How can you distinguish butanoic acid from ethyl ethanoate when both have molecular formula C₄H₈O₂ and nominal molecular mass 88? The quickest useful evidence is their functional groups. Butanoic acid contains a carboxylic acid group, –COOH, and usually exhibits a very broad carboxylic acid O–H infrared absorption over roughly 2500–3300 cm⁻¹, along with a carbonyl band often around 1710 cm⁻¹ in associated condensed-phase samples. Ethyl ethanoate is an ester, –COO–, with a characteristic carbonyl commonly near 1735 cm⁻¹ plus C–O bands; it does not contain an acid O–H bond.
Their proton NMR patterns offer another strong distinction. Butanoic acid, CH₃CH₂CH₂COOH, has a terminal 3H triplet, two distinct 2H methylene environments, and an exchangeable acidic OH proton commonly far downfield. Ethyl ethanoate, CH₃COOCH₂CH₃, has a 3H acetyl singlet near δ 2.0 ppm, an oxygen-bound 2H quartet near δ 4.1 ppm, and an ethyl 3H triplet near δ 1.2 ppm. The two structures can be separated by the combined logic of functional-group IR evidence and local hydrogen environments.
For JC families looking for an H3 Chemistry tutor in Hougang, this example puts the 2027 H3 Chemistry 9813 syllabus into practice. The prescribed skills include interpreting characteristic IR absorptions, chemical shifts, NMR integration and first-order splitting, and drawing structural inferences from evidence. These two isomers are used as an original integrative exercise; their names are not a special SEAB memorisation list.
- Shared formula: C₄H₈O₂, nominal molecular mass 88, DBE 1.
- Butanoic acid IR: very broad acid O–H feature around 2500–3300 cm⁻¹ plus carboxyl C=O commonly near 1710 cm⁻¹ in associated samples.
- Ethyl ethanoate IR: ester C=O often near 1735 cm⁻¹ and strong C–O-related absorptions, without a carboxylic acid OH band.
- Butanoic acid ¹H NMR: terminal 3H triplet, distinct 2H+2H methylenes and a variable downfield acid OH environment.
- Ethyl ethanoate ¹H NMR: 3H singlet near δ 2.0, 2H quartet near δ 4.1 and 3H triplet near δ 1.2.
- Do not rely on one number: carbonyl frequencies and acidic OH behaviour depend on structure and measurement conditions.
- Best conclusion: choose the candidate that satisfies formula, IR functional group and every meaningful NMR integration and splitting feature.
Official SEAB H3 Chemistry 9813 syllabus (2027) · OpenStax carboxylic-acid IR and NMR · OpenStax ester and acid carbonyl IR comparison · Hougang JC Chemistry learning hub.
Contents: Start with formula · Write both structures · Acid IR · Ester IR · Butanoic acid NMR · Ethyl ethanoate NMR · Put the evidence together · What can mislead · Worked unknown cases · Questions with answers · How to study · FAQs.
Step 1: Molecular Formula and DBE Cannot Decide the Isomer
The formula C₄H₈O₂ contains four carbons, eight hydrogens and two oxygens. Its nominal molecular mass is 4×12 + 8×1 + 2×16 = 48+8+32 = 88. This is consistent with both butanoic acid and ethyl ethanoate. A molecular-ion peak near m/z 88, if identifiable under a suitable ionisation method, cannot by itself distinguish them.
The conventional double-bond equivalent for an ordinary neutral C/H/O formula is DBE = (2C+2−H)/2. Here DBE = (8+2−8)/2 = 1. Both candidates contain one carbonyl C=O double bond, which accounts for the unsaturation. The formula therefore fits both an acid and an ester, and it can fit other structural possibilities as well.
A small carbon-13-associated M+1 molecular-ion satellite might suggest approximately four carbons from the familiar 4 × 1.1% ≈ 4.4% natural-abundance estimate, under the simplified conditions. But since both candidates contain four carbons, that clue also does not identify the isomer.
