JC2 H3 Chemistry Tutor | Preparing a Proton-NMR Assignment That Uses More Than Integration

Write the task as a comparison between methyl propanoate, CH₃CH₂C(=O)OCH₃, and ethyl ethanoate, CH₃C(=O)OCH₂CH₃. Both have molecular formula C₄H₈O₂, and the question asks which fits the supplied idealised proton-NMR data.

State that independent supplied evidence establishes an ester functional group and that the sample is treated as one compound. The first assignment is bounded to the two candidates. A carbonyl absorption alone should not be presented as a unique identification of every ester structure.

Specify ordinary achiral solution conditions and a simple first-order treatment for the chosen proton sets. The examples do not require resolving higher-order coupling or a mixture of overlapping compounds. Those complications can be introduced explicitly later.

Ask the student to compare full assignments rather than choose from one attractive feature. Every significant signal should have a structural home, and every proton set in the chosen candidate should have a corresponding signal under the stated model.

Prepare a key with separate columns for observation, proton set and structural implication. This keeps signal evidence distinct from an inferred fragment and makes it easier to identify the first unsupported step.

The formula C₄H₈O₂ gives one degree of unsaturation using (2C + 2 − H)/2 = (8 + 2 − 8)/2 = 1. In each supplied ester candidate, the carbonyl accounts for it. This checks formula–structure consistency without claiming that the formula alone establishes the ester functional group.

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In methyl propanoate, the terminal CH₃ of CH₃CH₂–, the CH₂ next to the carbonyl and the OCH₃ group are three distinct proton environments in the simple model. Their proton counts are three, two and three.

In ethyl ethanoate, the CH₃ next to the carbonyl, the OCH₂ group and the terminal CH₃ of –OCH₂CH₃ are also three environments, with counts three, two and three. Signal count and integration therefore do not distinguish the candidates by themselves.

A triplet consists of three lines within one signal pattern for one proton environment in this treatment. A quartet has four lines within another. Do not count a triplet, quartet and singlet as eight separate chemical environments.

Equivalent protons contribute together to a signal under the specified conditions. For these simple ester examples, the CH₂ protons are treated as equivalent in ordinary achiral solution. More complex stereochemical environments may require a different analysis.

Ask the student to mark the proton sets on both structural formulae before seeing the numerical shifts. This checks whether equivalence and connectivity are being reasoned through rather than inferred backwards from a remembered spectrum.

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Supply three integrated areas in increasing chemical-shift order: 0.60, 0.40 and 0.60 arbitrary area units. Their ratio is 3:2:3 after dividing by 0.20. The total ratio count is eight, consistent with the eight hydrogens in the supplied formula.

The integration refers to the area of the whole signal or multiplet. It does not mean that every line in a triplet independently represents three hydrogens. The splitting distributes one signal’s intensity among several lines.

Use a quartet as a bookkeeping example. If its total area is 0.40 and the ideal line-area ratio is 1:3:3:1, the individual areas are 0.05, 0.15, 0.15 and 0.05. Together they still represent the two-proton environment after normalisation.

A taller narrow peak can have less total area than a broader or split signal. The task should supply integration values or clearly readable integration traces rather than expect the student to count protons from peak height.

If the molecular formula were not supplied, 3:2:3 would initially be a relative proton ratio. Scaling it to an absolute count needs appropriate additional information. The given C₄H₈O₂ formula provides that check in this exercise.

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Give the lowest-shift signal as a triplet integrating to three hydrogens and the middle signal as a quartet integrating to two. In the intended first-order model, the CH₃ has two equivalent neighbouring hydrogens and the CH₂ has three.

This reciprocal pattern supports an ethyl fragment, CH₃CH₂–, with the two groups coupled to one another. Matching coupling constants, if supplied, would strengthen that pairing. In this simplified task, the candidate structures and first-order assumption provide the intended context.

The remaining three-proton singlet is consistent with a methyl set without adjacent hydrogens producing resolved first-order splitting in this model. Its chemical shift and structural position still need to be established.

Both candidates contain an ethyl fragment and a three-proton singlet. Methyl propanoate places the ethyl fragment next to the carbonyl; ethyl ethanoate places it on the oxygen side. Splitting alone therefore leaves the same two alternatives open.

Use n + 1 only within its appropriate simple setting of coupling to n equivalent neighbouring spin-half protons. Several non-equivalent neighbour sets or higher-order behaviour can give different patterns. Avoid teaching the rule as a universal count for every hydrogen nearby in a drawing.

For an added supplied-data check, give the triplet and quartet the same coupling constant, 7 Hz. That agreement supports their assignment to the mutually coupled ethyl group. A matching value is supporting evidence within this simple model, rather than proof that any two multiplets with similar spacing must belong to one fragment.

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Supply dataset P: δ 1.10 ppm, triplet, area 0.60; δ 2.30 ppm, quartet, area 0.40; δ 3.70 ppm, singlet, area 0.60. These are rounded idealised positions for the assignment exercise.

Provide a suitable shift guide with the task: alkyl CH₃ commonly appears in a lower-shift region; hydrogens on carbon adjacent to a carbonyl are shifted downfield; hydrogens on carbon directly bonded to oxygen commonly appear farther downfield in these examples.

The two-proton quartet at 2.30 ppm fits CH₂ next to the ester carbonyl, coupled to the terminal CH₃. The three-proton singlet at 3.70 ppm fits OCH₃. Together these assignments support CH₃CH₂C(=O)OCH₃, methyl propanoate.

