Edexcel A-Level Chemistry Paper 2, November 2021: Question 12

6 marks · Hard difficulty · Open Response

Devise a multi-step reaction scheme to synthesise 2,2-dimethylbutan-1-ol starting from 2-bromo-2-methylbutane, including reagents, conditions and intermediate structures.

Practise this question

Question

Question 12 shows the structural formula of 2,2-dimethylbutan-1-ol. The text asks to devise a reaction scheme for the synthesis of this alcohol starting from 2-bromo-2-methylbutane, including all reagents, conditions, and intermediate structures for 6 marks.
Question text

12 The alcohol 2,2-dimethylbutan-1-ol has the structure

H2 CH3 H2

C C

H3C C OH

CH3

Devise a reaction scheme for a synthesis of this alcohol starting from

2-bromo-2-methylbutane.

Include in your answer all reagents and conditions and the structures of any

intermediate compounds.

(6)

(Total for Question 12 = 6 marks)

Mark scheme

Show the mark scheme The mark scheme provides two alternative synthesis routes for 6 marks each: Route 1 using a Grignard reagent (reacting 2-bromo-2-methylbutane with Mg in dry ether, then reacting with methanal HCHO followed by dilute acid hydrolysis). Route 2 using cyanide substitution (reacting with KCN in ethanol to form a nitrile, hydrolysing the nitrile with dilute acid to form a carboxylic acid, and reducing it with LiAlH4).

Question Answer Additional Guidance Mark

Number

12 Note – award of reagent or (6)

• 2-bromo-2-methylbutane reacts with Mg (1) solvent marks must be in

context of attempt to carry

• Dry ether (1) out an appropriate reaction

e.g. use of ethanolic KCN to

• CH3CH2C(MgBr)(CH3)CH3 (1) react with a ketone would

not score OR M2

• react Grignard reagent with HCHO (1) do not award HCOH

• CH3CH2C(CH3)2CH2OMgBr (1)

• (hydrolyse) with (dilute) acid (1) Allow with water / H+

OR

• 2-bromo-2-methylbutane reacts with KCN (1) Ignore HCN

• ethanol (as solvent) (1) Allow methanol

• CH3CH2C(CN)(CH3)CH3 (1)

• nitrile (hydrolysed) with (dilute) acid (1) Allow H+

• CH3CH2C(COOH)(CH3)CH3 (1)

• carboxylic acid (reduced) with LiAlH4 (in dry ether) (1)

(Total for Question 12 = 6 marks)

TOTAL FOR PAPER = 90 MARKS

How to answer it

Organic Synthesis: 2,2-dimethylbutan-1-ol

What this question tests

This 6-mark synthesis question assesses your ability to design multi-step organic reaction pathways. You must link starting materials to target molecules by selecting appropriate carbon-chain-lengthening reagents, catalysts, solvents, and identifying correct intermediate structures.

Question 12 • 6 Marks

Devise a Reaction Scheme for Synthesis

Starting from 2-bromo-2-methylbutane , devise a reaction scheme to synthesise 2,2-dimethylbutan-1-ol . Include all reagents, conditions, and intermediate structures.

✅ Correct Answer (Pathway 1: Grignard Route)

  • Step 1 Reagents & Conditions: React 2-bromo-2-methylbutane with Mg in dry ether .
  • Intermediate 1: Grignard reagent CH₃CH₂C(MgBr)(CH₃)₂ .
  • Step 2 Reagents: React with methanal ( HCHO ).
  • Intermediate 2: Alkoxide CH₃CH₂C(CH₃)₂CH₂OMgBr .
  • Final Step: Hydrolyse with dilute acid ( H⁺(aq) or H₂O / H⁺ ) to form the primary alcohol.

✅ Correct Answer (Pathway 2: Nitrile Route)

  • Step 1 Reagents & Conditions: React 2-bromo-2-methylbutane with KCN in ethanol (solvent).
  • Intermediate 1: Nitrile CH₃CH₂C(CN)(CH₃)₂ .
  • Step 2 Reagents: Hydrolyse nitrile with dilute acid to form a carboxylic acid CH₃CH₂C(COOH)(CH₃)₂ .
  • Final Step: Reduce the carboxylic acid using LiAlH₄ in dry ether to yield the target primary alcohol.

💡 Key Knowledge

  • Carbon Chain Lengthening: The target molecule has 6 carbons in its longest backbone/branching system compared to the 5-carbon starting haloalkane. Both Grignard reactions with carbonyls and cyanide substitution mechanisms successfully increase carbon chain length.
  • Grignard Requirements: Grignard reagents are extremely sensitive to protic solvents and must be prepared and reacted in anhydrous/dry conditions ( dry ether ).

🧠 Exam Technique

  • Lay out your synthesis clearly in a linear reaction scheme format using arrows, clearly labeling reagents above/below the arrows and drawing structural formulas for every intermediate.
  • Ensure solvents are specified where required (e.g., dry ether or ethanol ). Missing a mandatory solvent loses an independent mark.

❌ Common Errors

  • Attempting to react the haloalkane directly with aqueous alkali via nucleophilic substitution (SN2) fails because 2-bromo-2-methylbutane is a tertiary haloalkane and undergoes rapid elimination instead, or reacts via an SN1 mechanism yielding a tertiary alcohol (wrong positional isomer).
  • Using wet reagents or failing to specify dry conditions for Grignard reactions destroys the organometallic reagent.
Mark Allocation: This is a 6-mark level of response / point-scored question. Each valid reagent, condition, and correctly drawn intermediate structure accounts for individual marks across either viable synthetic route.

Topics

Organic Chemistry · Topic 18: Organic Chemistry III · Topic 6: Organic Chemistry I · Topic 17: Organic Chemistry II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.