Concentrated sulfuric acid reacts with alkenes, alcohols and sodium halides - AQA - A-Level Chemistry - Question 2 - 2017 - Paper 3

1. Protonation: The concentrated sulfuric acid donates a proton (H⁺) to the double bond of propene, forming a carbocation.

2. Formation of Carbocation: The positive charge stabilizes the more substituted carbocation (secondary), leading to more stable product formation.

3. Nucleophilic Attack: The bisulfate ion (HSO₄⁻) then attacks the carbocation, leading to the formation of the major product—alkyl hydrogen sulfate.

4. Deprotonation: Finally, deprotonation takes place to yield the corresponding alkene.

The minor product can be represented by the following structure, which results from the hydrogen adding to the less substituted carbon:

    H   H
    |   |
CH3-C=C-OSO3H
    |   |
    H   H

This corresponds to an alkyl hydrogen sulfate formed with substitution occurring at the less stable site.

The major product is favored due to the stability of the carbocation intermediate formed during the reaction. The more substituted carbocation is more stable because it is stabilized by hyperconjugation and inductive effects from adjacent alkyl groups. This leads to a greater rate of formation of the major product compared to the minor product.

  H    H
  |    |
CH3-C=C-CH3
     |  
     H 

Name: 2-butene

  H    H  
  |    |
CH3-CH=C-CH3
     |  
     H 

Name: 1-butene

 H
  |  
CH3-C=C-CH3
     | 
     H  

Name: Buta-1,3-diene

By-product: The by-product of this reaction is likely to be butan-2-one (ketone).

Role of sulfuric acid: The sulfuric acid acts as a dehydrating agent, facilitating the formation of the ketone from the alcohol.

Name of separation method: The by-products could be separated via distillation.

Observation with sodium chloride: The reaction would produce acidic fumes and the solid sodium chloride may dissolve or react leading to the formation of a color change in solution.

Role of sulfuric acid: The sulfuric acid serves as a catalyst in this reaction, aiding in the substitution of the chloride ion, while also providing acidic conditions.

Black solid: The black solid is likely due to carbon disulfide or iodine precipitate formation in the reaction with iodide.

Yellow solid: The yellow solid could be caused by the production of sulphur or iodine compounds.

Gas: The gas with the smell of bad eggs is likely hydrogen sulfide (H₂S), arising from the reaction of sulfuric acid with iodide.