14.
A molecule has formula C4H8O2. Based on the following proton NMR spectrum, what is
the structure?
a. A
b. B
c. C
d. D

The correct structure is option A, the ester ethyl ethanoate (ethyl acetate).

Introduction

In many organic spectroscopy exams, a classic problem asks you to deduce the C4H8O2 proton NMR structure from a simple spectrum showing three signals with integrations 2H, 3H and 3H. This article walks through that exact style of question, explains why the answer is option A, and shows why options B, C and D are wrong using logical NMR analysis that is ideal for exam preparation and SEO‑friendly conceptual revision.

Step 1: Use formula and degree of unsaturation

The molecular formula C4H8O2 gives a degree of unsaturation of 1, meaning there is either one double bond (for example a carbonyl) or a ring. Esters and carboxylic acids with four carbons (like ethyl ethanoate or methyl propanoate) are common C4H8O2 isomers tested in NMR questions.

Step 2: Interpret the proton NMR data

The spectrum has three signals: one integrating to 2H and two integrating to 3H (total 8 protons), with approximate chemical shifts near 4 ppm (2H), 3 ppm (3H) and about 1–1.5 ppm (3H).

• A 2H signal near 4 ppm indicates a −CH2−−CH2− group attached to oxygen, such as −OCH2−−OCH2−.

• A 3H signal around 3–3.8 ppm suggests a methyl group bonded to oxygen, −OCH3−OCH3 or −COOCH3−COOCH3.

• A 3H signal near 1–1.5 ppm corresponds to a simple alkyl methyl group, −CH3−CH3 attached to carbon.

Because there are only three signals, the molecule contains three sets of equivalent protons, consistent with an ester that has two different methyl groups and one methylene group.

Option A: Why ethyl ethanoate fits

Option A shows an ester with structure CH3COOCH2CH3CH3COOCH2CH3 (ethyl ethanoate / ethyl acetate).

• The CH3CO−CH3CO− group gives a singlet of 3H around 2 ppm because its carbonyl neighbors deshield the protons and there are no adjacent hydrogens on the carbonyl carbon.

• The −OCH2−−OCH2− group gives 2H near 4 ppm, usually as a quartet because it is adjacent to a CH3CH3 group.

• The terminal −CH3−CH3 next to the CH2CH2 gives 3H near 1–1.3 ppm, normally as a triplet.

The problem’s simplified stick spectrum shows only positions and integrations (2H, 3H, 3H) without the detailed multiplet shape, and those numbers and shifts match ethyl ethanoate perfectly. Therefore option A is correct.

Option B: Why the carboxylic acid is wrong

Option B is a straight‑chain carboxylic acid (butanoic or propanoic acid‑type structure). Carboxylic acids of formula C4H8O2 contain an $-OH$ proton that appears as a broad singlet between 10–12 ppm.

• Such a structure would produce four distinct proton environments: acidic $-OH$, α−CH2α−CH2 next to COOH, internal CH2CH2 and terminal CH3CH3.

• The spectrum in the question shows only three signals and no downfield acidic proton, so a simple carboxylic acid like option B cannot match.

Thus, option B is inconsistent with both the number of signals and the absence of an $-OH$ resonance.

Option C: Why the cyclic ester is wrong

Option C appears to be a five‑membered cyclic ester (lactone) with a methyl substituent. Lactones with formula C4H8O2 would contain a ring with at least two nonequivalent methylene groups plus a methyl, giving more than three distinct proton environments.

• The ring methylenes adjacent to oxygen and carbonyl would appear as separate signals between about 2–4.5 ppm.

• Together with the methyl group, that would produce at least four separate signals, not three.

Because the question’s NMR shows exactly three signals, option C does not fit the integration pattern.

Option D: Why the dialkoxy species is wrong

Option D is an acetal‑like structure with two different alkoxy groups on the same carbonyl‑derived carbon (a di‑ether). Such an acetal would not contain a carbonyl; instead it would have a saturated carbon bound to two oxygens and two alkyl groups, giving a very different shift pattern.

• Protons on carbons next to two oxygens resonate strongly downfield and give multiple distinct environments for each alkyl fragment.

• A typical acetal of this type would show more than three signals because each alkoxy group has its own CH3CH3/CH2CH2 pattern with different chemical shifts.

Since the observed spectrum is simple (three signals total) and strongly suggests a single carbonyl (ester), option D cannot be correct.

Key takeaways for NMR exam questions

• For C4H8O2 proton NMR structure questions, quickly compute unsaturation and think “ester vs acid vs lactone vs acetal.”

• Three signals with integrations 3H, 2H, 3H and shifts near 2, 4 and 1 ppm are characteristic of an ethyl ester such as ethyl ethanoate, which corresponds to option A in this problem.