Propyne can be converted to pent-2-yne through a two-step process involving the formation of sodium prop-1-ynide, followed by its alkylation with ethyl chloride.
To transform propyne (CH₃-C≡CH) into pent-2-yne (CH₃CH₂-C≡C-CH₃), a standard alkyne synthesis route known as alkylation of terminal alkynes is employed. This method leverages the acidic nature of the terminal hydrogen atom in propyne.
Step 1: Formation of Sodium Prop-1-ynide
Propyne is a terminal alkyne, meaning it has a hydrogen atom directly attached to a triple-bonded carbon. This hydrogen is weakly acidic and can be abstracted by a very strong base to form a carbanion called an acetylide ion. This step is crucial as the acetylide ion is a powerful nucleophile, essential for the subsequent alkylation.
- Reagent: A strong base such as sodium amide (NaNH₂) is typically used.
- Conditions: The reaction is often carried out in liquid ammonia (NH₃(l)).
Reaction:
CH₃−C≡CH + NaNH₂ → CH₃−C≡C⁻Na⁺ + NH₃
- Propyne reacts with sodium amide to yield sodium prop-1-ynide (an acetylide) and ammonia.
Step 2: Alkylation with Ethyl Chloride
Once the sodium prop-1-ynide is formed, it can readily react with an alkyl halide (specifically, an unhindered primary alkyl halide) to extend the carbon chain. This reaction directly incorporates the ethyl group onto the alkyne, extending the chain from three carbons (propyne) to five carbons (pent-2-yne).
- Reagent: Ethyl chloride (C₂H₅Cl), also known as chloroethane, is used.
- Mechanism: This reaction proceeds via an S_N2 (bimolecular nucleophilic substitution) mechanism, where the acetylide anion attacks the electrophilic carbon of ethyl chloride, and the chloride ion departs as a leaving group.
Reaction:
CH₃−C≡C⁻Na⁺ + CH₃CH₂−Cl → CH₃−C≡C−CH₂CH₃ + NaCl
- Sodium prop-1-ynide reacts with ethyl chloride to form pent-2-yne and sodium chloride.
Summary of the Conversion
The conversion can be summarized as follows:
| Reactant | Reagent/Condition (Step 1) | Intermediate | Reagent (Step 2) | Product |
|---|---|---|---|---|
| Propyne | NaNH₂ / NH₃(l) | Sodium Prop-1-ynide | Ethyl Chloride | Pent-2-yne |
| CH₃−C≡CH | CH₃−C≡C⁻Na⁺ | CH₃CH₂−Cl | CH₃CH₂−C≡C−CH₃ |
This two-step approach is a fundamental method for synthesizing longer chain alkynes from shorter terminal alkynes.
