Hydrocarbons radical bromination

Other nonpolymeric radical-initiated processes include oxidation, autoxidation of hydrocarbons, chlorination, bromination, and other additions to double bonds. The same types of initiators are generally used for initiating polymerization and nonpolymerization reactions. Radical reactions are extensively discussed in the chemical Hterature (3—15). 

Free-radical bromination is an important method of selective functionalization of hydrocarbons. The process is a chain reaction involving the following steps ... 

Also, according to Equation 1.9, the overall reaction radical chlorination takes place on a given substrate considerably faster than the overall reaction radical bromination. If we consider this and the observation from Section 1.7.3, which states that radical chlorinations on a given substrate proceed with considerably lower regioselectivity than radical brominations, we have a good example of the so-called reactivity/selectivity principle. This states that more reactive reagents and reactants are less selective than less reactive ones. So selectivity becomes a measure of reactivity and vice versa. However, the selectivity-determining step of radical chlorination reactions of hydrocarbons takes place near the diffusion-controlled limit. Bromination is considerably slower. Read on. 

In this mechanism, bromine radicals are key intermediates producing hydrocarbon radicals via hydrogen abstraction [32]. The subsequent reactions are those typical for hydrocarbon autoxidation [3], Bromine radicals may terminate to Br but the bromide is replenished via phase... 

Further study of the Fe -H202 reaction showed that bromotrichloromethane and its congeners afford bromination instead of o.vidation. The selectivity is the same as the oxidation reaction and does not follow a typical radical bromination pattern. [4] Scheme 2 shows a series of hydrocarbons and their relative reactivity normalized per hydrogen for radical bromination and for Gif type Fe -picolinic acid (PA)-H202 bromination. In each case, the C-H bond reactivity in cyclohe. ane is set at 1.0. The two sets of numbers are completely different. Cyclohe. ane is the most active hydrocarbon in Gif bromination and the least active in radical bromination. The data were... 

Free radical bromination of hydrocarbons is an important method of selective... 

Write a mechanism for the radical bromination of the hydrocarbon benzene, CgHg (for structure, see Section 2-4). Use propagation steps similar to those in the halogenation of alkanes, as presented in Sections 3-4 through 3-6. Calculate AH° values for each step and for the reaction as a whole. How does this reaction compare thermodynamically with the bromination of other hydrocarbons Data DH° (CgHs-H) = 112 kcal mol DH° (C Hs-Br) = 81 kcal mol . Note the Caution in Exercise 3-5. 

Cyclopropyl carbanions are capable of maintaining their configuration whereas the CT-radical has been shown to reach inversion equlibrium with a rate constant of lO" s". ITie cyclopropyl bromide 13, and the corresponding iodide, are reduced in a single two-electron polarographic wave and the S +)-isomer yields the R(-)-hydrocarbon with 26% enantiomeric excess [67, 68]. Such a substantial retention of configuration during reduction of the carbon-bromine bond indicates a very fast second electron transfer process. Results from reduction of the cyclopropyl bro-... 

Bierbach, A., I. Barnes, and K. H. Becker, Rate Coefficients for the Gas-Phase Reactions of Bromine Radicals with a Series of Alkenes, Dienes, and Aromatic Hydrocarbons at 298 + 2 K, lnt.. J. Chem. Kinet., 28, 565-577 (1996). 

On the other hand, alkali metal atoms can be obtained by simple evaporation at moderate temperatures, and these may serve to abstract bromine or iodine from organic halides, thus providing access to radicals, biradicals,or highly strained hydrocarbons, such as small-ring propellanes. The technique has been described in some detail by Otteson and Michl. ... 

Free-radical halogenation of hydrocarbons induced thermally or photochemically can be performed with all four halogens, each exhibiting certain specificities. Because of the thermodynamics of the process, however, only chlorination (and bromination) are of practical importance.31,106-108 Fluorination with elemental fluorine is also possible. This reaction, as discussed above (see Section 10.1.1), follows an electrophilic mechanism in the solution phase.109,110 Under specific conditions, however, free-radical fluorination can be performed. 

Y. Kamiya illustrates the influence on catalytic activity of the form of the catalyst. Thus, in the cobalt-catalyzed oxidation of hydrocarbons in acetic acid solution, introduction of bromide ions increases the activity of the catalyst, especially when the metal ion concentration is fairly high. The presence of bromides also results in a marked increase in the proportion of carbonyl compounds among the products and it is believed that these are formed as a result of a propagation step in which bromine-containing cobaltous ions react with alkylperoxy radicals. 

This bromination reaction results exclusively in alpha substitution and therefore is limited to carboxylic acids with a hydrogens. Chlorine with a trace of phosphorus reacts similarily but with less overall specificity, because concurrent free-radical chlorination can occur at all positions along the chain (as in hydrocarbon halogenation see Section 4-6A). 

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