Nucleophilicity bromide reaction

The sulfur atom of the thiocarbonyl group is a good nucleophile, and reaction between benzyl bromide and l-(2-thiazolyl)thiourea yields the isothiouronium salt (496). The sulfur atom may also be engaged in a chelate, as exemplified by the Cu chelate of 2-thioureido-4-methylthiazole (491). These chelates with metal ions were thoroughly studied in acidic, neutral, and alkaline media for 66 metal ions in order to define their analytical use. They are formed in the molar ratio of 1 2 for metal II compounds (498). 

Primary carbocations are so high m energy that their intermediacy m nucleophilic substitution reactions is unlikely When ethyl bromide undergoes hydrolysis m aqueous formic acid substitution probably takes place by an 8 2 like process m which water is the nucleophile... 

Methyl bromide slowly hydrolyzes in water, forming methanol and hydrobromic acid. The bromine atom of methyl bromide is an excellent leaving group in nucleophilic substitution reactions and is displaced by a variety of nucleophiles. Thus methyl bromide is useful in a variety of methylation reactions, such as the syntheses of ethers, sulfides, esters, and amines. Tertiary amines are methylated by methyl bromide to form quaternary ammonium bromides, some of which are active as microbicides. 

Rates of debromination of bromonitro-thiophenes and -selenophenes with sodium thio-phenoxide and sodium selenophenoxide have been studied. Selenophene compounds were about four times more reactive than the corresponding thiophene derivatives. The rate ratio was not significantly different whether attack was occurring at the a- or /3-position. As in benzenoid chemistry, numerous nucleophilic displacement reactions are found to be copper catalyzed. Illustrative of these reactions is the displacement of bromide from 3-bromothiophene-2-carboxylic acid and 3-bromothiophene-4-carboxylic acid by active methylene compounds (e.g. AcCH2C02Et) in the presence of copper and sodium ethoxide (Scheme 77) (75JCS(P1)1390). 

A mechanism of this type permits substitution of certain aromatic and ahphatic nitro compounds by a variety of nucleophiles. These reactions were discovered as the result of efforts to explain the mechanistic basis for high-yield carbon alkylation of the 2-nitropropane anion by p-nitrobenzyl chloride. p-Nitrobenzyl bromide and iodide and benzyl halides that do not contain a nitro substituent give mainly the unstable oxygen alkylation product with this ambident anion ... 

Another approach is therefore to adopt a model process that is very similar to the reactions of interest. Swain and Scott ° selected as a standard reaction the nucleophilic substitution reaction of methyl bromide in water at 25°C. 

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

We shall now examine some applications of these ideas. Experiments were carried out on the reaction between isopropyl bromide and ethoxide ions. This process consists of competing elimination and nucleophilic substitution reactions,19... 

So the tertiary halide reacts by a different mechanism, which we call SnI- It s still a nucleophilic substitution reaction (hence the S and the N ) but this time it is a unimolecular reaction, hence the 1 . The rate-determining step during reaction is the slow unimolecular dissociation of the alkyl halide to form a bromide ion and a carbocation that is planar around the reacting carbon. 

To check this possibility we performed experiments with different concentrations of NaBr in the NaY zeolite. Table 2 presents the results. It can be seen that upon increasing the amount of NaBr impregnated on NaY, there is preference to formation of the cyclobutyl bromide over allylcarbinyl bromide, indicating that the relative position between the bromide ions and bicyclobutonium governs the product distribution. Hence, zeolites may act as solid solvent, favoring ionization of alkyl halides and nucleophilic substitution reactions. In contrast to liquid solvents, where solvation is mostly uniform, the zeolite surface seems to provide unsymmetrical solvation of the cations, leading to product distribution that is different from solution. 

In the absence of a nucleophile, the reaction of allenes with aryl bromides provides 1,3-dienes in good yield (Scheme 16.27) [32], The reaction is very sensitive to the reaction temperature, solvent, base and amount of phosphine used. The formation of a 1,3-diene may be explained by either /3-hydrogen elimination or deprotonation at the a-carbon of the Jt-allylpalladium intermediate. 

Quaternary ammonium salts are also known to promote nucleophilic substitution reactions in two-phase systems through the formation of micelles [15], but there is no evidence for micellar formation by bulky ammonium salts, such as tetra-n-buty-lammonium bromide, under liquidrliquid two-phase conditions [16]. 

From Chapter 7 it is apparent that the trichloromethyl anion is formed under basic conditions from chloroform, as a precursor of the carbene. The anion can also react with Jt-deficient alkenes (see Section 7.3) and participate in nucleophilic substitution reactions, e.g. 1,1-diacyloxy compounds are converted into 1,1,1-trichloroalkan-2-ols [58] (Scheme 6.35). Similarly, benzyl bromides are converted into (2-bromoethynyl)arenes via an initial nucleophilic displacement followed by elimination of hydrogen bromide [59] (Scheme 6.35). 

The rate-determining step in this process is the oxidative addition of methyl iodide to 1. Within the operating window of the process the reaction rate is independent of the carbon monoxide pressure and independent of the concentration of methanol. The methyl species 2 formed in reaction (2) cannot be observed under the reaction conditions. The methyl iodide intermediate enables the formation of a methyl rhodium complex methanol is not sufficiently electrophilic to carry out this reaction. As for other nucleophiles, the reaction is much slower with methyl bromide or methyl chloride as the catalyst component. 

Consider a proposed nucleophilic substitution reaction on the secondary alcohol shown using aqueous HBr. As a secondary alcohol, either Sn2 or SnI mechanisms are possible (see Section 6.2.3), but SnI is favoured because of the acidic environment and the large fert-butyl group hindering approach of the nucleophile. The expected SnI bromide product is formed, together with a smaller amount of the El-derived alkene in a competing reaction. 

Kenawy 64) immobilized ammonium and phosphonium peripheral functionalized dendritic branches on a montmorillonite supported chloromethylstyrene/methyl methacrylate copolymer (74-75). These polymer/montmorillonite-supported dendrimers were used as phase transfer catalysts (PTC) for the nucleophilic substitution reaction between -butyl bromide and thiocyanate, cyanide, and nitrite anions in a toluene or a benzene/water system. These PT catalysts could be recycled by filtration of the functionalized montmorillonite from the reaction mixture. Generally,... 

Pyridyl-phosphorus and -arsenic compounds have also been made by nucleophilic displacement reactions with, for example, 3-pyridinediazonium salts (74HC(14-2)489). Organomercury derivatives can be converted into bromides and iodides by standard methods, e.g. Scheme 147 (59JPR(8)156). 

Not all nucleophilic displacement reactions require lightly substituted onium ion catalysts for activity. For alkylation of 2-naphthoxide ion with benzyl bromide (Eq. (6)) 40-100% RS, 2% CL polystyrene catalysts 15 and 16 work well54). A 51 % RS catalyst 11 gave good yields in reactions of anionic oxygen and sulfur nucleophiles with alkyl halides 91). 

Consider the reversible transformation of the soil fumigant methyl bromide (CH3Br) to methyl chloride (CH3C1) in aqueous solution (a nucleophilic substitution reaction, see Chapter 13) ... 

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