Mechanism free-radical halogenation

N Bromosuccimmide provides a low concentration of molecular bromine which reacts with alkenes by a mechanism analogous to that of other free radical halogenations... 

Mechanism and selectivity of free-radical halogenation with cyclic N-haloimides 99MI19. 

Halogenation of alkanes had long been known, and in 1930 the kinetics of the chlorination of chloroform to carbon tetrachloride were reported by Schwab and Heyde (equation 40), while the kinetics of the chlorination of methane were described by Pease and Walz in 1931. Both of these studies showed the currently accepted mechanism, which was extended to reactions in solution by Hass et al. in 1936. The free radical halogenation mechanism of other alkanes was described by Kharasch and co-workers, ° including side chain halogenation of toluene. 

Cyclohexene was shown to react partly by a radical mechanism when chlorinated in the absence of oxygen even in the dark.247 The reaction is slightly less stereoselective than the ionic process (96% trans-1,2-dichlorocyclohexane vs. 99%). Isomeric butenes under similar conditions give products nonstereoselec-tively.248 Branched alkenes are less prone to undergo free-radical halogenation. 

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. 

Free-radical halogenation of an alkane occurs by a very different mechanism. The first step in these reactions is the homolytic splitting of a bond to give a pair of free radicals. 

Key Mechanism 4-1 Free-Radical Halogenation 136 4-4 Equilibrium Constants and Free Energy 138 4-5 Enthalpy and Entropy 140 4-6 Bond-Dissociation Enthalpies 142 4-7 Enthalpy Changes in Chlorination 143 4-8 Kinetics and the Rate Equation 145... 

Propose mechanisms and explain the steps for simple reactions such as free-radical halogenation. 

Q Propose a detailed mechanism for the free-radical halogenation of an alkane. 

Although we discussed its mechanism at length in Section 4-3, free-radical halogenation is rarely an effective method for the synthesis of alkyl halides. It usually produces mixtures of products because there are different kinds of hydrogen atoms that can be abstracted. Also, more than one halogen atom may react, giving multiple substitutions. For example, the chlorination of propane can give a messy mixture of products. 

The mechanism is similar to other free-radical halogenations. A bromine radical abstracts an allylic hydrogen atom to give a resonance-stabilized allylic radical. This radical reacts with Br2, regenerating a bromine radical that continues the chain reaction. 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

The hydrocarbon portion of an aliphatic acid can undergo the free-radical halogenation characteristic of alkanes, but because of the random nature of the substitution it is seldom used. The presence of a small amount of phosphorus, however, causes halogenation (by an ionic mechanism) to take place exclusively at the alpha position. T his reaction is known as the Hell-Volhard-Zelinsky reaction, and it is of great value in synthesis. 

Explain the mechanism and energetics of the free-radical halogenation of alkanes. 

Introduction to Mechanisms Using Free-Radical Halogenation... 

Now we are ready to introduce bromine at either C2 or C6. /nformation It is important here to remember the mechanism of free-radical halogenation The crucial intermediate is a radical center— in our case, at either C2 (E) or C6 (F)—that can he attacked by halogen from either side of the p orbital (Section 3-4). 

Cation (Section 1 2) Positively charged ion Cellobiose (Section 25 14) A disacchande in which two glu cose units are joined by a 3(1 4) linkage Cellobiose is oh tamed by the hydrolysis of cellulose Cellulose (Section 25 15) A polysaccharide in which thou sands of glucose units are joined by 3(1 4) linkages Center of symmetry (Section 7 3) A point in the center of a structure located so that a line drawn from it to any element of the structure when extended an equal distance in the op posite direction encounters an identical element Benzene for example has a center of symmetry Cham reaction (Section 4 17) Reaction mechanism m which a sequence of individual steps repeats itself many times usu ally because a reactive intermediate consumed m one step is regenerated m a subsequent step The halogenation of alkanes is a chain reaction proceeding via free radical intermediates... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

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