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Alkynes
Triple Bond, Terminal Alkynes, Internal Alkynes
Nomenclature
Suffix: "-yne"
• Find longest chain containing both carbons of the alkyne
• Number giving alkyne the lowest number
• Multiple Alkynes: -diyne (2), -triyne (3)
• Molecule with both Alkene and Alkyne Bonds: -enyne
The chain gets numbered to give the first point of
unsaturation at the lowest number
• Alkyne + Alcohol: -ynol (numbered with alcohol at lowest
number)
• Common Name: for ethyne (2 carbon alkyne) - acetylene
• As a substituent: ethynyl
Preparation
E2 reactions of geminal or vicinal dihalides
• Reagents - 2 possibilities
• NaNH2 (sodium amide - 2 equivalents)
• KOtBu (2 equivalents), DMSO, H2O
Hydrohalogenation
2 equivalents of H-X added
• Both hydrogens added to same carbon atom
• Markovnikov Addition seen with terminal alkynes
• Best for symmetrical or terminal alkynes - other wise see 2
products (constitutional/regio isomers)
• 1 equivalent of H-X will stop reaction at the alkene
Halogenation
• Addition of 2 equivalents of X2
Hydroboration-Oxidation
Terminal Alkyne: BH2 ends up on least substituted carbon
leading to an aldehyde product after oxidation
• Ketone/Aldehyde products form from tautomerization of enol
• Comparison with Hydration conditions with acid/Hg2+
• No difference for internal alkynes
• Terminal alkynes acidic conditions give ketone,
hydroboration conditions give aldehyde
Reactions of Acetylide Ions
Substitution/Elimination Reactions with Alkyl Halides
• Ring opening of epoxides (as strong nucleophiles)
Synthesis
• Retrosynthetic Analysis: planning a synthesis of a compound
involving more than one step - working backwards from
product to starting material
Hydrohalogenation 2
Add 2 equivalent of H-X to get geminal dihalide
Halogenation 2
Add 2 equivalent of X-X to get tetrahalide (anti addition of X2)
Hydration 2
Add H2O, H2SO3, HgSO4 gives ketone (enol intermediate)
Hydroboration-Oxidation2
1. R2BH, 2.H2O2 HO gives aldehyde (enol inteermediate)
Formation of acetylide anions
terminal alkyne + strong base (NaH or NaNH2) give acetylide anions
Acetylide anions react with electrophiles
Sn2
Acetylide anions and epoxides
Acetylide anions and epoxides in water make H-C(TB)C-CH2CH2OH