Edexcel A-Level Chemistry AS Paper 2, June 2019: Question 5

13 marks · Hard difficulty · Open Response

Investigate the elimination and substitution reactions of bromobutanes to produce butenes and butan-1-ol using experimental apparatus, calculations, mechanisms, and chemical tests.

Practise this question

Question

An exam question with multiple parts based on an experiment where bromobutane reacts with hot ethanolic potassium hydroxide in an apparatus featuring a round-bottom flask, hot oil bath, reflux condenser, and gas syringe. Parts (a) through (f) ask students to state the purpose of the condenser, describe a chemical test for the alkene gas, calculate percentage conversion and temperature of the gas, identify substitution reaction type, select the correct test for an alcohol, draw the nucleophilic substitution mechanism, and determine the number of isomeric alkenes formed from 2-bromobutane.
Question text

5 Bromobutanes react with hot ethanolic potassium hydroxide solution to produce

gaseous butenes.

C H9Br + OH− → C H8 + Br− + H O

44 2

Apparatus

gas

syringe

clamp to seal

gas syringe reflux

condenser

rubber seal for

injection of

bromobutane

hot ethanolic

potassium hydroxide

hot oil bath

Procedure

Ɣ 0.0080 mol of liquid 1-bromobutane was injected into a round bottom flask

containing hot ethanolic potassium hydroxide.

Ɣ After the reaction, the syringe was sealed using a clamp.

Ɣ The syringe was then removed from the apparatus and allowed to cool to

room temperature (298 K).

Result

The final volume of but-1-ene collected was 22.0 cm3.

(a) State the purpose of the condenser.

(1)

(b) Describe a chemical test on the gas in the syringe to identify its functional group.

Include the expected result.

(2)

… *P55611A01228*

(c) Calculate the percentage of 1-bromobutane which was converted to but-1-ene.

[Molar volume of a gas at r.t.p. = 24 000 cm3 mol–1]

(2)

(d) Before cooling, the volume of but-1-ene in the gas syringe was 24.0 cm3.

Calculate the temperature of the gas in the syringe before it cooled.

[Assume no loss from the gas syringe during cooling, and a constant pressure]

(2)

(e) (i) Another compound formed from 1-bromobutane under these conditions is butan-1-ol.

Identify the type of reaction taking place to form butan-1-ol.

(1)

(ii) The functional group in butan-1-ol can be confirmed using a single chemical test.

What is the single chemical test and expected observation?

(1)

Chemical test reagent Observation 13

A sodium carbonate solution*P55611A01328*effervescence

B aqueous silver nitrate cream precipitate

C Fehling’s solution red precipitate

D phosphorus(V) chloride steamy fumes

(iii) Draw the mechanism for the reaction of 1-bromobutane with hydroxide ions

to form butan-1-ol.

Include curly arrows, and any appropriate lone pairs and dipoles.

(3)

(f ) Alkene molecules are formed by elimination from 2-bromobutane.

How many isomeric alkene products will be formed in this reaction?

(1)

A 1

B 2

C 3

D 4

(Total for Question 5 = 13 marks)

Mark scheme

Show the mark scheme The mark scheme providing detailed answers and guidance for all parts of Question 5, including accepted descriptions for the reflux condenser, chemical test details for alkenes, step-by-step example calculations for moles, percentage yield, and Charles's law temperature calculations, correct substitution and test identification, mechanism diagrams with curly arrows and dipoles, and multiple-choice answers.

How to answer it

Halogenoalkanes: Elimination, Substitution & Mechanisms

📋 What this question tests

This comprehensive question assesses your practical and theoretical understanding of halogenoalkane chemistry. It covers reflux apparatus design, functional group testing for alkenes and alcohols, gas molar volume and gas law calculations, reaction mechanisms (nucleophilic substitution / SN2), and structural isomerism in alkenes.

Part (a): Purpose of the Condenser

Exam Question Part (a)

✅ Correct Answer

Returns or condenses volatile reactants/evaporated gases (except the product gas, but-1-ene) back to the reaction mixture so they are not lost.

💡 Key Knowledge

Reflux setups prevent the escape of volatile organic starting materials (like 1-bromobutane) and solvents when heated strongly, ensuring complete reaction without losing reactants.

❌ Common Errors

Students often lose the mark by stating it "condenses the products" or "cools down gases" generally. You must specify that volatile reactants are returned, while the gaseous alkene product passes through into the syringe.

Mark: 1 mark

Part (b): Chemical Test for Alkenes

Exam Question Part (b)

✅ Correct Answer

Reagent: Bromine water ( Br₂(aq) ) or bromine in an organic solvent.
Observation: Goes from orange/brown to colourless.
Alternative: Acidified potassium manganate(VII) ( KMnO₄/H⁺ ), goes from purple to colourless.

🧠 Exam Technique

Always state both the specific reagent and the clear colour change (initial and final colours). Writing "it clears" is insufficient—specify the starting colour and the resulting colourless state.

Marks: 2 marks

Part (c): Percentage Conversion Calculation

Exam Question Part (c)

📐 Step-by-Step Calculation

  1. Find moles of gas collected:
    Moles = Volume / Molar Volume = 22.0 / 24000 = 9.167 × 10⁻⁴ mol
  2. Calculate percentage converted:
    Percentage = (Moles of product / Initial moles of reactant) × 100
    = ( 9.167 × 10⁻⁴ / 0.0080 ) × 100 = 11.5% (or 11.46%)

❌ Common Calculation Traps

Watch out for unit mismatches! Ensure volume units match (cm³ to cm³, or converting both to dm³). Do not prematurely round intermediate steps in your calculator.

Marks: 2 marks

Part (d): Gas Laws & Temperature Calculation

Exam Question Part (d)

📐 Step-by-Step Calculation

  1. Recognize proportional relationships: At constant pressure and constant moles of gas, V₁ / T₁ = V₂ / T₂ . Therefore, T₂ = T₁ × (V₂ / V₁) .
  2. Substitute values (remembering Kelvin):
    Initial temperature T₁ = 298 K .
    Initial volume before cooling V₁ = 24.0 cm³ , final cooled volume V₂ = 22.0 cm³ .
  3. Calculate final temperature:
    T₂ = 298 × (22.0 / 24.0) = 273.17 K (or 0.17 °C ). Alternatively, using pV = nRT directly yields credit.

💡 Key Knowledge

Always convert temperatures to Kelvin ( K = °C + 273 ) when applying gas laws or ideal gas equations.

Marks: 2 marks

Part (e): Substitution Reactions & Mechanisms

Exam Question Part (e)

✅ (i) Type of Reaction

Substitution (or hydrolysis / nucleophilic substitution). Do not accept elimination or electrophilic substitution.

✅ (ii) Functional Group Test

Answer: D (phosphorus(V) chloride | steamy fumes)
PCl₅ reacts with the hydroxyl group in alcohols to produce misty/steamy fumes of hydrogen chloride gas ( HCl ).

💡 (iii) Mechanism for Formation of Butan-1-ol (SN2)

  • Curly arrow 1: From the lone pair of electrons on the oxygen atom of the hydroxide ion ( OH⁻ ) to the carbon atom bonded to the halogen.
  • Curly arrow 2: From the C-Br bond directly to the bromine atom, resulting in departure of the bromide ion ( Br⁻ ).
  • Dipoles & Charges: Partial positive charge ( δ⁺ ) on carbon and partial negative charge ( δ⁻ ) on bromine, along with a complete negative charge on the leaving bromide ion.
Marks: 1 + 1 + 3 = 5 marks total

Part (f): Alkene Isomerism from Elimination

Exam Question Part (f)

✅ Correct Answer

Answer: C (3)

🧠 Exam Technique

Elimination from 2-bromobutane ( CH₃-CH(Br)-CH₂-CH₃ ) can yield hydrogen from carbon-1 or carbon-3. This produces:

  1. But-1-ene
  2. cis-but-2-ene (Z isomer)
  3. trans-but-2-ene (E isomer)
Mark: 1 mark

Topics

Organic Chemistry · Physical Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.