This question is about nitrobenzenes - AQA - A-Level Chemistry - Question 8 - 2017 - Paper 2
Question 8
This question is about nitrobenzenes.
Nitrobenzene reacts when heated with a mixture of concentrated nitric acid and concentrated sulfuric acid to form a mixture of... show full transcript
Worked Solution & Example Answer:This question is about nitrobenzenes - AQA - A-Level Chemistry - Question 8 - 2017 - Paper 2
Step 1
Write an equation for the reaction of concentrated nitric acid with concentrated sulfuric acid to form the species that reacts with nitrobenzene.
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Answer
The equation for the reaction is:
HNO3+H2SO4→NO2++HSO4−+H2O
Step 2
Name of mechanism
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Answer
Electrophilic substitution.
Step 3
Mechanism
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Answer
The mechanism involves the generation of the nitronium ion (NO2+) from the concentrated nitric acid and sulfuric acid, which then acts as the electrophile, reacting with nitrobenzene to form 1,3-dinitrobenzene.
Step 4
Use the chromatogram in Figure 3 to deduce the Rf value of 1,4-dinitrobenzene in this experiment.
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Answer
The Rf value for 1,4-dinitrobenzene is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front. From the chromatogram:
Spot distance: 2 cm
Solvent front distance: 4.88 cm
Therefore,
Rf=4.882≈0.41
The correct option is A 0.41.
Step 5
State in general terms what determines the distance travelled by a spot in TLC.
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Answer
The distance travelled by a spot in TLC is determined by the polarity of the compound relative to the solvent used; more polar compounds tend to interact more with the stationary phase, leading to reduced movement.
Step 6
Give one other practical requirement when placing the plate in the beaker.
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The TLC plate should be held by the edges to avoid contamination from fingerprints, which could affect results.
Step 7
Deduce the relative polarities of the 1,2-dinitrobenzene and 1,4-dinitrobenzene and explain why 1,4-dinitrobenzene has the greater Rf value.
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Answer
1,2-Dinitrobenzene is more polar compared to 1,4-dinitrobenzene due to the positions of the nitro groups, which results in stronger intermolecular interactions. Therefore, 1,4-dinitrobenzene, being less polar, has a greater Rf value since it interacts less with the stationary phase.
Step 8
Is the student correct? Justify your answer.
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Answer
The student is incorrect. The Rf value of 1,2-dinitrobenzene would likely be less than 1,4-dinitrobenzene in the presence of a more polar solvent mixture like hexane and ethyl ethanoate, as it would prefer to remain in the stationary phase due to its higher polarity.
