Halogenation Fluorination, lodination

Halocyclohexadienones—see also B romocy clohexadienones, Chlorocyclohexadienones, Fluorocyclohexadienones formation of 648, 650, 652, 654, 655 Halogenation 985—see also Bromination, Chlorination, Fluorination, lodination anodic 1168... 

Of the two chain-propagating steps, then, step (2) is more difficult than step (3) (see Fig. 2.8). Once formed, methyl radicals react easily with any of the halogens it is how fast methyl radicals are formed that limits the rate of overall reaction. Fluorination is fast because fluorine atoms rapidly abstract hydrogen atoms from methane E cx. is only 1 kcal. lodination does not take place because iodine atoms. And it virtually impossible to abstract hydrogen from methane tLCt is more than 33 kcal. 

See Halogens, or Interhalogens, above Chromy 1 chloride Organic solvents Fluorine nitrate Organic materials Hydrogen peroxide Diethyl ether lodine(Vll) oxide Diethyl ether Lithium perchlorate Diethyl ether Nitric acid Diethyl ether Nitrosyl perchlorate Organic materials Nitryl perchlorate Organic solvents Ozone Diethyl ether... 

All of the processes above have a negative value for A// and are exothermic except for iodin-ation. lodination of methane has a positive A//, which means that AG will also be positive for that reaction. As a result, iodination is not thermodynamically favorable, and the reaction simply does not occur. The other halogenation reactions are all thermodynamically favorable, but fluorination is so exothermic that the reaction is too violent to be of practical use. Therefore, only chlorination and bromination are practical in the laboratory. 

Halogenation. Direct halogenation of quinoline proceeds at the 2-position. Fluorination and bromination are known, but they proceed using forcing conditions (eqs 24 and 25). lodination proceeds under milder conditions, but requires an additional titanium-based reagent (eq 26). Chlorination of quinoline without prior activation has not been reported. 

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