Alkynes, with their triple bonds, are eager to react. They undergo , forming carbocations along the way. This process mirrors alkene reactions but with unique twists in stability and product formation.
and additions to alkynes follow specific rules. guides where protons land, while determines the final product's shape. These reactions transform alkynes from sp to sp2 , changing their very nature.
Addition of HX and X2 to Alkynes
Electrophilic addition to alkynes
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Top images from around the web for Electrophilic addition to alkynes
Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Organic chemistry 18: Electrophilic addition to alkenes View original
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Electrophiles add to alkynes in a two-step process involving a intermediate
Step 1: Electrophile (proton or halogen) adds to one of the carbons forming a vinylic carbocation
determined by the stability of the vinylic carbocation intermediate (more stable carbocation favored according to Markovnikov's rule)
Step 2: Nucleophile ( or solvent) attacks the vinylic carbocation leading to the final product
Stereochemistry of the product determined by the approach of the nucleophile from either the top or bottom face of the planar vinylic carbocation
The reaction involves breaking one of the pi bonds in the alkyne
Mechanism of HX alkyne addition
Proton (H+) from HX adds to one of the alkyne carbons forming a vinylic carbocation intermediate
Proton adds to the carbon that will yield the more stable carbocation (Markovnikov's rule)
Halide ion (X−) attacks the vinylic carbocation leading to the final product
Similar two-step mechanism to alkene addition involving carbocation intermediate
Alkynes yield vinylic carbocations while alkenes yield alkyl carbocations
Vinylic carbocations more stable than primary alkyl carbocations but less stable than secondary and tertiary alkyl carbocations (stability: 3° > 2° > vinylic > 1°)
Increased stability of vinylic carbocations compared to primary alkyl carbocations due to (positive charge delocalized between the two carbons of the vinylic system)
Products of alkyne HX and X2 reactions
Addition of HX to alkynes ():
Markovnikov's rule: Proton adds to the carbon that will yield the more stable carbocation intermediate
Major product: Halogen attached to the less substituted carbon (more hydrogens)
Minor product: Halogen attached to the more substituted carbon (fewer hydrogens)
Stereochemistry: Product is a mixture of E and Z isomers due to approach of the nucleophile from either face of the planar vinylic carbocation
Addition of X2 to alkynes:
Stereochemistry: with halogen atoms adding to opposite faces of the alkyne in a concerted mechanism
Product: with (halogens on opposite sides of the double bond)
Hybridization changes during alkyne reactions
Alkynes start with sp hybridization
After addition reactions, the product has sp2 hybridization
This change in hybridization affects the geometry and reactivity of the molecule