Alkynes | Organic Chemistry 2
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
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: