Bromoalkane, formation

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Method A The bromoalkane (0.39 mol) is refluxed in Me2CO (30 ml) with Amberlyst A-23 resin (in HC02 form) (3.87 g equivalent to 3.65 mmol formate per g) for 72 h. The mixture is filtered and the filtrate is evaporated to yield the formate ester, HC02R (e.g. 

A viable iron carbonyl-mediated reduction process converts acid chlorides and bromoalkanes into aldehydes [3, 6]. Yields are high, with the exception of nitro-benzoyl chloride, and the procedure is generally applicable for the synthesis of alkyl, aryl and a,(i-unsaturated aldehydes from the acid chlorides. The reduction proceeds via the initial formation of the acyl iron complex, followed by hydride transfer and extrusion of the aldehyde (cf. Chapter 8). 

Propane and cyclopentane give isopropyl chloride and cyclopentyl chloride, respectively, whereas isobutane is transformed to ferf-butyl chloride under the same reaction conditions (yields are 69%, 74%, and 76%, respectively). Neopentane undergoes isomerization to yield 2-chloro-2-butane (88%). When saturated, hydrocarbons were allowed to react with methylene bromide and SbF5 bromoalkanes were obtained in comparable yields (64-75%). Formation of the halogenated product can be best explained by the mechanistic pathway (I) depicted in Scheme 5.55. Since SbF5 always contains some HF, mechanism (II) may also contribute to product formation (Scheme 5.55). 

FIGURE F Linear correlation of the standard enthalpies of formation of gaseous bromoalkanes versus the standard enthalpies of formation of gaseous chloroalkanes... 

Van Bekkum and coworkers have investigated the catalyzed Markovnikov addition of hydrogen bromide to 1-olefins and obtained best results with AgA zeolite (68% Ag+ exchange) which gives 90% of the 2-bromoalkane and 10% of the 3-bromoalkane (resulting from initial isomerization of the olefin). Interestingly, in the absence of the zeolite only a minor conversion (12%) is obtained with formation of the 1-, 2- and 3-bromo isomers156. 

Irradiation of bromoalkanes leads to formation of Br atoms, which can form complexes with the solvent, e.g. Br CH2Br2. This complex was found to oxidize p-methoxyphenol very rapidly to form the corresponding phenoxyl radical. In a later stud)f, rate constants were determined for the oxidation of a series of -substituted phenols and found to... 

Decarboxylation of silver carboxylates is a well known thermal process and is involved in the Hunsdiecker76 or Kolbe77 reactions. The Hunsdiecker reaction is the thermal decarboxylation of silver salts of acids and is used for the formation of bromoalkanes and related compounds, while the Kolbe process involves electrolysis of carboxylates as a route to decarboxylated radicals that can dimerize. Silver carboxylates are also photochemically reactive and the irradiation has been described as a facile process for the formation of alkyl radicals, as illustrated in equation 678. Later experimentation has shown that the irradiation of silver trifluoroacetate can serve as a route to trifluoromethyl radicals. This development uses irradiation of silver trifluoroacetate in the presence of titanium dioxide as a photocatalyst. The reaction follows the usual path with the formation of metallic silver and the formation of radicals. However, in this instance the formation of metallic... 

Wojtczak have found that the photosensitized degradation of polyethylene glycols decreases in the order triethylene glycol > polyethylene glycol 400 mol. wt. > polyethylene glycol 4000 mol. wt. Sastre and Gonzalez have shown that bromoalkanes are powerful sensitizers for the photo-oxidation of polystyrene, and Rabek and Ranby have found that polynuclear aromatics are photosensitizers for polybutadiene. Aromatic carbonyls have been shown to induce free-radical formation in cellulosic materials. 

The reaction of triphenylphosphine with carbon tetrabromide in acetonitrile has been studied by conductimetric titration and found to be rapid, leading to the formation of the salt (87), which was isolated from the reaction mixture. Treatment of alcohols with the triphenylphosphine-carbon tetrabromide reagent in the presence of radiolabelled bromide ion gives a rapid, low-temperature procedure for the synthesis of radiolabelled bromoalkanes under neutral conditions. ... 

Additional proof for the difference between Gif- and radical bromination was obtained from bromination of cyclohexyl bromide. Radical induced hydrogen atom abstraction occurrs at the p-position of bromoalkanes and in accordance with "Skell-Walling effect" results in the formation of the /ra 5-l,2-dibromide60. This was confirmed qualitatively for the radical bromination of cyclohexyl bromide. In contrast, this was not the case in the GoAggll bromination reaction. The ra 5-1,2-dibromide was found to be only a minor product, while trans-lA- and cw-l,3-dibromocyclohexanes were the major products. Thus all this data illustrates the different nature of Gif-reactions and radical reactions. Here it is worth mentioning that tertiary C-H bonds appeared to be the least reactive in the bromination process, as is found in the Gif- oxidation reactions. 

Figure 3.11 shows the relative reactivity as a function of ring size for two intramolecular displacement reactions, namely, conversion of oi-bromoalkane-carboxylates to lactones and formation of ethers from w-bromoalkyl monoethers of 1,2-dihydroxybenzene. 

The same authors were also successful in the analogous reaction of 4-keto-1-bromoalkanes (see also [75]) resulting in the formation of cyclobutanols. The yields of the reaction (30-60%) were found to be dependent on the length of the alkyl chain... 

In a similar vein, the silver (Ag+) salts of carboxylic adds undergo decarboxylation in the presence of bromine (Br2) to produce silver bromide and bromoalkane (the Hunsdiecker reaction). The Hunsdiecker reaction also appears to involve radicals as shown in Scheme 9.101, where,in the first step,silver bromide precipitates from the reaction mixture with formation of an acyl hypobromite (RC(D2Br). Then, homolysis of the oxygen-bromine bond generates bromine atoms and the carboxyl radical, seen here as the same radical generated in Scheme 9.100. Following loss of carbon dioxide (CO2), it is held that the alkyl radical is captured by the bromine atom (Br ) to produce alkylbromide (1-bromoethane [CH3CH2Br]). 

A visual demonstration of relative Sn1 reactivity. The three test tubes contain, from left to right, solutions of 1 -bromobutane, 2-bromopropane, and 2-bromo-2-methylpropane in ethanol, respectively. Addition of a few drops of AgNOa solution to each causes immediate formation of a heavy AgBr precipitate from the fert-bromoalkane (right), less AgBr precipitation from the secondary substrate (center), and no AgBr formation from the primary halide (left). 

How do the reaction mechanisms and product formation differ when the structure of the substrate and reaction conditions change To begin to unravel the nuances of bimolecular and uni-molecular substitution and elimination reactions, focus on the treatment of bromoalkanes A through D under conditions (a) through (e). Divide the problem evenly among yourselves so that each of you tackles the questions of reaction mechanism(s) and qualitative distribution of product(s), if any. Reconvene to discuss your conclusions and come to a consensus. When you are explaining a reaction mechanism to the rest of the team, use curved arrows to show the flow of electrons. Label the stereochemistry of starting materials and prodncts as R or S, as appropriate. 

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