Free radical halogenation of alkanes

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. 

Wohl in 1919 reported that A -bromoacetamide (CH CONHBr) induced allylic bromination. Then iV-bromosuccinimide (30) was described in 1942 by Ziegler and co-workers to be useful in such free radical bromination reactions (equation 41), and this widely utilized procedure is known as the Wohl-Ziegler reaction. In 1963 the mechanism of the reaction was proposed to involve halogen atoms in the hydrogen abstraction step instead of succinimidyl radicals as had been commonly supposed. The halogen atom mechanism had previously been proposed by Gosselain et al. for reactions of yV-chlorosuccinimide.  

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Halogenation (Sections 4 14 and 12 5) Replacement of a hy drogen by a halogen The most frequently encountered ex amples are the free radical halogenation of alkanes and the halogenation of arenes by electrophilic aromatic substitution... 

Table 2-1 Selectivity of Chlorine and Bromine in Free-Radical Halogenation of Alkanes ...

Other methods for the preparation of alkyl halides are electrophilic addition of hydrogen halides (HX) to alkenes (see Section 5.3.1) and free radical halogenation of alkanes (see Section 5.2). 

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

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 ... 

F. Minisci, O. Porta, F. Recupero, C. Gambarotti, R. Paganelli, G. F. Pedulli, F. Fontana, New free-radical halogeneations of alkanes, catalysed by N-hydroxyphthalimide. Polar and enthlpic effects on the chemo- and regioselectivity, Tertrahedron Lett. 45 (2004) 1607. 

Free-radical halogenation of alkanes (Sections 4.15 through 4.19)... 

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

Free radical halogenation of alkanes uses CI2 or Bt2 and light or heat to produce haloalkanes. 

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. 

Calculate the exothermicity or endothermicity of the following hydrogen abstraction reactions involved in free radical halogenation of alkanes (the BDEsof HF, HQ, HBr, and HI are 136,103,87, and 71 kcal/mol, respectively). Which reaction would you predict is the least selective ... 

As in the free-radical halogenation of alkanes, chlorination of alkylbenzenes is less selective than bromination. Given the relative rates per hydrogen for hydrogen atom abstraction from 1-phenylbutane by chlorine for the elementary step shown, calculate the percentage of 1-chloro-1-phenylbutane in the C10H13CI product. 

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