Halogenalkanen

Halogenalkanen, or haloalkanes, are organic compounds in which one or more hydrogen atoms of an alkane are replaced by a halogen (fluorine, chlorine, bromine or iodine). They are commonly represented by the formula R-X, where R is an alkyl group and X is a halogen. Depending on the number of halogen substituents, they are mono-, di- or polyhalogenated; and depending on substitution, primary, secondary or tertiary halides are defined. Haloalkanes include fluoro-, chloro-, bromo- and iodoalkanes, with properties governed by the C–X bond polarity and the size of X.

Physical properties vary with the halogen. Heavier halogens raise boiling points and increase polarity, giving haloalkanes that are liquids or dense gases at room temperature. They are often relatively nonpolar but possess a polar C–X bond, making them reactive toward nucleophiles. Reactions of haloalkanes include nucleophilic substitution (SN1 and SN2) and elimination (E1 and E2), enabling conversion to alcohols, alkenes and other products. SN2 is favored by primary substrates and strong nucleophiles; SN1 and E1 by tertiary substrates and weak bases.

Synthesis and nomenclature: Haloalkanes are prepared by halogenation of alkanes under radical conditions, hydrohalogenation of alkenes, or substitution of alcohols with reagents such as thionyl chloride (SOCl2) or phosphorus tribromide (PBr3). Fluorination is more challenging and often requires specialized methods. IUPAC naming uses the alkyl group as the substituent and the halogen as the functional substituent: e.g., chloromethane, 1,2-dichloroethane.

Applications and hazards: Haloalkanes are used as solvents (dichloromethane, chloroform) and as intermediates in pharmaceuticals and agrochemicals. Some are refrigerants or propellants; many have been restricted due to ozone depletion or climate impact. They can be toxic and some are carcinogenic; several are persistent environmental pollutants. Safe handling, ventilation and regulatory compliance are important.