Reactivity effects halogenation

Regarding the substituent effect on reactivity of groups in positions 4 and 5 there is little information in the literature. The reactivity of halogen in position 5 seems to be increased when an amino group is present in position 2. Substitution products are easily obtained using neutral nucleophiles such as thiourea, thiophenols, and mercaptans (52-59). 

Steric effects are undoubtedly a factor in the reactivity of halogen substituted fumaric acid complexes to Cr+2 (18). The results of rate measurements for these species are shown in Table III. Specific rates are entered both for the (H+) independent path (path 1, ki) and for the path first order in (H+) (path 2, fo). 

The reactivity of halogen atoms in position 2 of oxazoles is considerably enhanced by quaternization. This is best illustrated by the synthetic utility of the salt (141) which, like other Ar-alkyl-2-halogeno-azolium and -azinium salts, effects a number of condensation reactions but it far surpasses these other salts in activity (79AG(E)707). Thus it activates carboxylic acids and it dehydrates formamides to isocyanides (Scheme 6). Amides are similarly converted into cyanides, ketones (RCH2COAr) into alkynes (RC=CAr), and cyanohydrins (RCH(OH)CN) are transformed into the corresponding chloro compounds <79CL1117>. 

Halogen compounds with two reactive carbon-halogen bonds can be bifunctional initiators for metal-catalyzed living radical polymerizations to give telechelic polymers. The effective bifunctional initiators include MI-1 to MI-26 with various spacers between the initiating sites. 

Substitution for hydrogen by halogen is an important electrophilic aromatic substitution reaction. The reactivity of the halogen molecules decreases in the order Cl2>Br2 >l2. The molecular halogens are not the only species that can effect halogenation, however. Many reactions are run in the presence of Lewis acids, in... 

---