haloalkanes

Haloalkanes, also known as alkyl halides, are a class of organohalogen compounds in which one or more hydrogen atoms in an alkyl group are replaced by halogen atoms. The general formula is RX, where R is an alkyl group and X is a halogen (fluorine, chlorine, bromine, or iodine; astatine occurs only rarely in laboratory samples). They vary from gases to liquids and solids depending on the substituent and chain length.

Physical properties are governed by the polar carbon–halogen bond. Haloalkanes are typically more polar than the corresponding alkanes and often have higher boiling points, increasing with the mass of X. They are largely immiscible with water but dissolve in organic solvents. Their volatility and stability depend on the halogen and substitution pattern.

Reactivity centers on nucleophilic substitution and elimination. SN2 reactions are common for primary haloalkanes with good nucleophiles, often proceeding with inversion at stereocenters. SN1 reactions predominately occur with tertiary haloalkanes, forming carbocations and leading to racemization. The leaving-group ability generally follows RI > RBr > RCl > RF, making fluorinated haloalkanes relatively unreactive in typical substitution reactions. Haloalkanes can also undergo elimination (E1, E2) to form alkenes, influenced by base strength and substrate structure.

Preparation and uses: Haloalkanes are prepared by halogenation of alkanes, hydrohalogenation of alkenes, or substitution of alcohols with reagents such as HX, thionyl chloride, or phosphorus tribromide. They are used as solvents, reagents in organic synthesis, intermediates in pharmaceuticals and agrochemicals, and as monomers or intermediates in polymer chemistry. Environmental and health considerations are important, as some haloalkanes are toxic or persistent pollutants and certain industrial halogenated solvents are restricted due to regulatory concerns.