Haloalkanes and Haloarenes Notes

Overview

Haloalkanes and haloarenes are organic compounds in which one or more hydrogen atoms of aliphatic or aromatic hydrocarbons are replaced by halogens. This chapter connects structure, polarity, preparation, physical properties, reactions, mechanisms and environmental effects. Haloalkanes mainly undergo nucleophilic substitution and elimination because the carbon-halogen bond is polar and carbon is electrophilic. Haloarenes are less reactive towards nucleophilic substitution due to resonance and partial double-bond character of the C-X bond, but they undergo electrophilic substitution at ortho and para positions. NCERT and NEET repeatedly test classification, IUPAC naming, order of reactivity, SN1/SN2 mechanisms, boiling point trends, Grignard reagent formation, and environmental issues caused by chloroform, carbon tetrachloride, freons and DDT.

Overview

Haloalkanes and haloarenes are classified by the number of halogen atoms, the type of carbon attached to halogen, and the structure of the carbon skeleton. Haloalkanes may be primary, secondary or tertiary depending on whether the halogen-bearing carbon is attached to one, two or three carbons. Haloarenes have halogen directly attached to an aromatic ring, while aralkyl halides have halogen in the side chain. In IUPAC nomenclature, halogens are treated as prefixes: fluoro, chloro, bromo and iodo. The carbon-halogen bond is polar because halogens are more electronegative than carbon. Bond strength decreases from C-F to C-I, which strongly affects reactivity.

Overview

Haloalkanes are prepared mainly from alcohols, alkanes, alkenes and halide exchange reactions. Alcohols form alkyl chlorides or bromides using concentrated hydrogen halides, phosphorus halides or thionyl chloride. Alkanes undergo free-radical halogenation, while alkenes add hydrogen halides or halogens to give alkyl halides and vicinal dihalides. Halide exchange reactions such as Finkelstein and Swarts are important for preparing iodides and fluorides. Haloarenes are prepared by direct halogenation of benzene in the presence of Lewis acids and by Sandmeyer or Gattermann reactions from diazonium salts. NEET commonly asks reagent choice, Markovnikov addition, allylic or benzylic halogenation and why aryl halides cannot be prepared easily from phenols using HX.

Overview

Physical properties of haloalkanes and haloarenes are controlled by molecular mass, polarity, surface area, symmetry and intermolecular forces. Lower alkyl halides such as methyl chloride, methyl bromide and ethyl chloride are gases, while higher members are liquids or solids. Boiling points increase with increasing molecular mass and halogen size due to stronger van der Waals forces. For isomeric haloalkanes, branching lowers boiling point by reducing surface area. Haloalkanes are only slightly soluble in water because they cannot form strong hydrogen bonds with water, although they dissolve well in organic solvents. Many bromides and iodides are denser than water. Melting point depends strongly on molecular symmetry; para isomers often melt higher than ortho and meta isomers.

Overview

Haloalkanes are highly important reaction intermediates because the polar C-X bond allows attack by nucleophiles and removal of halide as a leaving group. In nucleophilic substitution, SN2 occurs in a single step with backside attack and inversion of configuration, while SN1 occurs in two steps through a carbocation and can cause racemization. Strong bases and heat favour elimination, producing alkenes through E1 or E2 pathways. Haloalkanes also form organometallic compounds such as Grignard reagents. Haloarenes are much less reactive toward nucleophilic substitution because resonance gives partial double-bond character to the C-X bond, but they undergo electrophilic substitution at ortho and para positions. NEET focuses on mechanism choice, stereochemical outcome, reactivity order and reagent-product prediction.

Overview

Polyhalogen compounds contain more than one halogen atom and include important industrial chemicals such as dichloromethane, chloroform, carbon tetrachloride, freons and DDT. Dichloromethane is used as a solvent and paint remover but can harm the nervous system. Chloroform was used as an anaesthetic but is toxic and slowly oxidizes to poisonous phosgene in air and light. Carbon tetrachloride was used in fire extinguishers and cleaning but causes liver damage and contributes to ozone depletion. Freons are chlorofluorocarbons formerly used as refrigerants and aerosol propellants; they release chlorine radicals that destroy stratospheric ozone. DDT is an insecticide that helped control malaria but is persistent, bioaccumulative and harmful to ecosystems.