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Halogen substituents are an exception: they are resonance donors (+M). With the exception of the halides, they are meta directing groups. Halides are ortho, para directing groups but unlike most ortho, para directors, halides mildly deactivate the arene. This unusual behavior can be explained by two properties:
Groups that are electron-withdrawing by resonance decrease the electron density especially at positions 2, 4 and 6, leaving positions 3 and 5 as the ones with comparably higher reactivity, so these types of groups are meta directors (see below). Halogens are electronegative, so they are deactivating by induction, but they have lone pairs, so ...
The ortho to para selectivity is low: [7] No reaction takes place when the solvent is replaced by tetrachloromethane. In contrast, when the reactant is 2-phenylethylamine, it is possible to employ relatively apolar solvents with exclusive ortho-regioselectivity due to the intermediate formation of a chloramine, enabling the Intramolecular reaction.
The halogens (/ ˈ h æ l ə dʒ ə n, ˈ h eɪ-,-l oʊ-,-ˌ dʒ ɛ n / [1] [2] [3]) are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors [4] would exclude tennessine as its chemistry is unknown and is theoretically expected to ...
The ortho effect also occurs when a meta-directing group is positioned in a meta arrangement relative to an ortho–para-directing group, a new substituent introduced into the molecule tends to preferentially occupy the ortho position relative to the meta-directing group rather than the para position.
Acyl substitution occurs when a nucleophile attacks a carbon that is doubly bonded to one oxygen and singly bonded to another oxygen (can be N or S or a halogen), called an acyl group. The nucleophile attacks the carbon causing the double bond to break into a single bond. The double can then reform, kicking off the leaving group in the process.
In organic chemistry, a directing group (DG) is a substituent on a molecule or ion that facilitates reactions by interacting with a reagent.The term is usually applied to C–H activation of hydrocarbons, where it is defined as a "coordinating moiety (an 'internal ligand'), which directs a metal catalyst into the proximity of a certain C–H bond."
Because electron donating groups are both ortho and para directors, separation of these isomers is a common problem in synthetic chemistry. Several methods exist in order to separate these isomers: Column chromatography will often separate these isomers, as the ortho is more polar than the para in general.