Which of the following would give more than one product when treated with xs HCl? 2-pentyne 2-butyne
The correct answer and explanation is:
Answer:
2-pentyne would give more than one product when treated with excess (xs) HCl, whereas 2-butyne would generally give fewer products (mainly one major product).
Explanation:
When alkynes react with excess hydrogen halides (like HCl), they undergo electrophilic addition reactions. The addition proceeds via the formation of a vinyl carbocation intermediate, which then rearranges or is attacked by the halide ion. The regioselectivity of the addition follows Markovnikov’s rule: the proton (H⁺) adds to the carbon with more hydrogens, and the halide (Cl⁻) adds to the more substituted carbon.
Step 1: Understanding the substrates
- 2-Butyne: a symmetrical internal alkyne, with the structure CH₃–C≡C–CH₃. Both carbons of the triple bond have identical substituents (methyl groups).
- 2-Pentyne: an unsymmetrical internal alkyne, with the structure CH₃–CH₂–C≡C–CH₃. The two carbons of the triple bond have different substituents (ethyl on one side, methyl on the other).
Step 2: Reaction with xs HCl
2-Butyne:
- The triple bond is symmetrical, so Markovnikov addition of HCl happens equally on either side.
- First, HCl adds across the triple bond, forming a vinyl chloride intermediate.
- The second equivalent of HCl adds across the formed double bond, yielding a geminal dichloride (both chlorines on the same carbon).
- Since both sides of the triple bond are identical, the product is mainly one major compound (the gem-dichloride with chlorines on the same carbon).
2-Pentyne:
- The triple bond is unsymmetrical, so the proton can add to either carbon of the triple bond, leading to two different carbocation intermediates.
- Because the substituents differ (ethyl vs methyl side), the carbocation stability differs slightly, but both possible carbocations can form to some extent.
- This leads to two possible regioisomeric vinyl chlorides after the first addition.
- Subsequent addition of HCl to each of these intermediates yields more than one product (two different geminal dichlorides).
Summary:
- 2-Butyne gives mainly one product because of symmetrical structure.
- 2-Pentyne gives more than one product because the alkyne is unsymmetrical, leading to two possible Markovnikov additions and multiple products.
Additional Notes:
- The reaction proceeds via Markovnikov addition with no significant anti-Markovnikov products under normal conditions.
- If the alkyne were terminal, the product distribution would be simpler, but internal unsymmetrical alkynes often yield mixtures.
- Reaction conditions, temperature, and solvent can affect product ratios but generally do not prevent the formation of multiple products with unsymmetrical alkynes.