So a student should leave this first step with a disciplined conclusion: the atom counts are consistent, but the connectivity is unresolved. We now need evidence that responds to the way the atoms are bonded rather than simply counting them.
Step 2: Draw Both Functional Groups Accurately
Butanoic acid
Butanoic acid is CH₃–CH₂–CH₂–C(=O)–OH. Its carbonyl carbon is bonded to an –OH oxygen and to the alkyl chain. That makes it a carboxylic acid. The molecule contains three distinct carbon-bound proton groups: terminal CH₃, a middle CH₂ and the CH₂ adjacent to the carbonyl carbon; a fourth proton environment belongs to the acidic O–H group.
The distinction between CH₂ adjacent to a carbonyl and O–CH₂ is crucial. The butanoic acid CH₂ closest to the carboxyl group is bonded to the carbonyl carbon, not directly to oxygen. Its hydrogens therefore do not occupy the same chemical environment as the oxygen-bound ethyl group in an ester.
Ethyl ethanoate
Ethyl ethanoate is CH₃–C(=O)–O–CH₂–CH₃, sometimes called ethyl acetate. Its carbonyl carbon is bonded to an oxygen that is itself bonded to an ethyl group. That gives the ester linkage, –C(=O)–O–. It contains no acid O–H hydrogen.
Its three proton environments are the acetyl CH₃ attached to the carbonyl carbon, the oxygen-bound CH₂ and the terminal CH₃ of the ethyl group. Count three, two and three hydrogens respectively: 3+2+3 = 8, exactly the molecular formula’s total.
Both formulas contain two oxygen atoms, but only the acid contains one bonded to hydrogen. Only the ester has a C–O–C linkage. That connectivity distinction underlies the IR and NMR differences.
Step 3: Predict Butanoic Acid’s IR Spectrum
A carboxylic acid contains both a carbonyl C=O stretch and an O–H bond. When acid molecules hydrogen-bond strongly, as often happens in ordinary condensed-phase samples, the O–H stretching feature can be very broad over roughly 2500–3300 cm⁻¹. It can overlap part of the normal C–H stretching region, creating a recognisably wide envelope instead of a single narrow peak.
The carboxylic acid C=O feature in an associated, hydrogen-bonded sample is often near 1710 cm⁻¹, while a less associated acid in a different environment can have a higher carbonyl frequency. OpenStax notes the difference between free and hydrogen-bonded acid environments; that is why writing ‘all acids are exactly 1710’ would be an overstatement.
For butanoic acid, the broad OH feature is particularly diagnostic when combined with the carbonyl. The same candidate may show C–O-related fingerprint features, but one does not need to identify every fingerprint line to recognise a chemically plausible acid pattern in an appropriately supplied examination dataset.
Why the OH absorption is so wide
Hydrogen-bonded acid molecules do not all have identical instantaneous bond environments, and hydrogen-bond formation changes the OH stretching energy. The resulting range of environments and interactions produces a broad absorption rather than the sharp isolated OH peak drawn in some simplified sketches.
The important examination move is to link the broad band to hydrogen bonding and to explain why it supports a carboxylic acid group. A broad OH region alone can be affected by sample conditions; its coexistence with a carboxyl C=O band and the molecular formula strengthens the inference.
Step 4: Predict Ethyl Ethanoate’s IR Spectrum
A simple saturated ester often gives a strong carbonyl stretch near 1735 cm⁻¹. Ethyl ethanoate is a standard example of this functional-group region. It also has strong C–O-related absorptions commonly in parts of the 1300–1000 cm⁻¹ fingerprint region, reflecting the ester bonding environment.
Unlike butanoic acid, ethyl ethanoate contains no O–H group. A pure suitable sample should therefore not display the characteristic broad carboxylic-acid OH absorption spanning roughly 2500–3300 cm⁻¹. If a sample shows such a feature, contamination, water, an acid component or another OH-containing compound should be considered rather than changing the ester formula.
The carbonyl frequencies of acid and ester can be different, but they are not mathematical fingerprints. Solvent, conjugation, phase and other environmental factors can shift them. In an H3 answer, I would first look for the broad acid OH region and the full functional-group combination rather than declare an ester merely because one band appears a little above 1720.
