OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2017: Question 11
1 mark · Easy difficulty · Multiple Choice
Determine the number of peaks observed in the 1H NMR spectrum of pentanedioic acid.
Practise this questionQuestion
Question text
11 The compound below is analysed by 1H NMR spectroscopy.
O O
HO OH
How many peaks are observed in the 1H NMR spectrum?
A 5
B 4
C 3
D 2
Your answer
[1]
Mark scheme
Show the mark scheme
11 C 1 ALLOW 3 (This is the number of peaks in the NMR spectrum)
How to answer it
Predicting Number of Peaks in 1H NMR Spectra
What this question tests
This question assesses your understanding of molecular symmetry and chemical environments in proton ( 1H ) NMR spectroscopy. You are required to identify unique hydrogen environments by examining structural symmetry and planes of symmetry within an organic molecule (pentanedioic acid).
Determining NMR Environments
✅ Correct Answer
C (3 peaks)
The molecule is pentanedioic acid, HOOC-CH₂-CH₂-CH₂-COOH, which possesses a vertical plane of molecular symmetry.
💡 Key Knowledge
- Each chemically non-equivalent proton environment produces a distinct peak in a 1H NMR spectrum.
- Symmetrical molecules feature mirror-image carbon/hydrogen frameworks that share identical chemical environments.
- Functional groups like carboxylic acids ( -COOH ) contain exchangeable protons that still generate their own distinct environment peak.
🧠 Exam Technique
Draw out or trace lines of symmetry down the middle of straight-chain carbon skeletons. Because the left half mirrors the right half in pentanedioic acid, matching halves collapse into the exact same chemical shift environment.
❌ Common Errors
- Counting individual hydrogens: Mistaking the total number of hydrogen atoms (8) for the number of peaks.
- Ignoring symmetry: Treating every carbon in the chain as a separate unique environment, leading to an over-counted guess of 4 or 5 peaks.
- Forgetting the acidic proton: Overlooking the -COOH proton environment.
Step-by-Step Breakdown of Pentanedioic Acid (HOOC-CH₂-CH₂-CH₂-COOH):
- Environment 1: The two carboxylic acid protons ( 2 × -COOH ) located at both terminal ends are in identical chemical environments due to molecular symmetry. (Peak 1)
- Environment 2: The central methylene group ( -CH₂- flanked by two other -CH₂- units) forms a distinct environment near the middle of the chain. (Peak 2)
- Environment 3: The two intermediate methylene groups ( -CH₂- groups adjacent to the -COOH carbons) are symmetrically equivalent to one another on opposite sides of the chain center. (Peak 3)
Total unique environments = 3.
Topics
Module 6: Organic chemistry and analysis · 6.3 Analysis
Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2017. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.