Organic Chemistry 1 · Molecular evidence and structure
Infrared evidence: practice problems
32 problems, each with its worked solution. Open one to read every step, or filter by tag.
Predict the product 7
- Predict a diagnostic regionRelate an explicit O–H bond to hydrogen-bonded IR absorption.
- C₄H₁₀O Isomers by IR: 1-Butanol vs Diethyl EtherChoose the IR band that separates an alcohol from an isomeric ether (1-butanol vs diethyl ether).
- Aldehyde C–H stretch pairUse the aldehyde C–H stretch pair to tell an aldehyde from a ketone.
- Tell isomers apart by IRChoose the IR feature that distinguishes two constitutional isomers.
- Ring size and C=O frequencyPredict how ring strain shifts a cyclic ketone's C=O stretch.
- Terminal alkyne evidenceIdentify the band that proves a terminal alkyne.
- Missing C=C Band: trans-3-Hexene vs 1-HexeneApply the dipole-change selection rule to predict a missing C=C band (trans-3-hexene vs 1-hexene).
Classify 2
- Classify a drawn oxygen compoundDistinguish ketone and alcohol evidence from explicit connectivity.
- Match bands to a structurePick the structure consistent with a set of IR bands.
Compare 6
- Assign a triple-bond regionConnect an explicit nitrile group to its expected spectral region.
- Compare two carbonyl environmentsExplain the qualitative effect of conjugation on carbonyl stretching.
- Rank C–H, C–Cl, C–Br and C–I Stretching FrequenciesRank C–H and carbon–halogen stretching frequencies with Hooke's law (reduced mass and bond strength).
- Rank C=O stretching frequenciesRank carbonyl stretching frequencies across acid derivatives.
- Rank C–H Stretches: Alkyne, Alkene and Alkane C–HRank sp, sp² and sp³ C–H stretching frequencies from hybridization (1-hexyne, 1-hexene, hexane).
- Ester vs Ketone C=O: Ethyl Acetate vs 2-ButanoneTell an ester from a ketone by C=O position and the C–O stretch (ethyl acetate vs 2-butanone).
Calculate 3
- Estimate an isotope shiftUse the reduced-mass model while holding bond stiffness fixed.
- Amine degree from N–H bandsCount N–H stretching bands to classify an amine.
- Hooke's law isotope shiftUse Hooke's law to predict a stretching frequency after deuteration.
Find the error 3
- Diagnose a changing band shapeDistinguish reduced hydrogen bonding from loss of a functional group.
- Evaluate a negative observationState the conditions needed to use absence of a band as exclusion evidence.
- Diagnose Incomplete Ethyl Benzoate Hydrolysis by IRDiagnose an incomplete ester hydrolysis from IR carbonyl bands (ethyl benzoate to benzoic acid).
Interpret the evidence 4
- Interpret overlapping O–H evidenceCombine carbonyl and broad O–H observations.
- Intramolecular H-Bond: 2-Nitrophenol vs 4-NitrophenolUse a dilution study to tell intramolecular from intermolecular hydrogen bonding (2-nitrophenol vs 4-nitrophenol).
- Two C=O Bands in Acetic AnhydrideExplain the paired anhydride C=O bands by vibrational coupling (acetic anhydride).
- Aromatic C–H and Ring Bands: Toluene vs MethylcyclohexaneUse the sp² C–H and ring C=C bands to tell an arene from a cycloalkane (toluene vs methylcyclohexane).
Work backward 1
- Identify Cyclohexanol From Its IR BandsIdentify an alcohol from present and absent IR bands (cyclohexanol vs ketone, allylic alcohol and acid).
Plan a synthesis 2
- Plan a connectivity testRecognize the limitations of IR for distinguishing carbonyl isomers.
- IR Sample Prep for a Solid: Benzoic AcidChoose a suitable IR sample preparation for a solid (benzoic acid).
Explain why 4
- Explain a weak triple-bond bandApply the dipole-change selection rule to a symmetric alkyne.
- Acid Chloride C=O Near 1800 cm⁻¹: Acetyl ChlorideExplain the high acid chloride C=O stretching frequency (acetyl chloride vs acetone).
- Acid Dimer vs Monomer C=O: Hexanoic AcidExplain how hydrogen-bonded dimers lower a carboxylic acid C=O frequency (hexanoic acid).
- Amide C=O Near 1650 cm⁻¹: N,N-DimethylacetamideExplain the low amide C=O stretching frequency by nitrogen resonance (N,N-dimethylacetamide vs acetone).
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