Reactions a-halogenation

In a /3-elimination (dehydrohalogenation) reaction a halogen and a hydrogen atom are removed from adjacent carbon atoms to form a double bond between the two C s. The reagent commonly used to remove HX is the strong base KOH in ethanol (cf. Section 6.2). 

Krohnke reactions. A halogen compound reacts with pyridine in ethanol to form a pyridinium salt (1), which is treated with p-nitroso-N,N-dimethylaniline and alkali to produce a nitrone (2), which on acid hydrolysis affords an aldehyde. 

The Meerwein reaction was first observed for <%,/ -unsaturated carbonyl compounds.241 In these cases, and for the corresponding nitriles, halogen (X) appears in the -position to a group R1 if the unsaturated compound had an alkyl group in the -position to R1 (reaction a) halogen (X) appeared in the... 

In addition to the several reports of intermolecular Fujiwara-Moritani reactions, there have been a nnmber of examples of intramolecular reactions, both stoichiometric and catalytic in palladinm. In considering an intramolecular oxidative Heck reaction, one can again draw a direct analogy to the classical Heck reaction (Figure 9.3). In the standard Heck reaction, a halogenated arene nndergoes an oxidative addition by palladium(0), followed by alkene insertion and jS-hydride elimination. In an oxidative version, a C—H bond of... 

In these reactions a halogen atom replaces one or more of the hydrogen atoms of the alkane and the corresponding hydrogen halide is formed as a by-product. Only fluorine, chlorine, and bromine react this way with alkanes. Iodine is essentially unreactive, for a reason that we shall explain later. 

Gattermann s reaction A variation of the Sandmeyer reaction copper powder and hydrogen halide are allowed to react with the diazonium salt solution and halogen is introduced into the aromatic nucleus in place of an amino group. 

Many of the reactions of halogens can be considered as either oxidation or displacement reactions the redox potentials (Table 11.2) give a clear indication of their relative oxidising power in aqueous solution. Fluorine, chlorine and bromine have the ability to displace hydrogen from hydrocarbons, but in addition each halogen is able to displace other elements which are less electronegative than itself. Thus fluorine can displace all the other halogens from both ionic and covalent compounds, for example... 

If cold benzene is treated with bromine in the absence of sunlight, very little reaction occurs if, however, a halogen carrier, such as iron, iodine, pyridine, etc., is also present, a rapid reaction by substitution occurs, forming first... 

A halogen atom directly attached to a benzene ring is usually unreactive, unless it is activated by the nature and position of certain other substituent groups. It has been show n by Ullmann, however, that halogen atoms normally of low reactivity will condense with aromatic amines in the presence of an alkali carbonate (to absorb the hydrogen halide formed) and a trace of copper powder or oxide to act as a catalyst. This reaction, known as the Ullmant Condensation, is frequently used to prepare substituted diphenylamines it is exemplified... 

This preparation illustrates the Reformatsky reaction, which consists in the interaction of a carbonyl compound, an a-halogen ester (e.g., ethyl bromo-acetate) and zinc In the presence of ether or benzene, followed by hydrolysis. 

It is probable that the reaction proceeds through an organic zinc derivative, analogous to a Grignard reagent, formed by interaction of the a-halogen ester... 

The direct introduction of a halogen atom (usually bromine) by means of V-haloamine (generally iV-bromosuccinimide) in the allyl position is known as the Wohl-Ziegler reaction ... 

Many organic halides do not react satisfactorily with lithium to form RLi ecMnpounds or with metallic magnesium to form Grignard reagents. The desired organolithium compound can often be prepared by a halogen-metal interconversion reaction ... 

The conversion of indoles to oxindoles can be achieved in several ways. Reaction of indoles with a halogenaling agent such as NCS, NBS or pyridin-ium bromide perbromide in hydroxylic solvents leads to oxindoles[l]. The reaction proceeds by nucleophilic addition to a 3-haloindolenium intermediate. 

Thus a second method was envisaged, the reaction of a nitrile, hydrogen selenide, and an a-halogenated ketone in the presence of a condensation catalyst, which can be POCl, or POCI3 with a Lewis acid such as PCI3 or anhydrous ZnCl. The use of fresh AICI3 leads to the formation of tarry side-products. 

The replacement of selenoamide by selenourea in the Hantzsch s synthesis. (1st method) leads to 2-aminoselenazoles 2, 14. 15). This series of compounds has been well developed, mainlv because selenourea is much more easily accessible than the selenoamides, but also because a wide variety of a-halogenated carbonyl compounds are available for the Hantzsch s evdization reaction (Scheme 5). 2-Aminoselenazole itself was prepared from commercially available chloroacetaldehyde semihydrate... 

The synthesis of a-aminoketones can be achieved using a-halogenated ketones as starting material. These latter are converted into the hexamethylene tetraminium salts by the method of Mannich and Hahn (42). This reaction proceeds in two steps ... 

The reaction of an alcohol with a hydrogen halide is a substitution A halogen usually chlorine or bromine replaces a hydroxyl group as a substituent on carbon Calling the reaction a substitution tells us the relationship between the organic reactant and its prod uct but does not reveal the mechanism In developing a mechanistic picture for a par ticular reaction we combine some basic principles of chemical reactivity with experi mental observations to deduce the most likely sequence of steps... 

The elementary steps (1) through (3) describe a free radical chain mech anism for the reaction of an alkane with a halogen... 

The mechanism of the haloform reaction begins with a halogenation via the eno late The electron attracting effect of an a halogen increases the acidity of the protons on the carbon to which it is bonded making each subsequent halogenation at that car bon faster than the preceding one... 

The Hell-Volhard-Zehnsky reaction is of synthetic value m that the a halogen can be displaced by nucleophilic substitution... 

Hell-Volhard-Zelmsky reaction (Section 19 16) The phos phorus tnhahde catalyzed a halogenation of a carboxylic acid... 

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