OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2018: Question 16

✅ Mark Scheme Answers

• (i) Systematic name: 3-methylbutan-2-ol [1 mark]
• (ii) Structural formula: (CH₃)₂CHCH(OH)CH₃ or CH₃CH(CH₃)CH(OH)CH₃ [1 mark]

🧠 Exam Technique

• Longest Chain Priority: Find the longest continuous carbon chain containing the -OH group (4 carbons = butan ).
• Lowest Locant Rule: Number from the end nearest the principal functional group: the -OH must get priority over the methyl substituent (C2 for -OH, C3 for -CH₃).
• Brackets in structural formulae show branching: (CH₃)₂CH- or side-groups -CH(OH)- .

❌ Common Errors

• Writing 2-methylbutan-3-ol (giving the methyl branch the lower number rather than the principal -OH group).
• Omitting hyphens between numbers and words, e.g. 3 methyl butan 2 ol.
• Writing a molecular formula ( C₅H₁₂O ) instead of the required condensed structural formula.

💡 Mark Scheme Allowances

The mark scheme allows 3-methylbutane-2-ol (with 'e'), but strictly rejects incorrect numbering such as 2-methylbutan-3-ol . Brackets around the -OH in the formula are optional if connectivity remains unambiguous.

✅ Correct Structures

Two distinct alkenes are formed by removing an -H from adjacent carbons (C1 or C3):

• Alkene 1 (from C2 & C3): 2-methylbut-2-ene
  Skeletal: A double bond between C2 and C3 with two methyls on C2: (CH₃)₂C=CHCH₃ [1 mark]
• Alkene 2 (from C1 & C2): 3-methylbut-1-ene
  Skeletal: A terminal double bond at C1=C2: (CH₃)₂CH-CH=CH₂ [1 mark]

💡 Key Chemistry Principle

During dehydration of 3-methylbutan-2-ol :

• Elimination towards C1 (H removed from C1) gives 3-methylbut-1-ene.
• Elimination towards C3 (H removed from C3) gives 2-methylbut-2-ene (major product by Zaitsev's rule).
• Stereoisomerism check: Neither alkene exhibits E/Z isomerism because C1 in alkene 2 has two identical H atoms, and C2 in alkene 1 has two identical methyl groups!

✅ Balanced Chemical Equation

Alternative fully balanced equation with Na₂SO₄:

• Haloalkane product: 2-chloro-3-methylbutane [1 mark]
• Correct balancing with all inorganic products [1 mark]

❌ Common Errors

• Producing NaOH as a product (e.g., reacting with NaCl alone without acid). This is chemically impossible in acid solution and receives 0 marks.
• Swapping chlorine to the wrong carbon (must replace the -OH at C2).
• Forgetting water ( H₂O ) in the mass balance.

✅ Completed Balanced Equation

• Organic Product: 3-hydroxy-3-methyl-2-oxobutanoic acid [1 mark]
• Rest of Equation: Balanced with 3 [O] on LHS and 2 H₂O on RHS [1 mark]

❌ Common Errors

• Attempting to oxidise the tertiary -OH group at C3 by breaking C-C bonds.
• Only oxidising the primary alcohol to an aldehyde (-CHO) instead of carboxylic acid (-COOH) despite the question specifying reflux with excess oxidising agent.
• Incorrect balance of [O] (often writing 2 [O] or forgetting the water molecules).

1. Excess CH₃OH / H₂SO₄ (Top Box)

Reaction: Acid-catalysed esterification of both carboxylic acid groups.

Name: Dimethyl but-2-enedioate [1 mark]

• Both -COOH groups convert into methyl esters ( -COOCH₃ ).
• The C=C double bond remains unchanged.

2. Steam / H₃PO₄ Catalyst (Right Box)

Reaction: Electrophilic addition of H₂O across the alkene C=C bond.

Name: 2-hydroxybutanedioic acid (malic acid) [1 mark]

• Double bond becomes single: H adds to one carbon, OH adds to the other.
• Carboxylic acid groups are unaffected.

3. Polymerisation (Bottom Box)

Reaction: Addition polymerisation across the C=C double bond.

Repeat Unit Structure:

• Two backbone carbons with open extension bonds extending beyond the unit [1 mark].
• Each backbone carbon bears one -H and one -COOH .

🧠 Top Tip for Repeat Units

Always draw the continuous carbon-carbon backbone horizontally with open extension bonds crossing through brackets (or just trailing bonds). Never include double bonds in the backbone of an addition polymer repeat unit.

❌ Flowchart Pitfalls

• Esterifying only one of the two -COOH groups (prompt states excess methanol).
• Hydrating the alkene and simultaneously modifying carboxylic acid groups.
• Drawing condensation polymer repeat units instead of addition polymerisation through the C=C double bond.