The primary IR comparison
- Butanoic acid: carbonyl plus very broad acid O–H.
- Ethyl ethanoate: ester carbonyl plus C–O fingerprint bands, but no carboxylic O–H.
- The two carbonyl frequencies can provide additional support, but their ranges may vary and overlap.
- Infrared evidence should be checked against molecular formula and proton NMR, especially if the unknown is impure or the measurement conditions are unusual.
Step 5: Predict the Proton NMR of Butanoic Acid
For CH₃CH₂CH₂COOH, number the alkyl chain from the carbonyl end if that helps. The CH₂ nearest C=O is influenced by the electron-withdrawing carbonyl environment; the middle CH₂ is a more ordinary alkyl environment; the terminal methyl is further away from the acid group.
Terminal CH₃: a 3H triplet
The terminal CH₃ has two neighbouring middle-CH₂ protons, so under first-order coupling it often produces a 3H triplet. A representative signal is around δ 0.9 ppm. This alone is not a unique acid clue, because an ethyl group also commonly gives a methyl triplet.
Middle CH₂: a 2H multiplet, sometimes sextet-like
The middle CH₂ lies between a CH₃ group containing three hydrogens and a CH₂ adjacent to the carbonyl containing two hydrogens. It may display a multiplet, sometimes approximately sextet-like when the relevant coupling constants are sufficiently similar. The integrals of this environment total 2H; an illustrative shift can be near δ 1.6 ppm.
Because the two neighbour groups are chemically distinct, a rigid assertion of exactly six perfectly resolved lines is not always justified. In a measured spectrum the middle CH₂ may show a more complex multiplet. The student should separate the robust two-hydrogen integration from the details of splitting.
CH₂ next to carbonyl: a 2H triplet
The CH₂ directly adjacent to the carboxyl carbon is influenced by the carbonyl group and often appears near δ 2.2–2.4 ppm. It is split principally by two adjacent middle-CH₂ hydrogens under a simple first-order model, giving a 2H triplet.
The critical contrast with ethyl ethanoate is that butanoic acid does not have an oxygen-bound O–CH₂ fragment whose protons would be expected around 4 ppm. The acid’s carbonyl-adjacent CH₂ is adjacent to C=O through a carbon–carbon bond, not directly to an ester oxygen.
The carboxylic O–H: a variable downfield 1H resonance
A carboxylic acid proton is often observed far downfield, commonly roughly around δ 10–12 ppm in an ordinary teaching description, and may appear broad because of hydrogen bonding and proton exchange. The exact chemical shift can vary, and in some spectra the proton may be difficult to observe or integrate reliably.
Thus the idealised environment count is four with integrations 3:2:2:1. Its exchangeable OH feature can be diagnostically helpful but should not be treated as a guaranteed sharply resolved one-proton singlet in every measured sample.
Step 6: Predict Ethyl Ethanoate’s Proton NMR
Ethyl ethanoate is CH₃COOCH₂CH₃. Its three chemically different proton groups are easiest to handle one at a time. Two belong to the ethyl fragment bonded through oxygen, and the third is the acetyl methyl group attached to the carbonyl carbon.
The acetyl CH₃: a 3H singlet near δ 2.0 ppm
The methyl group directly bonded to the carbonyl carbon is deshielded relative to an ordinary terminal alkyl methyl and often appears around 2.0 ppm. The adjacent carbonyl carbon has no attached hydrogen, so under the simple first-order model the acetyl CH₃ produces a singlet integrating to 3H.
The O–CH₂: a 2H quartet near δ 4.1 ppm
The ethyl methylene is directly attached to oxygen in the ester linkage. Its protons are relatively downfield, often near δ 4.1 ppm. The neighbouring CH₃ group has three equivalent protons, yielding a 2H quartet under first-order coupling.
This O–CH₂ quartet is one of the most useful diagnostic features for ethyl ethanoate when comparing it against butanoic acid. It gives direct evidence for an oxygen-bound ethyl group, which the acid structure lacks.
