Haloarenes are the chemical compounds containing arenes, where one or more hydrogen atoms bonded to an aromatic ring are replaced with halogens. The nature of C-X bond depends on both the nature of carbon in the aromatic ring and the halogen attached. Halogens are generally denoted by “X”. As we know halogens are group 17 elements having high electronegativity namely, fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Out of them, Fluorine has the highest electronegativity. The elements in this group are just one electron short of completing their nearest noble gas configuration.
Carbon in haloarenes is a 14th group element with comparatively lesser electronegativity in comparison to halogen molecules. This is due to the fact that electronegativity increase across a period from left to right.
Salient points on the nature of C-X bond in haloarenes are:
- The C-X bond in haloarenes is polarized, as halogens are more electronegative than carbon. Due to high electronegativity of halogen, it attracts the electron cloud more towards itself and thus gains slight negative charge, on the other hand, carbon obtains a slight positive charge.
- As halogens need only one electron to achieve their nearest noble gas configuration, only one sigma bond is formed between one carbon and one halogen atom.
- Due to the increase in atomic size from fluorine to astatine, the C-X bond length in haloarenes increases from fluorine to astatine and bond dissociation strength decreases.
- Dipole moment depends on the difference in electronegativity of carbon and halogens and as we know that the electronegativity of halogens decreases down the group, the dipole moment also decreases. There is an exception of C-Cl and C-F dipole moments. Though the electronegativity of Cl is less than F, the dipole moment of a C-Cl bond is more than C-F.
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