Substitution a-halogenation

The reaction of an alcohol with a hydrogen halide is a substitution A halogen usually chlorine or bromine replaces a hydroxyl group as a substituent on carbon Calling the reaction a substitution tells us the relationship between the organic reactant and its prod uct but does not reveal the mechanism In developing a mechanistic picture for a par ticular reaction we combine some basic principles of chemical reactivity with experi mental observations to deduce the most likely sequence of steps... 

This compound is heated with caustic potash solution, yielding beta-hydroxy-coumarin. From this body, coumarin is obtained by substituting a halogen atom for the OH group, and then reducing the product in alcoholic solution with zinc-dust. 

The simplest possible halogen acid is the one derived from formic acid by substituting a halogen atom for the non-carboxyl hydrogen. 

Elimination of the 12a-OH function would extend the chromophore (by producing an additional DB at 12a-l) and eliminate activity. Substituting a halogen into position 11a would constitute a chromophoric block. Both of the preceding actions would predictably result in considerably decreased activity. 

In this substitution a halogen atom replaces one of the hydrogen atoms of the methyl group of propene. 

Reactions have already been mentioned in which during electrophilic substitution a halogen atom is replaced from the pyrrole nucleus by some... 

It is now well established that MAO plays a double role in a first step, it methylates the metallocene by substituting a halogen atom and then cationizes the transition metal, permitting the coordination of an olefin monomer acting as a Lewis base. The reaction pathway of these two steps may be represented as follows ... 

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