Terminal ethyl CH₃: a 3H triplet near δ 1.2 ppm
The terminal CH₃ is coupled to the two O–CH₂ hydrogens and thus often forms a 3H triplet, roughly near δ 1.2 ppm. The triplet and the O–CH₂ quartet share a vicinal coupling relationship in the ordinary first-order model. These two signals together support the ethyl fragment.
The expected integrated proton pattern is therefore 3:2:3 across the three environments, summing to eight. There is no acidic O–H proton in ethyl ethanoate, so the 8H total is entirely carbon-bound.
Step 7: One Table of Evidence—Two Very Different Interpretations
- Molecular formula: C₄H₈O₂ for both.
- Nominal molecular ion: near 88 for both when observed as a suitable singly charged molecular ion.
- DBE: one for both, accounted for by carbonyl C=O.
- Butanoic acid IR: very broad acidic OH absorption plus carboxyl C=O.
- Ethyl ethanoate IR: ester C=O and C–O features, but no inherent carboxylic-acid OH band.
- Butanoic acid NMR: 3H triplet, two distinct 2H methylenes and often a far-downfield exchangeable acid proton.
- Ethyl ethanoate NMR: 3H acetyl singlet near 2.0, 2H O–CH₂ quartet near 4.1 and 3H terminal triplet near 1.2.
- Decisive distinction: functional-group OH evidence and the oxygen-bound ethyl quartet versus the acid’s carbonyl-adjacent methylene and acid proton.
The correct order is to use the most discriminating clues first. A broad acid O–H absorption combined with a carbonyl and a far-downfield exchangeable proton supports the acid. A carbonyl with an O–CH₂ quartet near δ 4.1 and a separate acetyl methyl singlet supports the ester.
Do not overinterpret small numerical differences between reference carbonyl shifts if the functional-group and NMR evidence is strong. Both absorption positions and proton chemical shifts can vary slightly by sample preparation and molecular environment. The student should seek consistency across measurements rather than searching for exact number matches.
What Can Mislead a Student in the Laboratory or Examination?
A carboxylic acid OH signal may be broad or hard to integrate
Acidic hydrogens can exchange and experience changing hydrogen-bond environments. A poorly resolved OH peak does not disprove a carboxylic acid if the rest of the spectrum and IR support it. Conversely, a small unexpected resonance at very high ppm should not automatically be assigned to acid without verifying the sample and measurement.
Water and solvent contamination
An ester sample contaminated with water or other OH-containing material can display extra absorption or NMR features. A pure compound’s theoretical pattern and a real mixture’s measured spectrum are not the same. Before declaring a new functional group, check purity and sample background.
Carbonyl frequency variation
A carboxylic acid may have different C=O stretching frequencies depending on whether it is relatively free or hydrogen-bonded. Esters also vary with conjugation and substitution. It is unsafe to identify either solely because a band is precisely 1710 or 1735 cm⁻¹.
A middle methylene with complicated splitting
The butanoic acid middle CH₂ can couple to two different adjacent groups. Its observed line pattern depends on the coupling constants and overlap. Use the 2H integration and local connectivity; do not insist on a perfect sextet when the data show a complex multiplet.
A missing molecular ion in mass spectrometry
Some ionisation methods produce extensive fragmentation, and the molecular ion may be weak. The absence of a strong m/z 88 peak does not automatically invalidate the formula; the ionisation context and available fragment evidence matter. A base peak is simply the most intense observed ion and need not be the intact molecule.
Relying on smell rather than analytical evidence
Butanoic acid and ethyl ethanoate have different characteristic sensory associations, but students should not sniff unknown laboratory chemicals as a structure-identification method. Safe chemical identification uses controlled, appropriate analytical procedures and the laboratory’s safety rules.
Four Fully Worked Unknown-Compound Scenarios
Unknown A: Broad 2500–3300 cm⁻¹ IR feature
A fictional pure sample has established formula C₄H₈O₂ and a strong carbonyl feature near 1710 cm⁻¹ together with a very broad absorption spanning much of 2500–3300 cm⁻¹. Its proton NMR contains an exchangeable far-downfield signal and two different methylene environments. Which candidate is supported?
