Alkanes free-radical halogenation

From alkanes free-radical halogenation (synthetically useful only in certain cases) (Sections 4-13 and 6-6)... 

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 ...

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. 

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). 

Alkyl halides are important starting materials in the synthesis and manufacture of a variety of organic compounds. Free-radical halogenation (Scheme 4.12) is an important methodology for the synthesis of alkyl halides from alkanes. Brominations (X = Br) tend to be very selective because of the... 

Identifying this monochloride derivative gives us the carbon skeleton. The starting alkane (compound A) must be 2,2-dimethylbutane. Its free-radical halogenation gives three different monochlorides ... 

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. 

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

Alkylbenzenes undergo free-radical halogenation much more easily than alkanes because abstraction of a hydrogen atom at a benzylic position gives a resonance-stabilized benzylic radical. For example, ethylbenzene reacts with chlorine in the presence of light to give cr-chlon >e thy I benzene. Further chlorination can occur to give a dichlorinated product. 

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

Just as free-radical halogenation occurs at the carbon of an alkane, via hydrogen abstraction to form the radical, a similar reaction occurs at silicon. When triisopro-pylsilane (iPraSi—H) reacts with ferf-butyl hypochlorite at — 10°C, the product is triisopropylchlorosilane (iPraSi—Cl)... 

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. 

Elemental chlorine can be used in free-radical halogenation reactions, too, but these reactions are less easily controlled, because the Cl- radical is more reactive than the Br- radical and hence less selective. The reagents t-BuOCl and SO2CI2 are used as alternative chlorinating agents. The F- radical is so reactive, and the reaction F-F + C-H —> H-F + C-F is so exothermic, that free-radical fluorinations result in violent and uncontrollable exotherms (explosions). At the other extreme, free-radical iodinations of alkanes do not work well at all, as the H- abstraction step is too endothermic. 

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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