Class 12 ncert chemistry haloalkanes and haloarenes

• The replacement of hydrogen atom(s) in an aliphatic or aromatic hydrocarbon by halogen atom(s) results in the formation of alkyl halide (haloalkane) and aryl halide (haloarene), respectively
• Haloalkanes contain halogen atom(s) attached to the sp3 hybridised carbon atom of an alkyl group
• Haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group
• Chlorine containing antibiotic, chloramphenicol, is produced by microorganisms and is effective for the treatment of typhoid fever
• Our body produces iodine containing hormone, thyroxine, the deficiency of which causes a disease called goiter
• Synthetic halogen compounds like chloroquine are used for the treatment of malaria; halothane is used as an anaesthetic during surgery
• Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery
• Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria
• Alkyl halides are further classified as primary, secondary, or tertiary according to the nature of carbon to which halogen is attached
• Allylic halides have the halogen atom bonded to an sp3-hybridised carbon atom adjacent to a carbon-carbon double bond (C=C)
• Benzylic halides have the halogen atom bonded to an sp3-hybridised carbon atom attached to an aromatic ring
• Possible monochloro structural isomers expected to be formed on free radical monochlorination of (CH3)2CHCH2CH3:

  • (CH3)2CHCH2CH2Cl
  • (CH3)2CHCH(Cl)CH3
  • (CH3)2C(Cl)CH2CH3
  • CH3CH(CH2Cl)CH2CH3
• Alkenes can be converted to corresponding alkyl halides by:

  • Addition of hydrogen halides (HCl, HBr, HI)
  • Addition of halogens (e.g. bromine in CCl4)
  • Finkelstein reaction for alkyl iodides
  • Swarts reaction for alkyl fluorides
• Aryl chlorides and bromides can be easily prepared by electrophilic substitution of arenes with chlorine and bromine respectively in the presence of Lewis acid catalysts like iron or iron(III) chloride

  • Reactions with iodine are reversible and require an oxidising agent (HNO3, HIO4)
  • Fluoro compounds are not prepared due to high reactivity of fluorine
• Primary aromatic amines can be converted to aryl halides through Sandmeyer's reaction:

  • Dissolving or suspending the amine in cold aqueous mineral acid
  • Treating with sodium nitrite to form a diazonium salt
  • Mixing the diazonium salt with cuprous chloride or cuprous bromide to replace the diazonium group by -Cl or -Br
  • Diazonium group can be replaced by iodine by shaking with potassium iodide
• Haloalkanes are colourless when pure

  • Bromides and iodides develop colour when exposed to light
  • Many volatile halogen compounds have a sweet smell
• Haloalkanes are very slightly soluble in water due to the need to overcome attractions between haloalkane molecules and break hydrogen bonds in water

  • Haloalkanes tend to dissolve in organic solvents due to similar strength of intermolecular attractions
• Density of bromo, iodo, and polychloro derivatives of hydrocarbons increases with the number of carbon atoms, halogen atoms, and atomic mass of the halogen atoms

  • Haloalkanes reactions can be categorised into nucleophilic substitution, elimination reactions, and reactions with metals
• Nucleophilic substitution reactions involve a nucleophile replacing a leaving group in a haloalkane

  • Nucleophiles can act through different linkage points, resulting in various products
  • Ambident nucleophiles like cyanides and nitrites can form different products based on the linkage point
• Bimolecular nucleophilic substitution (SN2) reaction involves the interaction of an incoming nucleophile with an alkyl halide, causing the carbon-halide bond to break and a new bond to form between carbon and the attacking nucleophile
• Edward Davies Hughes and Sir Christopher Ingold proposed a mechanism for an SN2 reaction in 1937
• In an SN2 reaction, the configuration of the carbon atom under attack inverts during the transition state, resulting in unstable structures that cannot be isolated
• Steric hindrance from bulky substituents near the carbon atom dramatically inhibits SN2 reactions
• Methyl halides react most rapidly in SN2 reactions due to having only three small hydrogen atoms, while tertiary halides are the least reactive
• Substitution Nucleophilic Unimolecular (SN1) reactions are generally carried out in polar protic solvents like water, alcohol, and acetic acid
• SN1 reactions occur in two steps, with the slowest and reversible step involving the cleavage of the C-Br bond to produce a carbocation and a bromide ion
• The rate of SN1 reactions depends on the concentration of the alkyl halide and not on the concentration of the nucleophile
• Allylic and benzylic halides show high reactivity towards SN1 reactions due to the stabilisation of the carbocation through resonance
• The reactivity order of alkyl halides towards SN1 and SN2 reactions is: Primary halide > Secondary halide > Tertiary halide
• Optically active compounds rotate the plane of plane-polarised light, with dextrorotatory compounds rotating it to the right and laevo-rotatory compounds rotating it to the left
• Chiral molecules are optically active, while achiral molecules are optically inactive
• Propan-2-ol is an achiral molecule because it does not contain an asymmetric carbon, as all four groups attached to the tetrahedral carbon are not different
• When the mirror image of propan-2-ol is rotated by 180° and overlapped with the original structure, they completely overlap
• Butan-2-ol is a chiral molecule because it has four different groups attached to the tetrahedral carbon
• Enantiomers are stereoisomers that are non-superimposable mirror images of each other
• Enantiomers possess identical physical properties such as melting point, boiling point, and refractive index, but differ in the rotation of plane polarised light
• A racemic mixture contains equal proportions of two enantiomers, resulting in zero optical rotation
• Racemisation is the process of converting an enantiomer into a racemic mixture
• In a reaction at an asymmetric carbon atom, retention of configuration preserves the spatial arrangement of bonds, inversion changes the configuration, and racemisation results in an optically inactive product
• In S N 2 reactions of optically active alkyl halides, the product has an inverted configuration compared to the reactant