Bromine compounds, chain reaction

In order to induce the free-radical chain reaction, a starter compound such as dibenzoyl diperoxide, azo-Zj -(isobutyronitrile) or tcrt-butyl hydroperoxide or UV-light is used. The commercially available, technical grade N-bromosuccinimide contains traces of bromine, and therefore is of slight red-brown color. Since a small amount of elemental bromine is necessary for the radical... 

For instance, bromination of toluene in carbon tetrachloride did not proceed at reflux, even though pentamethylbenzene was brominated at 30°C to give bromopentamethylbenzene quantitatively. Toluene and copper(II) bromide reacted at reflux for 72 h. to give benzyl bromide as the main product. In a similar reaction with alumina-supported copper(II) bromide, bromotoluene (o/p = l) was obtained in good yield and no side-chain-brominated compounds were detected. 

An analogue of amitriptyline which contains an additional double bond in the central seven membered ring shows much the same activity as the prototype. Treatment of dibenzocycloheptanone 7 with N-bromosuccinimide followed by triethyl amine serves to introduce the additional double bond by the bromination-dehydrohalogenation sequence. Reaction of the carbonyl group with the Grignard reagent from 3-chloropropyl-N,N-dimethyl amine serves to introduce the side chain (73). Acid catalyzed dehydration affords the antidepressant compound cyclobenzaprine (74). ... 

The kinetics of the thermal and photochemical chain reactions of bromine with saturated, hydrogen-containing compounds (for example, Br2 + HR —> HBr + RBr), usually has the form "... 

In an earlier paper Barton and Onyon considered the unimolecular mechanism of dehydrochlorination to be of more universal application than the radical chain mechanism and postulated that a chloro-compound will decompose by a radical chain mechanism only so long as neither the compound itself nor the reaction products will be inhibitors for the chains . On the basis of this postulate the authors correctly predicted the mechanism of decomposition of a number of chlorine compounds. The postulate does not hold well for bromine compounds which show a greater tendency to decompose via radical chain mechanisms. However, from their early studies on 2-bromopropane 2-bromobutane, t-butyl bromide, and bromo-cyclohexane, Maccoll et a/.234,235,397,410,412 concluded that these compounds also decompose unimolecularly via a four-centre transition state similar to that proposed by Barton and Head. 

In the presence of a radical initiator, alkenes react with reactive molecules such as hydrogen bromide to give simple 1 1 adducts rather than a polymer. The initiator radical reacts rapidly with an HBr molecule to give a bromine atom (6.49), which starts the chain reaction. In the first propagation step, the bromine atom adds to the alkene 61 to give the adduct radical 62 (reaction 6.50). Since 62 abstracts a hydrogen atom from HBr by reaction (6.51) more rapidly than it would add to the alkene to form a polymer radical as in (6.43), the chain continues with reactions (6.50) and (6.51) as the propagating steps, and the product is the primary bromo compound 63. This anti-Markovniko addition is in the reverse direction to the polar addition discussed in Chapter 5. Since the radical chain reaction is faster than the polar reaction, the anti-Markovnikov product dominates if radicals are present. If the Markovnikov product is required, the reaction must be carried out in the dark, in the absence of free radical initiators, and preferably with a radical inhibitor present. 

Halogenated organic compounds act in the flame, by blocking the chain reactions which are characteristic for the flame [1, 6]. Thus, any organic compound containing chlorine or bromine is decomposed into the corresponding hydracids (HC1 or HBr). These acids react with the most reactive radical existing in the flame, the hydroxyl radical, HO ... 

Allylic bromination by NBS is a radical-chain reaction occurring on the surface of the NBS crystals. A solution of NBS in tetrachloroethane or nitro-methane adds bromide to a C=C group 354 solvents are therefore used in which NBS and, if possible, also the succinimide formed, are difficultly soluble dry CC14 is usually chosen, but for reactive substances cyclohexane or benzene may also be used. The more polarized the N-halogen is, i.e.9 the more positive the Br is rendered, the more easily does the A-halogenated compound cause addition to the C=C bond NBS is particularly suitable for allylic bromination because of its steric structure and the almost apolar nature of its N-Br bond. 

Alkanes are called saturated hydrocarbons because they do not contain any double or triple bonds. Since they also have only strong cr bonds and atoms with no partial charges, alkanes are very umeactive. Alkanes do undergo radical substitution reactions with chlorine (Cl 2) or bromine (Br2) at high temperatures or in the presence of light, to form alkyl chlorides or alkyl bromides. The substitution reaction is a radical chain reaction with initiation, propagation, and termination steps. Unwanted radical reactions are prevented by radical inhibitors—compounds that destroy reactive radicals by creating umeactive radicals or compounds with only paired electrons. 

Bromine compounds and to a lesser extent chlorine compounds are known to be inhibitors of the branching radical chain reactions that occur in flames. The most important of these involve oxygen and... 

Campaigne et al. have used 3-thenyl bromide obtained by benzoyl peroxide-catalyzed, side-chain bromination of 3-methylthiophene with A -bromosuccinimide, as a starting material for 3-substituted thiophenes. - 22 3-Methylthiophene is now prepared commercially from itaconic acid. The reactive halogen in 3-thenyl bromide could be directly reacted with a variety of nucleophiles, such as cyanide, or malonate, to give more complex 3-substituted compounds. 3-Thenyl bromide was converted by the Sommelet reaction to 3-thio-phenealdehyde which, with silver oxide, was oxidized to 3-thio-... 

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