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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.
Answer
The reaction can be represented as:
This equation shows that concentrated nitric acid reacts with concentrated sulfuric acid to generate the nitronium ion (), which is the active electrophile that reacts with nitrobenzene.
Step 3
Mechanism
Answer
The mechanism involves the following steps:
- Formation of the electrophile: The nitronium ion () is generated as shown above.
- Attack by the electrophile: The electron-rich aromatic ring of nitrobenzene attacks the nitronium ion, forming a sigma complex.
- Deprotonation: A hydrogen atom is removed from the carbon that bonded with the nitronium ion, restoring the aromaticity of the ring.
Step 4
Use the chromatogram in Figure 3 to deduce the Rf value of 1,4-dinitrobenzene in this experiment.
Answer
Based on the chromatogram in Figure 3, the Rf value of 1,4-dinitrobenzene can be determined as follows:
- The distance traveled by the spot of 1,4-dinitrobenzene divided by the distance traveled by the solvent front gives:
- Based on visual observations, we can deduce the Rf value that corresponds to the closest option, which is B: 0.46.
Step 5
State in general terms what determines the distance travelled by a spot in TLC.
Answer
The distance traveled by a spot in TLC is generally determined by the polarity of the compound in relation to the stationary phase and the solvent. More polar compounds tend to interact more strongly with the stationary phase, leading to shorter distance traveled, while less polar compounds are more soluble in the mobile phase, allowing them to travel further.
Step 6
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.
Answer
Relative polarities: 1,2-dinitrobenzene is more polar than 1,4-dinitrobenzene.
Explanation: 1,4-dinitrobenzene has the greater Rf value because it is less polar. Less polar substances are less attracted to the stationary phase, allowing them to move further up the plate in a TLC experiment, contributing to the higher Rf value.
Step 8
Is the student correct? Justify your answer.
Answer
The student is not correct. In the third experiment, the use of a solvent mixture of hexane and ethyl ethanol could potentially affect the polarities of the compounds. Since 1,2-dinitrobenzene is likely to be more soluble in a more polar solvent, the Rf value may not be greater than that of 1,2-dinitrobenzene in the previous experiment.