Chemistry

• 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.
• Halogen-containing organic compounds have various applications in industry and daily life, serving as solvents and starting materials for the synthesis of organic compounds.
• 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.
• Fully fluorinated compounds are being considered as potential blood substitutes in surgery.
• The 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 from their given structures.
• Students will be able to describe the reactions involved in the preparation of haloalkanes and haloarenes and understand various reactions that they undergo.
• Students will be able to correlate the structures of haloalkanes and haloarenes with various types of reactions.
• Stereochemistry will be used as a tool for understanding the reaction mechanism.
• The unit will also highlight the applications of organo-metallic compounds and 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 alkyl groups with a halogen atom bonded to them, and they are further classified as primary, secondary, or tertiary based on the carbon atom to which the halogen is attached.
• Primary alkyl halide or 1° alkyl halide refers to an alkyl halide with a halogen attached to a primary carbon atom.
• Secondary alkyl halide or 2° alkyl halide refers to an alkyl halide with a halogen attached to a secondary carbon atom.
• Tertiary alkyl halide or 3° alkyl halide refers to an alkyl halide with a halogen attached to a tertiary carbon atom.
• Allylic halides are compounds where the halogen atom is bonded to an sp3-hybridized carbon atom adjacent to a carbon-carbon double bond (C=C).
• Benzylic halides are compounds where the halogen atom is bonded to an sp3-hybridized carbon atom attached to an aromatic ring.
• Vinylic halides are compounds where the halogen atom is bonded to an sp2-hybridized carbon atom of a carbon-carbon double bond (C=C).
• Aryl halides are compounds where the halogen atom is bonded to an aromatic ring.
• Common names of alkyl halides are derived by naming the alkyl group followed by the name of 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, but in the IUPAC system, numerals 1,2; 1,3; and 1,4 are used.
• 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.
• Gem-dihalides are named as alkylidene halides in the common name system and as dihaloalkanes in the IUPAC system.
• Vic-dihalides are named as alkylene dihalides in the common name system and as dihaloalkanes in the IUPAC system.
• Examples of common and IUPAC names of some halides are provided in Table 6.1.
• The structure, common name, and IUPAC name of various halides are given in the text.
• The size of halogen atoms increases from fluorine (smallest) to iodine (largest), resulting in an increase in the carbon-halogen bond length from C-F to C-I.
• Alkyl halides are best prepared from easily accessible alcohols.
• Some typical bond lengths, bond enthalpies, and dipole moments are given in Table 6.2.
• The IUPAC names of the following compounds are: (i) 4-Bromopent-2-ene, (ii) 3-Bromo-2-methylbut-1-ene, (iii) 4-Bromo-3-methylpent-2-ene, (iv) 1-Bromo-2-methylbut-2-ene, (v) 1-Bromobut-2-ene, (vi) 3-Bromo-2-methylpropene.
• CH3-F has a bond length of 139 pm, a bond enthalpy of 452 kJmol-1, and a dipole moment of 1.847 Debye.
• CH3-Cl has a bond length of 178 pm, a bond enthalpy of 351 kJmol-1, and a dipole moment of 1.860 Debye.
• CH3-Br has a bond length of 193 pm, a bond enthalpy of 293 kJmol-1, and a dipole moment of 1.830 Debye.
• CH3-I has a bond length of 214 pm, a bond enthalpy of 234 kJmol-1, and a dipole moment of 1.636 Debye.
• Alkyl chlorides can be prepared by passing dry hydrogen chloride gas through a solution of alcohol or by heating a mixture of alcohol and concentrated aqueous halogen acid.