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 reaction can be represented as:
ightarrow ext{NO}_2^+ + ext{HSO}_4^- + ext{H}_2 ext{O}$$
The species that reacts with nitrobenzene is the nitronium ion, ext{NO}_2^+.
Step 2
Name of mechanism
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Answer
Electrophilic substitution.
Step 3
Mechanism
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The mechanism involves the formation of the nitronium ion, which then attacks the aromatic ring of nitrobenzene. The steps include:
Generation of the electrophile (nitronium ion).
Formation of the sigma complex (arenium ion).
Deprotonation to restore the aromaticity.
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
Based on the chromatogram, the Rf value of 1,4-dinitrobenzene can be deduced as 0.46 (option B).
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 substance relative to the stationary phase and the solvent. More polar substances tend to interact more with the stationary phase and travel less distance.
Step 6
Give one other practical requirement when placing the plate in the beaker.
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Answer
The TLC plate should be placed in such a way that the start line is above the level of the solvent to prevent the spots from dissolving directly into the solvent.
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 than 1,4-dinitrobenzene due to the positioning of the nitro groups.
Explanation: The greater Rf value of 1,4-dinitrobenzene indicates it is less polar, allowing it to travel further in a non-polar solvent like hexane, whereas 1,2-dinitrobenzene, being more polar, has stronger interactions 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 would likely decrease for 1,2-dinitrobenzene when using a mixture of hexane and ethyl ethanol in the third experiment because the increased polarity of the solvent would enhance the solubility of polar components, thus affecting their Rf values.
