Hydrohalogenation

A hydrohalogenation reaction is the <a href="/facts/Electrophilic_addition/XS6Ym9WV">electrophilic addition</a> of <a href="/facts/Hydrogen_halides/avUQ6Nwy">hydrogen halides</a> like <a href="/facts/Hydrogen_chloride/7Wzkhomd">hydrogen chloride</a> or <a href="/facts/Hydrogen_bromide/rUw1duLA">hydrogen bromide</a> to <a href="/facts/Alkene/RtYcS6kU">alkenes</a> to yield the corresponding <a href="/facts/Haloalkane/Enb7LQqt">haloalkanes</a>.

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 <a href="/facts/Markovnikov%2527s_rule/WJokRx6l">Markovnikov's rule</a>. This is due to the abstraction of a hydrogen atom by the <a href="/facts/Alkene/RtYcS6kU">alkene</a> from the hydrogen halide (HX) to form the most stable <a href="/facts/Carbocation/vIggUxTh">carbocation</a> (relative stability: 3°>2°>1°>methyl), as well as generating a halogen <a href="/facts/Anion/wrbwhF3z">anion</a>.
A simple example of a hydrochlorination is that of <a href="/facts/Indene/ENoRaBy8">indene</a> with <a href="/facts/Hydrogen_chloride/7Wzkhomd">hydrogen chloride</a> 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 <a href="/facts/Vinyl_cation/3rIUACwt">vinyl cation</a>, followed by attack of HX/X− to give the product (AdE2) (see <a href="/facts/Electrophile/1AHFg1zm">electrophile</a> for arrow pushing). 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 <a href="/facts/Vinyl_halide/Jp8gDGw4">alkenyl halide</a>, or the product of twice hydrohalogenation to form a dihaloalkane. In most cases, the main regioisomer formed is the gem-dihaloalkane. 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.

Hydrohalogenation open-in-new

Hydrohalogenation