Ahydrohalogenation reaction is theelectrophilic addition ofhydrogen halides likehydrogen chloride orhydrogen bromide toalkenes to yield the correspondinghaloalkanes.[1][2][3]
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 asMarkovnikov's rule. This is due to the abstraction of a hydrogen atom by thealkene from the hydrogen halide (HX) to form the most stablecarbocation (relative stability:3°>2°>1°>methyl), as well as generating a halogenanion.
A simple example of a hydrochlorination is that ofindene withhydrogen chloride gas (no solvent):[4]
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 avinyl cation, followed by attack of HX/X− to give the product (AdE2) (seeelectrophilefor arrow pushing).[5] 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 formedalkenyl halide, or the product of twice hydrohalogenation to form a dihaloalkane. In most cases, the main regioisomer formed is thegem-dihaloalkane.[6] 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.
In the presence ofperoxides, HBr adds to a givenalkene 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 thischain reaction resemblesfree radical halogenation, in which the peroxide promotes formation of thebromine radical. However, this process is restricted to addition of HBr. Of the otherhydrogen halides (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.)[7][8]
The resulting 1-bromoalkanes are versatilealkylating agents. By reaction withdimethyl amine, they are precursors tofatty tertiary amines. By reaction with tertiary amines, long-chain alkyl bromides such as 1-bromododecane, givequaternary ammonium salts, which are used asphase transfer catalysts.[9]
WithMichael acceptors the addition is also anti-Markovnikov because now a nucleophilic X− reacts in anucleophilic conjugate addition for example in the reaction of HCl withacrolein.[10]
Recent research has found that addingsilica gel oralumina to H-Cl (or H-Br) indichloromethane increases the rate of reaction making it an easy one to carry out.[citation needed]
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