Hydrogen bromide synthesis

Hydrogen Bromide Synthesis The global reaction Ha -I- Bra 2HBr proceeds through the following chain of reactions  

Assuming pseudo-steady state for the concentrations of the free radicals H and Br , the global rate equation becomes 

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A chain reaction is a reaction in which a small number of elementary steps are repeated a large nrrmber of times following each other through a small number of intermediate species that are constantly formed and destroyed. Such an example is given by the mechanism of the synthesis of hydrogen bromide, expressed in section 2.2. A repetitive secprence is called a repeat unit in the chain (like the two steps [2.R2a] and [2.R2b] in hydrogen bromide synthesis). 

Chain reactions are reactions that have a closed sequence in which the loop is traversed completely in one direction, such as the hydrogen bromide synthesis in which the sequence constituting the repeat unit (reaction sequence that is repeated indefinitely) is linear (see Figure 2.5). 

We will reason on hydrogen bromide synthesis (originally shown in [2.R2]) ... 

Figure 5.5. Variation of species concentrations during the chain reaction of hydrogen bromide synthesis...

To illustrate this, we apply this method to hydrogen bromide synthesis for which the volumetric speed-concentration relation is relation [7.54], which will be written as follows ... 

In this type of breaking, two parts of the molecule take an election away from the electron pair, which leads to the formation of two radicals, such as for example the dissociation of bromine during the first step of hydrogen bromide synthesis ... 

A chain reaction is a reaction in which a smaii nnmber of eiementaiy steps are repeated numerous times with the heip of a smaii number of intermediate species that are constantly formed and destroyed. The repetitive sequence is called a Unk in the chain. The best known example is the reaction of hydrogen bromide synthesis, according to reaction [12.R1], whose mechanism may be represented by steps [12.Rla] to [12.Rld] ... 

We see that the example of hydrogen bromide synthesis (see section 12.1) corresponds to the case of F and G identical to two HBr. 

The same general procedure was applied satisfactorily to the synthesis of 2-bromothiazoles using hydrogen bromide below 0°C (465, 507) (Table 11-29). 

A new synthesis of 2-bromo-4-aminothiazole was reported from a-cyanoalkylthiocyanates and hydrogen bromide (189), Rj = H... 

Isoprene has sometimes been used as a starting matenal in the laboratory synthesis of ter penes In one such synthesis the first step is the electrophilic addition of 2 moles of hydrogen bromide to isoprene to give 1 3 dibromo 3 methylbutane... 

Polypeptide Synthesis and Analysis. Sihca or controUed-pore glass supports treated with (chloromethyl)phenylethyltrimethoxysilane [68128-25-6] or its derivatives are replacing chloromethylated styrene—divinylbenzene (Merrifield resin) as supports in polypeptide synthesis. The sdylated support reacts with the triethyl ammonium salt of a protected amino acid. Once the initial amino acid residue has been coupled to the support, a variety of peptide synthesis methods can be used (34). At the completion of synthesis, the anchored peptide is separated from the support with hydrogen bromide in acetic acid (see Protein engineering Proteins). 

A considerable amount of hydrobromic acid is consumed in the manufacture of inorganic bromides, as well as in the synthesis of alkyl bromides from alcohols. The acid can also be used to hydrobrominate olefins (qv). The addition can take place by an ionic mechanism, usually in a polar solvent, according to Markownikoff s rule to yield a secondary alkyl bromide. Under the influence of a free-radical catalyst, in aprotic, nonpolar solvents, dry hydrogen bromide reacts with an a-olefin to produce a primary alkyl bromide as the predominant product. Primary alkyl bromides are useful in synthesizing other compounds and are 40—60 times as reactive as the corresponding chlorides (6). 

NKOH, EtOH, 20°, 5-10 min 80% yield. 5-2,2-Bis(carboethoxy)ethyl thioether, stable to acidic reagents such as trifluoroacetic acid and hydrogen bromide/acetic acid, has been used in a synthesis of glutathione. ... 

This synthetic tropidine was converted into bromodihydrotropidine by hydrogen bromide in aeetie aeid, and the solution heated with 10 per cent, sulphurie acid at 200-10°, when it passed into -tropine,and, sinee this may be partially converted into tropine by oxidation to tropinone and reduction of the latter by zinc dust and hydriodic acid, this series of reactions affords a complete synthesis of tropine and of the tropeines. Combining the formula given above for tropine with that of tropic acid, atropine and hyoscyamine are represented as follows ... 

