Alkynes | Organic Chemistry 2
Naming the Alkynes
Name: the -ane ending of the corresponding alkane is replaced by -yne.
The rules for naming alkenes also apply to alkynes.
Substituents bearing a triple bond are alkynyl groups.
The alkyne takes precedence over alkene but not over the alcohol.
Properties of Alkynes
Boiling and Melting points: similar to corresponding alkanes and alkenes.
Hybridization: sp (linear geometry).
CC bond: strong and short (1.20 A, shorter than a double bond).
Highly energetic compound with π bonds more reactive than those of alkenes.
Acidity: terminal alkynes are remarkably acidic pKa ~ 25.
NMR: 1H δ ~ 2-3 ppm with coupling constant J between 2-4 Hz; 13C δ ~ 65-95 ppm
IR: CC bond stretch ν ~ 2100 cm-1
Preparation of Alkynes
Double Elimination from 1,2-dihaloakanes:
Base: 2 equivalents of strong base (ex: NaNH2, liquid NH3).
Mechanism: double elimination reaction.
Starting compound: vicinal dihaloalkane ⇒ electrophilic halogenation of alkenes.
Alkylation of alkynyl anions:
Base: BuLi, RMgBr or LiNH2, liquid NH3.
Alkylation: electrophilic addition of haloalkane, carbonyl or epoxide.
Reduction of Alkynes
Electrophilic Addition Reactions
anti addition and Markovnikov product.
The first addition of HBr is often followed by a second addition ⇒ geminal dihaloalkanes formation.
anti addition
The first addition of Br2 is often followed by a second addition.
anti addition and Markovnikov product.
Anti-Markovnikov Additions
Mechanism: radical addition.
Products: cis-trans mixture.