Hydrohalogenation reactions

Butene reacts with hydrogen chloride in a hydrohalogenation reaction. Estimate the standard reaction enthalpy from bond enthalpies. 

Allenyl sulfones can also readily undergo hydrohalogenation reactions with MX (M = Na, Li X=C1, Br, I) to afford 2-haloallylic sulfones 236 [87, 126]. The reactivity depends greatly on the substitution pattern of the allene moiety. 

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],... 

The hydrohalogenation reaction of 2,3-allenoamide with MX also delivers /3-halo-/3,y-unsaturated enamides. The Z- and E-isomers can be easily separated [87]. 

Like the hydrohalogenation reaction, the hydration of unsymmetrlcal alkenes, such as propene, leads to two products. As well as propan-2-ol, propan-l-ol is also formed. 

The addition of hydrogen halides to unsaturated organic compounds is called here hydrohalogenation in order to stress that it is the reverse reaction to dehydrohalogenation. Two types of the hydrohalogenation reaction have to be considered, the addition to a carbon carbon triple bond... 

Only a few speculations concerning the mechanism of the hydrohalogenation reactions can be found in the literature. They are mostly based on the better knowledge of the reverse reaction, the decomposition of haloalkanes. It seems evident that catalytic hydrohalogenation must also involve transfer of paired valence electrons and that a free radical-like process is highly improbable. The existence of 7r-complex intermediates has been suggested [321,336,337] but the hypothesis lacks experimental evidence. 

Many hydrohalogenation reactions begin with an achiral reactant and form an achiral product. For example, the addition of HBr to cyclohexene, an achiral alkene. forms bromocyclohexane. an achiral alkyl halide. 

Apply Markovnikov s rule to predict the major and minor products of the hydration and hydrohalogenation reactions of unsymmetrical alkenes. 

Hydrohalogenation reactions are characterized by the addition of H and X across a it bond. 

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. 

Hydrohalogenation reactions are only efficient when carbocation rearrangements are not possible. 

PROBLEM 11.37 Predict the major product(s) for the radical hydrohalogenation reaction (HBr, peroxides, heat) with each of the following alkenes ... 

Select the Radical alkene hydrohalogenation reaction. The animation shows initiation steps and propagation steps for the overall process. Write out the steps of the reaction and indicate which are initiation steps and which are propagation steps. 

PEGs were reported to exhibit remarkably high activity as catalysts for de-hydrohalogenation reactions [151] ... 

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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