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QLDChemistryQuick questions
Unit 4: Structure, synthesis and design
Quick questions on Proton (1H) NMR spectroscopy (QCE Chemistry Unit 4)
14short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the molecular mass?Show answer
| MS (M+) | | Are these functional groups present? | IR (O-H, C=O, N-H) | | How many hydrogen environments? Where?
What is what a 1H NMR spectrum shows?Show answer
A 1H NMR spectrum has three diagnostic features:
What is counting hydrogen environments?Show answer
Two hydrogens are in the same environment if they are equivalent by symmetry (interchangeable by a molecular symmetry operation or by free rotation of an attached methyl group). They give a single combined signal.
What is chemical shift?Show answer
Each environment has a characteristic chemical shift range, determined by the inductive effects of attached groups. The QCAA syllabus uses the following reference ranges (the QCAA data booklet table for Unit 4 spectroscopy):
What is integration?Show answer
The area under each peak is proportional to the number of hydrogens in that environment. NMR spectrometers display this as a step curve or as a number near each peak. Integrations are relative ratios, not absolute counts; if a molecule has 6 hydrogens distributed 3 : 2 : 1, the spectrum shows three peaks with integration ratio 3 : 2 : 1.
What is splitting?Show answer
A peak is split into (n+1) sub-peaks (multiplicity) by the n equivalent hydrogens on the adjacent carbon(s). The pattern of split heights is given by Pascal's triangle:
What is combining the three pieces?Show answer
The systematic approach for an unknown 1H NMR:
What is nMR vs IR vs MS?Show answer
A typical QCAA IA3 / EA stimulus provides all three spectra. The expected workflow is MS for Mr -> molecular formula candidates; IR to narrow functional groups; 1H NMR to fix the structure.
What is common traps?Show answer
Confusing chemical shift with integration. Chemical shift identifies the type of H. Integration counts how many. They are read off different parts of the spectrum.
What is confusing chemical shift with integration?Show answer
Chemical shift identifies the type of H. Integration counts how many. They are read off different parts of the spectrum.
What is applying n+1 to non-adjacent hydrogens?Show answer
Splitting comes only from hydrogens on directly bonded carbons (or atoms attached to those carbons). Hydrogens separated by C=O or oxygen do not couple in basic 1H NMR; the singlet for -OCO-CH3 in ethyl ethanoate is the classic example.
What is forgetting that OH and NH protons exchange?Show answer
These often appear as broad singlets at variable chemical shift, regardless of their nominal n+1 neighbours, because exchange with solvent water decouples them.
What is treating equivalent hydrogens as separate peaks?Show answer
A methyl group's 3 hydrogens are equivalent by free rotation and appear as a single peak. Drawing 3 separate peaks for them is wrong.
What is skipping the integration check?Show answer
Total integration must match the molecular formula. If it does not, you have miscounted environments or assigned the wrong peak.