Butane radical bromination

When a bromine radical is the hydrogen-abstracting agent, the differences in reactivity are so great that the reactivity factor is vastly more important than the probability factor. For example, radical bromination of butane gives a 98% yield of... 

When radical halogenation produces a chiral center or takes place at a hydrogen on an existing chiral center, the product is an equal mixture of R and S enantiomers. Consider, for example, radical bromination of butane, which produces 2-bromobutane. 

For example, the radical bromination of butane gives a 98% yield of 2-bromobutane, compared with a 71% yield of 2-chlorobutane obtained when butane is chlorinated (Section 13.4). 

How can a molecule exist as two nonsuperimposable mirror images Consider the radical bromination of butane. This reaction proceeds mainly at one of the secondary carbons to furnish 2-bromobutane. A molecular model of the starting material seems to show that either of the two hydrogens on that carbon may be replaced to give only one form of 2-bromobutane (Figure 5-3). Is this really true, however ... 

The radical bromination of butane at C2 creates a chiral molecule (Figure 5-3). This happens because one of the methylene hydrogens is replaced by a new group, furnishing a stereocenter—a carbon atom with four different substituents. 

Figure 5-13 The creation of racemic 2-bromobutane from butane by radical bromination at C2. Abstraction of either methylene hydrogen by bromine gives an achirai radicai. Reaction of Br2 with this radicai is equaiiy iikeiy at either the top or the bottom face, a condition ieading to a racemic mixture of products.

Bromination of alkanes follows the same mechanism as chlorination. The only difference is the reactivity of the radical i.e., the chlorine radical is much more reactive than the bromine radical. Thus, the chlorine radical is much less selective than the bromine radical, and it is a useful reaction when there is only one kind of hydrogen in the molecule. If a radical substitution reaction yields a product with a chiral centre, the major product is a racemic mixture. For example, radical chlorination of n-butane produces a 71% racemic mixture of 2-chlorobutane, and bromination of n-butane produces a 98% racemic mixture of 2-bromobutane. 

The recombination of bromine atoms can not occur except on collision with a third body but reaction between the more complicated ethyl radicals might be expected without the aid of walls or triple collisions. In any reaction involving the production of free ethyl radicals one might expect to find butane, ethane, etc., but actually very little of these products is found. Apparently in this case radicals of this type are not likely to combine with each other. 

Hydrogen bromide underwent rapid addition to methylcyclopropane in the presence of bromine to give predominantly 2-bromobutane (la, 50% yield). A small amount (10%) of disubstituted butanes 3a and 4a was also isolated. A 100% excess of bromine over the cyclopropane was added in order to convert alkene products into dibromides. The reaction was carried out in the dark to avoid radical reactions of bromine. A similar result was obtained with ethylcy-clopropane. cis- and /ran5-l,2-Dimethylcyclopropane gave a mixture of four isomeric bro-mopentanes in 76% and 89% yield, respectively, on treatment with hydrogen bromide/bromine at -78°C. 3... 

The cobalt-catalyzed autoxidation of toluene in acetic acid at 363 K is accelerated by butan-2-one and benzaldehyde because peroxy radicals play a minor role in ratecontrolling propagation reactions. High rates of autoxidation are also obtained in the presence of Br because bromine atoms are important chain-propagating species. ... 

The carbojylation of unsaturated PHA has been performed through reactions that involved the conversion of the double bonds to thioethers via the free-radical addition of 11-mercaptoundecanoic acid or (R)-3-mercaptopropionic acid. Transesterification reactions of poly(3-hydro y butyrate) were carried out under reflux of 1,2-dichlorobenzene in the presence of 1,4-butane diol, poly(ethylene glycol) bis(2-aminopropyl ether) with molecular weights of 1000 and 2000, poly(ethylene glycol)methactylate or glycerol at 180 °C. Addition reactions of bromine and the -SH groups of 3-mercaptopropionic acid to the double bond of poly(3-hydro)y-10-undecenoate) were also carried out. The molecular weights of the modified polymers (despite the addition of mercapto acids to the double bonds) remained almost constant. " ... 

If R = R = H, R" = OCHj, then radical VII is oxidised by bromine. In the case of R = R = R" = OCHj, radical Vll is readily oxidised (even by iodine). The basic products of the vigorous reaction of radical 1 with ozone are nitro-t-butane and oxygen [27]. The oxidation of alcohols into carbonyl compounds can be carried out via the interaction of piperidinoxyl with copper (11) [28] ... 

The radical mechanism explains why the bromination of butane results in a racemate... 

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