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Hydrohalogenation
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A hydrohalogenation reaction is the electrophilic addition of like hydrogen chloride or to to yield the corresponding .

If the two carbon atoms at the double bond are linked to a different number of hydrogen atoms, the halogen is found preferentially at the carbon with fewer hydrogen substituents, an observation known as Markovnikov's rule. This is due to the abstraction of a hydrogen atom by the from the hydrogen halide (HX) to form the most stable (relative stability: 3°>2°>1°>methyl), as well as generating a halogen .

A simple example of a hydrochlorination is that of with hydrogen chloride gas (no solvent):

Alkynes also undergo hydrohalogenation reactions. Depending on the exact substrate, alkyne hydrohalogenation can proceed though a concerted protonation/nucleophilic attack (AdE3) or stepwise by first protonating the alkyne to form a , followed by attack of HX/X to give the product (AdE2) ( see for arrow pushing).
(1987). 9780060440848, Harper & Row.
As in the case of alkenes, the regioselectivity is determined by the relative ability of the carbon atoms to stabilize positive charge (either a partial charge in the case of a concerted transition state or a full formal charge for a discrete vinyl cation). Depending on reaction conditions, the main product could be this initially formed , or the product of twice hydrohalogenation to form a dihaloalkane. In most cases, the main regioisomer formed is the gem-dihaloalkane.
(2014). 9781464120275
This regioselectivity is rationalized by the resonance stabilization of a neighboring carbocation by a lone pair on the initially installed halogen. Depending on relative rates of the two steps, it may be difficult to stop at the first stage, and often, mixtures of the mono and bis hydrohalogenation products are obtained.


Anti-Markovnikov addition
In the presence of , HBr adds to a given in an anti-Markovnikov addition fashion. Regiochemistry follows from the reaction mechanism, which exhibits halogen attack on the least-hindered unsaturated carbon. The mechanism for this resembles free radical halogenation, in which the peroxide promotes formation of the radical. However, this process is restricted to addition of HBr. Of the other (HF, HCl, and HI), only HCl reacts similarly, and the process is too slow for synthetic use. (With HF and HI, the energy released in the halogen-carbon addition does not suffice to cleave another hydrogen-halogen bond. Consequently the chain cannot propagate.)
(2004). 9780471264187, Wiley.

The resulting 1-bromoalkanes are versatile . By reaction with , they are precursors to . By reaction with tertiary amines, long-chain alkyl bromides such as 1-bromododecane, give quaternary ammonium salts, which are used as phase transfer catalysts.

With the addition is also anti-Markovnikov because now a nucleophilic X reacts in a nucleophilic conjugate addition for example in the reaction of HCl with .


Scope
Recent research has found that adding or to H-Cl (or H-Br) in increases the rate of reaction making it an easy one to carry out.

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