Halides from aromatic halogenation

The old method632 of making acid halides from aromatic aldehydes and a halogen can be of preparative value when the aldehyde is more accessible than the carboxylic acid for instance, about 80% of o-chlorobenzoyl chloride is obtained when chlorine is led for about 15 h into vigorously stirred molten o-chlorobenzaldehyde (1 mole) at 140-160° (for details see Organic Syntheses).633... 

Pyrimidinyl halides are not only precursors for Pd-catalyzed reactions, but also important pharmaceuticals in their own right. One of the most frequently employed approaches for halopyrimidine synthesis is direct halogenation. When pyrimidinium hydrochloride and 2-aminopyrimidine were treated with bromine, 5-bromopyrimidine and 2-amino-5-bromopyrimidine were obtained, respectively, via an addition-elimination process instead of an aromatic electrophilic substitution [4, 5], Analogously, 2-chloro-5-bromopyrimidine (1) was generated from direct halogenation of 2-hydroxypyrimidine [6], Treating 1 with HI then gave to 2-iodo-5-bromopyrimidine (2). In the preparation of 5-bromo-4,6-dimethoxypyrimidine (4), N-bromosuccinimide was found to be superior to bromine for the bromination of 4,6-dimethoxypyrimidine (3) [7]. 

Several research groups ha ve been involved in the study of ET reactions from an electrochemically generated aromatic radical anion to alkyl halides in order to describe the dichotomy between ET and polar substitution (SN2). The mechanism for indirect reduction of alkyl halides by aromatic mediators has been described in several papers. For all aliphatic alkyl halides and most benzylic halides the cleavage of the carbon-halogen bond takes place concertedly with the... 

Aryl halides are distinguished from aromatic hydrocarbons by the presence of halogen, as shown by elemental analysis. Aryl halides are distinguished from most alkyl halides by their inertness toward silver nitrate in this respect they resemble vinyl halides (Sec. 25.5). 

Direct elimination of halide from the ring may occur by the hydrolytic, reductive, or decarboxylative displacement of halogen the products then undergo oxidation followed by disruption of the aromatic ring. 

There are three major classes of organohalogen compounds the alkyl halides, the vinyl halides, and the aryl halides. An alkyl halide simply has a halogen atom bonded to one of the sp hybrid carbon atoms of an alkyl group. A vinyl halide has a halogen atom bonded to one of the sp hybrid carbon atoms of an alkene. An aryl halide has a halogen atom bonded to one of the sp hybrid carbon atoms of an aromatic ring. The chemistry of vinyl halides and aryl halides is different from that of alkyl halides because their bonding and hybridization are different. We consider the reactions of vinyl halides and aryl halides in later chapters. The structures of some rqjresentative alkyl halides, vinyl halides, and aryl halides are shown here, with their most common names and uses. 

Aromatic methyl ketones can be halogenated at the a-position with Oxone and sodium halide, however, conpeting halogenation of the aromatic ring is significant. Q, o -Dichloroketones can be synthesized from alkynes by reaction with Oxone in HCl/DMF (eq 83). Oxone consistently gave better results than MCPBA for this transformation. 

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