Halonium ions participation

A number of Intermediates of halogen participation (halonium ions, e.g., 21 and... 

A number of intermediates of halogen participation (halonium ions),88 e g., 19 and 20, have been prepared as stable salts in SbF5-S02 or SbFs-SC ClF solutions.89 Some have even... 

Neighbouring halogen participation through halonium ions (p. 91) has been postulated to account for features of some substitution reactions. 5a-Bromo-6jS hydroxy steroids (17) are converted by thionyl chloride into the 5a-bromO"6 -chloro derivatives (19) with over-all retention of configuration [124], An intermediate 5a,6a bromonium ion (18) controls the stereochemistry of subsequent introduction of chloride ion. 

There is direct evidence for the existence of halonium ions such as those that are invoked in bromination and chlorination reactions. They can be formed from dihalides by ionization with neighboring-group participation and can be observed by NMR measurements in nonnucleophilic solvents ... 

Step 1 Make a new bond between a nucleophile and an electrophile and simultaneously break a bond to give stable molecules or ions. The reason for the extremely rapid hydrolysis of the sulfur mustards is neighboring group participation by sulfur in the ionization of the carbon-chlorine bond to form a cyclic sulfonium ion. This is the rate-determining step of the reaction although it is the slowest step, it is much faster than reaction of a typical primary chloroalkane with water. At this point, you should review halogenation of alkenes (Sections 6.3D and 6.3F) and compare the cyclic halonium ions formed there with the cyclic sulfonium ion formed here. 

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