Alkyl radicals halo-substituted

The reaction of alkyl- and halo-substituted phenols with Cr02Cl2 results mainly in the formation of quinones and diphenoquinones. Phenoxyl radicals are involved as intermediates. The mechanism of oxidation of a-hydroxycar-boxylic acids by pyridinium chlorochromate involves a rate-limiting hydride transfer. In the reaction of HOCD2CO2H, a kinetic isotope effect (W d) = 5.80 has been determined. Spectroscopic evidence for Cr(IV) and Cr(V) species has been obtained in the oxidation of alkylaromatics by chromyl acetate in acetic anhydride.Stopped-flow and esr studies show two stages (equations 22, 23) in the reactions of RCH2Ph, with both rates being decreased on deuteration at... 

SOLUTION We know that the first reaction has to be a radical halogenation because that is the only reaction that an alkane undergoes. Bromination will lead to a greater yield of the desired 2-halo-substituted compound than will chlorination because the bromine radical is more selective than a chlorine radical. To maximize the yield of substitution product (Section 11.8), the alkyl bromide is treated with acetate ion and the ester is then hydrolyzed to the alcohol, which forms the target compound when it is oxidized. 

Reaction sequences involving halogen transfer, followed by non-radical interception of the alkyl iodide or bromide formed can allow for the trapping of products arising from less exothermic or even endothermic atom transfer additions and are exemplified in Scheme 11. Yoon [30] and Curran [31] have demonstrated that the a-halo ethers formed upon addition to vinyl ethers can be trapped with alcohols, leading to formation of acetals. Substitution reactions on the heteroaromatics pyrrole and indole have been carried out through a sequence of steps involving I- or Br-... 

Most oxy or halo free radicals readily oxidize phenjienediamines to radical cations called Wurster salts (12), which are stable in water-ethanol solutions at pH 3. On the other hand, carbon and sulfur radicals generally do not produce Wurster salts. The intense color of Wurster salts can be used as a quick spot test for variously substituted phenylenediamines when oxidized with bromine in carbon tetrachloride solution. For example, Al,AT-diaIkyl substitution gives red, N,N,N N gives blue, and N-alkyl-TST-aryl gives light blue or green. 

Metzer et al. reported an enantioselective radical reduction of a-halo carboxylic acid with chiral alkyl-substituted binaphthyltin compounds namely, optically active a,a-disubstituted esters were obtained from the corresponding a-halo esters by the treatments with stoichiometric amounts of chiral Sn(IV) hydride and chiral Sn(IV) bromide as an additive (Scheme 10.45) [70]. 

---