Critical thinking question (20 Points) The triple bond in alkynes can be reduced by hydrogenation reactions. Depending on the reagent used, different reduction products are obtained as described below: Complete reduction -Hydrogenation by H2(g) in presence of an active catalyst (Pd or Pt) in ethanol at RT, adds two moles of hydrogen across the triple bond to completely saturate the triple bond of the alkyne and forms an alkane Partial stereospecific reduction – Hydrogenation by H2(g) in presence of a poisoned catalyst (Pd/BaSO4) in ethanol at RT, adds one mole of hydrogen, partially reducing the triple bond to a double bond; Where possible, a cis-alkene is produced. Partial stereospecific Reduction (Birch reduction) with metallic sodium in presence of liquid ammonia, at RT, adds one mole of hydrogen, partially reducing the triple bond to a double bond; Where possible, a trans-alkene is produced. Sodium is oxidized to NaNH2. The triple bond in alkynes can be oxidized using hot alkaline potassium permanganate. This oxidation leads to cleavage of the triple bond. The sp carbons of the alkyne are oxidized to carboxylic acid Use this information to answer the following questions 25 points Write the equations for the following reactions of the alkyne A. Indicate the reactants and product(s), reagent and reaction conditions in each case A Partial catalytic reduction of A Birch reduction of A (c) Complete hydrogenation of A Propose a synthesis for the following alkene starting from an alkyne: (e) An alkyne on oxidation by hot potassium permanganate produced two carboxylic acids shown below. Determine the structure of the starting alkyne
The Correct Answer and Explanation is:
Given: An alkyne undergoes various reactions—partial catalytic reduction, Birch reduction, complete hydrogenation, and oxidative cleavage.
Let’s assume alkyne A = 2-butyne (CH₃–C≡C–CH₃) for illustration purposes.
(a) Partial catalytic reduction of A (Lindlar’s catalyst)
Reaction:
CH₃–C≡C–CH₃ + H₂ → CH₃–CH=CH–CH₃ (cis-2-butene)
Reagents/Conditions: H₂, Pd/BaSO₄ (Lindlar’s catalyst), ethanol, RT
(b) Birch reduction of A
Reaction:
CH₃–C≡C–CH₃ + 2[H] → CH₃–CH=CH–CH₃ (trans-2-butene)
Reagents/Conditions: Na, NH₃(l), RT
(c) Complete hydrogenation of A
Reaction:
CH₃–C≡C–CH₃ + 2H₂ → CH₃–CH₂–CH₂–CH₃ (butane)
Reagents/Conditions: H₂, Pd or Pt, ethanol, RT
(d) Propose a synthesis for the following alkene from an alkyne:
Target alkene: cis-2-butene
Synthesis:
Start with 2-butyne (CH₃–C≡C–CH₃), use Lindlar’s catalyst for partial hydrogenation.
Reaction:
CH₃–C≡C–CH₃ + H₂ (1 mol) —(Pd/BaSO₄)→ CH₃–CH=CH–CH₃ (cis)
(e) Oxidation by hot KMnO₄ yields: CH₃COOH and HOOCCH₃
This indicates symmetric cleavage of an internal alkyne into two acetic acid molecules.
Reaction:
CH₃–C≡C–CH₃ + [O] → 2 CH₃COOH
Reagents/Conditions: KMnO₄ (hot, conc.), KOH, H₂O
Starting alkyne: 2-butyne
Explanation
Alkynes are hydrocarbons with a carbon-carbon triple bond, which can undergo a variety of chemical reactions. These reactions vary based on the reagents and conditions used. One important transformation is hydrogenation—the addition of hydrogen.
Complete hydrogenation involves adding two moles of hydrogen to the alkyne using catalysts like Pd or Pt. This fully saturates the triple bond, producing an alkane. For example, hydrogenation of 2-butyne yields butane.
In contrast, partial hydrogenation only adds one mole of hydrogen, reducing the alkyne to an alkene. Two distinct pathways exist:
- Catalytic partial reduction using Lindlar’s catalyst (Pd/BaSO₄) results in the cis-alkene. This is a stereoselective reaction due to the syn-addition of hydrogen on the same face of the molecule. For instance, 2-butyne becomes cis-2-butene.
- Birch reduction, which uses sodium in liquid ammonia, adds hydrogen atoms to opposite sides of the triple bond (anti-addition), producing the trans-alkene, such as trans-2-butene.
A different reaction is oxidation using hot, alkaline KMnO₄, which cleaves the triple bond. The sp-hybridized carbon atoms are oxidized to carboxylic acids. If oxidation of an alkyne yields two acetic acid molecules, the alkyne must be 2-butyne, indicating the original triple bond was internal and symmetric.
In synthesis, choosing the right reagents enables chemists to selectively create cis- or trans-alkenes from alkynes, or to fully reduce or cleave them. These predictable pathways make alkynes valuable intermediates in organic synthesis.
