halogen atom is replaced by a nucleophile.
• Bromoethane reacts with ethanolic sodium hydroxide when heated to form ethene.
• If NaOH(ethanol) is used, an elimination reaction takes place to form an alkene from a halogenoalkane.
• In the addition of halogens to alkenes, one of the halogen atoms acts as an electrophile and the other as a nucleophile.