Question

Using propyne as the starting material, carry out the necessary reaction sequences to give the following products: (a) 2 -butyne (b) 2 -butene (c) 2 -bromobutane (d) pentane.

Short answer

To synthesize the given products from propyne, follow these steps: (a) 2-butyne: Deprotonate propyne using NaNH2, then perform alkylation with 1-iodoethane. (b) 2-butene: Start with 2-butyne from (a), then perform partial hydrogenation using the Lindlar catalyst. (c) 2-bromobutane: Synthesize 2-butene as in (b), then perform halogenation with HBr. (d) Pentane: Synthesize 2-butyne as in (a), then perform complete hydrogenation using a metal catalyst (Pd, Pt, or Ni).

Step-by-step solution

(a) Synthesis of 2-butyne

: 1. Reaction with NaNH2: To synthesize 2-butyne, we first need to deprotonate the least substituted carbon in propyne, which is the terminal carbon with the triple bond. Treat the propyne with a strong base, such as NaNH2, in liquid ammonia (NH3) as a solvent. This reaction generates an acetylide ion, as shown below: CH≡C-CH3 + NaNH2 → CH≡C:^- + Na^+ + NH3 2. Alkylation: Now, to elongate the carbon chain and get to 2-butyne, react the acetylide ion obtained in step 1 with 1-iodoethane (CH3CH2I). The acetylide ion will act as a nucleophile, attacking the carbon bearing the iodine, and the iodine will leave as iodide ion. CH≡C:^- + CH3CH2I → CH≡C-CH2CH3 The final product is 2-butyne.

(b) Synthesis of 2-butene

: Perform a partial hydrogenation of 2-butyne, obtained from part (a), using a Lindlar catalyst and hydrogen gas. The Lindlar catalyst consists of a poisoned palladium catalyst, and it'll stop the hydrogenation at the alkene stage. CH≡C-CH2CH3 + H2 (Lindlar catalyst) → CH2=CH-CH2CH3 The final product is 2-butene.

(c) Synthesis of 2-bromobutane

: 1. Forming 2-butene: Follow the reaction steps from part (b) to synthesize 2-butene from propyne. 2. Halogenation: Treat 2-butene obtained from step 1 with HBr to obtain 2-bromobutane. CH2=CH-CH2CH3 + HBr → CH3CHBrCH2CH3 The final product is 2-bromobutane.

(d) Synthesis of pentane

: 1. Forming 2-butyne: Follow the reaction steps from part (a) to synthesize 2-butyne from propyne. 2. Complete hydrogenation: Fully hydrogenate 2-butyne using a metal catalyst (e.g., Pd, Pt, or Ni) and hydrogen gas. The triple bond will be transformed into a single bond. CH≡C-CH2CH3 + 2H2 (Pd, Pt, or Ni catalyst) → CH3CH2CH2CH3 The final product is pentane.

Key concepts

Synthesis of 2-Butyne

To understand the synthesis of 2-butyne from propyne, we start by considering the terminal alkyne, propyne, as our foundational molecule. In the laboratory, synthesizing 2-butyne involves a two-step reaction.

Initially, we treat propyne with sodium amide (NaNH₂), a strong base that abstracts a proton from the terminal carbon of the alkyne, forming an acetylide ion. This is strategically done in a solvent like liquid ammonia, and the process looks like this:
CH≡C-CH₃ + NaNH₂ → CH≡C:^- + Na⁺ + NH₃

Once we have this acetylide ion, it is now ready to undergo an alkylation reaction. We introduce an alkyl halide such as 1-iodoethane (CH₃CH₂I) to the reaction. The acetylide ion, being nucleophilic, attacks the electrophilic carbon bonded to iodine. The iodine then departs as an iodide ion, leaving the extended carbon chain of 2-butyne behind:
CH≡C:^- + CH₃CH₂I → CH≡C-CH₂CH₃

This sequence of deprotonation and alkylation is a classic approach to building carbon chains in organic synthesis, essentially forming carbon-carbon bonds to extend the molecule.

Partial Hydrogenation

Partial hydrogenation is a nuanced and controlled chemical reaction used to reduce alkynes to alkenes. It's a fundamental step to obtain various olefinic (containing carbon-carbon double bonds) compounds such as 2-butene from alkynes like 2-butyne.

The process uses a specially designed catalyst known as a Lindlar catalyst, which is palladium deposited on calcium carbonate and treated with lead acetate and quinoline. This catalyst is 'poisoned' to ensure that it selectively stops the hydrogenation at the alkene stage without further reducing the alkene to an alkane.
CH≥C-CH₂CH₃ + H₂ (Lindlar catalyst) → CH₂=CH-CH₂CH₃

The use of a (Lindlar catalyst) ensures that the alkyne undergoes an addition of hydrogen (H₂) only once across the triple bond to form a cis-alkene. Understanding the specifics of the catalyst and reaction conditions is key for chemists to control the outcome of hydrogenation reactions in synthetic organic chemistry.

Alkylation

Alkylation is a fundamental organic chemistry reaction where an alkyl group is transferred from one molecule to another. It is often employed in extending carbon chains and creating more complex molecules from simpler ones.

For instance, in the synthesis of 2-butyne, the alkylation step involves adding an ethyl group to the acetylide ion generated from propyne. This is performed via a nucleophilic substitution reaction, where the negatively charged acetylide ion acts as the nucleophile and the electrophilic carbon (attached to iodine) in the alkyl halide serves as the substrate.
CH≡C:^- + CH₃CH₂I → CH≡C-CH₂CH₃

Alkylation reactions can sometimes face challenges such as rearrangements or multiple substitutions, and thus careful consideration of reaction conditions and the nature of the alkylating agent is crucial for reaching the desired outcome. The success of this method helps chemists to effectively build larger organic molecules in a stepwise fashion.

Halogenation

Halogenation is another key reaction in functional group transformation within organic compounds. It involves the addition of halogens, such as bromine or chlorine, to other molecules. Specifically, in the synthesis of 2-bromobutane, we use the halogenation of 2-butene with hydrogen bromide (HBr).

During halogenation, the pi electrons of the alkene double bond attack the electrophilic bromine atom in HBr, leading to the formation of a carbocation intermediate. Bromide ion (Br^-) then attacks the carbocation, resulting in the substituted bromoalkane.
CH₂=CH-CH₂CH₃ + HBr → CH₃CHBrCH₂CH₃

The halogenation reaction can proceed through different mechanisms, such as free radical halogenation or electrophilic addition, depending on the nature of the starting alkene and the reaction conditions. Halogenation is widely used to introduce halogen atoms into organic molecules, which can further serve as versatile intermediates for other chemical transformations.