Haloalkanes are a crucial bridge between hydrocarbons and hydrocarbon derivatives in organic synthesis. Their core role is to achieve functional group transformation and carbon skeleton construction. Their main applications are as follows:
Achieving functional group transformation and introducing various target groups.
Through different reactions of haloalkanes, halogen atoms can be transformed into various common functional groups:
Hydroxy group (-OH): Haloalkanes undergo hydrolysis under heating conditions with NaOH aqueous solution. The halogen is replaced by the hydroxyl group to obtain an alcohol. This is a common method for introducing hydroxyl groups in organic synthesis, such as the hydrolysis of bromoethane to produce ethanol.
Amino group (-NH₂): Haloalkanes undergo ammonolysis, where the halogen is replaced by the amino group to obtain amine compounds.
Carbon-carbon double/triple bond: Haloalkanes undergo elimination reactions under heating conditions with NaOH alcoholic solution, removing the HX to form an unsaturated bond. This can be used to prepare alkenes and alkynes from saturated hydrocarbons. For example, the elimination of 1,2-dibromoethane yields acetylene. This is an essential step in the conversion of saturated hydrocarbons to unsaturated hydrocarbons.
Cyano group (-CN): When a haloalkane is heated with sodium cyanide, substitution occurs, replacing the halogen atom with the cyano group to form a nitrile, providing a basis for subsequent conversion to carboxylic acids, amines, etc.
Changing the number and position of functional groups to adjust molecular structure
Changing the number of functional groups: Alkenes first undergo addition reactions with halogen elements to form dihaloalkanes, then hydrolyze to give diols. Monofunctional groups can be converted to difunctional groups, for example, ethylene → 1,2-dibromoethane → ethylene glycol, providing raw materials for subsequent synthesis of polyesters and polyethers.
Adjusting the position of functional groups: If the position of the hydroxyl group in an alcohol does not meet the synthetic requirements, the alcohol can be first converted to a haloalkane, then eliminated to obtain an alkene, and finally added back in to reintroduce the halogen atom/hydroxyl group, thus changing the position of the functional group.
Constructing a carbon skeleton to lengthen or couple carbon chains
Haloalkanes are important intermediates for extending carbon chains in organic synthesis:
Haloalkanes undergo substitution reactions with sodium cyanide, sodium acetylacetonate, etc., directly adding carbon atoms to the molecule, achieving carbon chain lengthening.
Grignard reagents (RMgX) are prepared by reacting halogenated hydrocarbons with magnesium in anhydrous diethyl ether. Grignard reagents can undergo addition reactions with aldehydes, ketones, and carbon dioxide to yield alcohols or carboxylic acids with more carbon atoms than the original haloalkane, which is an important method for constructing carbon-carbon bonds.
Halogenated hydrocarbons can undergo a Woods reaction with metallic sodium to couple long-chain alkanes with double the number of carbon atoms.