OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2023: Question 18
11 marks · Medium difficulty · Structured Questions
Outline the nitration mechanism of nitrobenzene to form 1,3-dinitrobenzene, determine its percentage yield, and describe the recrystallization purification method.
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Question text
18 1,3-dinitrobenzene is a solid at room temperature.
A chemist prepares 1,3-dinitrobenzene as outlined below.
Step 1 12.5 cm3 of nitrobenzene (density = 1.20 g cm–3) is refluxed with concentrated nitric acid
in the presence of concentrated sulfuric acid as a catalyst.
Step 2 The mixture is cooled. Impure crystals of 1,3-dinitrobenzene appear.
Step 3 The impure crystals are purified to obtain pure 1,3-dinitrobenzene.
The chemist obtains 15.0 g of pure 1,3-dinitrobenzene.
(a) Outline the mechanism for this reaction, including the role of H2SO4 as a catalyst.
[5]
(b) Determine the percentage yield of 1,3-dinitrobenzene.
Give your answer to 3 significant figures.
percentage yield = … % [3]
(c) Describe how to purify the impure crystals in Step 3.
… [3]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
18 (a) Role of H2SO4 catalyst 2 marks 5 ANNOTATE ANSWER WITH TICKS AND
CROSSES
Forming electrophile
HNO + H SO ⎯→ H O + HSO – + NO + ✓ AO1.2 ALLOW
32 4 2 4 2
HNO + 2H SO → H O+ + 2HSO – + NO +
32 4 3 4 2
Reforming catalyst AO1.2
H+ + HSO – ⎯→ H SO ✓ ALLOW
42 4
HNO + H SO → H NO + + HSO –
32 4 2 3 4
Electrophilic attack 1 mark then H NO + → H O + NO +
23 2 2
AO1.2
Curly arrow from -bond to NO + ✓ ALLOW +NO OR NO +
22 2
NOTE: curly arrows can be straight, snake-
like, etc.
but NOT double headed or half headed
arrows
1st curly arrow must
• start from, OR close to circle of
------------------------------------------------------------------- benzene ring
Correct intermediate 1 mark AND
• go to anywhere on NO +
AO2.5
✓ DO NOT ALLOW mark for intermediate if
additional NO2 is missing
17 AO
element
------------------------------------------------------------------------- IGNORE connectivity to NO2 groups (mark is
Reforming benzene ring 1 mark AO1.2 for correct substitution position and position
of -ring)
Curly arrow from C–H bond to reform -ring
DO NOT ALLOW the following intermediate:
-ring should cover approximately 4 of the 6
Curly arrow must start from, OR be traced back to,
sides of the benzene ring structure
any part of C–H bond and go inside the ‘hexagon’
AND
the correct orientation, i.e. gap towards C
with NO2 and H
ALLOW + sign anywhere inside the
‘hexagon’ of intermediate
18 AO
element
18 (b) FIRST CHECK ANSWER ON ANSWER LINE 3 ALLOW 3SF up to calculator value
If answer = 73.2 award 3 marks throughout working
---------------------------------------------------------------------------------
IGNORE rounding errors past 3SF
TAKE CARE as value written down may be
truncated but with value stored in calculator,
depending on rounding, either can be
credited.
Theoretical moles
n(C6H5NO2) OR n(C6H4(NO2)2)
12.5 1.20
= 123.0 OR 0.12195... (mol) ✓ AO2.8 Calculator = 0.1219512195
Actual moles
15.0
n(C6H4(NO2)2) = 168.0 OR 0.0892857(mol) ✓ AO2.8 Calculator = 0.08928571429
0.0892857… ALLOW ECF except for final mark if value is
% yield = 0.12195…. 100 AO1.2 ≥100%
= 73.2 % to 3SF ✓ ------------------------------------------------
Alternative method using mass
1. Theoretical moles = 0.12195…. mol
2. Mass = 0.12195… 168.0 OR 20.4878... g
3. % yield = 100 = 73.2%
20.4878…
----------------------------------------------
Common errors
87.9% → 2 marks
12.5
• From 123 = 0.101626…..(no density)
19 AO
element
18 (c) Dissolve in the minimum quantity of hot water/solvent ✓ 3 AO3.3 ALLOW any solvent
IGNORE
Cool (to allow crystals form) • Initial filtering
AND • Filtration between dissolving and cooling
Then filter (under reduced pressure) ✓ (implies hot filtration)
• Washing with cold solvent
DO NOT ALLOW use of drying agent (e.g.
