OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 25

9 marks · Medium difficulty · Structured Questions

Deduce the products of alcohol reactions, name the alcohol, outline the haloalkane hydrolysis mechanism, and explain IR spectroscopy changes for the functional group conversion.

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Question

Question text

25 This question is about reactions involving alcohols.

(a) Three reactions of an alcohol E are shown in Fig. 25.1.

(i) Complete Fig. 25.1 to show the structures of the organic products formed in the reactions.

H2SO4 + heat

CH3 H

H3C C C OH

H H

H2SO4/K2Cr2O7 H2SO4/K2Cr2O7

distil reflux

G H

Fig. 25.1

[3]

(ii) What is the systematic name of alcohol E?

… [1]

(b) An alcohol can be prepared by hydrolysing the haloalkane C2H5CHBrCH3 with aqueous

sodium hydroxide.

(i) Outline the mechanism for this reaction.

Show curly arrows and relevant dipoles.

[3]

(ii) The infrared (IR) spectrum for C2H5CHBrCH3 is shown in Fig. 25.2.

The C–Br bond absorption is labelled.

transmittance

(%)

C–Br

4000 3000 2000 1500 1000 500

wavenumber / cm–1

Fig. 25.2

Outline how IR spectroscopy could be used to show that the bromoalkane functional

group has reacted and that the alcohol functional group has formed.

… [2]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

25 (a) (i) H3C H 3 ALLOW correct structural OR displayed

OR skeletal formulae OR mixture of the above

C C (as long as unambiguous)

H3C H IGNORE molecular formula

ALLOW CH3–

CH3 CH3

H3C C CHO H3C C COOH

H H

ALLOW 1 mark for G AND H combined is structures are

G H correct but in wrong boxes

(a) (ii) 2-methylpropan-1-ol 1 IGNORE absence of hyphen or use of dots or commas as

separators

Both numbers required

DO NOT ALLOW 2-methylprop-1-ol

OR 2-methpropan-1-ol

OR 2-methypropan-1-ol

(b) (i) ANNOTATE ANSWER WITH TICKS AND CROSSES 15 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

start from, OR be traced back to any point

across width of lone pair on O of OH–

dipole shown on C–Br bond, C + and Br −,

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–)

2nd curly arrow must start from, OR be traced back to,

IGNORE incorrect R groups for curly arrow marks any part of C–Br bond and go to Br

IGNORE presence of Na+/Na but OH– needed

i.e. Na+OH–; NaOH– can be allowed with correct use of

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

curly arrow

ALLOW SN1 mechanism for 2 curly arrow marks

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

Products 1 mark First mark

correct organic product AND Br– Dipole shown on C–Br bond, C + and Br −,

AND curly arrow from C−Br bond to Br atom

+ – Second mark

IGNORE presence of Na but Br needed –

+ – – Curly arrow from OH AND to correct carbocation

i.e. Na Br /NaBr can be allowed

BUT NaBr does NOT show Br–

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

but NOT double headed or half headed arrows Use curly arrow criteria in guidance above

Question Answer 16 Marks Guidance

(b) (ii) Disappearance of 2

peak at 500–800 cm–1 OR C–Br peak ALLOW value within range 500–800 cm–1

Appearance of

peak at 3200–3600 cm–1 OR alcohol O–H peak ALLOW value within range 3200–3600 cm–1

DO NOT ALLOW responses that only describe the

spectrum shown

Total 9

How to answer it

Reactions Involving Alcohols Study Guide

What this question tests

This question assesses your knowledge of alcohol chemistry, including oxidation pathways (distillation vs. reflux), acid-catalysed dehydration reactions, IUPAC nomenclature, nucleophilic substitution mechanisms (S_N2/S_N1) with curly arrows and dipoles, and the interpretation of infrared (IR) spectroscopy data to track functional group transformations.

Question 25 (a)(i) — Organic Products of Alcohol E

Reactions of 2-methylpropan-1-ol

✅ Correct Products (F, G, H)

Product F (Dehydration): (CH₃)₂C=CH₂ (2-methylpropene) or structural equivalent showing the alkene double bond.
Product G (Distillation): (CH₃)₂CHCHO (2-methylpropanal). Formed by the partial oxidation of a primary alcohol.
Product H (Reflux): (CH₃)₂CHCOOH (2-methylpropanoic acid). Formed by the complete oxidation under reflux.

💡 Key Knowledge

Reflux vs Distillation: Primary alcohols oxidise to aldehydes on distillation (removed before further reaction) and carboxylic acids under reflux (ensuring complete oxidation).
Dehydration: Heating a primary alcohol with concentrated H₂SO₄ eliminates water to form an alkene.

❌ Common Errors

Confusing the conditions for aldehydes and carboxylic acids (putting the carboxylic acid in the distillation box).
Drawing incorrect carbon skeletons or misplacing methyl branches during oxidation.

🧠 Exam Technique

Ensure structural or displayed formulae clearly show connectivity (e.g., the -CHO group for the aldehyde and -COOH for the carboxylic acid).

Mark Allocation: 3 marks total (1 mark for each correct structure F, G, and H).

Question 25 (a)(ii) — IUPAC Nomenclature

Naming Alcohol E

✅ Correct Answer

2-methylpropan-1-ol (Both numbers required).

❌ Common Errors

Writing 2-methylpropan-1-ol without the locator numbers, or writing 2-methylpropanol (which lacks the -1- position locator).

Mark Allocation: 1 mark.

Question 25 (b)(i) — Nucleophilic Substitution Mechanism

Hydrolysis of a Haloalkane

✅ Correct Mechanism Requirements

Dipole & First Arrow: C^(δ+)—Br^(δ-) dipole shown clearly. First curly arrow starting from the lone pair/charge of the ⁻OH ion attacking the C^(δ+) carbon.
Second Arrow: Curly arrow originating from the C—Br bond and going directly to the Br atom.
Products: Correct alcohol product shown alongside the bromide ion ( Br⁻ ).

💡 Key Knowledge

This is an S_N2 mechanism (or S_N1 via a carbocation intermediate, both accepted by mark scheme if correctly annotated).
Hydroxide acts as a nucleophile donating an electron pair.

❌ Common Errors

Starting the curly arrow from the negative sign incorrectly instead of the lone pair, or drawing the bond-breaking arrow starting from the bromine atom instead of the covalent bond.

🧠 Exam Technique

Make sure arrowheads point precisely to atoms or bonds. Double-headed or half-headed radical arrows will lose credit.

Mark Allocation: 3 marks (2 marks for curly arrows and dipoles; 1 mark for organic product and Br⁻ ).

Question 25 (b)(ii) — IR Spectroscopy

Tracking Functional Group Changes

✅ Correct Answer Points

Disappearance: Peak at 500–800 cm⁻¹ (representing the C—Br bond) disappears.
Appearance: Broad peak at 3200–3600 cm⁻¹ (representing the alcohol O—H group) appears.

❌ Common Errors

Simply describing the spectrum shown rather than stating what happens to the specific peaks during the reaction (e.g., omitting the words "disappearance" or "appearance").

Mark Allocation: 2 marks (1 mark for disappearance of C–Br peak; 1 mark for appearance of O–H peak).

Topics

Module 4: Core organic chemistry · Module 6: Organic chemistry and analysis · 4.2 Alcohols, haloalkanes and analysis · 6.3 Analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.