Hydrohalogenation regioselectivity

The hydrohalogenation reaction can also be performed with MX in HOAc [207a], The reaction of 4-substituted-2,3-allenoates affords a mixture of ( )- and (Z)-/3-halo-/8,y-unsaturated enoates [87]. With 2-substituted-2,3-allenoates, the reaction should be carried out in H0Ac-CF3C02H (1 1) or CF3C02H to form /8-halo-/3,y-unsaturated enoates with high regioselectivity [255],... 

For unsymmetrical alkenes, the placement of the halogen represents an issue of regiochemistry. Hydrohalogenation reactions are regioselective, because the halogen is generally placed at the more substituted position, called Markovnikov addition. 

So far we have only looked at hydrohalogenations of symmetric alkenes, in which the sense of addition, H to one, X to the other carbon, was immaterial. What about unsymmetric alkenes Will there be regioselectivity To answer these questions, let us consider the reaction of propene with hydrogen chloride. Two products are possible 2-chloropropane and 1-chloropropane. However, the only product observed is 2-chloropropane. 

Hydrohalogenation Markovnikov regioselectivity, except HBr + peroxides (ROOR) (12-3,13)... 

Hydrohalogenation reactions are regioselective, because the halogen is generally placed at the substituted position, called addition. 

Since cation B is resonance-stabilized, it is much more stable than a typical vinyl carbocation. That is, cation B is more stable than cation A, e q)laining the regioselectivity observed in this series of hydrohalogenation reactions. This explains why Cl is installed at the benzylic position (the position next to the aromatic ring), because that is the location of the most stable carbocation. 

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