(Leave to) dry ✓
MgSO4)
How to answer it
Preparation and Purification of 1,3-dinitrobenzene
What this question tests
This multi-step synoptic question assesses your knowledge of aromatic chemistry mechanisms (electrophilic substitution of nitrobenzene), quantitative chemistry calculations (density, moles, and percentage yield), and practical organic chemistry techniques (purification of an impure solid via recrystallisation and filtration under reduced pressure).
Electrophilic Substitution Mechanism & Catalyst Role
💡 Key Knowledge
- Electrophile Generation: Concentrated nitric acid reacts with concentrated sulfuric acid to generate the nitronium ion (NO₂⁺): HNO₃ + 2H₂SO₄ → H₃O⁺ + 2HSO₄⁻ + NO₂⁺
- Catalyst Regeneration: H⁺ + HSO₄⁻ → H₂SO₄
- Already present nitro group ( -NO₂ ) is electron-withdrawing and meta-directing.
✅ Correct Mechanism Breakdown (5 Marks)
- Marks 1 & 2: Showing the generation of the electrophile and regeneration of the H₂SO₄ catalyst.
- Mark 3 (Attack): Curly arrow starting from the benzene pi-system ring and pointing directly to the NO₂⁺ ion.
- Mark 4 (Intermediate): Horseshoe intermediate with a positive charge located centrally inside the incomplete circle, and both H and NO₂ shown bonded at carbon-3.
- Mark 5 (Reforming Ring): Curly arrow starting from the C-H bond (or inside the hexagon near it) and pointing back into the ring to restore aromaticity.
🧠 Exam Technique
- Ensure your curly arrow for electrophilic attack clearly originates from the circle/pi-bond, not from a single carbon atom.
- The horseshoe shape in the intermediate must be open towards the top/side, and the positive charge must reside inside the horseshoe.
❌ Common Errors
- Drawing the first curly arrow starting from the NO₂⁺ ion instead of the ring.
- Failing to show the regeneration step of the H₂SO₄ catalyst, losing easy marks.
- Omitting the hydrogen atom on the carbon bearing the incoming NO₂ group in the intermediate.
Percentage Yield Calculation
📐 Step-by-Step Calculation
- Find theoretical moles of reactant (nitrobenzene, C₆H₅NO₂):
Mass = Volume × Density = 12.5 cm³ × 1.20 g cm⁻³ = 15.0 g
Molar mass of C₆H₅NO₂ = 123.0 g mol⁻¹
Moles = 15.0 / 123.0 = 0.12195... mol - Find actual moles of product obtained (1,3-dinitrobenzene, C₆H₄(NO₂)_{2}):
Actual mass = 15.0 g
Molar mass of C₆H₄(NO₂)₂ = 168.0 g mol⁻¹
Moles = 15.0 / 168.0 = 0.089285... mol - Calculate percentage yield:
Percentage Yield = ( Actual moles / Theoretical moles ) × 100
= ( 0.089285... / 0.12195... ) × 100 = 73.2%
❌ Common Calculation Traps
- Ignoring density: Forgetting to multiply the volume ( 12.5 cm³ ) by density ( 1.20 g cm⁻³ ) and treating it directly as mass. This gives a major error ( 87.9% ).
- Rounding too early: Rounding intermediate mole values can cause rounding errors in the final percentage digit. Keep full calculator values until the very last step.
Purification of Impure Crystals (Recrystallisation)
💡 Key Knowledge (3 Marks)
- Step 1: Dissolve the impure solid in a minimum volume of hot solvent / water. (Minimum volume ensures a saturated solution so crystals precipitate on cooling).
- Step 2: Cool the mixture to allow crystals to reform, then filter under reduced pressure (using a Buchner funnel and flask).
- Step 3: Wash with a little cold solvent and leave to dry .
🧠 Examiner Guidance & Pitfalls
- Crucial keyword: You must state minimum volume of hot solvent. Saying "hot water" without "minimum" loses the marking point.
- Filtering method: Standard gravity filtration is penalized if cooling occurs during filtration; suction/reduced pressure filtration is expected for fast, efficient separation.
- Do NOT include: Adding a drying agent like anhydrous MgSO₄ to the solid crystals (drying agents are used for organic liquids, not wet solid crystals).
Topics
Module 6: Organic chemistry and analysis · Practical Activity Groups · 6.1 Aromatic compounds, carbonyls and acids · PAG 6: Synthesis of an organic solid
Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.