Radical Halogenation of Alkanes

Alkane halogenation is a poor method of alkyl halide synthesis because mixtures of products invariably result. Por example, chlorination of methane does not stop cleanly at the monochlorinated stage. Rather, the reaction continues on to give a mixture of dichloro, trichloro, and even tetrachloi products  

The situation is even worse for chlorination of alkanes that have more than one type of hydrogen. For example, chlorination of butane gives two monochlorinated products in addition to dichiorobutane, trichiorobutane, and so on. Thirty percent of the monochloro product is 1-chlorobutane, ani 70% is 2-chlorobutane  

As another example, 2-methylpropane yields 2 Chloro-2-methylpropane and l-chloro-2-methylpropane in the ratio 35 65, along with more highly chlorinated products  

From these and similar reactions, it s possible to calculate a reactivity order toward chlorination for different types of hydrogen atoms in a molecule. Take the butane chlorination, for instance. Butane has six equivalent primary hydrogens (-CH3) and four equivalent secondary hydrogens (-CH,-). The fact that butane yields 30% of 1-chlorobutane product means that each one of the six primary hydrogens is responsible for 30% 6 = 5%j of the 

A similar calculation for the chlorination of 2-methylpropane indicates that each of the nine primary hydrogens accounts for 65% 9 = 7.2% of 

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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... 

Simple alkyl halides can be prepared by radical halogenation of alkanes, but mixtures of products usually result. The reactivity order of alkanes toward halogenation is identical to the stability order of radicals R3C- > R2CH- > RCH2-. Alkyl halides can also be prepared from alkenes by reaction with /V-bromo-succinimide (NBS) to give the product of allylic bromination. The NBS bromi-nation of alkenes takes place through an intermediate allylic radical, which is stabilized by resonance. 

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). 

Radical Halogenations of Alkanes 542 Peter R. Schreiner and Andrey A. Fokin... 

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

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 ... 

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. 

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