Chapter 11 - Alkynes

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