OCR A-Level Chemistry Synthesis and analytical techniques (02), November 2020: Question 9

1 mark · Medium difficulty · Multiple Choice

Identify the correct structural formula for ethyl 3-methylbutanoate from the given multiple-choice options.

Practise this question

Question

Multiple choice question 9 asks for the structural formula of ethyl 3-methylbutanoate with four options labeled A, B, C, and D showing various ester formulas: A is CH3CH2COOCH2CH2CH(CH3)2, B is CH3CH2COOCH(CH3)CH2CH3, C is CH3CH2CH(CH3)COOCH2CH3, and D is (CH3)2CHCH2COOCH2CH3. An answer box and a mark allocation of [1] are shown at the bottom.
Question text

9 What is the structural formula of ethyl 3-methylbutanoate?

A CH3CH2COOCH2CH2CH(CH3)2

B CH3CH2COOCH(CH3)CH2CH3

C CH3CH2CH(CH3)COOCH2CH3

D (CH3)2CHCH2COOCH2CH3

Your answer

[1]

Mark scheme

Show the mark scheme Mark scheme table showing question number 9 with the correct answer D.

9 D 1 1.2

How to answer it

Structural Formula of Esters

What this question tests

This question assesses your ability to interpret IUPAC nomenclature for esters and translate it into a condensed structural formula. You must correctly identify the alkyl chain attached to the oxygen atom (from the alcohol part, named first) and the principal acid chain containing the ester functional group (named second, with any substituents numbered relative to the carbonyl carbon).

Question 9

Exam Part: Multiple Choice Question 9

✅ Correct Answer

D: (CH₃)₂CHCH₂COOCH₂CH₃

Awarded 1 mark for selecting option D.

💡 Key Knowledge

  • Esters structure: Named as alkyl alkanoate . The first word comes from the alcohol ( ethyl = -CH₂CH₃ attached to oxygen).
  • Carboxylic acid chain: 3-methylbutanoate indicates a 4-carbon main chain including the carbonyl carbon ( butano- ), with a methyl branch on carbon-3.
  • Numbering rule: Carbon-1 is always the carbonyl carbon ( C=O ) of the ester group. Counting along: C1 is COO , C2 is CH₂ , C3 is CH(CH₃)₂ .

🧠 Exam Technique

Deconstruct the name from right to left (or work outwards from the ester linkage -COO- ):

  1. Locate the ester link ( -COO- ). The right-hand side must be ethyl ( -CH₂CH₃ ), eliminating options A and B.
  2. Look at the left-hand side for 3-methylbutanoate . A butyl chain has 4 carbons. Option C has the methyl group on carbon-2 relative to the carbonyl ( CH₃CH₂CH(CH₃)COOR ), whereas Option D correctly places the branching at the 3-position: (CH₃)₂CHCH₂- .

❌ Common Errors

  • Reversing the ester halves: Confusing which part belongs to the alcohol vs. the carboxylic acid.
  • Misnumbering the carbon chain: Starting the carbon numbering from the wrong end of the acid chain or forgetting to count the carbonyl carbon itself as carbon-1.
  • Branching confusion: Selecting option C because of a failure to carefully trace out positional isomerism and methyl branching along the longer chain.

Topics

Module 6: Organic chemistry and analysis · 6.1 Aromatic compounds, carbonyls and acids

Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.