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Chapter 8: ALKYNES - Coggle Diagram
Chapter 8: ALKYNES
Reactivity of Alkanes
2.1. Acidity of Terminal Alkynes
2.2. Electrophilic Addition
2.2.1. Hydrohalogenation
2.2.2. Hydration
Catalyzed by H2SO4/HgSO4
Follow Markovnikov rule
Enol product is not stable → tautomerize to ketone
For terminal alkyne → give ketone, not aldehyde
For asymmetric alkyne → give mixture of ketones
Enol-ketone tautomerization
Catalyzed by acid
Catalyzed by base
2.2.3. Hydroboration
Similar to hydroboration of alkenes
syn addition
anti-Markovnikov rule
Enol product is not stable → tautomerize to aldehyde
2.2.4. Hydrogenation
Normal catalyst → give alkane
Poisoned catalyst → give alkene (syn addition → cis-alkene)
To get trans-alkene → use Na/NH3 (anti addition)
Interconverting alkanes, alkenes, and alkynes
Similar to halogenation of alkenes
Cyclic ion intermediate → anti Addition
If cyclic ion is asymmetric → Nucleophile add to less substituted carbon
If excess X2 → Add 2 times
2.3. Oxidation
2.3.1. Ozonolysis
Terminal alkynes → give carboxylic acid and CO2
Other alkynes → give mixture of carboxylic acids
Preparation of Alkynes
1.1. From Inorganic Source
1.2. Elimination reaction
Vicinal dihalide
Geminal dihalide
Terminal alkynes are weak acids
3 equivalents of NaNH2/NH3
Alkynide ion need to be protonated to give alkyne
Weak bases (pKa of conjugated acid < 25) cannot deprotonate terminal alkynes, e.g.
RCOO- (carboxylate): pKa ≈ 4
HO- (hydroxide): pKa = 14
RO- (alkoxide): pKa ≈ 16-17
Alkynide ion can be a nucleophile or base for various reactions