Bromination Experimental Procedure

Cross-linked polystyrene can be directly brominated in carbon tetrachloride using bromine in the presence of Lewis acids (Experimental Procedure 6.2 [55-58]). Thal-lium(III) acetate is a particularly suitable catalyst for this reaction [59]. Harsher bro-mination conditions should be avoided, because these can lead to decomposition of the polymer. Considering that isopropylbenzene is dealkylated when treated with bromine to yield hexabromobenzene [60], the expected products of the extensive bromi-nation of cross-linked polystyrene would be soluble poly(vinyl bromide) and hexabromobenzene. In fact, if the bromination of cross-linked polystyrene is attempted using bromine in acetic acid, the polymer dissolves and apparently depolymerizes [61]. 

Bromination of 14 is achieved by reaction with three equiv of W-bromo-succinimide in boiling benzene under light irradiation. This reaction gives a mixture of the desired tribromide derivative 15a and also the dibromide and tetrabromide derivatives, 15b and 15c respectively (Scheme 9.12). The separation of these compounds is very difficult, and so the mixture is best used without purification for the next step. The experimental procedure is given in Protocol 10. 

For experimental procedures for fluorinations with iodine monofluoridc and bromine trifluoride, sec the reactions of hydrazones (Section 1.1.9.1.). 

This pe r discusses the chaniced reactions involved in the curing of fluoroelastomers with bis- henols and with peroxides. The mechanism of enuring with peroxides is based on cxir work where bromine was introduced in the polymer by cxjpolymeriza-tion. The experimental procedures and materials have been described in detail in previous publiceticxis (5,6c). 

To a solution of 0.5 mL CCI4 containing 0.0358 g 2-buten-2-yl tosylate (0.01585 mmol) cooled in an ice bath was added 0.025 g bromine (0.1585 mmol) the mixture was kept cold until the completion of the addition. Evaporation of solvent gave the crude product. The flask containing erythro-2,3-dibromo-2-butyl tosylate was added the 0.125 M buffer of formic acid-sodium formate, and the reaction was monitored to completion. Then the solution was neutralized with NaHCOs and extracted with CCI4. Removal of the solvent afforded 3-bromo-2-butanone. (Note No complete experimental procedure was given in the original literature.)... 

In macroscopic chemistry, the experimental procedures for the bromination of aromatic compounds depend greatly on the nature and reactivity of the starting material. Activated aromatics such as phenol and aniline can be brominated to the tri-and tetrabrominated derivatives by using dilute aqueous solutions of bromine, whereas a controlled monobromination is very challenging and often requires cryogenic conditions. On the other hand, thermally controlled brominations of less activated aromatics such as toluene are rather slu ish reactions. Th often require photoinitiation and the use of Lewis adds as catalysts. 

The solution reactions were carried out with or without cyclodextrins in (CH3)2S0, (CH3)2SO-d0 or carbon tetrachloride. The typical experimental procedure was as follows. tra/7S-Cinnamic acid (11 mg, 0.08 mmol), powdered cx-cyclodextrin complex (162 mg, contained 0.08 mmol of the acid), and 3-cyclodextrin complex (100 mg, contained 0.08 mmol of the acid) were dissolved in 0.4 ml of (CH3)2S0-d6, to which 0.1 ml of the same solvent containing bromine (0.08 mmol) was added at 25°C. At 2 h intervals, the conversion of the acid was determined by NMR spectra. Then the reaction mixtures were poured into 20 ml of 15 wt % aqueous sodium chloride soluticn, followed by extraction with diethyl ether. The products were obtained as white solid by evaporation of the ether layer and their optical rotations were measured in ethanol on a polarimeter. 

Apply the procedure given for bromination of (E)-stilbene to study this reaction with (Z)-stilbene. Develop a protocol whereby you could demonstrate the stereochemical outcome of the dibromide(s) produced. Consult with your instructor before undertaking any experimental procedures. 

Although benzene (4) is a potential starting point of our synthesis (Scheme 21.4), it is an alleged carcinogen, so the sequence of reactions begins with nitrobenzene (5). Discussion of and experimental procedures for the reduction of nitrobenzene and the conversion of aniline (6) to acetanilide (7) are found in Section 21.2, so the present description starts with the bromination of acetanilide. 

A number of other sulphoxide reduction reactions bear mentioning. The first, due to Marchelli and coworkers , is a very simple procedure whereby the sulphoxide is refluxed with t-butyl bromide and chloroform. A useful range of sulphoxides was studied and distillation of the reaction mixture (or percolation through a column of silica gel) gave pure sulphides in yields of > 90%. The procedure is appealing because of its experimental simplicity, and its use of a relatively inexpensive reagent. It may not be very successful with sterically hindered sulphoxides and the authors do not comment on this possibility. The mechanism of this reduction reaction is akin to that of BBrj (cf. Section II.A.3), except that the bromine trap is provided by a second mole of t-butyl bromide, as shown in equation (13) ... 

Since Condon s original application, this procedure has been extended to many additional reactions. Highly successful treatments of the rates of reaction of the polymethylbenzenes were observed for mercuration (Brown and McGary, 1955c), bromination (Brown and Stock, 1957a), chlorination (Brown and Stock, 1957b Baciocchi and Illuminati, 1958) and protodesilylation (Eaborn and Moore, 1959). A comparison of two sets of calculated and experimental relative rates is presented for selective bromination and non-selective mercuration in Table 28. 

The procedure for the elimination of HBr from the dibromo ester is a modification of the method of Lawton and co-workers for sui generis generation of the methyl or ethyl ester during a reaction. Methyl a-(bromomethyl)acrylate has also been prepared by bromination of methyl methacrylate in 700°C steam and by dehydrohalogenation with sodium acetate in acetic acid. Ethyl a-(bromomethyl)acrylate has been prepared by dehydrohalogenation with the monosodium salt of ethylene glycoP and ethyl diisopropylamine." The latter reaction was reported by Ohler et al. with no experimental details for the elimination reaction. The use of triethylamine as reported in this procedure appears to be the most efficient and convenient method for dehydrobromination to these acrylate esters. 

The procedure described for the preparation of 9-bromo-phenanthrene is an adaptation of that described by Henstock, who effected the bromination at various temperatures and in different solvents but gave little experimental detail. Other methods of preparation involve the formation and isolation of phenanthrene dibromide and its subsequent conversion to 9-bromophenanthrene by heating. 

The procedure involved two steps, bromination and hydrolysis. The poor yield might depend on either or both of these steps Due to steric hindrance, the bromination reaction might not have gone to completion when water was introduced, and/or the hydrolysis was slow and was not completed when the mixture was worked up. To clarify this, the important variables must be identified. It was known from other similar reaction, that the bromination must be effected at low temperature to suppress side reactions, and that the hydrolysis should be carried out not above room temperature. During the hydrolysis step, bromomethyl ketones are susceptible to acid catalyzed rearrangements to the isomeric 3-bromoketone. With these limitations taken into account, the following experimental variables were considered to be able to influence the result. 

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