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9.7: Electrophilic Addition to Alkynes: Hydrohalogenation

JoVE Core
Organic Chemistry

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Electrophilic Addition to Alkynes: Hydrohalogenation

9.7: Electrophilic Addition to Alkynes: Hydrohalogenation

Electrophilic addition of hydrogen halides, HX (X = Cl, Br or I) to alkenes forms alkyl halides as per Markovnikov's rule, where the hydrogen gets added to the less substituted carbon of the double bond. Hydrohalogenation of alkynes takes place in a similar manner, with the first addition of HX forming a vinyl halide and the second giving a geminal dihalide.


Addition of HCl to an Alkyne

Mechanism I – Vinylic carbocation Intermediate

The mechanism begins with a proton transfer from HCl to the alkyne. Here, the π electrons attack the hydrogen atom of hydrogen chloride and displace the chloride ion. This gives a stable secondary vinylic carbocation, which is further attacked by the chloride ion to form a vinyl chloride.


The addition of a second equivalent of hydrogen chloride proceeds with the protonation of vinyl chloride, giving two possible carbocations. In the secondary carbocation the positive charge is delocalized through resonance. As a result, it is more stable and favored over the primary carbocation.



A second nucleophilic attack by the chloride ion gives a geminal dichloride.


Mechanism II – Concerted Termolecular Process

In a vinylic carbocation, the positive charge resides on an electronegative sp-hybridized carbon making it unstable. The second mechanism avoids the formation of this carbocation. Instead, it proceeds via a termolecular (three molecules) process where the alkyne interacts simultaneously with two equivalents of a hydrogen halide like HCl. This leads to a transition state with a partially broken C–C π bond and partially formed C–Cl and C–H σ bonds. The net result is a trans addition of hydrogen from one HCl and a chloride from the other HCl to form a chloroalkene. This further reacts with the displaced HCl to form a geminal dichloride as the final product.


Halogenation of Alkynes with Peroxides

When treated specifically with HBr in the presence of peroxides, terminal alkynes undergo anti-Markovnikov's addition. The Br gets added to the less substituted carbon forming a mixture of E and Z alkenes.



Electrophilic Addition Alkynes Hydrohalogenation Hydrogen Halides Markovnikov's Rule Alkyl Halides Vinyl Halide Geminal Dihalide HCl Vinylic Carbocation Intermediate Proton Transfer Chloride Ion Vinyl Chloride Secondary Carbocation Primary Carbocation Geminal Dichloride Concerted Termolecular Process

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