Answer: Butanoic acid. The broad IR O–H and carbonyl combination supports a carboxylic acid, and the carbon-bound NMR pattern matches a three-carbon alkyl chain next to COOH.
Unknown B: The O–CH₂ quartet
Another C₄H₈O₂ sample has a strong ester-like carbonyl absorption and no broad acid OH region. Its NMR shows approximate shifts of δ 1.2 (triplet, 3H), δ 2.0 (singlet, 3H) and δ 4.1 (quartet, 2H). Identify the candidate.
Answer: Ethyl ethanoate. The 2H quartet near 4.1 ppm supports O–CH₂ coupled to a terminal CH₃ triplet. The acetyl CH₃ gives the 3H singlet near 2.0 ppm, and the absence of a carboxylic OH region agrees with an ester.
Unknown C: Same mass, no IR
A third substance gives only a plausible molecular ion at nominal m/z 88 and an M+1 signal consistent with four carbon atoms. Is there enough information to choose between butanoic acid and ethyl ethanoate?
Answer: No. Both share the same formula, nominal mass and carbon count. The evidence does not establish connectivity. Obtain suitable IR and proton NMR information.
Unknown D: The acid proton is not resolved
A fourth sample lacks a clearly visible far-downfield acid proton in its NMR because of exchange or measurement conditions, but has a strong carbonyl and a very broad acid-type OH IR envelope. Its carbon-bound proton signals are consistent with CH₃–CH₂–CH₂–CO–. What should the student conclude?
Answer: The evidence can still support butanoic acid despite a poorly resolved acid proton. The OH feature and carbon-bound connectivity should be evaluated together. If sample purity or measurement quality is uncertain, acknowledge the limitation.
Twelve Original Practice Questions With Answers
Question 1: Formula
Calculate the nominal molecular mass of C₄H₈O₂.
Answer: 88.
Question 2: DBE
What is the DBE of C₄H₈O₂?
Answer: (8+2−8)/2 = 1.
Question 3: Acid IR
Which broad IR feature commonly supports a carboxylic acid?
Answer: The wide O–H stretching region roughly 2500–3300 cm⁻¹, considered with an appropriate carbonyl band.
Question 4: Ester IR
Which carbonyl position is a useful reference for a simple saturated ester such as ethyl ethanoate?
Answer: Around 1735 cm⁻¹, with structural and experimental variation.
Question 5: 3H singlet
What group in ethyl ethanoate contributes the 3H singlet around δ 2.0?
Answer: The acetyl methyl group CH₃–C(=O)–.
Question 6: Oxygen-bound quartet
What group contributes the 2H quartet around δ 4.1 in ethyl ethanoate?
Answer: The O–CH₂ part of the –O–CH₂CH₃ ester ethyl group.
Question 7: Butanoic acid middle CH₂
Why might the middle methylene group have a multiplet rather than a neat sextet?
Answer: It couples to two chemically different neighbouring groups whose coupling constants need not be identical.
Question 8: Acid OH NMR
What type of proton often appears far downfield around δ 10–12 in a carboxylic acid?
Answer: The acidic O–H proton, subject to hydrogen-bonding and exchange variation.
Question 9: Two 2H groups
Which candidate has two different carbon-bound methylene environments?
Answer: Butanoic acid.
Question 10: No acid OH
Which candidate has no intrinsic O–H hydrogen in its pure structure?
Answer: Ethyl ethanoate.
Question 11: Molecular ion ambiguity
Does m/z 88 uniquely identify the ester rather than the acid?
Answer: No. Both share molecular formula C₄H₈O₂ and nominal mass 88.
Question 12: Contamination
An apparent ester sample shows a small broad OH absorption. Is it definitely butanoic acid?
Answer: No. Water, impurity or sample conditions can contribute. Use the full evidence and suitable controls.
Question 13: Compare two ester isomers
Can the same C₄H₈O₂ formula also describe methyl propanoate?