The malonic ester required for synthesis of cyclopal (107) can be obtained by alkylation of diethyl allylmalonate (115) with 1,2-dibromocyclopentane in the presence of excess base. It is probable that the reaction proceeds by elimination of hydrogen bromide from the dihalide as the first step. The resulting allilic halide (116) would be the most reactive electrophile in the reaction mixture and thus would quickly alkylate the anion of the malonate to afford 117. 

Cyclization of the two pendant alkyl side chains on barbiturates to form a spiran is consistent with sedative-hypnotic activity. The synthesis of this most complex barbiturate starts by alkylation of ethyl acetoacetate with 2-chloropentan-3-one to give 152. Hydrolysis and decarboxylation under acidic conditions gives the diketone, 153. This intermediate is then reduced to the diol (154), and that is converted to the dibromide (155) by means of hydrogen bromide. Double Internal alkylation of ethyl... 

Condensation of adipic acid derivative 17 with phenylethylamine in the presence of carbo-nyldiimidazole affords the bis-adipic acid amide 18. The synthesis is completed by reduction of the carbonyl groups with diborane followed by demethylation of the aromatic methoxy groups with hydrogen bromide the afford dopexamine (19) [3]. 

In extending this direct method of synthesis, we next investigated the possibility of preparing similarly constituted halides from 2-deoxy-D-arabino-hexose (2-deoxy-D-glucose) (21). The hexose was subjected to a partial anomerization procedure described by Bergmann and co-workers (1). The solid material obtained by this procedure is a mixture of the anomeric forms of 2-deoxy-D-arabino-hexose low temperature p-nitro-benzoylation of the latter in pyridine resulted in a mixture of crystalline, anomeric tetrakis-p-nitrobenzoates in a ratio of approximately 1 1. They were readily separable by fractional recrystallization, and treatment of either with an excess of hydrogen bromide in dichloromethane, or with... 

The structural homology between intermediate 4 and isostrych-nine I (3) is obvious intermediates 3 and 4 are simply allylic isomers and the synthetic problem is now reduced to isomerizing the latter substance into the former. Treatment of 4 with hydrogen bromide in acetic acid at 120°C results in the formation of a mixture of isomeric allylic bromides which is subsequently transformed into isostrychnine I (3) with boiling aqueous sulfuric acid. Following precedent established in 194810 and through the processes outlined in Scheme 8a, isostrychnine I (3) is converted smoothly to strychnine (1) upon treatment with potassium hydroxide in ethanol. Woodward s landmark total synthesis of strychnine (1) is now complete. 

Polyphosphoric acid is a commonly used catalyst for this reaction however, in some cases a mixture of hydrogen bromide/acetic acid gives better results. Acylation of the S-phenyl-, V-(4-tolyl)- or S-(l-naphthyl)-substituted thiobenzenepyruvic acids 3a-c affords the corresponding dibenzo[A,/]thiepins in satisfactory yields, while reaction of the S-(4-methoxyphenyl) or S-(2-naphthyl) derivatives fails to provide any thiepin.60 The intramolecular Friedel-Crafts acylation of 2-(arylsulfanyl)benzeneacetic acids also yields the corrresponding dibenzothiepins in this case the use of hydrogen fluoride sometimes results in purer products.38 The applicability of this method is restricted to the synthesis of stable bisannulated thiepins. 

Halo-l-benzothiepins 4 can be synthesized by the treatment of 7a-halobenzo[fe]cyclopropa-[e]thiopyran-7-ols 2 with hydrogen bromide and subsequent hydrogen bromide elimination from the 2,4-dihalo-2,3-dihydro-l-benzothiepins 3 by l,5-diazabicyclo[4.3.0]non-5-ene (DBN).9 The alcohols 2 are prepared by Grignard reaction of the corresponding 7a-halobenzo[ft]cyclopropa[c]thiopyran-7-ones l18 and are used for the synthesis without purification. The intermediate dihalodihydro-l-benzothiepins 3 are not isolated due to their thermal lability related and more stable compounds are described in Section 2.1.2.1. 

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