Concentrated sulfuric acid reacts with alkenes, alcohols and sodium halides - AQA - A-Level Chemistry - Question 2 - 2017 - Paper 3
Question 2
Concentrated sulfuric acid reacts with alkenes, alcohols and sodium halides.
1. Name the mechanism for the reaction of concentrated sulfuric acid with an alkene.
2... show full transcript
Worked Solution & Example Answer:Concentrated sulfuric acid reacts with alkenes, alcohols and sodium halides - AQA - A-Level Chemistry - Question 2 - 2017 - Paper 3
Step 1
Name the mechanism for the reaction of concentrated sulfuric acid with an alkene.
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Answer
The mechanism for the reaction of concentrated sulfuric acid with an alkene is known as electrophilic addition.
Step 2
Outline the mechanism for the reaction of concentrated sulfuric acid with propene to show the formation of the major product.
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Answer
The concentrated sulfuric acid acts as an electrophile and donates a proton (H⁺) to the alkene (propene). This forms a carbocation intermediate.
The most stable carbocation will form, which is the secondary carbocation.
The bisulfate ion (HSO₄⁻) then attacks the carbocation, resulting in the formation of the major product, isopropyl hydrogen sulfate.
Finally, upon hydrolysis, the product converts to isopropanol.
Step 3
Draw the structure of the minor product of the reaction between concentrated sulfuric acid and propene.
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Answer
The minor product, which is formed under specific conditions, is likely to be propyl hydrogen sulfate or a less stable alkyl sulfate.
Step 4
Explain why the product shown in your answer to Question 2.2 is the major product.
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Answer
The major product is favored due to the stability of the secondary carbocation formed during the reaction. Secondary carbocations are more stable than primary carbocations due to hyperconjugation and inductive effects, which explains the preferential formation of the isopropyl hydrogen sulfate.
Step 5
Draw the skeletal formula of each of the three isomeric alkenes formed by the reaction of butan-2-ol with concentrated sulfuric acid.
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Answer
But-2-ene:
Isobutylene (2-methylpropene):
But-1-ene:
Step 6
Give the full IUPAC name of each isomer.
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Answer
But-2-ene
2-Methylpropene (Isobutylene)
But-1-ene
Step 7
Name this by-product, identify the role of the sulfuric acid in its formation and suggest the name of a method that could be used to separate the products of this reaction.
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Answer
The by-product is butan-2-ol. The role of sulfuric acid is to protonate the alcohol, facilitating the elimination reaction leading to alkene formation. A suitable method for separation could be distillation.
Step 8
Observation with sodium chloride:
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The observation would include the formation of a white precipitate of sodium bisulfate.
Step 9
Role of sulfuric acid:
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The role of sulfuric acid in this reaction is to act as a dehydrating agent, promoting the reaction between sodium chloride and the release of hydrogen chloride.
Step 10
Black solid:
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Answer
The black solid observed is likely iodine.
Step 11
Yellow solid:
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Answer
The yellow solid is likely sulfur.
Step 12
Gas:
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Answer
The gas produced with the smell of bad eggs is hydrogen sulfide (H₂S).
