Edexcel A-Level Chemistry AS Paper 2, November 2020: Question 2

13 marks · Medium difficulty · Short Open Response

Identify alcohol isomers, explain elimination reactions, define stereoisomers and geometrical isomerism, write oxidation and substitution equations for alcohols and halogenoalkanes.

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Question

Question text

2 This question is about alcohols and their reactions.

The table gives some of the names and skeletal formulae of isomers having the

formula C4H9OH.

(a) Complete the table.

(2)

Name Skeletal formula

butan‑2‑ol

2‑methylpropan‑1‑ol

2‑methylpropan‑2‑ol

(b) (i) Some alcohols react with concentrated phosphoric acid to form alkenes.

What is the type of this reaction?

(1)

A addition

B elimination

C oxidation

D substitution

(ii) When butan‑2‑ol reacts with concentrated phosphoric acid, two stereoisomers

are formed.

Explain what is meant by the term stereoisomers.*P62307A0528*

(2)

(iii) Draw the structures and give the names of the two stereoisomers.

(2)

Stereoisomer 1 Stereoisomer 2

Name: Name:

(iv) Name this type of stereoisomerism.

(1)

2‑bromo‑2‑methylpropane and aqueous potassium hydroxide.

What is the role of the hydroxide ions in this reaction?

(1)

A alkali

B catalyst

C electrophile

D nucleophile

(d) (i) CH3CH2CH2CH2OH reacts with the oxidising agent potassium dichromate(VI) in

dilute sulfuric acid.

Two organic products can be formed, depending on the conditions.

Write a balanced equation for the formation of one of these products, giving

its name and the condition required to achieve this product in high yield.

Use [O] in the equation to represent each oxygen atom from the oxidising agent.

(3)

Equation

Name …

Condition …

(ii) The colour of the solution at the end of the reaction in (d)(i) will be

(1)

A brown

B green

C orange

D yellow

(Total for Question 2 = 13 marks)

Mark scheme

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How to answer it

Alcohols and Their Reactions

Edexcel AS Level Chemistry

What this question tests

This question assesses your knowledge of alcohol isomers (structural and stereoisomers), mechanisms of elimination and nucleophilic substitution, oxidation reactions of primary alcohols (including distinction between distillation and reflux), and associated practical colour changes.

Part (a): Isomers of C₄H₉OH

Completing the table of names and skeletal formulae

✅ Correct Answers

Row 1 Name: butan-1-ol (or 1-butanol)
Row 3 Skeletal Formula: A branched chain with 3 carbons in the main chain, a methyl group on carbon 2, and an -OH group attached to carbon 1 ( CH₃CH(CH₃)CH₂OH ).

❌ Common Errors

Writing just butanol without the position number (penalised by examiners).
Drawing horizontal bonds directly into the HO atom instead of connecting to the O .

Total for (a): 2 marks

Part (b)(i): Reaction Type

Dehydration of alcohols using concentrated phosphoric acid

✅ Correct Answer

B (elimination)

💡 Key Knowledge

Reacting an alcohol with concentrated acid removes a water molecule (elements of -H and -OH on adjacent carbons), creating a carbon-carbon double bond (an alkene). This is an elimination reaction.

Total for (b)(i): 1 mark

Part (b)(ii): Definition of Stereoisomers

Explaining stereoisomerism in butan-2-ol dehydration products

✅ Correct Answer

Compounds with the same structural formula, but with atoms/groups having a different arrangement in space.

🧠 Exam Technique

You must mention both parts to secure both marks: (1) same structural formula/connectivity, and (2) different 3D spatial arrangement. Vague answers like "different arrangement" without specifying "in space" lose marks.

Total for (b)(ii): 2 marks

Parts (b)(iii) & (iv): Drawing and Naming Stereoisomers

E/Z (Geometric) Isomerism

✅ Correct Answers

Stereoisomer 1: Z-but-2-ene (or cis-but-2-ene)
Structure features a C=C double bond with -CH₃ groups on the same side.

Stereoisomer 2: E-but-2-ene (or trans-but-2-ene)
Structure features a C=C double bond with -CH₃ groups on opposite sides.

Part (iv): Geometric isomerism (accept cis-trans or E-Z ).

❌ Common Errors

Confusing optical and geometric isomerism, or drawing structural isomers (like but-1-ene) instead of stereoisomers of but-2-ene.

Total for (b)(iii) & (iv): 3 marks

Part (c): Mechanism Role

Hydroxide ion role in haloalkane substitution

✅ Correct Answer

D (nucleophile)

💡 Key Knowledge

The OH⁻ ion possesses a lone pair of electrons which it donates to a partially positive carbon atom on the haloalkane, acting as an electron-pair donor (nucleophile).

Total for (c): 1 mark

Part (d)(i): Oxidation of Butan-1-ol

Equations, Names, and Conditions

✅ Correct Answers (Choose one route)

Route 1 (Aldehyde):

CH₃CH₂CH₂CH₂OH + [O] → CH₃CH₂CH₂CHO + H₂O

Name: butanal | Condition: distillation (distil off immediately)

Route 2 (Carboxylic Acid):

CH₃CH₂CH₂CH₂OH + 2[O] → CH₃CH₂CH₂COOH + H₂O

Name: butanoic acid | Condition: heat under reflux

🧠 Exam Technique

The condition mark is strictly dependent on getting a matching equation and product correct. Always use [O] for the oxidising agent as requested in the prompt. Ensure molecular formulae like C₃H₇CHO are used carefully if condensing, though full structural representations are safest.

Total for (d)(i): 3 marks

Part (d)(ii): Colour Change

Observation of acidified potassium dichromate(VI) after oxidation

✅ Correct Answer

B (green)

💡 Key Knowledge

During oxidation of primary/secondary alcohols, orange dichromate(VI) ions ( Cr₂O₇²⁻ ) are reduced to green chromium(III) ions ( Cr³⁺ ).

Total for (d)(ii): 1 mark

Topics

Organic Chemistry · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.