Halogenation of hydrocarbons

Walling, Free Radicals in Solution, J. Wiley, New York (1957), pp 347-396 Quart Rev 14 336 (I960) 

Free Radicals, McGraw-Hill, New York (1966), pp 179-213 M. L. Poutsma, Methods in Free Radical Chemistry, Ed. E. S. Huyser, Vol 1, Marcel Dekker, New York (1969), pp 79-193 

---

Free-radical halogenation of hydrocarbons induced thermally or photochemically can be performed with all four halogens, each exhibiting certain specificities. Because of the thermodynamics of the process, however, only chlorination (and bromination) are of practical importance.31,106-108 Fluorination with elemental fluorine is also possible. This reaction, as discussed above (see Section 10.1.1), follows an electrophilic mechanism in the solution phase.109,110 Under specific conditions, however, free-radical fluorination can be performed. 

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... 

Let us assume that only the reaction steps listed in this scheme participate in the radical chain halogenations of hydrocarbons. Let us thus disregard the fact that chain termination can also occur owing to the radical-consuming reactions R- + Hal-— R—Hal and 2R- —> R—R and possibly also by disproportionation of alkyl radicals R- to give the alkane, which has one H atom more, and the olefin, which has one H... 

Halogenation and Dehalogenation Reactions. Usually, reactions of this type take place readily without utilizing catalysts. However, when selectivity of the desired product is low or it is necessary to run the reaetion at a lower temperature, the use of a catalyst is desirable. Supported copper and silver halides can be used for the halogenation of hydrocarbons. Hydrochlorination reactions can be carried out with mercury copper or zinc halides. 

On an industrial scale alkyl halides—chiefly the chlorides because of the cheapness of chlorine—are most often prepared by direct halogenation of hydrocarbons at the high temperatures needed for these free-radical reactions (Secs. 3.19, 6.21, and 12.12-12.13). Even though mixtures containing isomers and compounds of different halogen content are generally obtained, these reactions are useful industrially since often a mixiure can be used as such or separated into its components by distillation. 

Many operations in chemical engineering require the contact of two liquid phases between which mass and heat transfer with reaction occurs. Examples are hydrometallurgical solvent extraction, nitrations and halogenations of hydrocarbons, hydrodesulfurization of crude stocks, emulsion polymerizations, hydrocarbon fermentations for single-cell proteins, glycerolysis of fats, and phase-transfer catalytic reactions. A most common method of bringing about the contact of the two phases is to disperse droplets of one within the other by mechanical agitation. 

Halogenation of hydrocarbons. Walling found /-butyl hypochlorite useful for light- or radical-induced chain chlorination of toluene. Thus, in the presence of azobisisobutyronitrile as initiator, the reaction with toluene at 40° affords benzyl chloride (84%), /-butanol (97%), and 1-3% each of chlorololuenes, methyl chloride, and acetone. 

Zeolites can also function as reservoirs for selective halogenation of hydrocarbons 114) and as convenient media for addition of chlorine to olefins, with subsequent dehydrohalogenation to form reaction mixtures rich in highly halogenated alkenes 108). Similarly, highly chlorinated benzenes have been prepared in low yield by passing cyclohexane over NaY in the presence of NOCl and HCI, NO and CI2, or CI2 alone at 149°-260° 115). 

---