Answer: Yes. It is a different ester constitutional isomer, with different ¹H NMR connectivity. Formula and functional-group class still do not determine one structure.
Question 14: Scientific confidence
What makes a more reliable conclusion: a lone carbonyl wavenumber or a consistent formula, IR and NMR assignment?
Answer: The consistent combined assignment. Independent evidence removes alternatives and permits better-supported inference.
How I Would Teach an Acid–Ester Unknown in a Three-Student H3 Group
First, I give the two structural formulas without their names. Each learner counts the hydrogen environments and predicts broad IR functional-group features. We do not consult a reference table until the structure has supplied a prediction. That is how a student learns that spectroscopy responds to bonds and environments rather than to the name written above a diagram.
Second, I provide an IR spectrum showing a broad 2500–3300-cm⁻¹ feature. The student should explain why that supports an acid rather than simply say ‘the first answer is butanoic acid’. I then remove the broad feature and add a resolved 2H quartet at δ 4.1: the logic should switch toward an ester with an oxygen-bound ethyl group.
Third, I deliberately make the NMR imperfect by obscuring the acid OH resonance. A student who has learned to use complementary evidence should still be able to reason from the other protons and the IR spectrum. This separates actual understanding from dependence on one conveniently labelled peak.
Finally, I introduce the third candidate methyl propanoate with the same formula and ester functionality. The student must distinguish the two esters using chemical shifts and proton splitting, not only the broad absence of acid OH. That exercise connects this lesson to ethyl ethanoate versus methyl propanoate without duplicating that separate structure-elucidation topic.
The eduKate JC2 H3 Chemistry small-group guide outlines a tutorial model focused on observing and correcting each learner’s reasoning. Families searching from Hougang should confirm H3 specialist class availability, current timetable and the teaching venue near Sixth Avenue MRT. The Hougang keyword does not imply a separate classroom in the neighbourhood.
Why Combining IR and NMR Matters After JC
Carboxylic acids and esters appear widely in organic synthesis, chemical manufacturing, laboratory solvents, flavours, materials and pharmaceutical intermediates. Knowing whether a product contains an acid or ester linkage can be essential to understanding what reaction occurred and whether a substance meets a specification.
Professional chemists often combine IR, NMR, mass spectrometry and chromatography in structural analysis. A single measurement rarely gives every fact required about a complex unknown. The JC exercise previews that professional practice: predict a spectrum from a molecular hypothesis, compare it with observation, and change the hypothesis when the evidence does not fit.
Butanoic Acid vs Ethyl Ethanoate FAQs
Do they have the same molecular formula?
Yes. Both are C₄H₈O₂ constitutional isomers.
What IR feature is most useful for butanoic acid?
A very broad carboxylic acid O–H stretching absorption around 2500–3300 cm⁻¹, alongside a suitable carbonyl band.
Does ethyl ethanoate contain OH?
No. It is an ester, CH₃COOCH₂CH₃, without a carboxylic O–H bond.
Which has a 2H quartet near δ 4.1?
Ethyl ethanoate, from the oxygen-bound CH₂ of its ethyl group.
Which has two distinct 2H methylene environments?
Butanoic acid, CH₃CH₂CH₂COOH.
What is a typical acid proton NMR region?
Often far downfield, roughly δ 10–12, but the resonance can be broad or difficult to observe because of exchange and hydrogen bonding.
Can mass spectrometry alone distinguish them?
Their shared nominal molecular mass does not. Fragmentation may differ, but an appropriately interpreted full spectrum and complementary data are required for structural confidence.
Are these named compulsory compounds in H3 9813?
They are illustrative structures applying the 2027 syllabus requirements for IR functional-group identification and proton NMR analysis, not a separate named memorisation list.
What matters is not remembering the name of a four-carbon acid or ester. It is recognising that the same eight hydrogens and two oxygens can be connected in different ways—and that different bonds generate different infrared and proton NMR evidence. Once the student can reconstruct that argument, unfamiliar spectra become problems they can solve rather than puzzles they must guess.
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