Alkanes radical halogenation, mechanism

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. 

The elementary steps (1) through (3) describe a free-radical chain mechanism for the reaction of an alkane with a halogen. 

A significant observation concerning bromine addition is that it and many of the other reactions listed on page 360 proceed in the dark and are not influenced by radical inhibitors. This is evidence against a radical-chain mechanism of the type involved in the halogenation of alkanes (Section 4-4D). However, it does not preclude the operation of radical-addition reactions under other conditions, and, as we shall see later in this chapter, bromine, chlorine, and many other reagents that commonly add to alkenes by ionic mechanisms also can add by radical mechanisms. 

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. 

Alkanes are fuels they burn in air if ignited. Complete combustion gives carbon dioxide and water less complete combustion gives carbon monoxide or other less oxidized forms of carbon. Alkanes react with halogens (chlorine or bromine) in a reaction initiated by heat or light. One or more hydrogens can be replaced by halogens. This substitution reaction occurs by a free-radical chain mechanism. 

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

Rather than using the halogens themselves, other halo n radical donors are more commonly used in laboratory scale synthesis. One of the simplest of these is CCU, which can chlorinate alkanes by a free radical chain mechanism.The chain lengths are not very long (equations 76-78), because of their slightly endothermic nature and in part because the reaction is also kinetically rather slow. Elevated temperatures are therefore normally required. Nitrosylchloride at 1(X) C has also been used for these reactions. ... 

Radical halogenation of alkanes was discussed in Chapter 15. The mechanism of radical halogenation at an allylic carbon was given in Section 15.10. 

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. 

Physical Properties of Haloalkanes Preparation of Haloalkanes by Halogenation of Alkanes Mechanism of Halogenation of Alkanes Allylic Halogenation Radical Autoxidation Radical Addition of HBr to Alkenes... 

In this chapter, we begin with the structure and physical properties of haloalkanes. We then study radical halogenation of alkanes as a vehicle to introduce an important type of reaction mechanism, namely the mechanism of radical chain reactions. Reactions of oxygen with alkenes and a radical mechanism for HBr addition to alkenes complete the chapter. 

From detailed studies of the conditions and products for halogenation of alkanes, chemists have concluded that these reactions occur by a type of mechanism called a radical chain mechanism. 

To account for the products fornaed from halogenation of alkanes, chemists propose a radical chain mechanism involving three types of steps (1) chain initiation, (2) chain propagation, and (3) chain termination. We illustrate radical halogenation of alkanes by the reaction of chlorine with ethane. 

Allylic bromination and chlorination proceed by a radical chain mechanism involving the same type of chain initiation, chain propagation, and chain termination steps involved in the radical halogenation of alkanes. 

In order to assess the sign of AG for any process, we must evaluate the signs of both the enthalpy and entropy terms. At the beginning of this chapter, we posed this type of thermodynamic argument to explore the halogenation of alkanes. In this section, we will explore each propagation step of the radical addition mechanism separately. 

Alkanes react with halogens (except iodine) by a radical chain mechanism to give haloalkanes. The mechanism consists of initiation to create a halogen atom, two propagation steps, and various termination steps. 

The mechanism of the addition reaction under these conditions is not an ionic sequence rather, it is a much faster radical chain sequence. The reason is that the activation energies of the component steps of radical reactions are very small, as we observed earlier during the discussion of the radical halogenation of alkanes (Section 3-4). Consequently, in the presence of radicals, anti-Markovnikov hydrobromination simply outpaces the regular addition pathway. The initiation steps are... 

As in the radical halogenation of alkanes (Section 3-8), the exothermic nature of aromatic halogenation decreases down the periodic table. Huorination is so exothermic that direct reaction of fluorine with benzene is explosive. Chlorination, on the other hand, is controllable but requires the presence of an activating catalyst, such as aluminum chloride or ferric chloride. The mechanism of this reaction is identical with that of bromination. Finally, electrophilic iodination with iodine is endothermic and thus not normally possible. Much like the radical halogenation of alkanes, electrophilic chlorination and bromination of benzene (and substituted benzenes. Chapter 16) introduces functionality that can be utilized in further reactions, in particular C-C bond formations through organometallic reagents (see Problem 54, Section 13-9, and Real Life 13-1). 

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... 

Chain reaction (Section 4.17) Reaction mechanism in which a sequence of individual steps repeats itself many times, usually because a reactive intermediate consumed in one step is regenerated in a subsequent step. The halogenation of alkanes is a chain reaction proceeding via free-radical intermediates. 

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