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The halogens (ˈhælədʒən,ˈheɪ-,-loʊ-,_-ˌdʒɛn) are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At), and tennessine (Ts), though some authors would exclude tennessine as its chemistry is unknown and is theoretically expected to be more like that of gallium. In the modern IUPAC nomenclature, this group is known as group (XVII) or group (VII). The word "halogen" means "salt former" or "salt maker". When halogens react with metals, they produce a wide range of salts, including calcium fluoride, sodium chloride (common table salt), silver bromide and potassium iodide. The group of halogens is the only periodic table group that contains elements in three of the main states of matter at standard temperature and pressure, though not far above room temperature the same becomes true of groups (I) and 15, assuming white phosphorus is taken as the standard state. All of the halogens form acids whe

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2,4-Difluoro-, 2,4,6-trifluoro-, and 2,3,4,6-tetra(fluoro)pyridine undergo nucleophilic substitution preferentially if not exclusively at the 4-position. However, after the introduction of a trialkylsilyl group at C-3 or C-5, the halogen at the 6-(2-)position is displaced selectively. Bulky trialkylsilyl groups were found to suppress the nucleofugal mobility of neighboring halogen atoms. This synthetically valuable regiocontrol can also be realized with other halopyridines such as 2,4-dichloro- and 2,4,6-tri(chloro)pyridine. [on SciFinder (R)]

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