OCR A-Level Chemistry Unified chemistry (03), November 2020: Question 1

16 marks · Hard difficulty · Structured Questions

Complete organic preparation diagrams, explain amine boiling points, determine the molecular formula and structure of an amine using ideal gas calculations and NMR data, and complete an organic reaction equation.

Practise this question

Question

Question text

1 This question is about organic chemistry.

(a) This part is about two practical techniques used in organic preparations.

(i) Complete the missing labels on the diagram and name the technique.

Heat

Name of technique: … [2]

(ii) Draw a labelled diagram to show apparatus set up for filtration under reduced pressure

(vacuum filtration).

[2]

(b) This part is about amines.

(i) The table shows the structures and boiling points of three amines, which are structural

isomers of C3H9N.

Amine CH3CH2CH2NH2 (CH3)2CHNH2 (CH3)3N

Skeletal formula NH2

NH2

Boiling point / °C 48–49 °C 33–34 °C 3–4 °C

Explain the difference in the boiling points of the three amines.

… [4]

(ii) Amine A is a liquid at room temperature and pressure.

When vaporised, 0.202 g of the amine produces 72.0 cm3 of gas at 1.00 × 105 Pa and

100 °C. The 13C NMR spectrum of amine A has 3 peaks.

Determine the molecular formula of A and suggest a possible structure for amine A.

Molecular formula of A …

Structure of A

[6]

formula C3H3NO and one other product.

Complete the equation for this reaction.

H2N COOH

C C +

H H

[2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

1 (a) (i) 2 1.2 × 2

Pear-shaped /

Round-bottom

flask

Water flow AND condenser

Water in at bottom and out at top

AND condenser

Flask and technique

Pear-shaped/round-bottom flask DO NOT ALLOW conical flask, volumetric flask,

AND reflux beaker in place of round bottom/pear shaped flask

element

(ii) Diagram showing knowledge of filtration under 2 Labels NOT required for diagram

reduced pressure

Diagram showing Buchner flask ALLOW diagram of a conical flask with a filtering

must have ONE side arm setup above

AND AND

Buchner/Hirsh funnel on top of flask 2.3 Side arm either in conical flask OR between flask

Labels not required and filter paper of funnel

IGNORE absence of seals

---------------------------------- ----------------------------------

Further details:

• Funnel sealed or stoppered to flask MUST imply some type of seal between filter setup

and flask. ALLOW small gaps

AND

• Apparatus capable of filtering under reduced pressure

AND

• Label for setup from side arm to indicate reduced Examples of suitable labels (may have arrow from

pressure side arm or tube attached)

• to pump

AND 2.7 • to vacuum

• Label for Buchner flask OR Buchner/Hirsh funnel • air out

ALLOW slips in spelling of ‘Buchner’ • suction

• reduced pressure

• etc.

For Buchner flask and Buchner funnel

DO NOT ALLOW just ‘flask OR ‘funnel’

Flask and funnel used in normal filtration

Question Answer 7 Marks Guidance

element

(b) (i) 5 → ANNOTATE WITH TICKS AND CROSSES, etc.

Comparison of branching and points of contact 4 max ------------------------------------------------------------------

e.g. CH3CH2CH2NH2 has longer chain / straight ALLOW ORA throughout

chain / no branches ALLOW ‘The straighter the chain, the more points

AND of contact ….’

e.g. CH3CH2CH2NH2 has more points of contact /

more surface interaction (between molecules) 1.2 IGNORE comparison using ‘primary’, ‘secondary’

and ‘tertiary’.

Relative strength of force Comparison of branching is required.

e.g. CH3CH2CH2NH2 has stronger/more

induced dipole(–dipole) interactions For London forces,

OR London forces 2.1 • ALLOW induced dipole(–dipole) interactions

• IGNORE IDID OR van der Waals’ forces/VDW

--------------------------------------------------------------

Hydrogen bonds 3CH2CH2NH2 has more

DO NOT ALLOW CH

CH3CH2CH2NH2 OR (CH3)2CHNH2 have electrons

hydrogen/H bonds (number of electrons are the same)

(CH3)3N has no hydrogen/H bonds

Relative strength of force 1.2

Hydrogen bonds are stronger than London forces

/permanent dipole interactions

------------------------------------------------------------------

Comparison of energy required to break force DO NOT ALLOW ‘more energy to break covalent

e.g. More energy to break/overcome London

bonds

f orces/intermolecular f orces in CH3CH2CH2NH2

ALLOW little energy is required to break London

More energy is needed to break H bonds (than 2.1

forces (compared with H bonds)

London forces)

8 element

(b) (ii) FIRST CHECK MOLECULAR FORMULA and 6

STRUCTURE

IF molecular formula = C5H13N AND correct structure

AND evidence of ideal gas equation → 6 marks

Correct up to 87 AND C5H13N → 5 marks

Correct up to 87 → 4 marks

--------------------------------------------------------------------------

Rearranging ideal gas equation pV

IF n = is omitted, ALLOW when values are

pV RT

n = 2.2×4 substituted into rearranged ideal gas equation.

Unit conversion AND substitution into n = :

• R = 8.314 OR 8.31

• V = 72(.0) × 10–6

• T in K: 373 K

1.00 × 105 × 72.0 × 10–6

e.g.

8.314 × 373

Calculation of n

n = 2.32 × 10–3 (mol)

Calculator: n = 2.321740325 × 10–3 from 8.314

From 8.31, n = 2.322857889 × 10–3

Calculation of M

0.202

M = –3 = 87

2.32 × 10

Molecular formula

C5H13N 3.2 ALLOW elements in any order

Molecular formula required

ALLOW molecular formula = C3H9N3

ALLOW other molecular formulae of an amine that

has M = 87, e.g. C4H9NO

9 AO

element

Structure of amine A from C5H13N 3.2 ALLOW any combination of skeletal OR structural

OR displayed formula as long as unambiguous

ALLOW structures below from

molecular formula = C3H9N3

OR ALLOW ECF but only if structure has calculated Mr

AND has 3 peaks in 13C NMR spectrum.

OR OR

Use of 3 marks max possible for use of 72.0 cm3 OR 0.720 dm3 by ECF

24000 Calculation

72.0 –3

e.g. n = = 3.00 × 10 No mark (calculation much simpler)

24000

0.202

M = –3 = 67.3 OR 67 ECF

3.00 × 10

Molecular formula = C4H5N ECF

Structure

ECF

10 element

OR displayed formula as long as unambiguous

ALLOW

Organic product and water marked independently.

NOTE: For ECF, any structure must have correct

1st mark correct organic product OR water number of bonds to C, H, O and N

IGNORE balancing numbers

DO NOT ALLOW structure of dimer

2nd mark BOTH products AND correctly balanced. Question states molecular formula = C3H3NO

Total 16

How to answer it

Organic Chemistry Practical Techniques, Amines & Synthesis Study Guide

What this question tests

This comprehensive question assesses core practical skills (reflux apparatus setup and vacuum filtration), organic intermolecular forces (boiling point trends in amines), gas law calculations combined with molecular mass determination, interpretation of carbon-13 NMR spectra, and structural organic reaction balancing.

Question 1(a)

Practical Techniques in Organic Chemistry

✅ Correct Answers (Part i)

Technique name: Reflux
Flask: Pear-shaped or round-bottom flask
Condenser labels: Water-in at the bottom, water-out at the top

💡 Key Knowledge

Reflux allows continuous boiling and condensation of a reaction mixture without losing volatile reactants or products. Water must enter the condenser from the bottom jacket to ensure it completely fills up and maintains efficient cooling.

✅ Correct Answers (Part ii - Vacuum Filtration)

Diagram must show a Buchner flask connected via a side arm to a vacuum line/pump.
A Buchner or Hirsch funnel must sit securely on top of the flask with a flat base and filter paper.

🧠 Exam Technique & Common Errors

Errors to avoid: Do NOT draw a standard conical flask or standard funnel (gravity filtration). The side-arm flask must be clearly identifiable as a filter flask, and the seal/connection must be air-tight to maintain reduced pressure.

Question 1(b)(i)

Boiling Point Trends in Isomeric Amines ($\text{C}_3\text{H}_9\text{N}$)

✅ Correct Answers & Marking Points

Primary amine ($\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$): Longest straight chain, greatest surface area of contact, strongest London forces (or van der Waals' forces).
Hydrogen bonding: Both primary and secondary amines form hydrogen bonds between molecules. Tertiary amine ($\text{(CH}_3\text{)_3}\text{N}$) has no hydrogen atoms bonded to nitrogen, so it cannot form hydrogen bonds.
Energy comparison: More energy is required to break the stronger intermolecular forces (hydrogen bonds and/or extensive London forces) in the primary/secondary amines compared to the tertiary amine.

❌ Common Student Errors

Stating that tertiary amines "have no hydrogen bonds" instead of correctly saying they "cannot form hydrogen bonds with themselves" (due to lacking an H attached to electronegative N).
Confusing intermolecular forces with covalent bonds (never say "breaking covalent bonds" when explaining boiling points!).
Using outdated terminology like "IDID" or "van der Waals" where specific terms like "London forces" or "induced dipole-dipole interactions" are required by the mark scheme.

Question 1(b)(ii)

Determining Molecular Formula and Structure from Gas Data

📐 Step-by-Step Calculation of Molar Mass ($M$)

Rearrange the Ideal Gas Equation: pV = nRT becomes n = pV / (RT)
Convert Units carefully:
- Pressure ($p$) = $1.00 \times 10^5 \text{ Pa}$
- Volume ($V$) = $72.0 \text{ cm}^3 = 72.0 \times 10^{-6} \text{ m}^3$
- Temperature ($T$) = $100 \text{ °C} + 273.15 = 373 \text{ K}$
- Gas constant ($R$) = $8.314 \text{ J mol}^{-1}\text{ K}^{-1}$
Calculate moles ($n$):
n = (1.00 × 10⁵ × 72.0 × 10⁻⁶) / (8.314 × 373) = 2.32 × 10⁻³ mol
Calculate Molar Mass ($M$):
M = mass / n = 0.202 g / (2.32 × 10⁻³ mol) = 87.0 g mol⁻¹
Deduce Molecular Formula: An amine with $M = 87$ corresponds to molecular formula C₅H₁₃N .

✅ Structure of Amine A

The structure must match molecular formula C₅H₁₃N and produce exactly 3 peaks in its ¹³C NMR spectrum (showing high molecular symmetry).

Accepted structures include:

(CH₃)₂CHCH₂NH₂ (or similar symmetrical skeletons)
Di-isopropylamine derivatives or symmetrical tertiary structures that fit the 3-environment symmetry constraint.

🧠 Exam Technique: NMR & ECF

Even if your calculated molar mass had a minor arithmetic slip, Examiners apply Error Carried Forward (ECF) for the structure and formula provided your structure is consistent with your calculated $M$ and spectral data.

Question 1(c)

Organic Synthesis & Equation Balancing

✅ Correct Chemical Equation

Reaction of the amino acid ( Z-H₂NCH=CHCOOH ) to form a cyclic compound + water ( H₂O ):

C₃H₅NO₂ (cyclic amide/lactam ring) + H₂O

Mark Breakdown:

Mark 1: Correct cyclic organic product structure AND water formula.
Mark 2: Balanced equation overall.

❌ Common Errors

Attempting to balance atoms by altering the core carbon skeleton of the cyclic product instead of accounting for simple condensation (loss of H₂O ).

Topics

Module 6: Organic chemistry and analysis · Practical Activity Groups · PAG 5: Synthesis of an organic liquid · 6.2 Nitrogen compounds, polymers and synthesis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.