Chapter 3

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:

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