Propagation steps alkane radical halogenation

Recall from Section 5.3 that radical substitution reactions require three kinds of steps initiation, propagation, and termination. Once an initiation step has started the process by producing radicals, the reaction continues in a self-sustaining cycle. The cycle requires two repeating propagation steps in which a radical, the halogen, and the alkane yield alkyl halide product plus more radical to carry on the chain. The chain is occasionally terminated by the combination of two radicals. 

An unusual radical halogenation of hydrocarbons by phase-transfer catalysis may be performed by reacting alkanes with tetrahalomethanes. The reaction is initiated by single-electron oxidation of OH- by CHlg4. The tetrahalomethane radical anion formed decomposes to the 12 trihalomethyl radical [Eq. (10.56)], which is then involved in C—H activation and propagation steps 286... 

The free radical halogenation of alkanes takes place in three steps initiation, propagation and termination (Scheme 2.35). 

Halogenation of Alkanes - Reaction Mechanism Section 4.4D The ethyl radicals formed from the decomposition of tetraethyllead can react with methane to form methyl radicals or with chlorine to form chlorine radicals. Both of these are part of the propagation steps. 

The mechanism for the halogenation of an alkane is well understood. The high temperature (or light) supplies the energy required to break the Cl—Cl or Br—Br bond homolytically. Homolytic bond cleavage is the initiation step of the reaction because it creates the radical that is used in the first propagation step (Section 4.10). Recall that an arrowhead with one barb signifies the movement of one electron (Section 3.6). 

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. 

A classic example of how reactivity is related to selectivity is concerned with the free radical halogenation of alkanes by CI2 and Br2. In this free radical chain reaction, the step that sets the position of the halogen in the alkane is a hydrogen atom abstraction step. The carbon based radical created in this first propagation step then abstracts a halogen atom from the CI2 or Br2, giving the alkyl halide (see below). In free radical halogenation by either CI2 or Br2, tertiary alkyl halides are created preferentially to secondary, which in turn are formed preferentially to primary alkyl halides. This reflects the fact that the order of radical stability decreases from tertiary to secondary to primary. Yet, the extent of the selectivity for tertiary over secondary over primary is quite different for chlorination and bromination. 

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. 

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. 

Overall, the reaction between alkanes and halogens, involving initiation, propagation and termination steps, is called free-radical substitution. 

Radical reactions are often called chain reactions. All chain reactions have three steps chain initiation, chain propagation and chain termination. For example, the halogenation of alkane is a free radical chain reaction. 

Write all steps in the free-radical chain reaction between a halogen and an alkane, and identify the initiation, propagation, and termination steps. 

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