Organic Chemistry 1 · Molecular evidence and structure
Molecular formulas, mass and carbon NMR: practice problems
28 problems, each with its worked solution. Open one to read every step, or filter by tag.
Predict the product 3
- Count carbon environments by symmetryDistinguish carbon atom count from carbon resonance count.
- α-Cleavage of butan-2-onePredict the acylium fragment from α-cleavage of a ketone.
- McLafferty ion of hexan-2-onePredict the even-mass McLafferty fragment of a methyl ketone with a γ-hydrogen.
Classify 4
- Infer an isotope contributorDistinguish one bromine from one chlorine using an idealized isotope pair.
- A carbon signal at δ 208Assign a ¹³C chemical shift to its carbon type.
- Match DEPT data to a C₄H₁₀O alcoholUse DEPT multiplicities to choose among C₄H₁₀O alcohols.
- Odd molecular ion at m/z 59Apply the nitrogen rule to an odd molecular ion.
Compare 2
- Match a multiplicity-edited spectrumCombine a complete carbon spectrum with DEPT-135 carbon types.
- Rank mass-spec fragment cations: tropylium to ethylRank EI fragment cations by stability using aromaticity and alkyl substitution.
Calculate 8
- Count unsaturation with nitrogen and halogenApply the DBE formula including N and a halogen.
- Separate nominal-mass formulasUse given accurate-mass sums to distinguish isobaric formulas.
- Carbon signals of ethylbenzeneCount ¹³C environments using ring symmetry.
- Exact mass of C₃H₈ (nominal mass 44)Calculate a monoisotopic exact mass from atomic masses.
- HRMS ppm error for benzaldehyde, C₇H₆OCalculate an HRMS mass error in parts per million.
- Carbons from the M+1 peakEstimate the carbon count from the ¹³C isotope (M+1) peak.
- McLafferty ion m/z of butanalCalculate the m/z of an aldehyde's McLafferty ion from the neutral alkene lost.
- Neutral lost from propylbenzene to give m/z 91Calculate the radical lost when an alkylbenzene forms the tropylium ion.
Find the error 3
- Recover neutral nominal massAccount for the observed ion type before assigning molecular mass.
- Diagnose a misleading carbon integralUse molecular symmetry rather than routine carbon peak areas.
- No McLafferty peak: find the ketone without a γ-HFind the ketone with no γ-hydrogen and so no McLafferty peak.
Interpret the evidence 2
- Interpret a one-degree budgetRecognize that a formula does not distinguish a ring from a double bond.
- Assign a formula to an exact mass of 72.0575Choose the molecular formula that matches a high-resolution exact mass.
Work backward 2
- DEPT puzzle: identify a C₄H₈O₂ esterUse DEPT-135 and chemical shifts to tell C₄H₈O₂ ester isomers apart.
- DEPT puzzle: a C₅H₁₀O ketone with three carbon signalsUse signal count and DEPT-135 to identify a symmetric C₅H₁₀O ketone.
Plan a synthesis 2
- Plan a molecular-mass checkAvoid assigning the base peak as molecular mass without ion evidence.
- Plan a DEPT test: butanal vs 2-methylpropanalChoose the single DEPT observation that distinguishes two C₄H₈O aldehydes.
Explain why 2
- Build a multi-isotope patternCombine isotope probabilities for two equivalent chlorine contributors.
- Tropylium ion at m/z 91 from ethylbenzeneExplain the m/z 91 base peak of an alkylbenzene through the tropylium ion.
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