The 1.10 ppm triplet then accounts for the terminal CH₃ of the propanoate-side ethyl fragment. All eight hydrogens are assigned across the three environments, and the proposed connectivity explains their integration and splitting.

The table records the full dataset and the assignments. The conclusion is supported by the combined pattern within the supplied candidate set; it does not come from the 3:2:3 ratio alone or from treating a shift value as an exact universal fingerprint.

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Ethyl ethanoate has OCH₂CH₃, so its two-proton quartet belongs to a carbon directly bonded to oxygen. In a suitable supplied comparison, that quartet lies around 4.10 ppm rather than the 2.30 ppm position assigned to the carbonyl-adjacent CH₂ in dataset P.

Its three-proton singlet belongs to CH₃C(=O)– and is supplied around 2.00 ppm, rather than the 3.70 ppm position of OCH₃ in P. The alternative swaps the environments responsible for the distinctive singlet and quartet shifts.

Do not reject ethyl ethanoate because it has the wrong integration ratio or the wrong number of signals; those features are shared. State the actual mismatch between its proton environments and the supplied chemical shifts.

An assignment should explain all major data. Matching the low-shift triplet in both candidates does not settle the structure when the quartet and singlet require different local bonding. Keep the full comparison visible.

Chemical shifts depend on conditions and are approximate guides, so the question supplies a clearly separated teaching pattern. If actual peaks overlap or fall in ambiguous regions, preserve that uncertainty and ask for complementary evidence rather than force a unique answer.

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Give a fresh dataset Q with a three-proton triplet at 1.20 ppm, a three-proton singlet at 2.00 ppm and a two-proton quartet at 4.10 ppm. The integration ratio is again 3:3:2 in this increasing-shift order.

The oxygen-bonded CH₂ quartet and carbonyl-adjacent methyl singlet now support ethyl ethanoate. The student should change the assignment because the shifts have changed, even though the overall proton counts and ethyl splitting pattern remain familiar.

State that these two esters contain no O–H or N–H proton in their structures. A disappearing exchangeable-proton signal is therefore not an expected requirement for assigning the eight carbon-bound hydrogens in these examples.

For a separate supplied alcohol example, an O–H signal may vary in position, breadth and observed coupling, and may disappear from the proton spectrum after suitable deuterium exchange. That observation supports a labile proton under the stated conditions.

Do not force an O–H proton to obey the same reliable neighbouring-proton splitting pattern as a simple alkyl fragment. Exchange can alter the observation. Keep that separate example clearly labelled so that an ester assignment is not complicated by an invented hydroxyl group.

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If the student counts eight signals from a triplet, quartet and singlet, ask for the number of distinct chemical-shift groups before counting individual lines. The repair distinguishes environments from splitting within an environment.

If the quartet is assigned eight hydrogens because it has four lines and integrates to two, return to the total area. Its four line areas add to one two-proton signal. The line count does not multiply the integrated proton count.

If ethyl ethanoate is chosen from P because an ethyl fragment is present, ask where its OCH₂ signal should lie using the supplied guide. The fragment identification is useful; its attachment point still needs checking.

If the singlet is assigned as an OH proton despite integrating to three and the supplied ester candidates, ask the student to account for the molecular formula and each candidate’s functional groups. A singlet shape alone does not identify a hydroxyl group.

If an exact structure is claimed from integration alone, compare the two candidates’ identical count patterns. Ask which additional feature resolves them here. This repairs the strength of the inference without discarding the correct proton counting.

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Give methyl ethanoate, CH₃C(=O)OCH₃, with its formula C₃H₆O₂, as a fresh supplied structure. In the simple model it has two distinct three-proton methyl sets and two singlets, rather than an ethyl triplet–quartet pair.

Ask for a relative integration of 3:3, which simplifies to 1:1. The formula establishes that the two equal groups each represent three hydrogens. This reinforces the distinction between a relative area ratio and an absolute proton count.

Use the supplied shift guide to assign the carbonyl-adjacent methyl at a lower shift than the oxygen-bonded methyl in this comparison. The exact numerical positions need appropriate supplied or reference data; the structural reasoning can be checked first.

Then return to P without the worked table and ask for a complete assignment in a few sentences. The student should identify the ethyl fragment, place its CH₂ next to the carbonyl and assign the downfield singlet to OCH₃.

For a parent update, describe whether the student can combine counts, splitting and shifts and reject an alternative for the correct reason. Mention if equivalence or integration still needs prompting, so that the next spectroscopy task addresses a specific gap.

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Check formulae, structures, signal counts, total integrated hydrogens and all supplied shifts. Ensure the student copy includes the shift guide and assumptions used in the key. A missing reference range can turn a reasoning task into unstated recall.

Verify the splitting assumptions and avoid requiring advanced treatment of non-equivalent CH₂ hydrogens in a task explicitly framed as a simple achiral first-order example. More demanding spectra need appropriately expanded instructions and data.

Keep idealised teaching spectra distinct from reported experimental measurements. Actual solvent signals, impurities, overlap, exchange and resolution may complicate an assignment. Include those features when they serve a defined next learning purpose.

Choose repair when signal-versus-line counting is insecure, stabilise with the two ester datasets and extend through complementary IR, mass-spectral or stereochemical evidence when the basic assignment is independent.

Retain the initial choice, the corrected signal map and a fresh prediction from structure. Together they show whether the student can use spectroscopy as a connected argument about molecular structure, rather than as a list of isolated peak labels.

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