Alkynes | Organic Chemistry 2

Alkynes are studied in this chapter: name and properties of alkynes, preparation of alkynes, reduction reactions, electrophilic addition reactions, anti-Markovnikov additions

Nomenclature of 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
Alkynes take precedence over alkenes but not over alcohols

 

Properties of Alkynes

Boiling and melting points: similar to corresponding alkanes and alkenes
Acidity: terminal alkynes are remarkably acidic (pKa ~ 25)
 

Absorption spectroscopy:

NMR: 1H δ ~ 2-3 ppm (coupling constant J between 2-4 Hz); 13C δ ~ 65-95 ppm
IR: C ≡ C bond stretching frequency ν ~ 2100 cm-1
 

C ≡ C bond of alkynes:

1 σ bond ⇒ formed from 2 sp orbitals of carbon atoms
2 π bonds ⇒ formed from 4 p orbitals of carbon atoms

The π bonds of alkynes more reactive than those of alkenes ⇒ highly energetic compound
C ≡ C bonds are stronger and shorter than a double bond
 

Geometry:

The carbons of the C ≡ C bond of an alkyne are sp hybridized ⇒ linear geometry

Preparation of Alkynes

Double Elimination from 1,2-dihaloakanes:
 


Mechanism: 

Double elimination reaction from a vicinal dihaloalkane ⇒ 2 equivalents of strong base are necessary (ex: NaNH2, liquid NH3). The starting vicinal dihaloalkane can be formed by electrophilic halogenation of alkenes​​​​​​​

 

Alkylation of alkynyl anions:
​​​​​​​


​​​​​​​Mechanism:

  1. Deprotonation of an alkyne with a strong base (BuLi, RMgBr or LiNH2, liquid NH3)
  2. Alkylation by electrophilic addition of haloalkane, carbonyl or epoxide

Reduction of Alkynes

Electrophilic Addition Reactions

Addition of hydrogen halide:
 


anti addition and Markovnikov product
The first addition of HBr is often followed by a second addition ⇒ geminal dihaloalkanes formation

 

Halogenation:
 


anti addition
The first addition of Br2 is often followed by a second addition

 

Hydration:
 


anti addition and Markovnikov product

Anti-Markovnikov Additions

Addition of HBr with radicals:
 

Radical addition of HBr
A cis-trans mixture is obtained

 

Hydroboration - Oxidation: