OCR A-Level Chemistry AS Depth in chemistry (02), November 2020: Question 5

18 marks · Hard difficulty · Structured Questions

Identify secondary alcohols among structures A-F, write combustion equations, calculate atom economy, outline nucleophilic substitution mechanisms, explain hydrolysis rate differences between haloalkanes, and deduce unknown alcohol and oxidation product structures using mass spectrum and IR data.

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Question

Question text

5 This question is about the alcohols A–F shown below.

OH OH

A B C

D E F

(a) Which of the alcohols A–F are secondary alcohols?

… [2]

(b) Complete a balanced equation for the complete combustion of alcohol C.

CH3CH2CH(OH)CH3 + … + … [1]

… [1]

(d) Alcohol A can be prepared by the alkaline hydrolysis of the bromoalkane, (CH3)2CHCH2CH2Br.

The hydrolysis with aqueous NaOH is shown in equation 5.1.

(CH3)2CHCH2CH2Br + NaOH (CH3)2CHCH2CH2OH + NaBr equation 5.1

alcohol A

A student gently heats a mixture of (CH3)2CHCH2CH2Br and NaOH(aq) for 25 minutes.

(i) Calculate the atom economy for the preparation of alcohol A in equation 5.1.

atom economy = … % [2]

(ii) Outline the mechanism for the alkaline hydrolysis of (CH3)2CHCH2CH2Br.

The structure of (CH3)2CHCH2CH2Br has been provided.

Show curly arrows, relevant lone pairs and dipoles, and the products.

(CH3)2CHCH2 C Br

[3]

(iii) Name this type of mechanism.

… [1]

(e) The student decides to prepare alcohol A using the same method as in (d) but using the

chloroalkane (CH3)2CHCH2CH2Cl instead of the bromoalkane, (CH3)2CHCH2CH2Br.

State and explain how the rates of hydrolysis of the chloroalkane and the bromoalkane would

differ.

… [2]

(f)* The structures of A–F are repeated below.

OH OH

A B C

D E F

Compound X is one of the alcohols A–F.

A student refluxes compound X with acidified potassium dichromate(VI) as an oxidising

agent. A pure sample of the organic product Y is obtained from the resulting mixture.

The mass spectrum and IR spectrum of Y are shown below.

Mass spectrum of Y

relative 60

intensity

10 20 30 40 50 60 70 80 90

m/z

IR spectrum of Y

transmittance 50

(100%)

4000 3000 2000 1500 1000 500

15wavenumber / cm–1

Using this information, identify compound X and product Y, and write an equation for the

formation of product Y from compound X. You may use [O] to represent the oxidising agent.

In your answer you should make clear how your conclusions are linked to the evidence. [6]

Additional answer space if required

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

5 (a) C, E AND F 2 1.1 ×1 If >2 alcohols are shown lose 1 mark for each

Three correct alcohols → 2 marks 2.1 ×1 incorrect response

Two correct alcohols → 1 mark

(b) (CH3CH2CHOHCH3 +) 6O2 → 4CO2 + 5H2O 1 2.6 ×1 DO NOT ALLOW [O]

(d) (i) FIRST, CHECK THE ANSWER ON ANSWER LINE 2

IF atom economy = 46.1(%) award 2 marks

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

Atom economy

Mr of (CH3)2CHCH2CH2OH Mr (CH3)2CHCH2CH2OH

= × 100 ALLOW × 100

Mr (CH3)2CHCH2CH2OH + Mr NaBr Mr (CH3)2CHCH2CH2Br + Mr NaOH

OR = × 100 1.2 ×1

190.9 ALLOW 46% up to calculator value (46.09743321)

2.2 ×1

= 46.1(%) ALLOW ECF from incorrect Mr values

Question Answer Marks AO Guidance

element

(ii) ANNOTATE ANSWER WITH TICKS AND CROSSES 3 1st curly arrow must

----------------------------------------------------------------------- • go to the C of C–Br

Curly arrows 2 marks AND

curly arrow from OH– to C atom of C−Br bond 2.5 ×1 • start from, OR be traced back to any point

across width of lone pair on O of OH–

dipole shown on C–Br bond, Cδ+ and Brδ−, 1.1 ×1

AND curly arrow from C−Br bond to Br atom

• OR start from – charge on O of –OH ion

(Lone pair NOT needed if curly arrow shown from

O–)

IGNORE incorrect R groups for curly arrow 2nd curly arrow must start from, OR be traced

marks back to, any part of C–Br bond and go to Br

IGNORE presence of Na+ but OH– needed

i.e. Na+OH–can be allowed if criteria met

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

ALLOW SN1 mechanism for 2 curly arrow marks

Products 1 mark

correct organic product AND Br– First mark

2.5 ×1 δ+ δ−

Dipole shown on C–Br bond, C and Br ,

AND curly arrow from C−Br bond to Br atom

IGNORE presence of Na+ but Br– needed

i.e. Na+Br–can be allowed Second mark

BUT NaBr does NOT show Br– Curly arrow from OH– AND to correct carbocation

NOTE: curly arrows can be straight, snake-like, etc.

but NOT double headed or half headed arrows

15 element

Use curly arrow criteria in guidance above

(iii) Nucleophilic substitution 1 1.1 ×1

(e) Rate slower with chloroalkane ORA 2 3.1 ×1 IGNORE reference to bond polarity

C–Cl bond is stronger than C–Br bond 2.5 ×1

OR C–Cl bond has greater bond enthalpy

OR more energy needed to break C–Clbond

element

(f) Please refer to the marking instructions on page 4 of this 6 2.5 ×1 Indicative scientific points

mark scheme for guidance on how to mark this question. 3.1 ×2

3.2 ×3 LOOK AT THE SPECTRA f or labelled peaks

Level 3 (5–6 marks) Mass Spectrum

The candidate gives thorough explanations of both spectra, • M+ or molecular ion of 86

and correctly identifies X and Y with a correct equation. + +

• m/z = 43 shows CH3CO OR C3H7

There is a well-developed line of reasoning which is clear IR Spectrum

• IR shows no broad absorption at 2500–3300 cm–

and logically structured. The information presented is

1 so no O–H bond AND not a carboxylic acid

relevant and substantiated.

• IR shows absorption at 1700 cm–1 for C=O bond

Level 2 (3–4 marks) OR indicates a ketone/aldehyde present

The candidate attempts all three scientific points but Identification and Equation

explanations are incomplete. • X must be a secondary alcohol, since refluxing a

OR secondary alcohol with acidified potassium

Explains two scientific points thoroughly with few dichromate (VI) forms a ketone

omissions. OR primary alcohol → carboxylic acid AND

AND tertiary alcohol would not be oxidised.

Attempts a f easible structure based on deduction from • X is (CH3)2CHCHOHCH3 OR compound E

correct Mr. OR 3-methylbutan-2-ol

There is a line of reasoning presented with some structure.

The information presented is relevant and supported by • Y is (CH3)2CHCOCH3 OR 3-methylbutan-2-one

some evidence 16 Equation

(CH3)2CHCHOHCH3 + [O] → (CH3)2CHCOCH3 + H2O

Level 1 (1–2 marks)

The candidate gives a simple description based on at least

two of the main scientific points.

Gives a thorough description and explanation of one of the

scientific points.

There is an attempt at a logical structure with a line of

reasoning. The information is in the most part relevant.

0 marks

No response or no response worthy of credit.

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How to answer it

Alcohols, Halogenoalkanes, and Analytical Chemistry

OCR AS Level Chemistry • Comprehensive Study Guide

What this question tests

This multi-part exam question covers core AS organic chemistry principles: alcohol classification, combustion equations, IUPAC nomenclature, atom economy calculations, nucleophilic substitution mechanisms (curly arrows, dipoles, and bond enthalpies), rates of hydrolysis for halogenoalkanes, and structural elucidation using combined Mass Spectrometry and IR Spectroscopy.

Part (a): Classifying Alcohols

Identifying secondary alcohols from structures A–F

✅ Correct Answer

C, E and F

2 marks available: 3 correct = 2 marks; 2 correct = 1 mark. Penalty: If >2 alcohols are listed, lose 1 mark for each incorrect response.

💡 Key Knowledge

Primary (1°): The C bonded to -OH is attached to 1 (or zero) other carbon atoms (e.g., A, D).
Secondary (2°): The C bonded to -OH is attached to 2 other carbon atoms (C, E, F).
Tertiary (3°): The C bonded to -OH is attached to 3 other carbon atoms (B).

Part (b): Combustion Equation

Balanced equation for complete combustion of alcohol C

✅ Correct Answer

CH₃CH₂CH(OH)CH₃ + 6O₂ → 4CO₂ + 5H₂O

1 mark. Note: Do NOT allow [O] in combustion equations.

🧠 Exam Technique

Balance elements systematically: Balance Carbon first (gives 4 CO₂), then Hydrogen (gives 5 H₂O), and finally balance Oxygen last using O₂ molecules.

Part (c): IUPAC Nomenclature

Systematic naming of alcohol B

✅ Correct Answer

2-methylbutan-2-ol

1 mark. Accept hyphen/number formatting variants that follow IUPAC rules strictly.

❌ Common Errors

Missing hyphens between numbers and letters.
Incorrect longest carbon chain counting (selecting 4 carbons properly as butane, but misplacing the branch locant).

Part (d)(i): Atom Economy Calculation

Calculating atom economy for equation 5.1

📐 Calculation Steps

Identify desired product: (CH₃)₂CHCH₂CH₂OH (Mᵣ = 88.0)
Identify all products: (CH₃)₂CHCH₂CH₂OH + NaBr (Mᵣ = 102.9)
Total Mᵣ of all products: 88.0 + 102.9 = 190.9
Apply Formula:
Atom Economy = (Mᵣ of desired product / Total Mᵣ of all products) × 100
Compute: (88 / 190.9) × 100 = 46.1%

2 marks. Full marks awarded for correct final answer. ECF allowed from incorrect Mᵣ values.

❌ Common Errors

Using the reactant masses instead of the total products in the denominator, or forgetting to multiply by 100.

Part (d)(ii) & (iii): Mechanism of Nucleophilic Substitution

Alkaline hydrolysis of (CH₃)₂CHCH₂CH₂Br

💡 Mechanism Details ((ii) - 3 marks)

Dipole & C-Br Bond: Partial positive charge on C (δ⁺) and partial negative charge on Br (δ⁻).
Curl Arrow 1: Starts from the lone pair (or negative charge) on the oxygen of the hydroxide ion ( ⁻OH ) and points to the electron-deficient carbon (C-Br carbon).
Curl Arrow 2: Starts from the C–Br bond and points to the bromine atom (heterolytic fission).
Products: The alcohol (CH₃)₂CHCH₂CH₂OH and a bromide ion ( Br⁻ ).

✅ Mechanism Type ((iii) - 1 mark)

Nucleophilic substitution

Total 4 marks across (ii) and (iii). Make sure curly arrows originate precisely from bonds/lone pairs and point directly to atoms.

Part (e): Comparing Rates of Hydrolysis

Chloroalkane vs. Bromoalkane reactivity

✅ Correct Answer

The chloroalkane reacts slower than the bromoalkane (or ORA).

💡 Explanation & Marks

The C–Cl bond is stronger than the C–Br bond (C–Cl has a greater bond enthalpy).
More energy is required to break the C–Cl bond, leading to a slower rate of hydrolysis.

2 marks. Must link bond strength/enthalpy directly to activation energy and rate.

Part (f): Structural Elucidation (6-Mark Extended Response)

Identifying compound X, product Y, and writing the reaction equation

✅ Final Identification & Equation

Compound X: (CH₃)₂CHCHOHCH₃ (Alcohol E)
Product Y: (CH₃)₂CHCOCH₃ (3-methylbutan-2-one)
Equation: (CH₃)₂CHCHOHCH₃ + [O] → (CH₃)₂CHCOCH₃ + H₂O

Marked using levels of response (Level 3: 5–6 marks). Requires logical connection between spectroscopic evidence and chemical deductions.

🧠 Step-by-Step Analytical Breakdown

Mass Spectrum: Molecular ion peak ( M⁺ ) at m/z = 86 corresponds to a relative molecular mass of 86. Peak at m/z = 43 represents a fragment CH₃CO⁺ or C₃H₇⁺ .
IR Spectrum: There is a sharp, strong absorption peak around 1700 cm⁻¹ , confirming the presence of a C=O (carbonyl) group. Crucially, there is no broad absorption in the 2500–3300 cm⁻¹ region, ruling out a carboxylic acid and showing that the original alcohol was oxidized into a ketone.
Conclusion: Product Y is a ketone, meaning starting compound X must be a secondary alcohol. Matching molecular mass 86 and structural options A–F points directly to alcohol E ( (CH₃)₂CHCHOHCH₃ ).

Topics

Module 2: Foundations in chemistry · Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 2.1 Atoms and reactions · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.