Question

2-Hexyne gives trans -2-Hexene on treatment with (A) \(\mathrm{LiAlH}_{4}\) (B) \(\mathrm{Pt} / \mathrm{H}_{2}\) (C) \(\mathrm{Li} / \mathrm{NH}_{3}\) (D) \(\mathrm{Pd} / \mathrm{BaSO}_{4}\)

Short answer

The suitable reducing agent for converting 2-hexyne to trans-2-hexene is \(C) \: Li/NH_3\), as it selectively reduces the triple bond to a double bond, forming the desired product.

Step-by-step solution

Identify the functional group in 2-hexyne

The molecular formula of 2-hexyne is \((CH_3)_2 C \equiv C(CH_2)_2CH_3\). It has a triple bond between the second and third carbons, which indicates that it is an alkyne.

Analyze the reaction with Option A: LiAlH4

Lithium aluminium hydride (LiAlH4) is a highly reducing agent capable of reducing carbonyl compounds like ketones, aldehydes, esters, and amides to alcohols. However, LiAlH4 does not reduce alkynes like 2-hexyne. Therefore, Option A is not suitable for the reaction.

Analyze the reaction with Option B: Pt/H2

Platinum under hydrogen gas (Pt/H2) is a common catalyst used in the hydrogenation reactions of alkenes and alkynes. However, Pt/H2 will reduce 2-hexyne to hexane, a saturated hydrocarbon, rather than trans-2-hexene. As the desired product is trans-2-hexene, Option B is not suitable for the reaction.

Analyze the reaction with Option C: Li/NH3

Lithium in liquid ammonia (Li/NH3) is a mild reducing agent known for reducing alkynes to trans-alkenes by selectively reducing the triple bond to a double bond. Treatment of 2-hexyne with Li/NH3 gives trans-2-hexene as the desired product. Therefore, option C is suitable for the reaction.

Analyze the reaction with Option D: Pd/BaSO4

Palladium on barium sulfate (Pd/BaSO4) is another catalyst used in hydrogenation reactions. However, like Pt/H2, in the reagent Pd/BaSO4, there is no chemo-selectivity for the reduction of the triple bond to a double bond, so Pd/BaSO4 is not suitable for this reaction. In conclusion, the correct option for the reduction of 2-hexyne to trans-2-hexene is: \(C) \: Li/NH_3 \)

Key concepts

Alkyne Functional Group

Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond, and their general formula is C_nH_2n-2. This functional group defines the chemical properties of the compound and is highly reactive due to the presence of the triple bond. The triple bond consists of one sigma bond and two pi bonds, which makes alkynes more unsaturated than alkenes and alkanes. Alkynes present unique reactions like addition, where the triple bond opens up to accommodate more atoms, leading to more saturated compounds.

For example, 2-hexyne is a molecule with a six-carbon chain, and the alkyne functional group is located between the second and third carbon atoms, indicated by the bond formula C≡C. This setup plays a crucial role in the reactivity and properties of the molecule, being a characteristic feature used in various organic synthesis reactions, including hydrogenation.

Hydrogenation Reactions

Hydrogenation reactions involve the addition of hydrogen (H₂) to other molecules, typically unsaturated organic compounds such as alkenes and alkynes. These reactions aim to reduce or saturate the compound by breaking the multi-bonds between carbon atoms and adding hydrogen atoms.

In the context of alkynes, hydrogenation can completely saturate the alkyne to form an alkane when using catalysts like platinum or palladium under hydrogen gas. These catalysts allow for the addition of hydrogen across the triple bond, converting it successively into a single bond, as they give up their pi bonds to form new sigma bonds with hydrogen atoms.

Important Considerations

• Catalyst Choice: The metal used as a catalyst plays a crucial role in the reaction outcome. For complete saturation, heavy metals like Pt or Pd are commonly used.
• Reaction Conditions: Temperature, pressure, and the presence of other additives can modify the reactivity and selectivity of the hydrogenation process.
• Product Type: Depending on the desired product, different catalysts and techniques are employed to control the outcome, with some methods allowing partial reduction to form alkenes.

Selective Reduction

Selective reduction is a targeted form of reaction where a specific functional group in a molecule is reduced without affecting other parts. This is crucial when dealing with multifunctional molecules where a single undesired reaction can lead to a different product.

The significance of selectivity is evident in the reduction of alkynes to alkenes. If a reduction is non-selective, an alkyne can become fully saturated to an alkane. However, certain techniques and reagents, like the use of lithium in liquid ammonia (Li/NH₃), allow for partial reduction. This selective process involves the careful addition of hydrogen to just one of the triple bonds, converting it to a cis or trans double bond, which results in an alkene. The stereospecificity of the reaction can lead to either cis or trans products, with Li/NH₃ typically leading to trans alkenes due to the reaction mechanism.

Selective reduction is not only critical for synthesizing specific compounds but is also advantageous for creating compounds with desired properties, such as selectivity between cis and trans forms of an alkene.