Alkynes | Organic Chemistry 2

The alkynes are studied in this chapter: the naming and properties of alkynes, the preparation of alkynes, the reduction reactions, the electrophilic addition reactions, the anti-Markovnikov additions.

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).
C≡C 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: C≡C 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.