Acetylide

Acetylides are synthesised by reacting terminal alkynes (RC≡CHRC \equiv CHRC≡CH) with strong bases (e.g., sodium amide) or metals (e.g., copper or silver salts). Acetylides are nucleophilic and commonly used in reactions like alkynylation and coupling to extend carbon chains. Certain acetylides (e.g., silver and copper acetylides) are used as qualitative tests for terminal alkynes.

1.0Acetylide

Acetylides are compounds where a metal is bonded to an alkyne, with the general formula RC≡CM (R = organic group, M = metal). Acetylides are used as reagents in organic synthesis and intermediates in coupling reactions.

2.0Structure and Bonding of Acetylide

Ionic Properties:

• When bonded with alkali or alkaline earth metals, acetylides show ionic behaviour.
• Hydrolysis: Produces acetylene and metal oxides.
• Example: HC≡CK + H_2O → HC≡CH + KOH

Covalent Properties:

• Observed when acetylides are bonded with transition metals.
• They may form polymeric structures where the acetylide group is a bridging ligand.
• Similarity: Shares properties with doubly substituted analogues (disubstituted alkynes).

3.0 Synthesis of Acetylides

From Alkynes

1. Superbases Used:

• Organometallic Superbases: e.g., Organolithium reagents (e.g., Ethyl lithium).
• Inorganic Superbases: e.g., Sodium amide (NaNH₂), Lithium amide (LiHMDS).

2. General Reaction: $\text{RC}\equiv \text{CH}+\text{R-M}\rightleftharpoons \text{R-H}+\text{RC}\equiv \text{CM}$
3. Example: $\text{H-C}\equiv \text{C-H}+\text{EtLi}\to \text{H-C}\equiv \text{C-Li}+\text{Et-H}(At-78 °C)$

From Alkenes

1. Solvents Used:

• Liquid Ammonia
• Ethereal Solvents

Using Specific Reagents

1. Lithium Acetylide:

• Reagents: Lithium amide (LiHMDS), Organolithium reagents (e.g., Ethyl lithium).

2. Sodium/Potassium Acetylide:

• Reagents: Sodium amide (NaNH₂) or Potassium hydroxide (KOH) at room temperature.
• Reaction:  HC≡CH+KOH⇌HC≡C-K+H₂​O

3. Copper(I) Acetylide:

• Method: Pass Copper(I) chloride (CuCl) through acetylene gas.

4. Silver Acetylide:

• Method: React Acetylene with Silver nitrate (AgNO₃).

5. Calcium Carbide:

• Method: Heat Calcium oxide (CaO) with Carbon at ~2000 °C.

6. Lithium Carbide:

• Method: Heat Lithium oxide (Li₂O) with Carbon at ~2000 °C.

4.0Chemical Reactions of Acetylide

1. Propargyl Alcohol Formation

• Reaction: Acetylide reacts with alcohol or Aldehydes.

2. Deprotonation:

• Reagent: N-Butyllithium deprotonates Ethyl propiolate to form the corresponding acetylide.

Favorskii Reaction

• Under fundamental conditions, an alkyne and a carbonyl compound reaction produces an alkynol.
• Discovered By: Russian chemist Alexei Favorski in the early 20th century.
• General Reaction: $RC\equiv CH+R^{\prime }COR"\stackrel{Base}{\to }R-C\equiv C-CH(OH)-R"$

Coupling Reactions

Reactions involve joining two chemical species to form a new bond.

Common Example: Organic Halide + Organometallic Compound → Coupled Product

Role of Acetylides: Intermediates in coupling reactions, facilitating the formation of carbon-carbon bonds.

Alkynylation

• Used in introducing alkynyl groups into molecules.

Nucleophilic Reactions

• Acetylides act as nucleophiles, reacting with electrophiles or unsaturated substrates.

5.0Applications of Acetylides

1. Explosives: High sensitivity to water makes them useful as explosives.
2. Industrial Testing:

Copper Acetylide:

• Tests for the presence of acetylene gas.
• Forms inside pipes, used in petrochemistry.

3. Organic Synthesis:

• Detects terminal alkynes.
• Acts as reagents/intermediates in forming complex organic molecules

4. Hybridisation & Bonding: Understand the sp, sp², and sp³ hybridisation in related compounds.
5. Synthesis Techniques: Memorise reagents and conditions for synthesising different acetylides.
6. Reaction Mechanisms: Grasp the mechanism of Favorskii and Coupling reactions involving acetylides.
7. Applications: Relate practical applications to theoretical concepts, especially in synthesis and industrial processes.
8. Equilibrium Reactions: Analyse reversible reactions, such as acetylide formation with bases.