Haloalkanes and Haloarenes

• Halogen-containing organic compounds have various applications in industry and daily life, serving as solvents and starting materials for the synthesis of organic compounds.
• 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, while haloarenes contain halogen atom(s) attached to sp2 hybridised carbon atom(s) of an aryl group.
• Examples of clinically useful halogen-containing compounds include the chlorine-containing antibiotic chloramphenicol for treating typhoid fever and the iodine-containing hormone thyroxine produced by the body.
• Halothane is used as an anaesthetic during surgery.
• Certain fully fluorinated compounds are being considered as potential blood substitutes in surgery.
• This unit covers the methods of preparation, physical and chemical properties, and uses of organohalogen compounds.
• After studying this unit, students will be able to name haloalkanes and haloarenes according to the IUPAC system of nomenclature, describe their preparation and reactions, understand the correlation between their structures and reactions, use stereochemistry to understand reaction mechanisms, appreciate the applications of organometallic compounds, and highlight the environmental effects of polyhalogen compounds.
• Halogenated compounds persist in the environment due to their resistance to breakdown by soil bacteria.
• Haloalkanes and haloarenes may be classified as mono, di, or polyhalogen compounds depending on the number of halogen atoms in their structures.
• Alkyl halides or haloalkanes (R—X) are a class of haloalkanes where the halogen atom is bonded to an alkyl group (R).
• Alkyl halides can be further classified as primary, secondary, or tertiary based on the nature of the carbon atom to which the halogen is attached.
• Primary alkyl halide or 1° alkyl halide: Halogen is attached to a primary carbon atom.
• Secondary alkyl halide or 2° alkyl halide: Halogen is attached to a secondary carbon atom.
• Tertiary alkyl halide or 3° alkyl halide: Halogen is attached to a tertiary carbon atom.
• Allylic halides: Halogen is bonded to an sp3-hybridized carbon atom adjacent to a carbon-carbon double bond (C=C).
• Benzylic halides: Halogen is bonded to an sp3-hybridized carbon atom attached to an aromatic ring.
• Vinylic halides: Halogen is bonded to an sp2-hybridized carbon atom of a carbon-carbon double bond (C=C).
• Aryl halides: Halogen is bonded to an sp2-hybridized carbon atom attached to an aromatic ring.
• Common names of alkyl halides are derived by naming the alkyl group followed by the name of the halide.
• In the IUPAC system, alkyl halides are named as halosubstituted hydrocarbons.
• For mono halogen substituted derivatives of benzene, common and IUPAC names are the same.
• For dihalogen derivatives, the prefixes "o-", "m-", "p-" are used in the common system, while the numerals 1,2; 1,3; and 1,4 are used in the IUPAC system.
• Dihaloalkanes with the same type of halogen atoms are named as alkylidene or alkylene dihalides.
• Dihalo-compounds with both halogen atoms on the same carbon atom are classified as geminal halides or gem-dihalides.
• In the common name system, gem-dihalides are named as alkylidene halides and vic-dihalides are named as alkylene dihalides.
• In the IUPAC system, gem-dihalides and vic-dihalides are named as dihaloalkanes.
• Examples of common and IUPAC names of some halides are provided in Table 6.1.
• Example 6.1 provides a solution for a specific case related to halides.
• 1-Bromopentane (1o)
• 2-Bromopentane (2o)
• 3-Bromopentane (2o)
• 1-Bromo-3-methylbutane (1o)
• The size of halogen atoms increases from fluorine to iodine, with fluorine being the smallest and iodine being the largest.
• Consequently, the carbon-halogen bond length also increases from C-F to C-I.
• Alkyl halides are best prepared from alcohols.
• Some typical bond lengths, bond enthalpies, and dipole moments are given in Table 6.2.
• IUPAC names of compounds are provided, such as 4-Bromopent-2-ene, 3-Bromo-2-methylbut-1-ene, etc.
• Example 6.2 provides a solution to a specific problem related to hydrocarbons.
• CH3-F has a bond length of 139 pm, a bond enthalpy of 452 kJmol-1, and a dipole moment of 1.847 Debye.